Prolonged antigen presentation is required for optimal CD8+ T cell responses against malaria liver stage parasites

Domain-based mRNA vaccines encoding spike protein N-terminal and receptor binding domains confer protection against SARS-CoV-2

With the success of messenger RNA (mRNA) vaccines against coronavirus disease 2019, strategies can now focus on improving vaccine potency, breadth, and stability. We designed and evaluated domain-based mRNA vaccines encoding the wild-type spike protein receptor binding domain (RBD) or N-terminal domain (NTD) alone or in combination. An NTD-RBD-linked candidate vaccine, mRNA-1283, showed improved antigen expression, antibody responses, and stability at refrigerated temperatures (2° to 8°C) compared with the clinically available mRNA-1273, which encodes the full-length spike protein. In BALB/c mice administered mRNA-1283 as a primary series, booster, or variant-specific booster, similar or greater immune responses from viral challenge were observed against wild-type, beta, delta, or omicron (BA.1) viruses compared with mRNA-1273-immunized mice, especially at lower vaccine dosages. K18-hACE2 mice immunized with mRNA-1283 or mRNA-1273 as a primary series demonstrated similar degrees of protection from challenge with SARS-CoV-2 Delta and Omicron variants at all vaccine dosages. These results support clinical assessment of mRNA-1283, which has now entered clinical trials (NCT05137236).

Development of SARS-CoV-2 mRNA vaccines encoding spike N-terminal and receptor binding domains

With the success of mRNA vaccines against coronavirus disease 2019 (COVID-19), strategies can now focus on improving vaccine potency, breadth, and stability. We present the design and preclinical evaluation of domain-based mRNA vaccines encoding the wild-type spike-protein receptor-binding (RBD) and/or N-terminal domains (NTD). An NTD-RBD linked candidate vaccine, mRNA-1283, showed improved antigen expression, antibody responses, and stability at refrigerated temperatures (2-8°C) compared with the clinically available mRNA-1273, which encodes the full-length spike protein. In mice administered mRNA-1283 as a primary series, booster, or variant-specific booster, similar or greater immune responses and protection from viral challenge were observed against wild-type, beta, delta, or omicron (BA. 1) compared with mRNA-1273 immunized mice, especially at lower vaccine dosages. These results support clinical assessment of mRNA-1283 ( NCT05137236 ).

One Sentence Summary

A domain-based mRNA vaccine, mRNA-1283, is immunogenic and protective against SARS-CoV-2 and emerging variants in mice.

Splenic Dendritic Cells and Macrophages Drive B Cells to Adopt a Plasmablast Cell Fate

Upon encountering cognate antigen, B cells can differentiate into short-lived plasmablasts, early memory B cells or germinal center B cells. The factors that determine this fate decision are unclear. Past studies have addressed the role of B cell receptor affinity in this process, but the interplay with other cellular compartments for fate determination is less well understood. Moreover, B cell fate decisions have primarily been studied using model antigens rather than complex pathogen systems, which potentially ignore multifaceted interactions from other cells subsets during infection. Here we address this question using a Plasmodium infection model, examining the response of B cells specific for the immunodominant circumsporozoite protein (CSP). We show that B cell fate is determined in part by the organ environment in which priming occurs, with the majority of the CSP-specific B cell response being derived from splenic plasmablasts. This plasmablast response could occur independent of T cell help, though gamma-delta T cells were required to help with the early isotype switching from IgM to IgG. Interestingly, selective ablation of CD11c⁺ dendritic cells and macrophages significantly reduced the splenic plasmablast response in a manner independent of the presence of CD4 T cell help. Conversely, immunization approaches that targeted CSP-antigen to dendritic cells enhanced the magnitude of the plasmablast response. Altogether, these data indicate that the early CSP-specific response is predominately primed within the spleen and the plasmablast fate of CSP-specific B cells is driven by macrophages and CD11c⁺ dendritic cells.

Approaches of the Innate Immune System to Ameliorate Adaptive Immunotherapy for B-Cell Non-Hodgkin Lymphoma in Their Microenvironment

A dominant paradigm being developed in immunotherapy for hematologic malignancies is of adaptive immunotherapy that involves chimeric antigen receptor (CAR) T cells and bispecific T-cell engagers. CAR T-cell therapy has yielded results that surpass those of the existing salvage immunochemotherapy for patients with relapsed/refractory diffuse large B-cell lymphoma (DLBCL) after first-line immunochemotherapy, while offering a therapeutic option for patients with follicular lymphoma (FL) and mantle cell lymphoma (MCL). However, the role of the innate immune system has been shown to prolong CAR T-cell persistence. Cluster of differentiation (CD) 47-blocking antibodies, which are a promising therapeutic armamentarium for DLBCL, are novel innate immune checkpoint inhibitors that allow macrophages to phagocytose tumor cells. Intratumoral Toll-like receptor 9 agonist CpG oligodeoxynucleotide plays a pivotal role in FL, and vaccination may be required in MCL. Additionally, local stimulator of interferon gene agonists, which induce a systemic anti-lymphoma CD8⁺ T-cell response, and the costimulatory molecule 4-1BB/CD137 or OX40/CD134 agonistic antibodies represent attractive agents for dendritic cell activations, which subsequently, facilitates initiation of productive T-cell priming and NK cells. This review describes the exploitation of approaches that trigger innate immune activation for adaptive immune cells to operate maximally in the tumor microenvironment of these lymphomas.

Chemoprophylaxis vaccination with a Plasmodium liver stage autophagy mutant affords enhanced and long-lasting protection

Genetically attenuated sporozoite vaccines can elicit long-lasting protection against malaria but pose risks of breakthrough infection. Chemoprophylaxis vaccination (CVac) has proven to be the most effective vaccine strategy against malaria. Here, we demonstrate that a liver stage-specific autophagy mutant of Plasmodium berghei (ATG8 overexpressor), when used as a live vaccine under a CVac regimen, provides superior long-lasting protection, in both inbred and outbred mice, as compared to WT-CVac. Uniquely, the protection elicited by this mutant is predominantly dependent on a CD8⁺ T-cell response through an IFN-γ-independent mechanism and is associated with a stable population of antigen-experienced CD8⁺ T cells. Jointly, our findings support the exploitation of liver-stage mutants as vaccines under a CVac protocol. This vaccination strategy is also a powerful model to study the mechanisms of protective immunity and discover new protective antigens.

CpG Oligonucleotides as Vaccine Adjuvants

CpG Oligonucleotides (ODN) are immunomodulatory synthetic oligonucleotides specifically designed to stimulate Toll-like receptor 9. TLR9 is expressed on human plasmacytoid dendritic cells and B cells and triggers an innate immune response characterized by the production of Th1 and pro-inflammatory cytokines. This chapter reviews recent progress in understanding the mechanism of action of CpG ODN and provides an overview of human clinical trial results using CpG ODN to improve vaccines for the prevention/treatment of cancer, allergy, and infectious disease.

