The Deadly Dance of B Cells with Trypanosomatids

Effect of Leishmania infantum infection on B cell lymphopoiesis and memory in the bone marrow and spleen

Visceral leishmaniasis (VL) is characterized by an uncontrolled infection of internal organs such as the spleen, liver and bone marrow (BM) and can be lethal when left untreated. No effective vaccination is currently available for humans. The importance of B cells in infection and VL protective immunity has been controversial, with both detrimental and protective effects described. VL infection was found in this study to increase not only all analyzed B cell subsets in the spleen but also the B cell progenitors in the BM. The enhanced B lymphopoiesis aligns with the clinical manifestation of polyclonal hypergammaglobulinemia and the occurrence of autoantibodies. In line with earlier reports, flow cytometric and microscopic examination identified parasite attachment to B cells of the BM and spleen without internalization, and transformation of promastigotes into amastigote morphotypes. The interaction appears independent of IgM expression and is associated with an increased detection of activated lysosomes. Furthermore, the extracellularly attached amastigotes could be efficiently transferred to infect macrophages. The observed interaction underscores the potentially crucial role of B cells during VL infection. Additionally, using immunization against a fluorescent heterologous antigen, it was shown that the infection does not impair immune memory, which is reassuring for vaccination campaigns in VL endemic areas.

Leishmania donovani Exploits Tunneling Nanotubes for Dissemination and Propagation of B Cell Activation

Polyclonal B cell activation and the resulting hypergammaglobulinemia are a detrimental consequence of visceral leishmaniasis (VL); however, the mechanisms underlying this excessive production of nonprotective antibodies are still poorly understood. Here, we show that a causative agent of VL, Leishmania donovani, induces CD21-dependent formation of tunneling nanotubule (TNT)-like protrusions in B cells. These intercellular connections are used by the parasite to disseminate among cells and propagate B cell activation, and close contact both among the cells and between B cells and parasites is required to achieve this activation. Direct contact between cells and parasites is also observed in vivo, as L. donovani can be detected in the splenic B cell area as early as 14 days postinfection. Interestingly, Leishmania parasites can also glide from macrophages to B cells via TNT-like protrusions. Taken together, our results suggest that, during in vivo infection, B cells may acquire L. donovani from macrophages via TNT-like protrusions, and these connections are subsequently exploited by the parasite to disseminate among B cells, thus propagating B cell activation and ultimately leading to polyclonal B cell activation. IMPORTANCE Leishmania donovani is a causative agent of visceral leishmaniasis, a potentially lethal disease characterized by strong B cell activation and the subsequent excessive production of nonprotective antibodies, which are known to worsen the disease. How Leishmania activates B cells is still unknown, particularly because this parasite mostly resides inside macrophages and would not have access to B cells during infection. In this study, we describe for the first time how the protozoan parasite Leishmania donovani induces and exploits the formation of protrusions that connect B lymphocytes with each other or with macrophages and glides on these structures from one cell to another. In this way, B cells can acquire Leishmania from macrophages and become activated upon contact with the parasites. This activation will then lead to antibody production. These findings provide an explanation for how the parasite may propagate B cell activation during infection.

Leishmaniasis: Immune Cells Crosstalk in Macrophage Polarization

Leishmaniasis is a complex infectious parasitic disease caused by protozoa of the genus Leishmania, belonging to a group of neglected tropical diseases. It establishes significant global health challenges, particularly in socio-economically disadvantaged regions. Macrophages, as innate immune cells, play a crucial role in initiating the inflammatory response against the pathogens responsible for this disease. Macrophage polarization, the process of differentiating macrophages into pro-inflammatory (M1) or anti-inflammatory (M2) phenotypes, is essential for the immune response in leishmaniasis. The M1 phenotype is associated with resistance to Leishmania infection, while the M2 phenotype is predominant in susceptible environments. Notably, various immune cells, including T cells, play a significant role in modulating macrophage polarization by releasing cytokines that influence macrophage maturation and function. Furthermore, other immune cells can also impact macrophage polarization in a T-cell-independent manner. Therefore, this review comprehensively examines macrophage polarization's role in leishmaniasis and other immune cells' potential involvement in this intricate process.

Immunodominant surface epitopes power immune evasion in the African trypanosome

The African trypanosome survives the immune response of its mammalian host by antigenic variation of its major surface antigen (the variant surface glycoprotein or VSG). Here we describe the antibody repertoires elicited by different VSGs. We show that the repertoires are highly restricted and are directed predominantly to distinct epitopes on the surface of the VSGs. They are also highly discriminatory; minor alterations within these exposed epitopes confer antigenically distinct properties to these VSGs and elicit different repertoires. We propose that the patterned and repetitive nature of the VSG coat focuses host immunity to a restricted set of immunodominant epitopes per VSG, eliciting a highly stereotyped response, minimizing cross-reactivity between different VSGs and facilitating prolonged immune evasion through epitope variation.

