Pathogen-induced proapoptotic phenotype and high CD95 (Fas) expression accompany a suboptimal CD8+ T-cell response: reversal by adenoviral vaccine

Immunopathogenesis in Trypanosoma cruzi infection: a role for suppressed macrophages and apoptotic cells

During Trypanosoma cruzi infection, macrophages phagocytose parasites and remove apoptotic cells through efferocytosis. While macrophage 1 (M1) produces proinflammatory cytokines and NO and fights infection, M2 macrophages are permissive host cells that express arginase 1 and play a role in tissue repair. The regulation of M1 and M2 phenotypes might either induce or impair macrophage-mediated immunity towards parasite control or persistence in chronic Chagas disease. Here, we highlight a key role of macrophage activation in early immune responses to T. cruzi that prevent escalating parasitemia, heart parasitism, and mortality during acute infection. We will discuss the mechanisms of macrophage activation and deactivation, such as T cell cytokines and efferocytosis, and how to improve macrophage-mediated immunity to prevent parasite persistence, inflammation, and the development of chagasic cardiomyopathy. Potential vaccines or therapy must enhance early T cell-macrophage crosstalk and parasite control to restrain the pathogenic outcomes of parasite-induced inflammation in the heart.

Transcriptional and Clonal Characterization of Cytotoxic T Cells in Crescentic Glomerulonephritis

SIGNIFICANCE STATEMENT

T-cell infiltration is a hallmark of crescentic GN (cGN), often caused by ANCA-associated vasculitis. Pathogenic T-cell subsets, their clonality, and downstream effector mechanisms leading to kidney injury remain to be fully elucidated. Single-cell RNA sequencing and T-cell receptor sequencing revealed activated, clonally expanded cytotoxic CD4 + and CD8 + T cells in kidneys from patients with ANCA-associated cGN. In experimental cGN, kidney-infiltrating CD8 + T cells expressed the cytotoxic molecule, granzyme B (GzmB), which induced apoptosis in renal tissue cells by activation of procaspase-3, and aggravated disease pathology. These findings describe a pathogenic function of (clonally expanded) cytotoxic T cells in cGN and identify GzmB as a mediator and potential therapeutic target in immune-mediated kidney disease.

BACKGROUND

Crescentic GN (cGN) is an aggressive form of immune-mediated kidney disease that is an important cause of end stage renal failure. Antineutrophilic cytoplasmic antibody (ANCA)-associated vasculitis is a common cause. T cells infiltrate the kidney in cGN, but their precise role in autoimmunity is not known.

METHODS

Combined single-cell RNA sequencing and single-cell T-cell receptor sequencing were conducted on CD3 + T cells isolated from renal biopsies and blood of patients with ANCA-associated cGN and from kidneys of mice with experimental cGN. Functional and histopathological analyses were performed with Cd8a-/- and GzmB-/- mice.

RESULTS

Single-cell analyses identified activated, clonally expanded CD8 + and CD4 + T cells with a cytotoxic gene expression profile in the kidneys of patients with ANCA-associated cGN. Clonally expanded CD8 + T cells expressed the cytotoxic molecule, granzyme B (GzmB), in the mouse model of cGN. Deficiency of CD8 + T cells or GzmB ameliorated the course of cGN. CD8 + T cells promoted macrophage infiltration and GzmB activated procaspase-3 in renal tissue cells, thereby increasing kidney injury.

CONCLUSIONS

Clonally expanded cytotoxic T cells have a pathogenic function in immune-mediated kidney disease.

ASP-2/Trans-sialidase chimeric protein induces robust protective immunity in experimental models of Chagas' disease

Immunization with the Amastigote Surface Protein-2 (ASP-2) and Trans-sialidase (TS) antigens either in the form of recombinant protein, encoded in plasmids or human adenovirus 5 (hAd5) confers robust protection against various lineages of Trypanosoma cruzi. Herein we generated a chimeric protein containing the most immunogenic regions for T and B cells from TS and ASP-2 (TRASP) and evaluated its immunogenicity in comparison with our standard protocol of heterologous prime-boost using plasmids and hAd5. Mice immunized with TRASP protein associated to Poly-ICLC (Hiltonol) were highly resistant to challenge with T. cruzi, showing a large decrease in tissue parasitism, parasitemia and no lethality. This protection lasted for at least 3 months after the last boost of immunization, being equivalent to the protection induced by DNA/hAd5 protocol. TRASP induced high levels of T. cruzi-specific antibodies and IFNγ-producing T cells and protection was primarily mediated by CD8⁺ T cells and IFN-γ. We also evaluated the toxicity, immunogenicity, and efficacy of TRASP and DNA/hAd5 formulations in dogs. Mild collateral effects were detected at the site of vaccine inoculation. While the chimeric protein associated with Poly-ICLC induced high levels of antibodies and CD4⁺ T cell responses, the DNA/hAd5 induced no antibodies, but a strong CD8⁺ T cell response. Immunization with either vaccine protected dogs against challenge with T. cruzi. Despite the similar efficacy, we conclude that moving ahead with TRASP together with Hiltonol is advantageous over the DNA/hAd5 vaccine due to pre-existing immunity to the adenovirus vector, as well as the cost-benefit for development and large-scale production.

Recent Advances in the Development of Adenovirus-Vectored Vaccines for Parasitic Infections

Vaccines against parasites have lagged centuries behind those against viral and bacterial infections, despite the devastating morbidity and widespread effects of parasitic diseases across the globe. One of the greatest hurdles to parasite vaccine development has been the lack of vaccine strategies able to elicit the complex and multifaceted immune responses needed to abrogate parasitic persistence. Viral vectors, especially adenovirus (AdV) vectors, have emerged as a potential solution for complex disease targets, including HIV, tuberculosis, and parasitic diseases, to name a few. AdVs are highly immunogenic and are uniquely able to drive CD8+ T cell responses, which are known to be correlates of immunity in infections with most protozoan and some helminthic parasites. This review presents recent developments in AdV-vectored vaccines targeting five major human parasitic diseases: malaria, Chagas disease, schistosomiasis, leishmaniasis, and toxoplasmosis. Many AdV-vectored vaccines have been developed for these diseases, utilizing a wide variety of vectors, antigens, and modes of delivery. AdV-vectored vaccines are a promising approach for the historically challenging target of human parasitic diseases.

Axl receptor induces efferocytosis, dampens M1 macrophage responses and promotes heart pathology in Trypanosoma cruzi infection

Adaptive immunity controls Trypanosoma cruzi infection, but the protozoan parasite persists and causes Chagas disease. T cells undergo apoptosis, and the efferocytosis of apoptotic cells might suppress macrophages and exacerbate parasite infection. Nonetheless, the receptors involved in the efferocytosis of apoptotic lymphocytes during infection remain unknow. Macrophages phagocytose apoptotic cells by using the TAM (Tyro3, Axl, Mer) family of receptors. To address how the efferocytosis of apoptotic cells affects macrophage-mediated immunity, we employ here Axl receptor- and Mer receptor-deficient mouse strains. In bone marrow-derived macrophages (BMDMs), both Axl and Mer receptors play a role in the efferocytosis of proapoptotic T cells from T. cruzi-infected mice. Moreover, treatment with a TAM receptor inhibitor blocks efferocytosis and upregulates M1 hallmarks induced by immune T cells from infected mice. Remarkably, the use of Axl-/- but not Mer-/- macrophages increases T-cell-induced M1 responses, such as nitric oxide production and control of parasite infection. Furthermore, infected Axl-/- mice show reduced peak parasitemia, defective efferocytosis, improved M1 responses, and ameliorated cardiac inflammation and fibrosis. Therefore, Axl induces efferocytosis, disrupts M1 responses, and promotes parasite infection and pathology in experimental Chagas disease. Axl stands as a potential host-direct target for switching macrophage phenotypes in infectious diseases.

Bacteroides acidifaciens in the gut plays a protective role against CD95-mediated liver injury

The intestinal flora plays an important role in the development of many human and animal diseases. Microbiome association studies revealed the potential regulatory function of intestinal bacteria in many liver diseases, such as autoimmune hepatitis, viral hepatitis and alcoholic hepatitis. However, the key intestinal bacterial strains that affect pathological liver injury and the underlying functional mechanisms remain unclear. We found that the gut microbiota from gentamycin (Gen)-treated mice significantly alleviated concanavalin A (ConA)-induced liver injury compared to vancomycin (Van)-treated mice by inhibiting CD95 expression on the surface of hepatocytes and reducing CD95/CD95L-mediated hepatocyte apoptosis. Through the combination of microbiota sequencing and correlation analysis, we isolated 5 strains with the highest relative abundance, Bacteroides acidifaciens (BA), Parabacteroides distasonis (PD), Bacteroides thetaiotaomicron (BT), Bacteroides dorei (BD) and Bacteroides uniformis (BU), from the feces of Gen-treated mice. Only BA played a protective role against ConA-induced liver injury. Further studies demonstrated that BA-reconstituted mice had reduced CD95/CD95L signaling, which was required for the decrease in the L-glutathione/glutathione (GSSG/GSH) ratio observed in the liver. BA-reconstituted mice were also more resistant to alcoholic liver injury. Our work showed that a specific murine intestinal bacterial strain, BA, ameliorated liver injury by reducing hepatocyte apoptosis in a CD95-dependent manner. Determination of the function of BA may provide an opportunity for its future use as a treatment for liver disease.