Development of Plasmodium-specific liver-resident memory CD8+ T cells after heat-killed sporozoite immunization in mice

Malaria remains a major cause of mortality in the world and an efficient vaccine is the best chance of reducing the disease burden. Vaccination strategies for the liver stage of disease that utilise injection of live radiation-attenuated sporozoites (RAS) confer sterile immunity, which is mediated by CD8⁺ memory T cells, with liver-resident memory T cells (T_RM ) being particularly important. We have previously described a TCR transgenic mouse, termed PbT-I, where all CD8⁺ T cells recognize a specific peptide from Plasmodium. PbT-I form liver T_RM cells upon RAS injection and are capable of protecting mice against challenge infection. Here, we utilize this transgenic system to examine whether nonliving sporozoites, killed by heat treatment (HKS), could trigger the development of Plasmodium-specific liver T_RM cells. We found that HKS vaccination induced the formation of memory CD8⁺ T cells in the spleen and liver, and importantly, liver T_RM cells were fewer in number than that induced by RAS. Crucially, we showed the number of T_RM cells was significantly higher when HKS were combined with the glycolipid α-galactosylceramide as an adjuvant. In the future, this work could lead to development of an antimalaria vaccination strategy that does not require live sporozoites, providing greater utility.

Use of Clodronate Liposomes to Deplete Phagocytic Immune Cells in Drosophila melanogaster and Aedes aegypti

The innate immune system is the primary defense response to limit invading pathogens for all invertebrate species. In insects, immune cells are central to both cellular and humoral immune responses, however few genetic resources exist beyond Drosophila to study immune cell function. Therefore, the development of innovative tools that can be widely applied to a variety of insect systems is of importance to advance the study of insect immunity. Here, we have adapted the use of clodronate liposomes (CLD) to deplete phagocytic immune cells in the vinegar fly, Drosophila melanogaster, and the yellow fever mosquito, Aedes aegypti. Through microscopy and molecular techniques, we validate the depletion of phagocytic cell populations in both insect species and demonstrate the integral role of phagocytes in combating bacterial pathogens. Together, these data demonstrate the wide utility of CLD in insect systems to advance the study of phagocyte function in insect innate immunity.

Protection of Batf3-deficient mice from experimental cerebral malaria correlates with impaired cytotoxic T-cell responses and immune regulation

Excessive inflammatory immune responses during infections with Plasmodium parasites are responsible for severe complications such as cerebral malaria (CM) that can be studied experimentally in mice. Dendritic cells (DCs) activate cytotoxic CD8+ T-cells and initiate immune responses against the parasites. Batf3-/- mice lack a DC subset, which efficiently induces strong CD8 T-cell responses by cross-presentation of exogenous antigens. Here we show that Batf3-/- mice infected with Plasmodium berghei ANKA (PbA) were protected from experimental CM (ECM), characterized by a stable blood-brain barrier (BBB) and significantly less infiltrated peripheral immune cells in the brain. Importantly, the absence of ECM in Batf3-/- mice correlated with attenuated responses of cytotoxic T-cells, as their parasite-specific lytic activity as well as the production of interferon gamma and granzyme B were significantly decreased. Remarkably, spleens of ECM-protected Batf3-/- mice had elevated levels of regulatory immune cells and interleukin 10. Thus, protection from ECM in PbA-infected Batf3-/- mice was associated with the absence of strong CD8+ T-cell activity and induction of immunoregulatory mediators and cells.

A DNA Vaccine in Which the RSV-F Ectodomain Is Covalently Linked to the Burkholderia pseudomallei Antigens TssM and Hcp1 Augments the Humoral and Cytotoxic Response in Mice

DNA vaccines have great potential to control infectious disease, particularly those caused by intracellular organisms. They are inexpensive to produce and can be quickly modified to combat emerging infectious threats, but often fail to generate a strong immunologic response limiting enthusiasm for their use in humans and animals. To improve the immunogenic response, we developed a DNA vaccine in which the F protein ectodomain of Respiratory Syncytial Virus (RSV-F) was covalently linked to specific antigens of interest. The presence of the RSV-F ectodomain allowed secretion of the translated fusion product out of the originally transfected cells followed by its active binding to adjacent cells. This allowed the targeting of a greater number of cells than those originally transfected, enhancing both humoral and cytotoxic immune responses against the expressed antigen(s). We developed an engrafted mouse model that used antigen-expressing tumor cells to assess the in vivo cytotoxic immune response to specific antigens. We then used this model to demonstrate that a DNA vaccine in which the RSV-F ectodomain is fused to two antigens expressed by Burkholderia pseudomallei, the intracellular gram-negative organism that causes melioidosis, generated a stronger cytotoxic response than a DNA vaccine that lacked the RSV-F sequence while still generating a robust humoral response.

Attenuated plasmodium sporozoite expressing MAGE-A3 induces antigen-specific CD8+ T cell response against lung cancer in mice

Objective

Cancer vaccines that rely on tumor antigen-specific CD8⁺ T cell responses, are promising anti-cancer adjuvant immunotherapies. This study investigated whether genetically attenuated Plasmodium sporozoite (GAS) could be used as a novel vector to induce antigen-specific CD8⁺ T cell responses against lung cancer.

Methods

We constructed GAS/MAGE-A3, a recombinant GAS engineered to express the lung cancer-specific antigen, melanoma-associated antigen 3 (MAGE-A3), and assessed its therapeutic effects against lung cancer.

Results

Robust parasite-specific CD8α^lowCD11a^high and CD49d^highCD11a^high CD4⁺ T cell responses as well as a MAGE-A3-specific CD8⁺ T cell response were induced in GAS/MAGE-A3-immunized mice. Adoptive transfer of GAS/MAGE-A3-induced CD8⁺ T cells from HLA-A2 transgenic mice into lung cancer-bearing nude mice inhibited tumor growth and prolonged survival.

Conclusions

These findings demonstrate that GAS/MAGE-A3 induces a strong MAGE-A3-specific CD8⁺ T cell response against lung cancer in vivo, and indicate that GAS is a novel and efficacious antigen delivery vector for antitumor immunotherapy.

Innate Lymphocytes and Malaria - Players or Spectators?

Malaria remains an important global disease. Despite significant advances over the past decade in reducing disease morbidity and mortality, new measures are needed if malaria is to be eliminated. Significant advances in our understanding about host immune responses during malaria have been made, opening up opportunities to generate long-lasting antiparasitic immunity through vaccination or immune therapy. However, there is still much debate over which immune cell populations contribute to immunity to malaria, including innate lymphocytes that comprise recently identified innate lymphoid cells (ILCs) and better known innate-like T cell subsets. Here, we review research on these immune cell subsets and discuss whether they have any important roles in immunity to malaria or if they are redundant.

Prime and target immunization protects against liver-stage malaria in mice

Despite recent advances in treatment and vector control, malaria is still a leading cause of death, emphasizing the need for an effective vaccine. The malaria life cycle can be subdivided into three stages: the invasion and growth within liver hepatocytes (pre-erythrocytic stage), the blood stage (erythrocytic stage), and, finally, the sexual stage (occurring within the mosquito vector). Antigen (Ag)-specific CD8+ T cells are effectively induced by heterologous prime-boost viral vector immunization and known to correlate with liver-stage protection. However, liver-stage malaria vaccines have struggled to generate and maintain the high numbers of Plasmodium-specific circulating T cells necessary to confer sterile protection. We describe an alternative “prime and target” vaccination strategy aimed specifically at inducing high numbers of tissue-resident memory T cells present in the liver at the time of hepatic infection. This approach bypasses the need for very high numbers of circulating T cells and markedly increases the efficacy of subunit immunization against liver-stage malaria with clinically relevant Ags and clinically tested viral vectors in murine challenge models. Translation to clinical use has begun, with encouraging results from a pilot safety and feasibility trial of intravenous chimpanzee adenovirus vaccination in humans. This work highlights the value of a prime-target approach for immunization against malaria and suggests that this strategy may represent a more general approach for prophylaxis or immunotherapy of other liver infections and diseases.