The outcomes of polyparasitism in stray cats from Brazilian Midwest assessed by epidemiological, hematological and pathological data

We evaluated the epidemiological, hematological, and pathological data of Leishmania spp., Toxoplasma gondii, Platynosomum illiciens, feline immunodeficiency virus (FIV), and feline leukemia virus (FeLV) infections and the coinfections in stray cats of an endemic area for leishmaniasis. The diagnosis was performed by serological tests and necropsy. We described gross lesions and histopathological findings. We used immunohistochemistry and chromogenic in situ hybridization for L. infantum detection. We found infection in 27 out of 50 sampled cats, among them, 14 presented coinfections. A strong correlation between splenomegaly and lymphadenomegaly with FeLV, and an association between hepatic lesions and cachexia with parasitism due to P. illiciens were observed. Moreover, we found a significant increase in the monocyte count in the FeLV-infected and a decrease in the red blood cell count in the FIV-infected animals. Amastigote forms of Leishmania spp. and tissue changes were detected in lymphoid organs of an animal coinfected with P. illiciens, T. gondii, and FIV. Polyparasitism recorded in stray cats of the Brazilian Midwest should be considered in effective control strategies for public health diseases. Moreover, stray cats of Campo Grande may be a source of infection of FIV, FeLV and P. illiciens for populations of domiciled cats.

Trypanosoma cruzi Induces B Cells That Regulate the CD4+ T Cell Response

Trypanosoma cruzi infection induces a polyclonal B cell proliferative response characterized by maturation to plasma cells, excessive generation of germinal centers, and secretion of parasite-unrelated antibodies. Although traditionally reduced to the humoral response, several infectious and non-infectious models revealed that B lymphocytes could regulate and play crucial roles in cellular responses. Here, we analyze the trypomastigote-induced effect on B cells, their effects on CD4+ T cells, and their correlation with in vivo findings. The trypomastigotes were able to induce the proliferation and the production of IL-10 or IL-6 of naïve B cells in co-culture experiments. Also, we found that IL-10-producing B220lo cells were elicited in vivo. We also found up-regulated expression of FasL and PD-L1, proteins involved in apoptosis induction and inhibition of TCR signaling, and of BAFF and APRIL mRNAs, two B-cell growth factors. Interestingly, it was observed that IL-21, which plays a critical role in regulatory B cell differentiation, was significantly increased in B220+/IL-21+ in in vivo infections. This is striking since the secretion of IL-21 is associated with T helper follicular cells. Furthermore, trypomastigote-stimulated B-cell conditioned medium dramatically reduced the proliferation and increased the apoptotic rate on CD3/CD28 activated CD4+ T cells, suggesting the development of effective regulatory B cells. In this condition, CD4+ T cells showed a marked decrease in proliferation and viability with marginal IL-2 or IFNγ secretion, which is counterproductive with an efficient immune response against T. cruzi. Altogether, our results show that B lymphocytes stimulated with trypomastigotes adopt a particular phenotype that exerts a strong regulation of this T cell compartment by inducing apoptosis, arresting cell division, and affecting the developing of a proinflammatory response.

Anti-Leishmania infantum Antibody-Producing Plasma Cells in the Spleen in Canine Visceral Leishmaniasis

The spleen is involved in visceral leishmaniasis immunopathogenesis, and presents alterations in white-pulp microenvironments that are associated with an increased susceptibility to coinfections and patient death. Plasmacytosis in splenic red pulp (RP) is one observed alteration, but the specificity of antibody-secreting cells and the distribution of them has not yet been evaluated. We biotinylated soluble L. infantum membrane antigens (bSLMA) used as probes in modified immunohistochemistry, and detected the presence of anti-L. infantum antibody-secreting cells. Were used spleens from eight dogs from the endemic area for canine visceral leishmaniasis (CanL), and three healthier controls. The spleen sections were cryopreserved, and we performed modified immunohistochemistry. The ratio of plasma cells which were reactive to bSLMA (Anti-Leish-PC) in the spleen RP and periarteriolar lymphatic sheath (PALS) were calculated. Dogs with CanL present hyperglobulinemia and more plasma cells in their RP than the controls. Furthermore, dogs with CanL presented a lower proportion of Anti-Leish-PC in their RP than in PALS. Likewise, dysproteinemia was related to RP and PALS plasmacytosis, and a more severe clinical profile.