Management of cell death in parasitic infections

For a long time, host cell death during parasitic infection has been considered a reflection of tissue damage, and often associated with disease pathogenesis. However, during their evolution, protozoan and helminth parasites have developed strategies to interfere with cell death so as to spread and survive in the infected host, thereby ascribing a more intriguing role to infection-associated cell death. In this review, we examine the mechanisms used by intracellular and extracellular parasites to respectively inhibit or trigger programmed cell death. We further dissect the role of the prototypical “eat-me signal” phosphatidylserine (PtdSer) which, by being exposed on the cell surface of damaged host cells as well as on some viable parasites via a process of apoptotic mimicry, leads to their recognition and up-take by the neighboring phagocytes. Although barely dissected so far, the engagement of different PtdSer receptors on macrophages, by shaping the host immune response, affects the overall infection outcome in models of both protozoan and helminth infections. In this scenario, further understanding of the molecular and cellular regulation of the PtdSer exposing cell-macrophage interaction might allow the identification of new therapeutic targets for the management of parasitic infection.

CXCR3 chemokine receptor contributes to specific CD8+ T cell activation by pDC during infection with intracellular pathogens

Chemokine receptor type 3 (CXCR3) plays an important role in CD8⁺ T cells migration during intracellular infections, such as Trypanosoma cruzi. In addition to chemotaxis, CXCR3 receptor has been described as important to the interaction between antigen-presenting cells and effector cells. We hypothesized that CXCR3 is fundamental to T. cruzi-specific CD8⁺ T cell activation, migration and effector function. Anti-CXCR3 neutralizing antibody administration to acutely T. cruzi-infected mice decreased the number of specific CD8⁺ T cells in the spleen, and those cells had impaired in activation and cytokine production but unaltered proliferative response. In addition, anti-CXCR3-treated mice showed decreased frequency of CD8⁺ T cells in the heart and numbers of plasmacytoid dendritic cells in spleen and lymph node. As CD8⁺ T cells interacted with plasmacytoid dendritic cells during infection by T. cruzi, we suggest that anti-CXCR3 treatment lowers the quantity of plasmacytoid dendritic cells, which may contribute to impair the prime of CD8⁺ T cells. Understanding which molecules and mechanisms guide CD8⁺ T cell activation and migration might be a key to vaccine development against Chagas disease as those cells play an important role in T. cruzi infection control.

Inflammatory chemokine receptors such as CXCR3 play an important role in T lymphocytes migration into an infected tissue during Th1 response. Recently, the role of CXCR3 as a co-stimulatory molecule was demonstrated, and T lymphocytes from CXCR3 deficient mice had impaired effector function. CXCR3 receptor was highly expressed on specific CD8⁺ T cells after challenge with T. cruzi, and the hypothesis of that molecule is important for CD8⁺ T cells activation, migration and functionality was raised. We used the anti-CXCR3 neutralizing antibody approach and demonstrated that C57BL/6 treated mice died very quickly due to T. cruzi infection, and specific CD8⁺ T cells had decreased effector phenotyping, cytokine production, and cytotoxicity. In addition, anti-CXCR3 treatment decreased the number of dendritic plasmacytoid cells in the lymphoid tissues. The lower quantity of dendritic plasmacytoid cells in those tissues might contribute to the decrease in CD8⁺ T cells activation. Overall, CXCR3 molecule seems to be an important molecule to be explored during vaccine against Chagas disease strategies.

São Paulo School of Advanced Sciences on Vaccines: an overview

Two years ago, we held an exciting event entitled the São Paulo School of Advanced Sciences on Vaccines (SPSASV). Sixty-eight Ph.D. students, postdoctoral fellows and independent researchers from 37 different countries met at the Mendes Plaza Hotel located in the city of Santos, SP - Brazil to discuss the challenges and the new frontiers of vaccinology. The SPSASV provided a critical and comprehensive view of vaccine research from basics to the current state-of-the-art techniques performed worldwide. For 10 days, we discussed all the aspects of vaccine development in 36 lectures, 53 oral presentations and 2 poster sessions. At the end of the course, participants were further encouraged to present a model of a grant proposal related to vaccine development against individual pathogens. Among the targeted pathogens were viruses (Chikungunya, HIV, RSV, and Influenza), bacteria (Mycobacterium tuberculosis and Streptococcus pyogenes), parasites (Plasmodium falciparum or Plasmodium vivax), and the worm Strongyloides stercoralis. This report highlights some of the knowledge shared at the SPSASV.

Chagas disease vaccine design: the search for an efficient Trypanosoma cruzi immune-mediated control

Chagas disease is currently endemic to 21 Latin-American countries and has also become a global concern because of globalization and mass migration of chronically infected individuals. Prophylactic and therapeutic vaccination might contribute to control the infection and the pathology, as complement of other strategies such as vector control and chemotherapy. Ideal prophylactic vaccine would produce sterilizing immunity; however, a reduction of the parasite burden would prevent progression from Trypanosoma cruzi infection to Chagas disease. A therapeutic vaccine for Chagas disease may improve or even replace the treatment with current drugs which have several side effects and require long term treatment that frequently leads to therapeutic withdrawal. Here, we will review some aspects about sub-unit vaccines, the rationale behind the selection of the immunogen, the role of adjuvants, the advantages and limitations of DNA-based vaccines and the idea of therapeutic vaccines. One of the main limitations to advance vaccine development against Chagas disease is the high number of variables that must be considered and the lack of uniform criteria among research laboratories. To make possible comparisons, much of this review will be focused on experiments that kept many variables constant including antigen mass/doses, type of eukaryotic plasmid, DNA-delivery system, mice strain and sex, lethal and sublethal model of infection, and similar immunogenicity and efficacy assessments.

CXCR3 chemokine receptor guides Trypanosoma cruzi-specific T-cells triggered by DNA/adenovirus ASP2 vaccine to heart tissue after challenge

CD8+ T lymphocytes play an important role in controlling infections by intracellular pathogens. Chemokines and their receptors are crucial for the migration of CD8+ T-lymphocytes, which are the main IFNγ producers and cytotoxic effectors cells. Although the participation of chemokine ligands and receptors has been largely explored in viral infection, much less is known in infection by Trypanosoma cruzi, the causative agent of Chagas disease. After T. cruzi infection, CXCR3 chemokine receptor is highly expressed on the surface of CD8+ T-lymphocytes. Here, we hypothesized that CXCR3 is a key molecule for migration of parasite-specific CD8+ T-cells towards infected tissues, where they may play their effector activities. Using a model of induction of resistance to highly susceptible A/Sn mice using an ASP2-carrying DNA/adenovirus prime-boost strategy, we showed that CXCR3 expression was upregulated on CD8+ T-cells, which selectively migrated towards its ligands CXCL9 and CXCL10. Anti-CXCR3 administration reversed the vaccine-induced resistance to T. cruzi infection in a way associated with hampered cytotoxic activity and increased proapoptotic markers on the H2KK-restricted TEWETGQI-specific CD8+ T-cells. Furthermore, CXCR3 receptor critically guided TEWETGQI-specific effector CD8+ T-cells to the infected heart tissue that express CXCL9 and CXCL10. Overall, our study pointed CXCR3 and its ligands as key molecules to drive T. cruzi-specific effector CD8+ T-cells into the infected heart tissue. The unveiling of the process driving cell migration and colonization of infected tissues by pathogen-specific effector T-cells is a crucial requirement to the development of vaccine strategies.

TLR3 Is a Negative Regulator of Immune Responses Against Paracoccidioides brasiliensis

Toll-like receptors (TLRs) comprise the best-characterized pattern-recognition receptor (PRR) family able to activate distinct immune responses depending on the receptor/adaptor set assembled. TLRs, such as TLR2, TLR4 and TLR9, and their signaling were shown to be important in Paracoccidioides brasiliensis infections. However, the role of the endosomal TLR3 in experimental paracoccidioidomycosys remains obscure. In vitro assays, macrophages of the bone marrow of WT or TLR3^−/− mice were differentiated for evaluation of their microbicidal activity. In vivo assays, WT or TLR3^−/− mice were infected intratracheally with Paracoccidioides brasiliensis yeasts for investigation of the lung response type induced. The cytotoxic activity of CD8⁺ T cells was assessed by cytotoxicity assay. To confirm the importance of CD8⁺ T cells in the control of infection in the absence of tlr3, a depletion assay of these cells was performed. Here, we show for the first time that TLR3 modulate the infection against Paracoccidioides brasiliensis by dampening pro-inflammatory response, NO production, IFN⁺CD8⁺T, and IL-17⁺CD8⁺T cell activation and cytotoxic function, associated with granzyme B and perforin down regulation. As conclusion, we suggest that TLR3 could be used as an escape mechanism of the fungus in an experimental paracoccidioidomycosis.

Class-B CpG-ODN Formulated With a Nanostructure Induces Type I Interferons-Dependent and CD4+ T Cell-Independent CD8+ T-Cell Response Against Unconjugated Protein Antigen

There is a need for new vaccine adjuvant strategies that offer both vigorous antibody and T-cell mediated protection to combat difficult intracellular pathogens and cancer. To this aim, we formulated class-B synthetic oligodeoxynucleotide containing unmethylated cytosine-guanine motifs (CpG-ODN) with a nanostructure (Coa-ASC16 or coagel) formed by self-assembly of 6-0-ascorbyl palmitate ester. Our previous results demonstrated that mice immunized with ovalbumin (OVA) and CpG-ODN formulated with Coa-ASC16 (OVA/CpG-ODN/Coa-ASC16) elicited strong antibodies (IgG1 and IgG2a) and Th1/Th17 cellular responses without toxic systemic effects. These responses were superior to those induced by a solution of OVA with CpG-ODN or OVA/CpG-ODN formulated with aluminum salts. In this study, we investigated the capacity of this adjuvant strategy (CpG-ODN/Coa-ASC16) to elicit CD8+ T-cell response and some of the underlying cellular and molecular mechanisms involved in adaptive response. We also analyzed whether this adjuvant strategy allows a switch from an immunization scheme of three-doses to one of single-dose. Our results demonstrated that vaccination with OVA/CpG-ODN/Coa-ASC16 elicited an antigen-specific long-lasting humoral response and importantly-high quality CD8+ T-cell immunity with a single-dose immunization. Moreover, Coa-ASC16 promoted co-uptake of OVA and CpG-ODN by dendritic cells. The CD8+ T-cell response induced by OVA/CpG-ODN/Coa-ASC16 was dependent of type I interferons and independent of CD4+ T-cells, and showed polyfunctionality and efficiency against an intracellular pathogen. Furthermore, the cellular and humoral responses elicited by the nanostructured formulation were IL-6-independent. This system provides a simple and inexpensive adjuvant strategy with great potential for future rationally designed vaccines.