A Plasmodium Parasite with Complete Late Liver Stage Arrest Protects against Preerythrocytic and Erythrocytic Stage Infection in Mice

Genetically attenuated malaria parasites (GAP) that arrest during liver stage development are powerful immunogens and afford complete and durable protection against sporozoite infection. Late liver stage-arresting GAP provide superior protection against sporozoite challenge in mice compared to early live stage-arresting attenuated parasites. However, very few late liver stage-arresting GAP have been generated to date. Therefore, identification of additional loci that are critical for late liver stage development and can be used to generate novel late liver stage-arresting GAPs is of importance. We further explored genetic attenuation in Plasmodium yoelii by combining two gene deletions, PlasMei2 and liver-specific protein 2 (LISP2), that each cause late liver stage arrest with various degrees of infrequent breakthrough to blood stage infection. The dual gene deletion resulted in a synthetic lethal phenotype that caused complete attenuation in a highly susceptible mouse strain. P. yoeliiplasmei2^-lisp2^- arrested late in liver stage development and did not persist in livers beyond 3 days after infection. Immunization with this GAP elicited robust protective antibody responses in outbred and inbred mice against sporozoites, liver stages, and blood stages as well as eliciting protective liver-resident T cells. The immunization afforded protection against both sporozoite challenge and blood stage challenge. These findings provide evidence that completely attenuated late liver stage-arresting GAP are achievable via the synthetic lethal approach and might enable a path forward for the creation of a completely attenuated late liver stage-arresting P. falciparum GAP.

Induction of immunity following vaccination with a chemically attenuated malaria vaccine correlates with persistent antigenic stimulation

Objectives

Blood stage malaria parasites attenuated with seco-cyclopropyl pyrrolo indole (CPI) analogues induce robust immunity in mice to homologous and heterologous malaria parasites and are being considered for the development of a human vaccine. However, it is not understood how attenuated parasites induce immunity. We showed that following vaccination, parasite DNA persisted in blood for several months, raising the possibility that ongoing immune stimulation may be critical. However, parasites were not seen microscopically beyond 24 h postvaccination. We aimed to provide a mechanistic understanding of immune induction.

Methods

Mice were vaccinated with chemically attenuated Plasmodium chabaudi parasites. PCR and adoptive transfer studies were used to determine the presence of parasites and antigen in vivo. In other experiments, Plasmodium falciparum parasitised red blood cells were attenuated in vitro and RNA and antigen expression studied.

Results

We show that blood transferred from vaccinated mice into naïve mice activates T cells and induces complete protective immunity in the recipient mice strongly suggesting that there is persistence of parasite antigen postvaccination. This is supported by the presence of parasite RNA in vaccinated mice and both RNA and antigen expression in P. falciparum cultures treated with CPI drugs in vitro. In addition, drugs that block parasite growth also prevent the induction of immunity in vaccinated mice, indicating that some growth of attenuated parasites is required for immune induction.

Conclusions

Attenuated parasites persist at submicroscopic levels in the blood of mice postvaccination with the ability to activate T cells and induce ongoing protective immune responses.

T-cell responses against Malaria: Effect of parasite antigen diversity and relevance for vaccine development

The on-going agenda for global malaria elimination will require the development of additional disease control and prevention measures since currently available tools are showing signs of inadequacy. Malaria vaccines are seen as one such important addition to the control arsenal since vaccines have proven to be highly effective public health tools against important human diseases. Both cell-mediated and antibody responses are generally believed to be important for malaria parasite control, although the exact targets of T and B cell responses against malaria have not been clearly defined. However, our current understanding of the immune response to malaria suggests that T cell responses against multiple antigenic targets may potentially be key for the development of a highly efficacious malaria vaccine. This review takes a comprehensive look at the available literature on T cell-mediated immunity against all human stages of the malaria parasite and the effect of antigen diversity on these responses. The implications of these interrelationships for the development of an effective vaccine for malaria are also highlighted.

Targeting Angiotensin II Type-1 Receptor (AT1R) Inhibits the Harmful Phenotype of Plasmodium-Specific CD8+ T Cells during Blood-Stage Malaria

CD8⁺ T-cell response is critical in the pathogenesis of cerebral malaria during blood-stage. Our group and other have been shown that angiotensin II (Ang II) and its receptor AT₁ (AT₁R), a key effector axis of renin-angiotensin system (RAS), have immune regulatory effects on T cells. Previously, we showed that inhibition of AT₁R signaling protects mice against the lethal disease induced by Plasmodium berghei ANKA infection However, most of the Ang II/AT₁R actions were characterized by using only pharmacological approaches, the effects of which may not always be due to a specific receptor blockade. In addition, the mechanisms of action of the AT₁R in inducing the pathogenic activity of Plasmodium-specific CD8⁺ T cells during blood-stage were not determined. Here, we examined how angiotensin II/AT₁R axis promotes the harmful response of Plasmodium-specific CD8⁺ T-cell during blood-stage by using genetic and pharmacological approaches. We evaluated the response of wild-type (WT) and AT₁R^−/−Plasmodium-specific CD8⁺ T cells in mice infected with a transgenic PbA lineage expressing ovalbumin; and in parallel infected mice receiving WT Plasmodium-specific CD8⁺ T cells were treated with losartan (AT₁R antagonist) or captopril (ACE inhibitor). Both, AT₁R^−/− OT-I cells and WT OT-I cells from losartan- or captopril-treated mice showed lower expansion, reduced IL-2 production and IL-2Rα expression, lower activation (lower expression of CD69, CD44 and CD160) and lower exhaustion profiles. AT₁R^−/− OT-I cells also exhibit lower expression of the integrin LFA-1 and the chemokine receptors CCR5 and CXCR3, known to play a key role in the development of cerebral malaria. Moreover, AT₁R^−/− OT-I cells produce lower amounts of IFN-γ and TNF-α and show lower degranulation upon restimulation. In conclusion, our results show the pivotal mechanisms of AT₁R-induced harmful phenotype of Plasmodium-specific CD8⁺ T cells during blood-stage malaria.

The role of liver sinusoidal cells in local hepatic immune surveillance

Although the liver's function as unique immune organ regulating immunity has received a lot of attention over the last years, the mechanisms determining hepatic immune surveillance against infected hepatocytes remain less well defined. Liver sinusoidal cells, in particular, liver sinusoidal endothelial cells (LSECs) and Kupffer cells (KCs), serve as physical platform for recruitment and anchoring of blood-borne immune cells in the liver. Liver sinusoidal cells also function as portal of entry for infectious microorganisms targeting the liver such as hepatotropic viruses, bacteria or parasites. At the same time, liver sinusoidal cells actively contribute to achieve immune surveillance against bacterial and viral infections. KCs function as adhesion hubs for CD8 T cells from the circulation, which initiates the interaction of virus-specific CD8 T cells with infected hepatocytes. Through their phagocytic function, KCs contribute to removal of bacteria from the circulation and engage in cross talk with sinusoidal lymphocyte populations to achieve elimination of phagocytosed bacteria. LSECs contribute to local immune surveillance through cross-presentation of viral antigens that causes antigen-specific retention of CD8 T cells from the circulation. Such cross-presentation of viral antigens activates CD8 T cells to release TNF that in turn triggers selective killing of virus-infected hepatocytes. Beyond major histocompatibility complex (MHC)-restricted T-cell immunity, CD1- and MR1-restricted innate-like lymphocytes are found in liver sinusoids whose roles in local immune surveillance against infection need to be defined. Thus, liver sinusoidal cell populations bear key functions for hepatic recruitment and for local activation of immune cells, which are both required for efficient immune surveillance against infection in the liver.