Trypanosoma cruzi trans-Sialidase as a Potential Vaccine Target Against Chagas Disease

Chagas' disease is caused by the protozoan Trypanosoma cruzi, described in the early 20th century by the Brazilian physician Dr. Carlos Chagas. There was a great amount of research devoted to diagnosis, treatment and prevention of the disease. One of the most important discoveries made since then, impacting the understanding of how the parasite interacts with the host's immune system, was the description of trans-sialidase. It is an unique enzyme, capable of masking the parasite's presence from the host, while at the same time dampening the activation of CD8+ T cells, the most important components of the immune response. Since the description of Chagas' disease in 1909, extensive research has identified important events in the disease in order to understand the biochemical mechanism that modulates T. cruzi-host cell interactions and the ability of the parasite to ensure its survival. The importance of the trans-sialidase enzyme brought life to many studies for the design of diagnostic tests, drugs and vaccines. While many groups have been prolific, such efforts have encountered problems, among them: the fact that while T. cruzi have many genes that are unique to the parasite, it relies on multiple copies of them and the difficulty in providing epitopes that result in effective and robust immune responses. In this review, we aim to convey the importance of trans-sialidase as well as to provide a history, including the initial failures and the most promising successes in the chasing of a working vaccine for a disease that is endemic in many tropical countries, including Brazil.

Critical Roles of Phosphoinositide 3-Kinase δ in the Humoral Immune Response to Trypanosoma congolense Infection

PI3Kδ is critical in generating humoral and regulatory immune responses. In this study, we determined the impact of PI3Kδ in immunity to Trypanosoma congolense, an African trypanosome that can manipulate and evade Ab responses critical for protection. Upon infection with T. congolense, PI3Kδ^D910A mice lacking PI3Kδ activity paradoxically show a transient enhancement in early control of parasitemia, associated with impaired production of regulatory IL-10 by B cells in the peritoneum. C57BL/6 wild-type (WT) mice treated with the PI3Kδ inhibitor (PI3Kδi) Idelalisib showed a similar transient decrease in parasitemia associated with reduced IL-10. Strikingly, however, we find that PI3Kδ^D910A mice were ultimately unable to control this infection, resulting in uncontrolled parasitemia and death within 2 wk. Assessment of humoral responses revealed delayed B cell activation, impaired germinal center responses, and compromised Ab responses to differing degrees in PI3Kδ^D910A and PI3Kδi-treated mice. To test the role of Abs, we administered serum from WT mice to PI3Kδ^D910A mice and found that lethality was prevented by postinfection serum. Interestingly, serum from naive WT mice provided partial protection to PI3Kδ^D910A mutants, indicating an additional role for natural Abs. Together our findings suggest that although PI3Kδ drives immune regulatory responses that antagonize early control of parasite growth in the peritoneum, it is also required for generation of Abs that are critical for protection from systemic trypanosome infection. The essential role of PI3Kδ for host survival of African trypanosome infection contrasts with findings for other pathogens such as Leishmania, underlining the critical importance of PI3Kδ-dependent humoral immunity in this disease.

VLP-Based Vaccines as a Suitable Technology to Target Trypanosomatid Diseases

Research on vaccines against trypanosomatids, a family of protozoa that cause neglected tropical diseases, such as Chagas disease, leishmaniasis, and sleeping sickness, is a current need. Today, according to modern vaccinology, virus-like particle (VLP) technology is involved in many vaccines, including those undergoing studies related to COVID-19. The potential use of VLPs as vaccine adjuvants opens an opportunity for the use of protozoan antigens for the development of vaccines against diseases caused by Trypanosoma cruzi, Leishmania spp., and Trypanosoma brucei. In this context, it is important to consider the evasion mechanisms of these protozoa in the host and the antigens involved in the mechanisms of the parasite–host interaction. Thus, the immunostimulatory properties of VLPs can be part of an important strategy for the development and evaluation of new vaccines. This work aims to highlight the potential of VLPs as vaccine adjuvants for the development of immunity in complex diseases, specifically in the context of tropical diseases caused by trypanosomatids.