Therapeutical effects of vaccine from Trypanosoma cruzi amastigote surface protein 2 by simultaneous inoculation with live parasites

The aim of this study was to evaluate the efficacy of vaccine using replication-deficient human recombinant Type 5 replication-defective adenoviruses (AdHu5) carrying sequences of the amastigote surface protein 2 (ASP2) (AdASP2) in mice infected with the Trypanosoma cruzi ( T cruzi) Y strain. A total of 16 A/Sn mice female were distributed into four groups, as follows (n = 4 per group): Group 1 - Control Group (CTRL); Group 2 - Infected Group (TC): animals were infected by subcutaneous route with 150 bloodstream trypomastigotes of T cruzi Y strain; Group 3 - Immunized Group (AdASP-2): animals were immunized by intramuscular injection (im) route with 50 µL of AdSP-2 (2 × 10 ⁸ plaque forming units [pfu]/cam) at day 0; Group 4-Immunized and Infected Group (AdASP-2+TC): animals were immunized by im route with 50 µL of ASP-2 (2 × 10 ⁸  pfu/cam) and infected by T cruzi at the same day (day 0). It was observed a significant decrease of nests in the group that was immunized with AdASP-2 and infected on the same day. Tumor necrosis factor alpha (TNF-α) and inducible nitric oxide synthase (iNOS) gene expressions showed a significant increase in the AdASP-2+TC group when compared to TC group, but it was noted that Cyclooxygenase-2 (Cox-2) was increased in TC group when compared to AdASP-2+TC group. Increase of matrix metalloproteinases-2 (MMP-2) and decrease of MMP-9 immunoexpression in the AdASP-2+TC group was noticed as well. Oxidative DNA damage was present in myocardium for AdASP-2+TC group as a result of 8-hydroxydeoxyguanosine immunoexpression. Taken together, our results highlighted an increased oxidative stress, MMP-2 activity and inflammatory host response promoted by AdASP-2 against T cruzi infection.

Therapeutic effects of vaccine derived from amastigote surface protein-2 (ASP-2) against Chagas disease in mouse liver

This study investigated the efficacy of the vaccine in liver of mice infected with the Trypanosoma cruzi (T. cruzi) and immunized with AdASP-2. For this purpose, histopathological analysis and gene expression of COX-2, TNF-alpha, TNFR, iNOS, cytochrome C, caspase-3, TLR4, IL-6 and IL10 were evaluated. The following groups were used in this study: Group 1 - Control Group (CTRL) animals received AdβGal vehicle; Group 2 - Infected Group (TC) animals were infected with T. cruzi; Group 3 - Immunized Group (AdASP-2): animals were immunized by AdASP-2 vaccine; Group 4 - Immunized and Infected Group (AdASP-2+TC) animals were infected with T. cruzi and immunized by AdSP-2 vaccine. A significant decrease of amastigote nests was noticed in the group of animals that were immunized with AdASP-2 and infected on the same day. COX-2 and TNF-alpha gene expressions increased in TC group, whereas TNF-alpha decreased in the TC+AdASP-2 group. TNFR expression was high in AdASP-2+TC group. iNOS expression was high for all experimental groups whereas cytochrome C decreased for all experimental groups. Caspase 3 increased in TC and TC+AdASP-2 groups. The gene expression of TLR4 and IL-10 showed an increase in AdASP-2+TC group. Finally, hepatic fibrosis was noticed to TC and AdASP-2 + TC groups. Taken together, our results demonstrated that vaccination with AdASP-2 was effective against the acute phase of experimental Chagas disease as a result of a more powerful and rapid immune response closely related to expression of some inflammatory genes, such as iNOS, TNF-alpha, TLR 4, and IL-10.

Vaccination With Recombinant Filamentous fd Phages Against Parasite Infection Requires TLR9 Expression

Recombinant filamentous fd bacteriophages (rfd) expressing antigenic peptides were shown to induce cell-mediated immune responses in the absence of added adjuvant, being a promising delivery system for vaccination. Here, we tested the capacity of rfd phages to protect against infection with the human protozoan Trypanosoma cruzi, the etiologic agent of Chagas Disease. For this, C57BL/6 (B6) and Tlr9^−/− mice were vaccinated with rfd phages expressing the OVA_257–264 peptide or the T. cruzi-immunodominant peptides PA8 and TSKB20 and challenged with either the T. cruzi Y-OVA or Y-strain, respectively. We found that vaccination with rfd phages induces anti-PA8 and anti-TSKB20 IgG production, expansion of Ag-specific IFN-γ, TNF-α, and Granzyme B-producing CD8⁺ T cells, as well as in vivo Ag-specific cytotoxic responses. Moreover, the fd-TSKB20 vaccine was able to protect against mortality induced by a high-dose inoculum of the parasite. Although vaccination with rfd phages successfully reduced both parasitemia and parasite load in the myocardium of WT B6 mice, Tlr9^−/− animals were not protected against infection. Thus, our data extend previous studies, demonstrating that rfd phages induce Ag-specific IgG and CD8⁺ T cell-mediated responses and confer protection against an important human parasite infection, through a TLR9-dependent mechanism.

Proteomic and functional analysis identifies galectin-1 as a novel regulatory component of the cytotoxic granule machinery

Secretory granules released by cytotoxic T lymphocytes (CTLs) are powerful weapons against intracellular microbes and tumor cells. Despite significant progress, there is still limited information on the molecular mechanisms implicated in target-driven degranulation, effector cell survival and composition and structure of the lytic granules. Here, using a proteomic approach we identified a panel of putative cytotoxic granule proteins, including some already known granule constituents and novel proteins that contribute to regulate the CTL lytic machinery. Particularly, we identified galectin-1 (Gal1), an endogenous immune regulatory lectin, as an integral component of the secretory granule machinery and unveil the unexpected function of this lectin in regulating CTL killing activity. Mechanistic studies revealed the ability of Gal1 to control the non-secretory lytic pathway by influencing Fas–Fas ligand interactions. This study offers new insights on the composition of the cytotoxic granule machinery, highlighting the dynamic cross talk between secretory and non-secretory pathways in controlling CTL lytic function.

LFA-1 Mediates Cytotoxicity and Tissue Migration of Specific CD8+ T Cells after Heterologous Prime-Boost Vaccination against Trypanosoma cruzi Infection

Integrins mediate the lymphocyte migration into an infected tissue, and these cells are essential for controlling the multiplication of many intracellular parasites such as Trypanosoma cruzi, the causative agent of Chagas disease. Here, we explore LFA-1 and VLA-4 roles in the migration of specific CD8⁺ T cells generated by heterologous prime-boost immunization during experimental infection with T. cruzi. To this end, vaccinated mice were treated with monoclonal anti-LFA-1 and/or anti-VLA-4 to block these molecules. After anti-LFA-1, but not anti-VLA-4 treatment, all vaccinated mice displayed increased blood and tissue parasitemia, and quickly succumbed to infection. In addition, there was an accumulation of specific CD8⁺ T cells in the spleen and lymph nodes and a decrease in the number of those cells, especially in the heart, suggesting that LFA-1 is important for the output of specific CD8⁺ T cells from secondary lymphoid organs into infected organs such as the heart. The treatment did not alter CD8⁺ T cell effector functions such as the production of pro-inflammatory cytokines and granzyme B, and maintained the proliferative capacity after treatment. However, the specific CD8⁺ T cell direct cytotoxicity was impaired after LFA-1 blockade. Also, these cells expressed higher levels of Fas/CD95 on the surface, suggesting that they are susceptible to programmed cell death by the extrinsic pathway. We conclude that LFA-1 plays an important role in the migration of specific CD8⁺ T cells and in the direct cytotoxicity of these cells.

Vaccine Containing the Three Allelic Variants of the Plasmodium vivax Circumsporozoite Antigen Induces Protection in Mice after Challenge with a Transgenic Rodent Malaria Parasite

Plasmodium vivax is the most common species that cause malaria outside of the African continent. The development of an efficacious vaccine would contribute greatly to control malaria. Recently, using bacterial and adenoviral recombinant proteins based on the P. vivax circumsporozoite protein (CSP), we demonstrated the possibility of eliciting strong antibody-mediated immune responses to each of the three allelic forms of P. vivax CSP (PvCSP). In the present study, recombinant proteins representing the PvCSP alleles (VK210, VK247, and P. vivax-like), as well as a hybrid polypeptide, named PvCSP-All epitopes, were generated. This hybrid containing the conserved C-terminal of the PvCSP and the three variant repeat domains in tandem were successfully produced in the yeast Pichia pastoris. After purification and biochemical characterization, they were used for the experimental immunization of C57BL/6 mice in a vaccine formulation containing the adjuvant Poly(I:C). Immunization with a recombinant protein expressing all three different allelic forms in fusion elicited high IgG antibody titers reacting with all three different allelic variants of PvCSP. The antibodies targeted both the C-terminal and repeat domains of PvCSP and recognized the native protein on the surface of P. vivax sporozoites. More importantly, mice that received the vaccine formulation were protected after challenge with chimeric Plasmodium berghei sporozoites expressing CSP repeats of P. vivax sporozoites (Pb/PvVK210). Our results suggest that it is possible to elicit protective immunity against one of the most common PvCSP alleles using soluble recombinant proteins expressed by P. pastoris. These recombinant proteins are promising candidates for clinical trials aiming to develop a multiallele vaccine against P. vivax malaria.