Angiotensin II type-1 receptor (AT1R) regulates expansion, differentiation, and functional capacity of antigen-specific CD8+ T cells

Angiotensin II (Ang II) and its receptor AT₁ (AT₁R), an important effector axis of renin-angiotensin system (RAS), have been demonstrated to regulate T-cell responses. However, these studies characterized Ang II and AT₁R effects using pharmacological tools, which do not target only Ang II/AT₁R axis. The specific role of AT₁R expressed by antigen-specific CD8⁺ T cells is unknown. Then we immunized transgenic mice expressing a T-cell receptor specific for SIINFEKL epitope (OT-I mice) with sporozoites of the rodent malaria parasite Plasmodium berghei expressing the cytotoxic epitope SIINFEKL. Early priming events after immunization were not affected but the expansion and contraction of AT₁R-deficient (AT₁R^-/-) OT-I cells was decreased. Moreover, they seemed more activated, express higher levels of CTLA-4, PD-1, LAG-3, and have decreased functional capacity during the effector phase. Memory AT₁R^-/- OT-I cells exhibited higher IL-7Rα expression, activation, and exhaustion phenotypes but less cytotoxic capacity. Importantly, AT₁R^-/- OT-I cells show better control of blood parasitemia burden and ameliorate mice survival during lethal disease induced by blood-stage malaria. Our study reveals that AT₁R in antigen-specific CD8⁺ T cells regulates expansion, differentiation, and function during effector and memory phases of the response against Plasmodium, which could apply to different infectious agents.

Development of whole sporozoite malaria vaccines

INTRODUCTION

Despite recent advances, malaria remains a major health threat both to populations in endemic areas as well travelers, including military personnel, to these areas. Subunit vaccines have not yet achieved sufficient efficacy needed for use in any of these at risk populations. Areas covered: This review discusses the current status of various whole sporozoite vaccine approaches and is mainly focused on current clinical trials. Expert commentary: Nearly 100% efficacy was achieved by administering multiple bites of radiation-attenuated sporozoite (RAS) Plasmodium falciparum-infected mosquitoes; this is impractical for widespread use. Now, this high level efficacy has been reproduced using purified, metabolically active RAS (PfSPZ Sanaria® Vaccine), which is undergoing extensive clinical testing. Alternative whole sporozoite vaccines include immunization with fully infectious sporozoites under chloroquine prophylaxis (CPS) or as genetically-attenuated parasites (GAP). By also manufacturing purified infectious sporozoites, it is now possible to combine these with CPS and GAP, as well as perform challenge studies using controlled doses of sporozoites.

Route of administration of attenuated sporozoites is instrumental in rendering immunity against Plasmodia infection

Whole sporozoite vaccine (WSV) approach has been shown to induce efficient CD8(+) T cell response, critical for developing of long-lasting sterile protection against Plasmodium. Although WSV was initiated over four decades ago, we still do not fully understand about the absolute requirements for the generation of liver-stage specific CD8(+) T memory cells. For more than a decade intravenous (IV) route of immunization has been shown to be protective in pre-clinical studies. However, the intradermal (ID) route is preferred over IV route by many researchers as it is perceived to mimic the natural route of parasite delivery through mosquito bite. Various clinical studies have shown that ID route provokes poor protective responses compared to those seen with IV route of administration. The present study highlights the importance of circumsporozoite (CS) protein in preventing sporozoite entry to the hepatocytes, which however, it is not necessarily sufficient to ensure sterile protection. Instead, this article favors the idea that liver-stage development is a prime requirement for generation of antigen specific CD8(+) T cells and suggests the conditions favored by IV inoculation of sporozoite.

Polymorphisms in Plasmodium vivax Circumsporozoite Protein (CSP) Influence Parasite Burden and Cytokine Balance in a Pre-Amazon Endemic Area from Brazil

Mechanisms involved in severe P. vivax malaria remain unclear. Parasite polymorphisms, parasite load and host cytokine profile may influence the course of infection. In this study, we investigated the influence of circumsporozoite protein (CSP) polymorphisms on parasite load and cytokine profile in patients with vivax malaria. A cross-sectional study was carried out in three cities: São Luís, Cedral and Buriticupu, Maranhão state, Brazil, areas of high prevalence of P. vivax. Interleukin (IL)-2, IL-4, IL-10, IL-6, IL-17, tumor necrosis factor alpha (TNF-α, interferon gamma (IFN-γ and transforming growth factor beta (TGF-β were quantified in blood plasma of patients and in supernatants from peripheral blood mononuclear cell (PBMC) cultures. Furthermore, the levels of cytokines and parasite load were correlated with VK210, VK247 and P. vivax-like CSP variants. Patients infected with P. vivax showed increased IL-10 and IL-6 levels, which correlated with the parasite load, however, in multiple comparisons, only IL-10 kept this association. A regulatory cytokine profile prevailed in plasma, while an inflammatory profile prevailed in PBMC culture supernatants and these patterns were related to CSP polymorphisms. VK247 infected patients showed higher parasitaemia and IL-6 concentrations, which were not associated to IL-10 anti-inflammatory effect. By contrast, in VK210 patients, these two cytokines showed a strong positive correlation and the parasite load was lower. Patients with the VK210 variant showed a regulatory cytokine profile in plasma, while those infected with the VK247 variant have a predominantly inflammatory cytokine profile and higher parasite loads, which altogether may result in more complications in infection. In conclusion, we propose that CSP polymorphisms is associated to the increase of non-regulated inflammatory immune responses, which in turn may be associated with the outcome of infection.

Recent evidences have associated P. vivax infections with clinical complications, previously only attributed to P. falciparum malaria. The interaction between host and parasite may contribute to severity of the disease, however, the specific contribution of each factor remains unclear. Previous studies have shown that polymorphisms in Plasmodium vivax CSP may interfere in systemic reactions, response to drug treatments, leading to different symptoms as well as humoral responses. In this study, we investigate whether these polymorphisms could influence the parasite load and cytokine profile, which altogether may influence the malaria outcome. In this sense, studies have shown that subjects with high parasitic loads, responding with production of pro-inflammatory cytokines develop a more severe disease. The present data indicate that VK247 variant are associated with significant higher parasite loads and pro-inflammatory cytokine profile compared to VK210 variant. In this regard, this study demonstrates that P. vivax CSP polymorphisms have systemic effects in the host immune response, and the investigation of immunogenicity of parasite proteins may provide evidences for a better understanding of this infection.

The Timing of T Cell Priming and Cycling

The proliferation of specific lymphocytes is the central tenet of the clonal selection paradigm. Antigen recognition by T cells triggers a series of events that produces expanded clones of differentiated effector cells. TCR signaling events are detectable within seconds and minutes and are likely to continue for hours and days in vivo. Here, I review the work done on the importance of TCR signals in the later part of the expansion phase of the primary T cell response, primarily regarding the regulation of the cell cycle in CD4(+) and CD8(+) cells. The results suggest a degree of programing by early signals for effector differentiation, particularly in the CD8(+) T cell compartment, with optimal expansion supported by persistent antigen presentation later on. Differences to CD4(+) T cell expansion and new avenues toward a molecular understanding of cell cycle regulation in lymphocytes are discussed.