Understanding the immune responses involved in mediating protection or immunopathology during leishmaniasis

Leishmaniasis is a vector-borne Neglected Tropical Disease (NTD) transmitted by the sand fly and is a major public health problem worldwide. Infections caused by Leishmania clinically manifest as a wide range of diseases, such as cutaneous (CL), diffuse cutaneous (DCL), mucosal (MCL) and visceral leishmaniasis (VL). The host innate and adaptative immune responses play critical roles in the defense against leishmaniasis. However, Leishmania parasites also manipulate the host immune response for their survival and replication. In addition, other factors such as sand fly salivary proteins and microbiota also promote disease susceptibility and parasite spread by modulating local immune response. Thus, a complex interplay between parasite, sand fly and the host immunity governs disease severity and outcome. In this review, we discuss the host immune response during Leishmania infection and highlight the factors associated with resistance or susceptibility.

Innate Immune Sensing by Cells of the Adaptive Immune System

Sensing of microbes or of danger signals has mainly been attributed to myeloid innate immune cells. However, T and B cells also express functional pattern recognition receptors (PRRs). In these cells, PRRs mediate signaling cascades that result in different functions depending on the cell's activation and/or differentiation status, on the environment, and on the ligand/agonist. Some of these functions are beneficial for the host; however, some are detrimental and are exploited by pathogens to establish persistent infections. In this review, we summarize the available literature on innate immune sensing by cells of the adaptive immune system and discuss possible implications for chronic infections.

B-cell profile, B-cell activating factor concentration and IgG levels in human cutaneous and mucosal leishmaniasis

AIMS

The aim of this study was to evaluate characteristics of B cells in human tegumentary leismaniasis (TL) analysing cutaneous leishmaniasis (CL), most prevalent form and mucosal leishmaniasis (ML), aggressive form characterized by the destruction of the oral-nasal-pharyngeal cavities.

METHODS AND RESULTS

By flow cytometry analysis, we found decreased percentages of non-class-switched memory B cells in TL with the degree of the loss related to clinical severity. Using commercial ELISA, we reported high levels of B-cell activating factor (BAFF) and IgG preferentially in aggressive CL and markedly in ML together with decreased BAFF receptors in the latter. We also found lower levels of BAFF after clinical recovery suggesting a relation between BAFF and disease activity. Mucosal leishmaniasis history of therapeutic failure presented high levels of BAFF accompanied by detectable concentrations of IFN-γ and IL-6 (assayed by commercial ELISA and cytometric bead arrays respectively), cytokines involved in exaggerated inflammatory responses and tissue damage in TL.

CONCLUSION

We demonstrate B-cell disturbances in TL with the degree of the alterations related to clinical severity. We suggest a relation between excess of BAFF and disease activity and point towards a possible implication of BAFF in the inflammatory phenomenon of ML.

Characterization of Sv129 Mice as a Susceptible Model to Leishmania amazonensis

Leishmaniasis is a complex of neglected diseases caused by parasites of the genus Leishmania, such as Leishmania (Leishmania) amazonensis, the ethiologic agent of diffuse cutaneous leishmaniasis in Brazil. In this work, we investigated a new experimental model of infection for L. amazonensis: the Sv129 mouse. First, we subcutaneously infected Sv129 mice with 2 × 10⁵ or 2 × 10⁶L. amazonensis parasites of the Josefa strain. A progressive lesion developed for both inoculation doses, showing that Sv129 mice are susceptible, independent of parasite dose. We next investigated the mechanisms associated with the pathogenesis of infection. We did not observe an increase of frequency of interferon-gamma (IFN- γ)-producing CD4⁺ and CD8⁺ T cells, a phenotype similar to that seen in BALB/c mice. There was an increased of frequency and number of IL-17-producing γδ (gamma-delta) T cells in infected Sv129 mice compared to naïve SV129 and an increased frequency of this population compared to infected BALB/c mice. In addition, Sv129 mice presented high levels of both IgG1 and IgG2a, suggesting a mixed Th1 and Th2 response with a skew toward IgG1 production based on IgG1/IgG2a ratio. Susceptibility of the Sv129 mice was further confirmed with the use of another strain of L. amazonensis, LTB0016. In this work, we characterized the Sv129 mice as a new model of susceptibility to Leishmania amazonensis infection, during infection there was controlled IFN-γ production by CD4⁺ or CD8⁺ T cells and induced IL-17 production by γδ T cells.