Implication of Apoptosis for the Pathogenesis of Trypanosoma cruzi Infection

Apoptosis is induced during the course of immune response to different infectious agents, and the ultimate fate is the recognition and uptake of apoptotic bodies by neighboring cells or by professional phagocytes. Apoptotic cells expose specific ligands to a set of conserved receptors expressed on macrophage cellular surface, which are the main cells involved in the clearance of the dying cells. These scavenger receptors, besides triggering the production of anti-inflammatory factors, also block the production of inflammatory mediators by phagocytes. Experimental infection of mice with the parasite Trypanosoma cruzi shows many pathological changes that parallels the evolution of human infection. Leukocytes undergoing intense apoptotic death are observed during the immune response to T. cruzi in the mouse model of the disease. T. cruzi replicate intensely and secrete molecules with immunomodulatory activities that interfere with T cell-mediated immune responses and secretion of pro-inflammatory cytokine secretion. This mechanism of immune evasion allows the infection to be established in the vertebrate host. Under inflammatory conditions, efferocytosis of apoptotic bodies generates an immune-regulatory phenotype in phagocytes, which is conducive to intracellular pathogen replication. However, the relevance of cellular apoptosis in the pathology of Chagas’ disease requires further studies. Here, we review the evidence of leukocyte apoptosis in T. cruzi infection and its immunomodulatory mechanism for disease progression.

Analysis of novel meningococcal vaccine formulations

The protective effect of meningococcal vaccines targeting disease causing serogroups exemplified by the introduction of MenAfriVac™ in Africa, is well established and documented in large population-based studies. Due to the emergence of other meningococcal disease causing serogroups, novel vaccine formulations are needed. There is a high potential for novel nanotechnology-based meningococcal vaccine formulations that can provide wider vaccine coverage. The proposed meningococcal vaccine formulation contains spherical shaped micro and nanoparticles that are biological mimics of Niesseria meningitidis, therefore present to immune system as invader and elicit robust immune responses. Vaccine nanoparticles encapsulate meningococcal CPS polymers in a biodegradable material that slowly release antigens, therefore enhance antigen presentation by exerting antigen depot effect. The antigenicity of meningococcal vaccine delivered in nanoparticles is significantly higher when compared to vaccine delivered in solution. Preclinical studies are required to assess the immunogenicity of novel vaccine formulations. Therefore, implementing various in-vitro human immune cell-based assays that mimic in-vivo interactions, would provide good insight on optimal antigen dose, effective antigen presentation, facilitate screening of various vaccine and adjuvant combinations and predict in-vivo immunogenicity. This rapid approach is cost-effective and provides data required for the preclinical immunogenicity assessment of novel meningococcal vaccine formulations.

Engineered trivalent immunogen adjuvanted with a STING agonist confers protection against Trypanosoma cruzi infection

The parasite Trypanosoma cruzi is the causative agent of Chagas disease, a potentially life-threatening infection that represents a major health problem in Latin America. Several characteristics of this protozoan contribute to the lack of an effective vaccine, among them: its silent invasion mechanism, T. cruzi antigen redundancy and immunodominance without protection. Taking into account these issues, we engineered Traspain, a chimeric antigen tailored to present a multivalent display of domains from key parasitic molecules, combined with stimulation of the STING pathway by c-di-AMP as a novel prophylactic strategy. This formulation proved to be effective for the priming of functional humoral responses and pathogen-specific CD8⁺ and CD4⁺ T cells, compatible with a Th1/Th17 bias. Interestingly, vaccine effectiveness assessed across the course of infection, showed a reduction in parasite load and chronic inflammation in different proof of concept assays. In conclusion, this approach represents a promising tool against parasitic chronic infections.

An amalgamation of parasitic proteins may be the first effective vaccine against the as yet untreatable chronic phase of Chagas disease. The infliction, caused by the parasite Trypanosoma cruzi (T. cruzi), is the world’s leading cause of infectious cardiac inflammation and puts one-sixth of the population of Latin America at risk of infection. International collaborators led by Emilio Malchiodi, of the University of Buenos Aires, Argentina, constructed a vaccine (dubbed ‘Traspain’) comprised of key T. cruzi proteins alongside a novel ‘adjuvant’—designed to promote the efficacy of a vaccine by activating inflammatory responses. The chimera and adjuvant combination elicited a promising immune response and also showed the capacity to prevent tissue damage caused by chronic infection. Multi-part vaccines such as Traspain offer an attractive direction for research into vaccines against chronic parasitic infections.

Antiparasitic Treatment Induces an Improved CD8+ T Cell Response in Chronic Chagasic Patients

Chagas disease is a chronic infection caused by Trypanosoma cruzi, an intracellular protozoan parasite. Chronic chagasic patients (CCPs) have dysfunctional CD8⁺ T cells that are characterized by impaired cytokine production, high coexpression of inhibitory receptors, and advanced cellular differentiation. Most patients diagnosed in the chronic phase of Chagas disease already exhibit heart involvement, and there is no vaccination that protects against the disease. Antiparasitic treatment is controversial as to its indication for this stage of the disease. There is a lack of biological markers to evaluate the effectiveness of antiparasitic treatment, and little is known about the effect of the treatment on CD8⁺ T cells. Thus, the aim of the current study was to analyze the early effects of antiparasitic treatment on CD8⁺ T cells from CCPs with asymptomatic clinical forms of disease. To evaluate the CD8⁺ T cell subsets, expression of inhibitory receptors, and functionality of T cells in CCPs, PBMCs were isolated. The results showed that treatment of CCPs with the asymptomatic form of the disease induces an increase in the frequency of CD8⁺ central memory T cells and terminal effector T cells, a decrease in the coexpression of inhibitory receptors, an improved Ag-specific CD8⁺ T cell response exhibited by the individual production of IFN-γ or IL-2, and a multifunctional CD8⁺ T cell profile of up to four functions (IFN-γ⁺IL-2⁺Perforin⁺Granzyme B⁺). These findings suggest that, in CCPs, antiparasitic treatment improved the quality of Ag-specific CD8⁺ T cell responses associated with a decrease in inhibitory receptor coexpression, which could serve as biomarkers for monitoring the effectiveness of antiparasitic treatment.

Cannabidiol (CBD) induces functional Tregs in response to low-level T cell activation

Many effects of the non-psychoactive cannabinoid, cannabidiol (CBD), have been described in immune responses induced by strong immunological stimuli. It has also been shown that CBD enhances IL-2 production in response to low-level T cell stimulation. Since IL-2, in combination with TGF-β1, are critical for Treg induction, we hypothesized that CBD would induce CD4+CD25+FOXP3+ Tregs in response to low-level stimulation. Low-level T cell stimulation conditions were established based on minimal CD25 expression in CD4+ cells using suboptimal PMA/Io (4nM/0.05μM, S/o), ultrasuboptimal PMA/Io (1nM/0.0125μM, Us/o) or soluble anti-CD3/28 (400-800ng each, s3/28). CBD increased CD25+FOXP3+ cells from CD4+, CD4+CD25+, and CD4+CD25- T cells, as well as in CD4+ T cells derived from FOXP3-GFP mice. Most importantly, the Us/o+CBD-induced CD4+CD25+ Tregs robustly suppressed responder T cell proliferation, demonstrating that the mechanism by which CBD is immunosuppressive under low-level T cell stimulation involves induction of functional Tregs.

Chagas Disease Diagnostic Applications: Present Knowledge and Future Steps

Chagas disease, caused by the protozoan Trypanosoma cruzi, is a lifelong and debilitating illness of major significance throughout Latin America and an emergent threat to global public health. Being a neglected disease, the vast majority of Chagasic patients have limited access to proper diagnosis and treatment, and there is only a marginal investment into R&D for drug and vaccine development. In this context, identification of novel biomarkers able to transcend the current limits of diagnostic methods surfaces as a main priority in Chagas disease applied research. The expectation is that these novel biomarkers will provide reliable, reproducible and accurate results irrespective of the genetic background, infecting parasite strain, stage of disease, and clinical-associated features of Chagasic populations. In addition, they should be able to address other still unmet diagnostic needs, including early detection of congenital T. cruzi transmission, rapid assessment of treatment efficiency or failure, indication/prediction of disease progression and direct parasite typification in clinical samples. The lack of access of poor and neglected populations to essential diagnostics also stresses the necessity of developing new methods operational in point-of-care settings. In summary, emergent diagnostic tests integrating these novel and tailored tools should provide a significant impact on the effectiveness of current intervention schemes and on the clinical management of Chagasic patients. In this chapter, we discuss the present knowledge and possible future steps in Chagas disease diagnostic applications, as well as the opportunity provided by recent advances in high-throughput methods for biomarker discovery.

A Human Trypanosome Suppresses CD8+ T Cell Priming by Dendritic Cells through the Induction of Immune Regulatory CD4+ Foxp3+ T Cells

Although CD4⁺ Foxp3⁺ T cells are largely described in the regulation of CD4⁺ T cell responses, their role in the suppression of CD8⁺ T cell priming is much less clear. Because the induction of CD8⁺ T cells during experimental infection with Trypanosoma cruzi is remarkably delayed and suboptimal, we raised the hypothesis that this protozoan parasite actively induces the regulation of CD8⁺ T cell priming. Using an in vivo assay that eliminated multiple variables associated with antigen processing and dendritic cell activation, we found that injection of bone marrow-derived dendritic cells exposed to T. cruzi induced regulatory CD4⁺ Foxp3⁺ T cells that suppressed the priming of transgenic CD8⁺ T cells by peptide-loaded BMDC. This newly described suppressive effect on CD8⁺ T cell priming was independent of IL-10, but partially dependent on CTLA-4 and TGF-β. Accordingly, depletion of Foxp3⁺ cells in mice infected with T. cruzi enhanced the response of epitope-specific CD8⁺ T cells. Altogether, our data uncover a mechanism by which T. cruzi suppresses CD8⁺ T cell responses, an event related to the establishment of chronic infections.