An extrafollicular pathway for the generation of effector CD8(+) T cells driven by the proinflammatory cytokine, IL-12

The proinflammatory cytokine IL-12 drives the generation of terminally differentiated KLRG1⁺ effector CD8⁺ T cells. Using a Toxoplasma vaccination model, we delineate the sequence of events that naïve CD8⁺ T cells undergo to become terminal effectors and the differentiation steps controlled by IL-12. We demonstrate that direct IL-12 signaling on CD8⁺ T cells is essential for the induction of KLRG1 and IFN-γ, but the subsequent downregulation of CXCR3 is controlled by IL-12 indirectly through the actions of IFN-γ and IFN-γ-inducible chemokines. Differentiation of nascent effectors occurs in an extrafollicular splenic compartment and is driven by late IL-12 production by DCs distinct from the classical CD8α⁺ DC. Unexpectedly, we also found extensive proliferation of both KLRG1⁻ and KLRG1⁺ CD8⁺ T cells in the marginal zone and red pulp, which ceases prior to the final KLRG1^Hi CXCR3^Lo stage. Our findings highlight the notion of an extrafollicular pathway for effector T cell generation.

DOI:http://dx.doi.org/10.7554/eLife.09017.001

The immune system helps to protect us from cancer, infection by microbes and other diseases. There are several different types of immune cells that each have particular roles. For example, cytotoxic T cells can kill other cells in the body that are damaged or infected. These cells are found in various locations around the body—including a region of the spleen known as the white pulp—where they wait in an inactive state until they detect signals from a damaged or infected cell. These T cells divide and mature to produce populations of active T cells known as effector cytotoxic lymphoid cells (or CTLs for short), a process which is thought to occur within the white pulp.

A small protein called cytokine IL-12 is involved in the production of CTLs. The cytokine is released from other immune cells and causes the activated T cells to divide and mature. It has long been believed that IL-12 produced in the white pulp early on in the process is sufficient to drive this process, but more recent work suggests that sustained production of IL-12 in other areas of the spleen that are accessible to the bloodstream may be needed.

Here, Shah et al. studied the generation of cytotoxic T cells in mice that had been exposed to a vaccine against a disease called Toxoplasmosis. Their experiments show that IL-12 drives both the early and late stages of CTL production. In the early stages, the T cells respond to IL-12 that is secreted by a group of ‘lymphoid dendritic’ cells in the white pulp. However, in the later stages, the T cells move away from the white pulp to other parts of the spleen known as the marginal zone and red pulp, where a distinct group of ‘myeloid dendritic’ cells also produce IL-12 and direct the final maturation of the CTLs.

Shah et al.'s findings also show that the process in which cytotoxic T cells divide and later mature to produce CTLs involves a series of tightly controlled events that mostly occur outside of the white pulp. These observations provide a new perspective on how to develop vaccines and other treatments that more efficiently generate the CTLs needed to protect against infections and cancer.

DOI:http://dx.doi.org/10.7554/eLife.09017.002

From the draining lymph node to the liver: the induction and effector mechanisms of malaria-specific CD8+ T cells

Parasitic protozoa cause considerable disease in humans and, due to their intracellular life cycle, induce robust CD8(+) T cell responses. A greater understanding of the factors that promote and maintain CD8(+) T cell-mediated immunity against these pathogens is likely needed for the development of effective vaccines. Immunization with radiation-attenuated sporozoites, the infectious stage of the malaria parasite transmitted by mosquitoes, is an excellent model to study these questions as CD8(+) T cells specific for a single epitope can completely eliminate parasite infection in the liver. Furthermore, live, radiation-attenuated parasites represent the "gold standard" for malaria vaccination. Here, we will highlight recent studies aimed at understanding the factors required for the induction, recruitment, and maintenance of effector and memory CD8(+) T cells against malaria liver stages.

Chitosan hydrogel vaccine generates protective CD8 T cell memory against mouse melanoma

CD8(+) T cells are important in the control of viral infections and cancers because of their cytolytic activity. A vaccine able to generate these cells could be beneficial in the prevention or treatment of these diseases. Chitosan hydrogel is a promising vaccine formulation that has previously been shown to generate effector CD8(+) T cells in a mouse model. This vaccine promotes sustained release of antigen and adjuvant, which generates a robust effector response. For longer lasting immunity, a memory population of these CD8(+) T cells is required to control further disease. We found that vaccination with chitosan hydrogel or dendritic cells using ovalbumin protein as a model antigen and Quil-A adjuvant provided protection in a subcutaneous melanoma challenge 30 days later. Ovalbumin-specific memory CD8(+) T cells were detectable following vaccination with the chitosan hydrogel but not the dendritic cell vaccine and an in vivo cytotoxicity assay demonstrated specific lysis of target cells in chitosan hydrogel vaccinated mice but not those receiving dendritic cell vaccination. These results demonstrate that vaccination with chitosan hydrogel is equally effective as dendritic cell vaccination in tumour protection but has more readily detectable immune correlates of protection. This may be advantageous in predetermining protection in vaccinated individuals.

Malaria and the liver: immunological hide-and-seek or subversion of immunity from within?

During the pre-erythrocytic asymptomatic phase of malarial infection, sporozoites develop transiently inside less than 100 hepatocytes that subsequently release thousands of merozoites. Killing of these hepatocytes by cytotoxic T cells (CTLs) confers protection to subsequent malarial infection, suggesting that this bottleneck phase in the parasite life cycle can be targeted by vaccination. During natural transmission, although some CTLs are generated in the skin draining lymph nodes, they are unable to eliminate the parasite, suggesting that the liver is important for the sporozoite to escape immune surveillance. The contribution of the organ to this process is unclear. Based on the known ability of several hepatic antigen-presenting cells (APCs) to induce primary activation of CD8 T cells and tolerance, malarial antigens presented by both infected hepatocytes and/or hepatic cross-presenting APCs should result in tolerance. However, our latest model predicts that due to the low frequency of infected hepatocytes, some T cells recognizing sporozoite epitopes with high affinity should differentiate into CTLs. In this review, we discuss two possible models to explain why CTLs generated in the liver and skin draining lymph nodes are unable to eliminate the parasite: (1) sporozoites harness the tolerogenic property of the liver; (2) CTLs are not tolerized but fail to detect infected cells due to sparse infection of hepatocytes and the very short liver stage. We propose that while malaria sporozoites might use the ability of the liver to tolerize both naive and effector cells, they have also developed strategies to decrease the probability of encounter between CTLs and infected liver cells. Thus, we predict that to achieve protection, vaccination strategies should aim to boost intrahepatic activation and/or increase the chance of encounter between sporozoite-specific CTLs and infected hepatocytes.

The role of hepatic immune regulation in systemic immunity to viral infection

The liver has particular immune functions attributed by its unique microenvironment and its liver-resident cell populations. During autoimmunity and viral hepatitis, the liver serves as target for effector responses of immune cells. However, skewing of effector T cell functions through tolerogenic liver-resident antigen-presenting cells and through the immune regulatory hepatic microenvironment. Importantly, the liver also participates in shaping systemic antigen-specific immunity. Local antigen-presenting cell populations, in particular liver sinusoidal endothelial cells (LSECs), cross-present soluble, circulating or hepatocyte-derived antigens to naïve CD8 T cells. Upon priming by cross-presenting LSECs, naïve CD8 T cells develop into a unique population of antigen-experienced memory-like T cell population that can be reactivated in an inflammatory context to protect against infection with viruses or bacteria. Furthermore, upon prolonged inflammatory TNF-dependent signaling, the induction of intrahepatic myeloid cell aggregates for T cell population expansion (iMATEs) is observed in liver tissue. iMATEs are formed by inflammatory monocytes developing into dendritic cells and function to attract recently activated CD8 T cells. Those CD8 T cells located within the cocoon-like iMATE structure show strong proliferation initiated by co-stimulatory signaling. Locally expanded CD8 T cells are key to control acute and chronic viral infections. The mechanistic understanding of local hepatic T cell priming and local expansion of effector CD8 T cells will help to develop novel therapeutic vaccination strategies.

