Beyond the VSG layer: Exploring the role of intrinsic disorder in the invariant surface glycoproteins of African trypanosomes.

In the bloodstream of mammalian hosts, African trypanosomes face the challenge of protecting their invariant surface receptors from immune detection. This crucial role is fulfilled by a dense, glycosylated protein layer composed of variant surface glycoproteins (VSGs), which undergo antigenic variation and provide a physical barrier that shields the underlying invariant surface glycoproteins (ISGs). The protective shield's limited permeability comes at the cost of restricted access to the extracellular host environment, raising questions regarding the specific function of the ISG repertoire. In this study, we employ an integrative structural biology approach to show that intrinsically disordered membrane-proximal regions are a common feature of members of the ISG super-family, conferring the ability to switch between compact and elongated conformers. While the folded, membrane-distal ectodomain is buried within the VSG layer for compact conformers, their elongated counterparts would enable the extension beyond it. This dynamic behavior enables ISGs to maintain a low immunogenic footprint while still allowing them to engage with the host environment when necessary. Our findings add further evidence to a dynamic molecular organization of trypanosome surface antigens wherein intrinsic disorder underpins the characteristics of a highly flexible ISG proteome to circumvent the constraints imposed by the VSG coat.

Microarray profiling predicts early neurological and immune phenotypic traits in advance of CNS disease during disease progression in Trypanosoma. b. brucei infected CD1 mouse brains.

Human African trypanosomiasis (HAT), also known as sleeping sickness, is a major cause of mortality and morbidity in sub-Saharan Africa. We hypothesised that recent findings of neurological features and parasite brain infiltration occurring at much earlier stages in HAT than previously thought could be explained by early activation of host genetic programmes controlling CNS disease. Accordingly, a transcriptomal analysis was performed on brain tissue at 0, 7, 14, 21 and 28dpi from the HAT CD1/GVR35 mouse model. Up to 21dpi, most parasites are restricted to the blood and lymphatic system. Thereafter the trypanosomes enter the brain initiating the encephalitic stage. Analysis of ten different time point Comparison pairings, revealed a dynamic transcriptome comprising four message populations. All 7dpi Comparisons had by far more differentially expressed genes compared to all others. Prior to invasion of the parenchyma, by 7dpi, ~2,000 genes were up-regulated, denoted [7dpi↑] in contrast to a down regulated population [7dpi↓] also numbering ~2,000. However, by 14dpi both patterns had returned to around the pre-infected levels. The third, [28dpi↑] featured over three hundred transcripts which had increased modestly up to14dpi, thereafter were significantly up-regulated and peaked at 28dpi. The fourth, a minor population, [7dpi↑-28dpi↑], had similar elevated levels at 7dpi and 28dpi. KEGG and GO enrichment analysis predicted a diverse phenotype by 7dpi with changes to innate and adaptive immunity, a Type I interferon response, neurotransmission, synaptic plasticity, pleiotropic signalling, circadian activity and vascular permeability without disruption of the blood brain barrier. This key observation is consistent with recent rodent model neuroinvasion studies and clinical reports of Stage 1 HAT patients exhibiting CNS symptoms. Together, these findings challenge the strict Stage1/Stage2 phenotypic demarcation in HAT and show that that significant neurological, and immune changes can be detected prior to the onset of CNS disease.

Nanobody-mediated macromolecular crowding induces membrane fission and remodeling in the African trypanosome.

The dense variant surface glycoprotein (VSG) coat of African trypanosomes represents the primary host-pathogen interface. Antigenic variation prevents clearing of the pathogen by employing a large repertoire of antigenically distinct VSG genes, thus neutralizing the host's antibody response. To explore the epitope space of VSGs, we generate anti-VSG nanobodies and combine high-resolution structural analysis of VSG-nanobody complexes with binding assays on living cells, revealing that these camelid antibodies bind deeply inside the coat. One nanobody causes rapid loss of cellular motility, possibly due to blockage of VSG mobility on the coat, whose rapid endocytosis and exocytosis are mechanistically linked to Trypanosoma brucei propulsion and whose density is required for survival. Electron microscopy studies demonstrate that this loss of motility is accompanied by rapid formation and shedding of nanovesicles and nanotubes, suggesting that increased protein crowding on the dense membrane can be a driving force for membrane fission in living cells.