CD8⁺ T lymphocytes mediate immunity to intracellular pathogens by killing infected cells. However, some pathogens are able to evade the response of CD8⁺ T cells and, thus, establish chronic infections. This is the case of Trypanosoma cruzi, the protozoan parasite that causes Chagas disease. Here, we investigated the basis of the suboptimal response of CD8⁺ T cells during T. cruzi infection. We observed that cells incubated with the parasite and then adoptively transferred into mice are able to convert an optimal in vivo response of transgenic CD8⁺ T cells specific to an unrelated epitope into suboptimal. The mechanism of this disturbance relies on the induction of regulatory CD4⁺ Foxp3⁺ T cells that interfere with the priming of CD8⁺ T cells by dendritic cells. These findings illustrate the involvement of regulatory T cells in the regulation of CD8⁺ T cell priming and contribute to understand how T. cruzi evades host immunity to establish a chronic infection.

Apoptotic CD8 T-lymphocytes disable macrophage-mediated immunity to Trypanosoma cruzi infection

Chagas disease is caused by infection with the protozoan Trypanosoma cruzi. CD8 T-lymphocytes help to control infection, but apoptosis of CD8 T cells disrupts immunity and efferocytosis can enhance parasite infection within macrophages. Here, we investigate how apoptosis of activated CD8 T cells affects M1 and M2 macrophage phenotypes. First, we found that CD8 T-lymphocytes and inflammatory monocytes/macrophages infiltrate peritoneum during acute T. cruzi infection. We show that treatment with anti-Fas ligand (FasL) prevents lymphocyte apoptosis, upregulates type-1 responses to parasite antigens, and reduces infection in macrophages cocultured with activated CD8 T cells. Anti-FasL skews mixed M1/M2 macrophage profiles into polarized M1 phenotype, both in vitro and following injection in infected mice. Moreover, inhibition of T-cell apoptosis induces a broad reprogramming of cytokine responses and improves macrophage-mediated immunity to T. cruzi. The results indicate that disposal of apoptotic CD8 T cells increases M2-macrophage differentiation and contributes to parasite persistence.

The Combined Deficiency of Immunoproteasome Subunits Affects Both the Magnitude and Quality of Pathogen- and Genetic Vaccination-Induced CD8+ T Cell Responses to the Human Protozoan Parasite Trypanosoma cruzi

The β1i, β2i and β5i immunoproteasome subunits have an important role in defining the repertoire of MHC class I-restricted epitopes. However, the impact of combined deficiency of the three immunoproteasome subunits in the development of protective immunity to intracellular pathogens has not been investigated. Here, we demonstrate that immunoproteasomes play a key role in host resistance and genetic vaccination-induced protection against the human pathogen Trypanosoma cruzi (the causative agent of Chagas disease), immunity to which is dependent on CD8⁺ T cells and IFN-γ (the classical immunoproteasome inducer). We observed that infection with T. cruzi triggers the transcription of immunoproteasome genes, both in mice and humans. Importantly, genetically vaccinated or T. cruzi-infected β1i, β2i and β5i triple knockout (TKO) mice presented significantly lower frequencies and numbers of splenic CD8⁺ effector T cells (CD8⁺CD44^highCD62L^low) specific for the previously characterized immunodominant (VNHRFTLV) H-2K^b-restricted T. cruzi epitope. Not only the quantity, but also the quality of parasite-specific CD8⁺ T cell responses was altered in TKO mice. Hence, the frequency of double-positive (IFN-γ⁺/TNF⁺) or single-positive (IFN-γ⁺) cells specific for the H-2K^b-restricted immunodominant as well as subdominant T. cruzi epitopes were higher in WT mice, whereas TNF single-positive cells prevailed among CD8⁺ T cells from TKO mice. Contrasting with their WT counterparts, TKO animals were also lethally susceptible to T. cruzi challenge, even after an otherwise protective vaccination with DNA and adenoviral vectors. We conclude that the immunoproteasome subunits are key determinants in host resistance to T. cruzi infection by influencing both the magnitude and quality of CD8⁺ T cell responses.

CD8⁺ t lymphocytes are cells of the immune system that mediate control of intracellular infections by viruses, prokaryote as well as eukaryote pathogens. To confer protection, these lymphocytes need to be elicited by pathogen peptides that are presented in association with MHC class I molecules. The degradation of self and microbial proteins by catalytic domains of the cytosolic proteasome β1, β2 and β5 subunits is intimately linked to the generation of MHC class I-restricted epitopes, which in turn are important determinants of the kinetics, specificity and efficiency of CD8⁺ T cell-mediated immunity. Importantly, inflammatory stimuli trigger the expression of the inducible alternative β1i, β2i and β5i subunits that form the immunoproteasomes. The qualitative and quantitative importance of immunoproteasomes in generating CD8⁺ T cell epitopes has recently been demonstrated in mice that are simultaneously devoid of the β1i, β2i and β5i subunits. In this study, we explored the role of immunoproteasomes in host resistance to Trypanosoma cruzi, a protozoan parasite that causes Chagas disease. We found that β1i, β2i and β5i triply deficient mice have an impaired response of CD8⁺ T cells and are highly susceptible to primary infection with T. cruzi. We also demonstrated that host resistance induced by a genetic vaccine able to protect normal mice from T. cruzi challenge fails to do so in the immunoproteasome-deficient mice. Our study provides strong evidences that β1i, β2i and β5i immunoproteasome subunits are important determinants of both the magnitude and quality of CD8⁺ T cell responses as well as immune-mediated host resistance to a human pathogen.

Promiscuous Recognition of a Trypanosoma cruzi CD8+ T Cell Epitope among HLA-A2, HLA-A24 and HLA-A1 Supertypes in Chagasic Patients

Background

TcTLE is a nonamer peptide from Trypanosoma cruzi KMP-11 protein that is conserved among different parasite strains and that is presented by different HLA-A molecules from the A2 supertype. Because peptides presented by several major histocompatibility complex (MHC) supertypes are potential targets for immunotherapy, the aim of this study was to determine whether MHC molecules other than the A2 supertype present the TcTLE peptide.

Methodology/Principal Findings

From 36 HLA-A2-negative chagasic patients, the HLA-A genotypes of twenty-eight patients with CD8⁺ T cells that recognized the TcTLE peptide using tetramer (twenty) or functional (eight) assays, were determined. SSP-PCR was used to identify the A locus and the allelic variants. Flow cytometry was used to analyze the frequency of TcTLE-specific CD8⁺ T cells, and their functional activity (IFN-γ, TNFα, IL-2, perforin, granzyme and CD107a/b production) was induced by exposure to the TcTLE peptide. All patients tested had TcTLE-specific CD8⁺ T cells with frequencies ranging from 0.07–0.37%. Interestingly, seven of the twenty-eight patients had HLA-A homozygous alleles: A*24 (5 patients), A*23 (1 patient) and A*01 (1 patient), which belong to the A24 and A1 supertypes. In the remaining 21 patients with HLA-A heterozygous alleles, the most prominent alleles were A24 and A68. The most common allele sub-type was A*2402 (sixteen patients), which belongs to the A24 supertype, followed by A*6802 (six patients) from the A2 supertype. Additionally, the A*3002/A*3201 alleles from the A1 supertype were detected in one patient. All patients presented CD8⁺ T cells producing at least one cytokine after TcTLE peptide stimulation.

Conclusion/Significance

These results show that TcTLE is a promiscuous peptide that is presented by the A24 and A1 supertypes, in addition to the A2 supertype, suggesting its potential as a target for immunotherapy.

A Novel Vaccine Approach for Chagas Disease Using Rare Adenovirus Serotype 48 Vectors

Due to the increasing amount of people afflicted worldwide with Chagas disease and an increasing prevalence in the United States, there is a greater need to develop a safe and effective vaccine for this neglected disease. Adenovirus serotype 5 (Ad5) is the most common adenovirus vector used for gene therapy and vaccine approaches, but its efficacy is limited by preexisting vector immunity in humans resulting from natural infections. Therefore, we have employed rare serotype adenovirus 48 (Ad48) as an alternative choice for adenovirus/Chagas vaccine therapy. In this study, we modified Ad5 and Ad48 vectors to contain T. cruzi’s amastigote surface protein 2 (ASP-2) in the adenoviral early gene. We also modified Ad5 and Ad48 vectors to utilize the “Antigen Capsid-Incorporation” strategy by adding T. cruzi epitopes to protein IX (pIX). Mice that were immunized with the modified vectors were able to elicit T. cruzi-specific humoral and cellular responses. This study indicates that Ad48-modified vectors function comparable to or even premium to Ad5-modified vectors. This study provides novel data demonstrating that Ad48 can be used as a potential adenovirus vaccine vector against Chagas disease.