IFNγ Responses to Pre-erythrocytic and Blood-stage Malaria Antigens Exhibit Differential Associations With Past Exposure and Subsequent Protection

BACKGROUND

The malaria-specific T-cell response is believed to be important for protective immunity. Antimalarial chemoprevention may affect this response by altering exposure to malaria antigens.

METHODS

We performed interferon γ (IFNγ) ELISpot assays to assess the cellular immune response to blood-stage and pre-erythrocytic antigens longitudinally from 1 to 3 years of age in 196 children enrolled in a randomized trial of antimalarial chemoprevention in Tororo, Uganda, an area of high transmission intensity.

RESULTS

IFNγ responses to blood-stage antigens, particularly MSP1, were frequently detected, strongly associated with recent malaria exposure, and lower in those adherent to chemoprevention compared to nonadherent children and those randomized to no chemoprevention. IFNγ responses to pre-erythrocytic antigens were infrequent and similar between children randomized to chemoprevention or no chemoprevention. Responses to blood-stage antigens were not associated with subsequent protection from malaria (aHR 0.96, P = .83), but responses to pre-erythrocytic antigens were associated with protection after adjusting for prior malaria exposure (aHR 0.52, P = .009).

CONCLUSIONS

In this high transmission setting, IFNγ responses to blood-stage antigens were common and associated with recent exposure to malaria but not protection from subsequent malaria. Responses to pre-erythrocytic antigens were uncommon, not associated with exposure but were associated with protection from subsequent malaria.

Plasmodium and mononuclear phagocytes

Plasmodium, the causative agent of malaria, initially multiplies inside liver cells and then in successive cycles inside erythrocytes, causing the symptoms of the disease. In this review, we discuss interactions between the extracellular and intracellular forms of the Plasmodium parasite and innate immune cells in the mammalian host, with a special emphasis on mononuclear phagocytes. We overview here what is known about the innate immune cells that interact with parasites, mechanisms used by the parasite to evade them, and the protective or detrimental contribution of these interactions on parasite progression through its life cycle and pathology in the host.

Malaria immunity in man and mosquito: insights into unsolved mysteries of a deadly infectious disease

Malaria is a mosquito-borne disease caused by parasites of the obligate intracellular Apicomplexa phylum the most deadly of which, Plasmodium falciparum, prevails in Africa. Malaria imposes a huge health burden on the world's most vulnerable populations, claiming the lives of nearly one million children and pregnant women each year. Although there is keen interest in eradicating malaria, we do not yet have the necessary tools to meet this challenge, including an effective malaria vaccine and adequate vector control strategies. Here we review what is known about the mechanisms at play in immune resistance to malaria in both the human and mosquito hosts at each step in the parasite's complex life cycle with a view toward developing the tools that will contribute to the prevention of disease and death and, ultimately, to the goal of malaria eradication. In so doing, we hope to inspire immunologists to participate in defeating this devastating disease.

Exacerbation of autoimmune neuro-inflammation in mice cured from blood-stage Plasmodium berghei infection

The thymus plays an important role shaping the T cell repertoire in the periphery, partly, through the elimination of inflammatory auto-reactive cells. It has been shown that, during Plasmodium berghei infection, the thymus is rendered atrophic by the premature egress of CD4+CD8+ double-positive (DP) T cells to the periphery. To investigate whether autoimmune diseases are affected after Plasmodium berghei NK65 infection, we immunized C57BL/6 mice, which was previously infected with P. berghei NK65 and treated with chloroquine (CQ), with MOG35-55 peptide and the clinical course of Experimental Autoimmune Encephalomyelitis (EAE) was evaluated. Our results showed that NK65+CQ+EAE mice developed a more severe disease than control EAE mice. The same pattern of disease severity was observed in MOG35-55-immunized mice after adoptive transfer of P. berghei-elicited splenic DP-T cells. The higher frequency of IL-17+- and IFN-γ+-producing DP lymphocytes in the Central Nervous System of these mice suggests that immature lymphocytes contribute to disease worsening. To our knowledge, this is the first study to integrate the possible relationship between malaria and multiple sclerosis through the contribution of the thymus. Notwithstanding, further studies must be conducted to assert the relevance of malaria-induced thymic atrophy in the susceptibility and clinical course of other inflammatory autoimmune diseases.

Prolonged antigen presentation following an acute virus infection requires direct and then cross-presentation

Antiviral CD8(+) T cell recognition of MHC class I-peptide complexes on the surface of professional APCs is a requisite step in an effective immune response following many potentially lethal infections. Although MHC class I-peptide production is thought to be closely linked to the continued presence of virus, several studies have shown that the persistence of Ag presentation occurs for an extended period of time following the clearance of RNA viruses. However, the mechanism responsible for Ag presentation persistence following viral clearance was unknown until now. In this study, we used a recombinant DNA virus expressing different forms of a model Ag to study the mechanism of prolonged Ag presentation in mice. We determined that the persistence of Ag presentation consists of three distinct mechanistic phases, as follows: ongoing viral replication, persistence of virally infected cells, and cross-presentation of Ag. These data will allow manipulation of the form of Ag contained within viral vectors to produce the most effective and protective CD8(+) T cell response to be generated following vaccination.

Tissue signatures influence the activation of intrahepatic CD8(+) T cells against malaria sporozoites

Plasmodium sporozoites and liver stages express antigens that are targeted to the MHC-Class I antigen-processing pathway. After the introduction of Plasmodium sporozoites by Anopheles mosquitoes, bone marrow-derived dendritic cells in skin-draining lymph nodes are the first cells to cross-present parasite antigens and elicit specific CD8⁺ T cells. One of these antigens is the immunodominant circumsporozoite protein (CSP). The CD8⁺ T cell-mediated protective immune response against CSP is dependent on the interleukin loop involving IL-4 receptor expression on CD8⁺ cells and IL-4 secretion by CD4⁺ T cell helpers. In a few days, these CD8⁺ T cells re-circulate to secondary lymphoid organs and the liver. In the liver, the hepatic sinusoids are enriched with cells, such as dendritic, sinusoidal endothelial and Kupffer cells, that are able to cross-present MHC class I antigens to intrahepatic CD8⁺ T cells. Specific CD8⁺ T cells actively find infected hepatocytes and target intra-cellular parasites through mechanisms that are both interferon-γ-dependent and -independent. Immunity is mediated by CD8⁺ T effector or effector-memory cells and, when present in high numbers, these cells can provide sterilizing immunity. Human vaccination trials with recombinant formulations or attenuated sporozoites have yet to achieve the high numbers of specific effector T cells that are required for sterilizing immunity. In spite of the limited number of specific CD8⁺ T cells, attenuated sporozoites provided multiple times by the endovenous route provided a high degree of protective immunity. These observations highlight that CD8⁺ T cells may be useful for improving antibody-mediated protective immunity to pre-erythrocytic stages of malaria parasites.