CXCR6+CD4+ T cells promote mortality during Trypanosoma brucei infection.

Liver macrophages internalize circulating bloodborne parasites. It remains poorly understood how this process affects the fate of the macrophages and T cell responses in the liver. Here, we report that infection by Trypanosoma brucei induced depletion of macrophages in the liver, leading to the repopulation of CXCL16-secreting intrahepatic macrophages, associated with substantial accumulation of CXCR6+CD4+ T cells in the liver. Interestingly, disruption of CXCR6 signaling did not affect control of the parasitemia, but significantly enhanced the survival of infected mice, associated with reduced inflammation and liver injury. Infected CXCR6 deficient mice displayed a reduced accumulation of CD4+ T cells in the liver; adoptive transfer experiments suggested that the reduction of CD4+ T cells in the liver was attributed to a cell intrinsic property of CXCR6 deficient CD4+ T cells. Importantly, infected CXCR6 deficient mice receiving wild-type CD4+ T cells survived significantly shorter than those receiving CXCR6 deficient CD4+ T cells, demonstrating that CXCR6+CD4+ T cells promote the mortality. We conclude that infection of T. brucei leads to depletion and repopulation of liver macrophages, associated with a substantial influx of CXCR6+CD4+ T cells that mediates mortality.

The Trypanosome-Derived Metabolite Indole-3-Pyruvate Inhibits Prostaglandin Production in Macrophages by Targeting COX2.

The protozoan parasite Trypanosoma brucei is the causative agent of the neglected tropical disease human African trypanosomiasis, otherwise known as sleeping sickness. Trypanosomes have evolved many immune-evasion mechanisms to facilitate their own survival, as well as prolonging host survival to ensure completion of the parasitic life cycle. A key feature of the bloodstream form of T. brucei is the secretion of aromatic keto acids, which are metabolized from tryptophan. In this study, we describe an immunomodulatory role for one of these keto acids, indole-3-pyruvate (I3P). We demonstrate that I3P inhibits the production of PGs in activated macrophages. We also show that, despite the reduction in downstream PGs, I3P augments the expression of cyclooxygenase (COX2). This increase in COX2 expression is mediated in part via inhibition of PGs relieving a negative-feedback loop on COX2. Activation of the aryl hydrocarbon receptor also participates in this effect. However, the increase in COX2 expression is of little functionality, as we also provide evidence to suggest that I3P targets COX activity. This study therefore details an evasion strategy by which a trypanosome-secreted metabolite potently inhibits macrophage-derived PGs, which might promote host and trypanosome survival.

Trypanosoma brucei triggers a broad immune response in the adipose tissue.

Adipose tissue is one of the major reservoirs of Trypanosoma brucei parasites, the causative agent of sleeping sickness, a fatal disease in humans. In mice, the gonadal adipose tissue (AT) typically harbors 2-5 million parasites, while most solid organs show 10 to 100-fold fewer parasites. In this study, we tested whether the AT environment responds immunologically to the presence of the parasite. Transcriptome analysis of T. brucei infected adipose tissue revealed that most upregulated host genes are involved in inflammation and immune cell functions. Histochemistry and flow cytometry confirmed an increasingly higher number of infiltrated macrophages, neutrophils and CD4+ and CD8+ T lymphocytes upon infection. A large proportion of these lymphocytes effectively produce the type 1 effector cytokines, IFN-γ and TNF-α. Additionally, the adipose tissue showed accumulation of antigen-specific IgM and IgG antibodies as infection progressed. Mice lacking T and/or B cells (Rag2-/-, Jht-/-), or the signature cytokine (Ifng-/-) displayed a higher parasite load both in circulation and in the AT, demonstrating the key role of the adaptive immune system in both compartments. Interestingly, infections of C3-/- mice showed that while complement system is dispensable to control parasite load in the blood, it is necessary in the AT and other solid tissues. We conclude that T. brucei infection triggers a broad and robust immune response in the AT, which requires the complement system to locally reduce parasite burden.

Autoimmunity to phosphatidylserine and anemia in African Trypanosome infections.