A Two-Component DNA-Prime/Protein-Boost Vaccination Strategy for Eliciting Long-Term, Protective T Cell Immunity against Trypanosoma cruzi

In this study, we evaluated the long-term efficacy of a two-component subunit vaccine against Trypanosoma cruzi infection. C57BL/6 mice were immunized with TcG2/TcG4 vaccine delivered by a DNA-prime/Protein-boost (D/P) approach and challenged with T. cruzi at 120 or 180 days post-vaccination (dpv). We examined whether vaccine-primed T cell immunity was capable of rapid expansion and intercepting the infecting T. cruzi. Our data showed that D/P vaccine elicited CD4⁺ (30-38%) and CD8⁺ (22-42%) T cells maintained an effector phenotype up to 180 dpv, and were capable of responding to antigenic stimulus or challenge infection by a rapid expansion (CD8>CD4) with type 1 cytokine (IFNγ⁺ and TFNα⁺) production and cytolytic T lymphocyte (CTL) activity. Subsequently, challenge infection at 120 or 180 dpv, resulted in 2-3-fold lower parasite burden in vaccinated mice than was noted in unvaccinated/infected mice. Co-delivery of IL-12- and GMCSF-encoding expression plasmids provided no significant benefits in enhancing the anti-parasite efficacy of the vaccine-induced T cell immunity. Booster immunization (bi) with recombinant TcG2/TcG4 proteins 3-months after primary vaccine enhanced the protective efficacy, evidenced by an enhanced expansion (1.2-2.8-fold increase) of parasite-specific, type 1 CD4⁺ and CD8⁺ T cells and a potent CTL response capable of providing significantly improved (3-4.5-fold) control of infecting T. cruzi. Further, CD8⁺T cells in vaccinated/bi mice were predominantly of central memory phenotype, and capable of responding to challenge infection 4-6-months post bi by a rapid expansion to a poly-functional effector phenotype, and providing a 1.5-2.3-fold reduction in tissue parasite replication. We conclude that the TcG2/TcG4 D/P vaccine provided long-term anti-T. cruzi T cell immunity, and bi would be an effective strategy to maintain or enhance the vaccine-induced protective immunity against T. cruzi infection and Chagas disease.

Chagas disease, caused by Trypanosoma cruzi infection, represents the third greatest tropical disease burden in the world. No vaccine or suitable treatment is available for control of this infection. Based upon several studies we have conducted, we believe that TcG2 and TcG4 candidate antigens that are highly conserved in T. cruzi, expressed in clinically relevant forms of the parasite, and recognized by both B and T cell responses in multiple hosts, are an excellent choice for subunit vaccine development. In this study, we demonstrate that the delivery of TcG2 and TcG4 as a DNA-prime/protein-boost vaccine provided long-term protection from challenge infection, and this protection was associated with elicitation of long-lived CD8⁺ effector T cells. The longevity and efficacy of vaccine could be enhanced by booster immunization. We believe that this is the first report demonstrating a) a subunit vaccine can be useful in achieving long-term protection against T. cruzi infection and Chagas disease, and b) the effector T cells can be long-lived and play a role in vaccine elicited protection from parasitic infection.

Utilizing the antigen capsid-incorporation strategy for the development of adenovirus serotype 5-vectored vaccine approaches

Adenovirus serotype 5 (Ad5) has been extensively modified with traditional transgene methods for the vaccine development. The reduced efficacies of these traditionally modified Ad5 vectors in clinical trials could be primarily correlated with Ad5 pre-existing immunity (PEI) among the majority of the population. To promote Ad5-vectored vaccine development by solving the concern of Ad5 PEI, the innovative Antigen Capsid-Incorporation strategy has been employed. By merit of this strategy, Ad5-vectored we first constructed the hexon shuttle plasmid HVR1-KWAS-HVR5-His6/pH5S by subcloning the hypervariable region (HVR) 1 of hexon into a previously constructed shuttle plasmid HVR5-His6/pH5S, which had His6 tag incorporated into the HVR5. This HVR1 DNA fragment containing a HIV epitope ELDKWAS was synthesized. HVR1-KWAS-HVR5-His6/pH5S was then linearized and co-transformed with linearized backbone plasmid pAd5/∆H5 (GL) , for homologous recombination. This recombined plasmid pAd5/H5-HVR1-KWAS-HVR5-His6 was transfected into cells to generate the viral vector Ad5/H5-HVR1-KWAS-HVR5-His6. This vector was validated to have qualitative fitness indicated by viral physical titer (VP/ml), infectious titer (IP/ml) and corresponding VP/IP ratio. Both the HIV epitope and His6 tag were surface-exposed on the Ad5 capsid, and retained epitope-specific antigenicity of their own. A neutralization assay indicated the ability of this divalent vector to circumvent neutralization by Ad5-positive sera in vitro. Mice immunization demonstrated the generation of robust humoral immunity specific to the HIV epitope and His6. This proof-of-principle study suggested that the protocol associated with the Antigen Capsid-Incorporation strategy could be feasibly utilized for the generation of Ad5-vectored vaccines by modifying different capsid proteins. This protocol could even be further modified for the generation of rare-serotype adenovirus-vectored vaccines.

Prophylactic and therapeutic DNA vaccines against Chagas disease

Chagas disease is a zoonosis caused by Trypanosoma cruzi in which the most affected organ is the heart. Conventional chemotherapy has a very low effectiveness; despite recent efforts, there is currently no better or more effective treatment available. DNA vaccines provide a new alternative for both prevention and treatment of a variety of infectious disorders, including Chagas disease. Recombinant DNA technology has allowed some vaccines to be developed using recombinant proteins or virus-like particles capable of inducing both a humoral and cellular specific immune response. This type of immunization has been successfully used in preclinical studies and there are diverse models for viral, bacterial and/or parasitic diseases, allergies, tumors and other diseases. Therefore, several research groups have been given the task of designing a DNA vaccine against experimental infection with T. cruzi. In this review we explain what DNA vaccines are and the most recent studies that have been done to develop them with prophylactic or therapeutic purposes against Chagas disease.

A human type 5 adenovirus-based Trypanosoma cruzi therapeutic vaccine re-programs immune response and reverses chronic cardiomyopathy

Chagas disease (CD), caused by the protozoan Trypanosoma cruzi, is a prototypical neglected tropical disease. Specific immunity promotes acute phase survival. Nevertheless, one-third of CD patients develop chronic chagasic cardiomyopathy (CCC) associated with parasite persistence and immunological unbalance. Currently, the therapeutic management of patients only mitigates CCC symptoms. Therefore, a vaccine arises as an alternative to stimulate protective immunity and thereby prevent, delay progression and even reverse CCC. We examined this hypothesis by vaccinating mice with replication-defective human Type 5 recombinant adenoviruses (rAd) carrying sequences of amastigote surface protein-2 (rAdASP2) and trans-sialidase (rAdTS) T. cruzi antigens. For prophylactic vaccination, naïve C57BL/6 mice were immunized with rAdASP2+rAdTS (rAdVax) using a homologous prime/boost protocol before challenge with the Colombian strain. For therapeutic vaccination, rAdVax administration was initiated at 120 days post-infection (dpi), when mice were afflicted by CCC. Mice were analyzed for electrical abnormalities, immune response and cardiac parasitism and tissue damage. Prophylactic immunization with rAdVax induced antibodies and H-2K^b-restricted cytotoxic and interferon (IFN)γ-producing CD8⁺ T-cells, reduced acute heart parasitism and electrical abnormalities in the chronic phase. Therapeutic vaccination increased survival and reduced electrical abnormalities after the prime (analysis at 160 dpi) and the boost (analysis at 180 and 230 dpi). Post-therapy mice exhibited less heart injury and electrical abnormalities compared with pre-therapy mice. rAdVax therapeutic vaccination preserved specific IFNγ-mediated immunity but reduced the response to polyclonal stimuli (anti-CD3 plus anti-CD28), CD107a⁺ CD8⁺ T-cell frequency and plasma nitric oxide (NO) levels. Moreover, therapeutic rAdVax reshaped immunity in the heart tissue as reduced the number of perforin+ cells, preserved the number of IFNγ⁺ cells, increased the expression of IFNγ mRNA but reduced inducible NO synthase mRNA. Vaccine-based immunostimulation with rAd might offer a rational alternative for re-programming the immune response to preserve and, moreover, recover tissue injury in Chagas’ heart disease.

The idea that Chagas disease (CD) has an important autoimmune involvement contributed to delay the development of therapies and vaccines. CD is a parasitic neglected disease which afflicts millions of people mostly in Latin America. The cardiac form is the main clinical manifestation of CD. Currently, patients with access to therapy receive medicaments that only mitigate symptoms. Because of the limited prospect of treatment, vaccine reemerged as a strategy to prevent infection, interfere with CD progression and, moreover, reverse heart abnormalities. Here we tested a recombinant adenovirus carrying sequences of ASP2 and TS T. cruzi antigens (rAdVax) as prophylactic and therapeutic tool using a model of chronic Chagas’ heart disease. We showed that prophylactic vaccination reduced heart parasite load, inflammation and electrical abnormalities. The rAdVax therapeutic vaccination also reduced heart injury and improved electrical function, preserved specific IFNγ-mediated immunity but reduced response to polyclonal stimuli, CD107a⁺ CD8⁺ T-cell frequency and plasma nitric oxide levels. Moreover, therapeutic rAdVax preserved the number IFNγ⁺ cells, but decreased perforin⁺ cells in the heart tissue. Therefore, our results support the hypothesis that vaccination can modify the immunological unbalance that concurs to Chagas’ heart disease to improve prognosis.

Deficiency of antigen-specific B cells results in decreased Trypanosoma cruzi systemic but not mucosal immunity due to CD8 T cell exhaustion

Vaccines against mucosally invasive, intracellular pathogens must induce a myriad of immune responses to provide optimal mucosal and systemic protection, including CD4(+) T cells, CD8(+) T cells, and Ab-producing B cells. In general, CD4(+) T cells are known to provide important helper functions for both CD8(+) T cell and B cell responses. However, the relative importance of CD4(+) T cells, CD8(+) T cells, and B cells for mucosal protection is less clearly defined. We have studied these questions in detail using the murine model of Trypanosoma cruzi infection. Despite our initial hypothesis that mucosal Abs would be important, we show that B cells are critical for systemic, but not mucosal, T. cruzi protective immunity. B cell-deficient mice developed normal levels of CD8(+) effector T cell responses early after mucosal T. cruzi infection and T. cruzi trans-sialidase vaccination. However, after highly virulent systemic challenge, T. cruzi immune mice lacking T. cruzi-specific B cells failed to control parasitemia or prevent death. Mechanistically, T. cruzi-specific CD8(+) T cells generated in the absence of B cells expressed increased PD-1 and Lag-3 and became functionally exhausted after high-level T. cruzi systemic challenge. T. cruzi immune serum prevented CD8(+) T cell functional exhaustion and reduced mortality in mice lacking B cells. Overall, these results demonstrate that T. cruzi-specific B cells are necessary during systemic, but not mucosal, parasite challenge.