Hypergammaglobulinemia sustains the development of regulatory responses during chronic Leishmania donovani infection in mice

Visceral leishmaniasis, a chronic, potentially fatal disease, is characterized by high production of low-affinity antibodies. In humans, hypergammaglobulinemia is prediction of disease progression. Nevertheless, the contribution of hypermutated and/or class-switched immunoglobulins to disease pathogenesis has never been studied. Using Aicda^-/- mice and the experimental model of Leishmania donovani infection, we demonstrate that the absence of hypermutated and/or class-switched antibodies was associated with increased resistance to disease, stronger protective Th1 responses, and a lower frequency of regulatory IFNγ⁺ IL-10⁺ CD4 T cells. Interestingly, stronger Th1 responses and the absence of IFNγ⁺ IL-10⁺ CD4 T cells during chronic infection in infected Aicda^-/- mice were not caused by a T-cell intrinsic effect of AID, but by changes in the cytokine environment during chronic disease. Indeed TNF, IL-10 and IFN-ß expressions were only upregulated in the presence of hypermutated, class-switched antibodies and hypergammaglobulinemia at later stages of infection. Taken together, our results suggest that hypergammaglobulinemia sustains inhibitory responses during chronic visceral leishmaniasis.

Induction of protective cellular immune responses against experimental visceral leishmaniasis mediated by dendritic cells pulsed with the N-terminal domain of Leishmania infantum elongation factor-2 and CpG oligodeoxynucleotides

Leishmania elongation factor 2 (EF-2) has been previously identified as a T_H1-stimulatory protein. In this study, we assayed the protective potential of the N-terminal domain of EF-2 (N-LiEF-2, 1-357 aa) that has been predicted to contain several overlapping MHC class I and II-restricted epitopes injected in the form of dendritic cell (DC)-based vaccine. Ex vivo pulsing of DCs with the recombinant N-LiEF-2 domain along with CpG oligodeoxynucleotides (ODNs) resulted in their functional differentiation. BALB/c vaccinated with CpG-triggered DCs pulsed with N-LiEF-2 were found to be the most immune-reactive in terms of induction of DTH responses, increased T cell proliferation and IL-2 production. Moreover, vaccination induced antigen-specific T_H1 type immune response as evidenced by increased IFN-γ and TNFα levels followed by a significant increase of nitrite (NO) and reactive oxygen species (ROS) in splenocyte cultures. Vaccinated mice showed a pronounced decrease in parasite load in spleen and liver when challenged with L. infantum, increased expression of Stat1 and Tbx21 mRNA transcripts versus reduced expression of Foxp3 transcripts and were able to produce significantly elevated levels of IL-2, IFN-γ and TNFα but not IL-10 compared to non-vaccinated mice. Both antigen and parasite-specific CD4⁺ T and CD8⁺ T cells contributed to the IFN-γ production indicating that both subtypes contribute to the resistance to infection and correlated with robust nitrite generation, critical in controlling Leishmania infection. Together, these findings demonstrated the immunogenic as well as protective potential of the N-terminal domain of Leishmania EF-2 when given with CpG-triggered DCs representing a basis for the development of rationalized vaccine against leishmaniasis.

Analysis of the Antigenic and Prophylactic Properties of the Leishmania Translation Initiation Factors eIF2 and eIF2B in Natural and Experimental Leishmaniasis

Different members of intracellular protein families are recognized by the immune system of the vertebrate host infected by parasites of the genus Leishmania. Here, we have analyzed the antigenic and immunogenic properties of the Leishmania eIF2 and eIF2B translation initiation factors. An in silico search in Leishmania infantum sequence databases allowed the identification of the genes encoding the α, β, and γ subunits and the α, β, and δ subunits of the putative Leishmania orthologs of the eukaryotic initiation factors F2 (LieIF2) or F2B (LieIF2B), respectively. The antigenicity of these factors was analyzed by ELISA using recombinant versions of the different subunits. Antibodies against the different LieIF2 and LieIF2B subunits were found in the sera from human and canine visceral leishmaniasis patients, and also in the sera from hamsters experimentally infected with L. infantum. In L. infantum (BALB/c) and Leishmania major (BALB/c or C57BL/6) challenged mice, a moderate humoral response against these protein factors was detected. Remarkably, these proteins elicited an IL-10 production by splenocytes derived from infected mice independently of the Leishmania species employed for experimental challenge. When DNA vaccines based on the expression of the LieIF2 or LieIF2B subunit encoding genes were administered in mice, an antigen-specific secretion of IFN-γ and IL-10 cytokines was observed. Furthermore, a partial protection against murine CL development due to L. major infection was generated in the vaccinated mice. Also, in this work we show that the LieIF2α subunit and the LieIF2Bβ and δ subunits have the capacity to stimulate IL-10 secretion by spleen cells from naïve mice. B-lymphocytes were identified as the major producers of this anti-inflammatory cytokine. Taking into account the data found in this study, it may be hypothesized that these proteins act as virulence factors implicated in the induction of humoral responses as well as in the production of the down-regulatory IL-10 cytokine, favoring a pathological outcome. Therefore, these proteins might be considered markers of disease.