Local immune response to injection of Plasmodium sporozoites into the skin

Malarial infection is initiated when the sporozoite form of the Plasmodium parasite is inoculated into the skin by a mosquito. Sporozoites invade hepatocytes in the liver and develop into the erythrocyte-infecting form of the parasite, the cause of clinical blood infection. Protection against parasite development in the liver can be induced by injection of live attenuated parasites that do not develop in the liver and thus do not cause blood infection. Radiation-attenuated sporozoites (RAS) and genetically attenuated parasites are now considered as lead candidates for vaccination of humans against malaria. Although the skin appears as the preferable administration route, most studies in rodents, which have served as model systems, have been performed after i.v. injection of attenuated sporozoites. In this study, we analyzed the early response to Plasmodium berghei RAS or wild-type sporozoites (WTS) injected intradermally into C57BL/6 mice. We show that RAS have a similar in vivo distribution to WTS and that both induce a similar inflammatory response consisting of a biphasic recruitment of polymorphonuclear neutrophils and inflammatory monocytes in the skin injection site and proximal draining lymph node (dLN). Both WTS and RAS associate with neutrophils and resident myeloid cells in the skin and the dLN, transform inside CD11b(+) cells, and induce a Th1 cytokine profile in the dLN. WTS and RAS are also similarly capable of priming parasite-specific CD8(+) T cells. These studies delineate the early and local response to sporozoite injection into the skin, and suggest that WTS and RAS prime the host immune system in a similar fashion.

Clinical evaluation of CpG oligonucleotides as adjuvants for vaccines targeting infectious diseases and cancer

Synthetic oligonucleotides (ODN) that express unmethylated "CpG motifs" trigger cells that express Toll-like receptor 9. In humans this includes plasmacytoid dendritic cells and B cells. CpG ODN induce an innate immune response characterized by the production of Th1 and pro-inflammatory cytokines. Their utility as vaccine adjuvants was evaluated in a number of clinical trials. Results indicate that CpG ODN improve antigen presentation and the generation of vaccine-specific cellular and humoral responses. This work provides an up-to-date overview of the utility of CpG ODN as adjuvants for vaccines targeting infectious agents and cancer.

CD8+ T cells from a novel T cell receptor transgenic mouse induce liver-stage immunity that can be boosted by blood-stage infection in rodent malaria

To follow the fate of CD8⁺ T cells responsive to Plasmodium berghei ANKA (PbA) infection, we generated an MHC I-restricted TCR transgenic mouse line against this pathogen. T cells from this line, termed PbT-I T cells, were able to respond to blood-stage infection by PbA and two other rodent malaria species, P. yoelii XNL and P. chabaudi AS. These PbT-I T cells were also able to respond to sporozoites and to protect mice from liver-stage infection. Examination of the requirements for priming after intravenous administration of irradiated sporozoites, an effective vaccination approach, showed that the spleen rather than the liver was the main site of priming and that responses depended on CD8α⁺ dendritic cells. Importantly, sequential exposure to irradiated sporozoites followed two days later by blood-stage infection led to augmented PbT-I T cell expansion. These findings indicate that PbT-I T cells are a highly versatile tool for studying multiple stages and species of rodent malaria and suggest that cross-stage reactive CD8⁺ T cells may be utilized in liver-stage vaccine design to enable boosting by blood-stage infections.

Malaria is a disease caused by Plasmodium species, which have a highly complex life cycle involving both liver and blood stages of mammalian infection. To prevent disease, one strategy has been to induce CD8⁺ T cells against liver-stage parasites, usually by immunization with stage-specific antigens. Here we describe a T cell receptor specificity that recognizes an antigen expressed in both the liver and blood stages of several rodent Plasmodium species. We generated a T cell receptor transgenic mouse with this specificity and showed that T cells from this line could protect against liver-stage infection. We used this novel tool to identify the site and cell-type involved in priming to a recently developed intravenous attenuated sporozoite vaccine shown to have efficacy in humans. We showed that CD8⁺ T cells with this specificity could protect against liver-stage infection while causing pathology to the blood stage. Finally, we provided evidence that T cells with cross-stage specificity can be primed and boosted on alternative stages, raising the possibility that antigens expressed in multiple stages might be ideal vaccine candidates for generating strong immunity to liver-stage parasites.

Vaccines against tropical parasitic diseases: a persisting answer to a persisting problem

Live and live-attenuated whole organism vaccines against Plasmodium falciparum malaria and cutaneous leishmaniasis due to Leishmania major remain the most uniformly effective vaccines against human parasitic diseases. These vaccines are discussed in terms of the nature of the T cell populations that mediate the strong and durable localized immunity to these infections, and the requirement for persisting antigen to generate and maintain the protective response. The difficulties in developing subunit vaccines that fulfill this requirement argue that despite their own formidable problems in manufacture and delivery, live and live- attenuated whole organism vaccines against human parasitic diseases should be vigorously pursued.

Immune responses to vaccines involving a combined antigen-nanoparticle mixture and nanoparticle-encapsulated antigen formulation

Many physicochemical characteristics significantly influence the adjuvant effect of micro/nanoparticles; one critical factor is the kinetics of antigen exposure to the immune system by particle-adjuvanted vaccines. Here, we investigated how various antigen-nanoparticle formulations impacted antigen exposure to the immune system and the resultant antigen-specific immune responses. We formulated antigen with poly(lactic-co-glycolic acid) (PLGA) nanoparticles by encapsulating antigen within nanoparticles or by simply mixing soluble antigen with the nanoparticles. Our results indicated that the combined formulation (composed of antigen encapsulated in nanoparticles and antigen mixed with nanoparticles) induced more powerful antigen-specific immune responses than each single-component formulation. Mice immunized with the combined vaccine formulation displayed enhanced induction of antigen-specific IgG antibodies with high avidity, increased cytokine secretion by splenocytes, and improved generation of memory T cell. Enhanced immune responses elicited by the combined vaccine formulation might be attributed to the antigen-depot effect at the injection site, effective provision of both adequate initial antigen exposure and long-term antigen persistence, and efficient induction of dendritic cell (DC) activation and follicular helper T cell differentiation in draining lymph nodes. Understanding the effect of antigen-nanoparticle formulations on the resultant immune responses might have significant implications for rational vaccine design.

Memory T cells maintain protracted protection against malaria

Immunologic memory is one of the cardinal features of antigen-specific immune responses, and the persistence of memory cells contributes to prophylactic immunizations against infectious agents. Adequately maintained memory T and B cell pools assure a fast, effective and specific response against re-infections. However, many aspects of immunologic memory are still poorly understood, particularly immunologic memory inducible by parasites, for example, Plasmodium spp., the causative agents of malaria. For example, memory responses to Plasmodium antigens amongst residents of malaria endemic areas appear to be either inadequately developed or maintained, because persons who survive episodes of childhood malaria remain vulnerable to intermittent malaria infections. By contrast, multiple exposures of humans and laboratory rodents to radiation-attenuated Plasmodium sporozoites (γ-spz) induce sterile and long-lasting protection against experimental sporozoite challenge. Multifactorial immune mechanisms maintain this protracted and sterile protection. While the presence of memory CD4 T cell subsets has been associated with lasting protection in humans exposed to multiple bites from Anopheles mosquitoes infected with attenuated Plasmodium falciparum, memory CD8 T cells maintain protection induced with Plasmodium yoelii and Plasmodium berghei γ-spz in murine models. In this review, we discuss our observations that show memory CD8 T cells specific for antigens expressed by P. berghei liver stage parasites as an indispensable component for the maintenance of protracted protective immunity against experimental malaria infection; moreover, the provision of an Ag-depot assures a quick recall of memory T cells as IFN-γ-producing effector CD8 T cells and IL-4- producing CD4 T cells that collaborate with B cells for an effective antibody response.