Immunomodulation by Trypanosoma cruzi: toward understanding the association of dendritic cells with infecting TcI and TcII populations

Dendritic cells (DCs) are major immune components, and depending on how these cells are modulated, the protective host immune response changes drastically. Trypanosoma cruzi is a parasite with high genetic variability and modulates DCs by interfering with their capacity for antigen recognition, migration, and maturation. Despite recent efforts, the association between DCs and T. cruzi I (TcI) and TcII populations is unknown. Herein, it was demonstrated that AQ1.7 and MUTUM TcI strains present low rates of invasion of bone marrow-derived DCs, whereas the 1849 and 2369 TcII strains present higher rates. Whereas the four strains similarly induced the expression of PD-L1, the production and expression of IL-10 and TLR-2, respectively, in DCs were differentially increased. The production of TNF-α, IL-12, IL-6, and CCL2 and the expression of CD40, CD80, MHC-II, CCR5, and CCR7 changed depending on the strain. The 2369 strain yielded the most remarkable results because greater invasion correlated with an increase in the levels of anti-inflammatory molecules IL-10 and PD-L1 but not with a change in the levels of TNF-α, MHC-II, or CD40 molecules. These results suggest that T. cruzi strains belonging to different populations have evolved specific evasion strategies that subvert DCs and consequently the host response.

Immunization with Hexon modified adenoviral vectors integrated with gp83 epitope provides protection against Trypanosoma cruzi infection

BACKGROUND

Trypanosoma cruzi is the causative agent of Chagas disease. Chagas disease is an endemic infection that affects over 8 million people throughout Latin America and now has become a global challenge. The current pharmacological treatment of patients is unsuccessful in most cases, highly toxic, and no vaccines are available. The results of inadequate treatment could lead to heart failure resulting in death. Therefore, a vaccine that elicits neutralizing antibodies mediated by cell-mediated immune responses and protection against Chagas disease is necessary.

METHODOLOGY/PRINCIPAL FINDINGS

The "antigen capsid-incorporation" strategy is based upon the display of the T. cruzi epitope as an integral component of the adenovirus' capsid rather than an encoded transgene. This strategy is predicted to induce a robust humoral immune response to the presented antigen, similar to the response provoked by native Ad capsid proteins. The antigen chosen was T. cruzi gp83, a ligand that is used by T. cruzi to attach to host cells to initiate infection. The gp83 epitope, recognized by the neutralizing MAb 4A4, along with His6 were incorporated into the Ad serotype 5 (Ad5) vector to generate the vector Ad5-HVR1-gp83-18 (Ad5-gp83). This vector was evaluated by molecular and immunological analyses. Vectors were injected to elicit immune responses against gp83 in mouse models. Our findings indicate that mice immunized with the vector Ad5-gp83 and challenged with a lethal dose of T. cruzi trypomastigotes confer strong immunoprotection with significant reduction in parasitemia levels, increased survival rate and induction of neutralizing antibodies.

CONCLUSIONS/SIGNIFICANCE

This data demonstrates that immunization with adenovirus containing capsid-incorporated T. cruzi antigen elicits a significant anti-gp83-specific response in two different mouse models, and protection against T. cruzi infection by eliciting neutralizing antibodies mediated by cell-mediated immune responses, as evidenced by the production of several Ig isotypes. Taken together, these novel results show that the recombinant Ad5 presenting T. cruzi gp83 antigen is a useful candidate for the development of a vaccine against Chagas disease.

Induction of death receptor CD95 and co-stimulatory molecules CD80 and CD86 by meningococcal capsular polysaccharide-loaded vaccine nanoparticles

Neisseria meningitidis is a leading cause of bacterial meningitis and sepsis, and its capsular polysaccharides (CPS) are a major virulence factor in meningococcal infections and form the basis for serogroup designation and protective vaccines. We formulated a novel nanovaccine containing meningococcal CPS as an antigen encapsulated in albumin-based nanoparticles (NPs) that does not require chemical conjugation to a protein carrier. These nanoparticles are taken up by antigen-presenting cells and act as antigen depot by slowly releasing the antigen. In this study, we determined the ability of CPS-loaded vaccine nanoparticles to induce co-stimulatory molecules, namely CD80, CD86, and CD95 that impact effective antigen presentation. Co-stimulatory molecule gene induction and surface expression on macrophages and dendritic cells pulsed with meningococcal CPS-loaded nanoparticles were investigated using gene array and flow cytometry methods. Meningococcal CPS-loaded NP significantly induced the surface protein expression of CD80 and CD86, markers of dendritic cell maturation, in human THP-1 macrophages and in murine dendritic cells DC2.4 in a dose-dependent manner. The massive upregulation was also observed at the gene expression. However, high dose of CPS-loaded NP, but not empty NP, induced the expression of death receptor CD95 (Fas) leading to reduced TNF-α release and reduction in cell viability. The data suggest that high expression of CD95 may lead to death of antigen-presenting cells and consequently suboptimal immune responses to vaccine. The CPS-loaded NP induces the expression of co-stimulatory molecules and acts as antigen depot and can spare antigen dose, highly desirable criteria for vaccine formulations.

CD8(+) T cell-mediated immunity during Trypanosoma cruzi infection: a path for vaccine development?

MHC-restricted CD8⁺ T cells are important during infection with the intracellular protozoan parasite Trypanosoma cruzi, the causative agent of Chagas disease. Experimental studies performed in the past 25 years have elucidated a number of features related to the immune response mediated by these T cells, which are important for establishing the parasite/host equilibrium leading to chronic infection. CD8⁺ T cells are specific for highly immunodominant antigens expressed by members of the trans-sialidase family. After infection, their activation is delayed, and the cells display a high proliferative activity associated with high apoptotic rates. Although they participate in parasite control and elimination, they are unable to clear the infection due to their low fitness, allowing the parasite to establish the chronic phase when these cells then play an active role in the induction of heart immunopathology. Vaccination with a number of subunit recombinant vaccines aimed at eliciting specific CD8⁺ T cells can reverse this path, thereby generating a productive immune response that will lead to the control of infection, reduction of symptoms, and reduction of disease transmission. Due to these attributes, activation of CD8⁺ T lymphocytes may constitute a path for the development of a veterinarian or human vaccine.

Chagas disease: still many unsolved issues

Over the past 20 years, the immune effector mechanisms involved in the control of Trypanosoma cruzi, as well as the receptors participating in parasite recognition by cells of the innate immune system, have been largely described. However, the main questions on the physiopathology of Chagas disease remain unanswered: “Why does the host immune system fail to provide sterile immunity?” and “Why do only a proportion of infected individuals develop chronic pathology?” In this review, we describe the mechanisms proposed to explain the inability of the immune system to eradicate the parasite and the elements that allow the development of chronic heart disease. Moreover, we discuss the possibility that the inability of infected cardiomyocytes to sense intracellular T. cruzi contributes to parasite persistence in the heart and the development of chronic pathology.

Genetic vaccination against experimental infection with myotropic parasite strains of Trypanosoma cruzi

In earlier studies, we reported that a heterologous prime-boost regimen using recombinant plasmid DNA followed by replication-defective adenovirus vector, both containing Trypanosoma cruzi genes encoding trans-sialidase (TS) and amastigote surface protein (ASP) 2, provided protective immunity against experimental infection with a reticulotropic strain of this human protozoan parasite. Herein, we tested the outcome of genetic vaccination of F1 (CB10XBALB/c) mice challenged with myotropic parasite strains (Brazil and Colombian). Initially, we determined that the coadministration during priming of a DNA plasmid containing the murine IL-12 gene improved the immune response and was essential for protective immunity elicited by the heterologous prime-boost regimen in susceptible male mice against acute lethal infections with these parasites. The prophylactic or therapeutic vaccination of resistant female mice led to a drastic reduction in the number of inflammatory infiltrates in cardiac and skeletal muscles during the chronic phase of infection with either strain. Analysis of the electrocardiographic parameters showed that prophylactic vaccination reduced the frequencies of sinus arrhythmia and atrioventricular block. Our results confirmed that prophylactic vaccination using the TS and ASP-2 genes benefits the host against acute and chronic pathologies caused by T. cruzi and should be further evaluated for the development of a veterinary or human vaccine against Chagas disease.

Patients with tuberculosis disease have Mycobacterium tuberculosis-specific CD8 T cells with a pro-apoptotic phenotype and impaired proliferative capacity, which is not restored following treatment

CD8 T cells play a critical role in control of chronic viral infections; however, the role of these cells in containing persistent bacterial infections, such as those caused by Mycobacterium tuberculosis (Mtb), is less clear. We assessed the phenotype and functional capacity of CD8 T cells specific for the immunodominant Mtb antigens CFP-10 and ESAT-6, in patients with pulmonary tuberculosis (TB) disease, before and after treatment, and in healthy persons with latent Mtb infection (LTBI). In patients with TB disease, CFP-10/ESAT-6-specific IFN-γ+ CD8 T cells had an activated, pro-apoptotic phenotype, with lower Bcl-2 and CD127 expression, and higher Ki67, CD57, and CD95 expression, than in LTBI. When CFP-10/ESAT-6-specific IFN-γ+ CD8 T cells were detectable, expression of distinct combinations of these markers was highly sensitive and specific for differentiating TB disease from LTBI. Successful treatment of disease resulted in changes of these markers, but not in restoration of CFP-10/ESAT-6-specific CD8 or CD4 memory T cell proliferative capacity. These data suggest that high mycobacterial load in active TB disease is associated with activated, short-lived CFP-10/ESAT-6-specific CD8 T cells with impaired functional capacity that is not restored following treatment. By contrast, LTBI is associated with preservation of long-lived CFP-10/ESAT-6-specific memory CD8 T cells that maintain high Bcl-2 expression and which may readily proliferate.

Adenovirus vector-induced CD8⁺ T effector memory cell differentiation and recirculation, but not proliferation, are important for protective immunity against experimental Trypanosoma cruzi Infection

Heterologous prime-boost vaccination using plasmid DNA followed by replication-defective adenovirus vector generates a large number of specific CD8⁺ T effector memory (TEM) cells that provide long-term immunity against a variety of pathogens. In the present study, we initially characterized the frequency, phenotype, and function of these T cells in vaccinated mice that were subjected to infectious challenge with the human protozoan parasite Trypanosoma cruzi. We observed that the frequency of the specific CD8⁺ T cells in the spleens of the vaccinated mice increased after challenge. Specific TEM cells differentiated into cells with a KLRG1(High) CD27(Low) CD43(Low) CD183(Low)T-bet(High) Eomes(Low) phenotype and capable to produce simultaneously the antiparasitic mediators IFNγ and TNF. Using the gzmBCreERT2/ROSA26EYFP transgenic mouse line, in which the cells that express Granzyme B after immunization, are indelibly labeled with enhanced yellow fluorescent protein, we confirmed that CD8⁺ T cells present after challenge were indeed TEM cells that had been induced by vaccination. Subsequently, we observed that the in vivo increase in the frequency of the specific CD8⁺ T cells was not because of an anamnestic immune response. Most importantly, after challenge, the increase in the frequency of specific cells and the protective immunity they mediate were insensitive to treatment with the cytostatic toxic agent hydroxyurea. We have previously described that the administration of the drug FTY720, which reduces lymphocyte recirculation, severely impairs protective immunity, and our evidence supports the model that when large amounts of antigen-experienced CD8⁺ TEM cells are present after heterologous prime-boost vaccination, differentiation, and recirculation, rather than proliferation, are key for the resultant protective immunity.

Impairment of T cell function in parasitic infections

In mammals subverted as hosts by protozoan parasites, the latter and/or the agonists they release are detected and processed by sensors displayed by many distinct immune cell lineages, in a tissue(s)-dependent context. Focusing on the T lymphocyte lineage, we review our present understanding on its transient or durable functional impairment over the course of the developmental program of the intracellular parasites Leishmania spp., Plasmodium spp., Toxoplasma gondii, and Trypanosoma cruzi in their mammalian hosts. Strategies employed by protozoa to down-regulate T lymphocyte function may act at the initial moment of naïve T cell priming, rendering T cells anergic or unresponsive throughout infection, or later, exhausting T cells due to antigen persistence. Furthermore, by exploiting host feedback mechanisms aimed at maintaining immune homeostasis, parasites can enhance T cell apoptosis. We will discuss how infections with prominent intracellular protozoan parasites lead to a general down-regulation of T cell function through T cell anergy and exhaustion, accompanied by apoptosis, and ultimately allowing pathogen persistence.

Immunogenicity of a prime-boost vaccine containing the circumsporozoite proteins of Plasmodium vivax in rodents

Plasmodium vivax is the most widespread and the second most prevalent malaria-causing species in the world. Current measures used to control the transmission of this disease would benefit from the development of an efficacious vaccine. In the case of the deadly parasite P. falciparum, the recombinant RTS,S vaccine containing the circumsporozoite antigen (CSP) consistently protects 30 to 50% of human volunteers against infection and is undergoing phase III clinical trials in Africa with similar efficacy. These findings encouraged us to develop a P. vivax vaccine containing the three circulating allelic forms of P. vivax CSP. Toward this goal, we generated three recombinant bacterial proteins representing the CSP alleles, as well as a hybrid polypeptide called PvCSP-All-CSP-epitopes. This hybrid contains the conserved N and C termini of P. vivax CSP and the three variant repeat domains in tandem. We also generated simian and human recombinant replication-defective adenovirus vectors expressing PvCSP-All-CSP-epitopes. Mice immunized with the mixture of recombinant proteins in a formulation containing the adjuvant poly(I·C) developed high and long-lasting serum IgG titers comparable to those elicited by proteins emulsified in complete Freund's adjuvant. Antibody titers were similar in mice immunized with homologous (protein-protein) and heterologous (adenovirus-protein) vaccine regimens. The antibodies recognized the three allelic forms of CSP, reacted to the repeated and nonrepeated regions of CSP, and recognized sporozoites expressing the alleles VK210 and VK247. The vaccine formulations described in this work should be useful for the further development of an anti-P. vivax vaccine.

Vaccination using recombinants influenza and adenoviruses encoding amastigote surface protein-2 are highly effective on protection against Trypanosoma cruzi infection

In the present study we evaluated the protection raised by immunization with recombinant influenza viruses carrying sequences coding for polypeptides corresponding to medial and carboxi-terminal moieties of Trypanosoma cruzi ´s amastigote surface protein 2 (ASP2). Those viruses were used in sequential immunization with recombinant adenovirus (heterologous prime-boost immunization protocol) encoding the complete sequence of ASP2 (Ad-ASP2) in two mouse strains (C57BL/6 and C3H/He). The CD8 effector response elicited by this protocol was comparable to that observed in mice immunized twice with Ad-ASP2 and more robust than that observed in mice that were immunized once with Ad-ASP2. Whereas a single immunization with Ad-ASP2 sufficed to completely protect C57BL/6 mice, a higher survival rate was observed in C3H/He mice that were primed with recombinant influenza virus and boosted with Ad-ASP2 after being challenged with T. cruzi. Analyzing the phenotype of CD8+ T cells obtained from spleen of vaccinated C3H/He mice we observed that heterologous prime-boost immunization protocol elicited more CD8+ T cells specific for the immunodominant epitope as well as a higher number of CD8+ T cells producing TNF-α and IFN-γ and a higher mobilization of surface marker CD107a. Taken together, our results suggest that immunodominant subpopulations of CD8+ T elicited after immunization could be directly related to degree of protection achieved by different immunization protocols using different viral vectors. Overall, these results demonstrated the usefulness of recombinant influenza viruses in immunization protocols against Chagas Disease.

Challenge of chronically infected mice with homologous trypanosoma cruzi parasites enhances the immune response but does not modify cardiopathy: implications for the design of a therapeutic vaccine

Chagas disease is a Trypanosoma cruzi-induced zoonosis that has no natural cure. Local damage induced by the parasite and the immune response causes chronic heart and digestive lesions. Efforts to develop a therapeutic vaccine that boosts the immune response to completely clear the parasite are needed because there is no effective treatment for chronically infected patients. In an attempt to modify the host-parasite equilibrium to increase parasite destruction, we analyzed cardiopathy and the immune response in chronically infected mice that were challenged with live homologous parasites. Challenge with a single dose of parasite increased CD4(+) and CD8(+) T cell populations, gamma interferon (IFN-γ) production, and serum-specific IgG levels. However, subpatent parasitemias and cardiac tissue were not affected. Because of the short duration of the immune boost after a single challenge, we next evaluated the impact of four parasite doses, administered 3 weeks apart. At 1 to 2 months after the last dose, the numbers of CD4(+) T cells and IFN-γ-producing CD4(+) memory cells and the CD4(+) T cell proliferative response to T. cruzi antigen were increased in the spleen. The frequency of IFN-γ-producing CD8(+) memory cells in the blood was also increased. However, the sustained challenge did not favor TH1 development; rather, it induced an increase in serum-specific IgG1 levels and mixed TH1/TH2 cytokine production. Moreover, there were no significant changes in cardiac lesions and subpatent parasitemias. In conclusion, we believe that this study may help in elucidating the necessary elements for a successful therapeutic vaccine which may reduce cardiomyopathy in chronically infected human patients.

Relevance of long-lived CD8(+) T effector memory cells for protective immunity elicited by heterologous prime-boost vaccination

Owing to the importance of major histocompatibility complex class Ia-restricted CD8(+) T cells for host survival following viral, bacterial, fungal, or parasitic infection, it has become largely accepted that these cells should be considered in the design of a new generation of vaccines. For the past 20 years, solid evidence has been provided that the heterologous prime-boost regimen achieves the best results in terms of induction of long-lived protective CD8(+) T cells against a variety of experimental infections. Although this regimen has often been used experimentally, as is the case for many vaccines, the mechanism behind the efficacy of this vaccination regimen is still largely unknown. The main purpose of this review is to examine the characteristics of the protective CD8(+) T cells generated by this vaccination regimen. Part of its efficacy certainly relies on the generation and maintenance of large numbers of specific lymphocytes. Other specific characteristics may also be important, and studies on this direction have only recently been initiated. So far, the characterization of these protective, long-lived T cell populations suggests that there is a high frequency of polyfunctional T cells; these cells cover a large breadth and display a T effector memory (TEM) phenotype. These TEM cells are capable of proliferating after an infectious challenge and are highly refractory to apoptosis due to a control of the expression of pro-apoptotic receptors such as CD95. Also, they do not undergo significant long-term immunological erosion. Understanding the mechanisms that control the generation and maintenance of the protective activity of these long-lived TEM cells will certainly provide important insights into the physiology of CD8(+) T cells and pave the way for the design of new or improved vaccines.