Differential kinetics of antigen dependency of CD4+ and CD8+ T cells

Ag recognition via the TCR is necessary for the expansion of specific T cells that then contribute to adaptive immunity as effector and memory cells. Because CD4+ and CD8+ T cells differ in terms of their priming APCs and MHC ligands we compared their requirements of Ag persistence during their expansion phase side by side. Proliferation and effector differentiation of TCR transgenic and polyclonal mouse T cells were thus analyzed after transient and continuous TCR signals. Following equally strong stimulation, CD4+ T cell proliferation depended on prolonged Ag presence, whereas CD8+ T cells were able to divide and differentiate into effector cells despite discontinued Ag presentation. CD4+ T cell proliferation was neither affected by Th lineage or memory differentiation nor blocked by coinhibitory signals or missing inflammatory stimuli. Continued CD8+ T cell proliferation was truly independent of self-peptide/MHC-derived signals. The subset divergence was also illustrated by surprisingly broad transcriptional differences supporting a stronger propensity of CD8+ T cells to programmed expansion. These T cell data indicate an intrinsic difference between CD4+ and CD8+ T cells regarding the processing of TCR signals for proliferation. We also found that the presentation of a MHC class II-restricted peptide is more efficiently prolonged by dendritic cell activation in vivo than a class I bound one. In summary, our data demonstrate that CD4+ T cells require continuous stimulation for clonal expansion, whereas CD8+ T cells can divide following a much shorter TCR signal.

Fusion of antigen to a dendritic cell targeting chemokine combined with adjuvant yields a malaria DNA vaccine with enhanced protective capabilities

Although sterilizing immunity to malaria can be elicited by irradiated sporozoite vaccination, no clinically practical subunit vaccine has been shown to be capable of preventing the approximately 600,000 annual deaths attributed to this infection. DNA vaccines offer several potential advantages for a disease that primarily affects the developing world, but new approaches are needed to improve the immunogenicity of these vaccines. By using a novel, lipid-based adjuvant, Vaxfectin, to attract immune cells to the immunization site, in combination with an antigen-chemokine DNA construct designed to target antigen to immature dendritic cells, we elicited a humoral immune response that provided sterilizing immunity to malaria challenge in a mouse model system. The chemokine, MIP3αCCL20, did not significantly enhance the cellular infiltrate or levels of cytokine or chemokine expression at the immunization site but acted with Vaxfectin to reduce liver stage malaria infection by orders of magnitude compared to vaccine constructs lacking the chemokine component. The levels of protection achieved were equivalent to those observed with irradiated sporozoites, a candidate vaccine undergoing development for further large scale clinical trial. Only vaccination with the combined regimen of adjuvant and chemokine provided 80–100% protection against the development of bloodstream infection. Treating the immunization process as requiring the independent steps of 1) attracting antigen-presenting cells to the site of immunization and 2) specifically directing vaccine antigen to the immature dendritic cells that initiate the adaptive immune response may provide a rational strategy for the development of a clinically applicable malaria DNA vaccine.

CD8+ T Cell Responses to Plasmodium and Intracellular Parasites

Parasitic protozoa are major threats to human health affecting millions of people around the world. Control of these infections by the host immune system relies on a myriad of immunological mechanisms that includes both humoral and cellular immunity. CD8⁺ T cells contribute to the control of these parasitic infections in both animals and humans. Here, we will focus on the CD8⁺ T cell response against a subset of these protozoa: Plasmodium, Toxoplasma gondii, Leishmania and Trypanosoma cruzi, with an emphasis on experimental rodent systems. It is evident a complex interaction occurs between CD8⁺ T cells and the invading protozoa. A detailed understanding of how CD8⁺ T cells mediate protection should provide the basis for the development of effective vaccines that prevent and control infections by these parasites.

Early Decision: Effector and Effector Memory T Cell Differentiation in Chronic Infection

As effector memory T cells (Tem) are the predominant population elicited by chronic parasitic infections, increasing our knowledge of their function, survival and derivation, as phenotypically and functionally distinct from central memory and effector T cells will be critical to vaccine development for these diseases. In some infections, memory T cells maintain increased effector functions, however; this may require the presence of continued antigen, which can also lead to T cell exhaustion. Alternatively, in the absence of antigen, only the increase in the number of memory cells remains, without enhanced functionality as central memory. In order to understand the requirement for antigen and the potential for longevity or protection, the derivation of each type of memory must be understood. A thorough review of the data establishes the existence of both memory (Tmem) precursors and effector T cells (Teff) from the first hours of an immune response. This suggests a new paradigm of Tmem differentiation distinct from the proposition that Tmem only appear after the contraction of Teff. Several signals have been shown to be important in the generation of memory T cells, such as the integrated strength of “signals 1-3” of antigen presentation (antigen receptor, co-stimulation, cytokines) as perceived by each T cell clone. Given that these signals integrated at antigen presentation cells have been shown to determine the outcome of Teff and Tmem phenotypes and numbers, this decision must be made at a very early stage. It would appear that the overwhelming expansion of effector T cells and the inability to phenotypically distinguish memory T cells at early time points has masked this important decision point. This does not rule out an effect of repeated stimulation or chronic inflammatory milieu on populations generated in these early stages. Recent studies suggest that Tmem are derived from early Teff, and we suggest that this includes Tem as well as Tcm. Therefore, we propose a testable model for the pathway of differentiation from naïve to memory that suggests that Tem are not fully differentiated effector cells, but derived from central memory T cells as originally suggested by Sallusto et al. in 1999, but much debated since.

Looking under the skin: the first steps in malarial infection and immunity

Malaria, which is caused by Plasmodium spp., starts with an asymptomatic phase, during which sporozoites, the parasite form that is injected into the skin by a mosquito, develop into merozoites, the form that infects erythrocytes. This pre-erythrocytic phase is still the most enigmatic in the parasite life cycle, but has long been recognized as an attractive vaccination target. In this Review, we present what has been learned in recent years about the natural history of the pre-erythrocytic stages, mainly using intravital imaging in rodents. We also consider how this new knowledge is in turn changing our understanding of the immune response mounted by the host against the pre-erythrocytic forms.

Dynamics of the major histocompatibility complex class I processing and presentation pathway in the course of malaria parasite development in human hepatocytes: implications for vaccine development

Control of parasite replication exerted by MHC class I restricted CD8+ T-cells in the liver is critical for vaccination-induced protection against malaria. While many intracellular pathogens subvert the MHC class I presentation machinery, its functionality in the course of malaria replication in hepatocytes has not been characterized. Using experimental systems based on specific identification, isolation and analysis of human hepatocytes infected with P. berghei ANKA GFP or P. falciparum 3D7 GFP sporozoites we demonstrated that molecular components of the MHC class I pathway exhibit largely unaltered expression in malaria-infected hepatocytes until very late stages of parasite development. Furthermore, infected cells showed no obvious defects in their capacity to upregulate expression of different molecular components of the MHC class I machinery in response to pro-inflammatory lymphokines or trigger direct activation of allo-specific or peptide-specific human CD8+ T-cells. We further demonstrate that ectopic expression of circumsporozoite protein does not alter expression of critical genes of the MHC class I pathway and its response to pro-inflammatory cytokines. In addition, we identified supra-cellular structures, which arose at late stages of parasite replication, possessed the characteristic morphology of merosomes and exhibited nearly complete loss of surface MHC class I expression. These data have multiple implications for our understanding of natural T-cell immunity against malaria and may promote development of novel, efficient anti-malaria vaccines overcoming immune escape of the parasite in the liver.