Anemia caused by trypanosome infection is poorly understood. Autoimmunity during Trypanosoma brucei infection was proposed to have a role during anemia, but the mechanisms involved during this pathology have not been elucidated. In mouse models and human patients infected with malaria parasites, atypical B-cells promote anemia through the secretion of autoimmune anti-phosphatidylserine (anti-PS) antibodies that bind to uninfected erythrocytes and facilitate their clearance. Using mouse models of two trypanosome infections, Trypanosoma brucei and Trypanosoma cruzi, we assessed levels of autoantibodies and anemia. Our results indicate that acute T. brucei infection, but not T. cruzi, leads to early increased levels of plasma autoantibodies against different auto antigens tested (PS, DNA and erythrocyte lysate) and expansion of atypical B cells (ABCs) that secrete these autoantibodies. In vitro studies confirmed that a lysate of T. brucei, but not T. cruzi, could directly promote the expansion of these ABCs. PS exposure on erythrocyte plasma membrane seems to be an important contributor to anemia by delaying erythrocyte recovery since treatment with an agent that prevents binding to it (Annexin V) ameliorated anemia in T. brucei-infected mice. Analysis of the plasma of patients with human African trypanosomiasis (HAT) revealed high levels of anti-PS antibodies that correlated with anemia. Altogether these results suggest a relation between autoimmunity against PS and anemia in both mice and patients infected with T. brucei.

Role of T cells during the cerebral infection with Trypanosoma brucei.

The infection by Trypanosoma brucei brucei (T.b.b.), a protozoan parasite, is characterized by an early-systemic stage followed by a late stage in which parasites invade the brain parenchyma in a T cell-dependent manner. Here we found that early after infection effector-memory T cells were predominant among brain T cells, whereas, during the encephalitic stage T cells acquired a tissue resident memory phenotype (TRM) and expressed PD1. Both CD4 and CD8 T cells were independently redundant for the penetration of T.b.b. and other leukocytes into the brain parenchyma. The role of lymphoid cells during the T.b.b. infection was studied by comparing T- and B-cell deficient rag1-/- and WT mice. Early after infection, parasites located in circumventricular organs, brain structures with increased vascular permeability, particularly in the median eminence (ME), paced closed to the sleep-wake regulatory arcuate nucleus of the hypothalamus (Arc). Whereas parasite levels in the ME were higher in rag1-/- than in WT mice, leukocytes were instead reduced. Rag1-/- infected mice showed increased levels of meca32 mRNA coding for a blood /hypothalamus endothelial molecule absent in the blood-brain-barrier (BBB). Both immune and metabolic transcripts were elevated in the ME/Arc of WT and rag1-/- mice early after infection, except for ifng mRNA, which levels were only increased in WT mice. Finally, using a non-invasive sleep-wake cycle assessment method we proposed a putative role of lymphocytes in mediating sleep alterations during the infection with T.b.b. Thus, the majority of T cells in the brain during the early stage of T.b.b. infection expressed an effector-memory phenotype while TRM cells developed in the late stage of infection. T cells and parasites invade the ME/Arc altering the metabolic and inflammatory responses during the early stage of infection and modulating sleep disturbances.

Multiplex Bead Assay for the Detection of Human IgG Antibody Responses to African Trypanosomes.

The recent introduction of large-scale, population-based serologic surveys in several nations where human African trypanosomiasis (HAT) remains endemic could provide an opportunity to better map the remaining disease foci and to identify asymptomatic, seropositive individuals who are infected with the more chronic form of the parasite, Trypanosoma brucei gambiense (gHAT). We have incorporated a soluble form of variant surface glycoprotein 117 and a recombinant invariant surface glycoprotein 65.1 into a multiplex bead assay (MBA) method that is commonly used for the detection of IgG antibody responses to other neglected tropical diseases. A positive result was defined as reactivity to both antigens. MBA sensitivity and specificity for gHAT infection were 92% and 96%, respectively. Assay specificity for the acute form of disease caused by T.b. rhodesiense (rHAT) was 94%, but the sensitivity was only 63.6%. In the future, additional antigens could be incorporated into the multiplex assay to improve rHAT sensitivity.

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, PI3KδD910A 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 PI3KδD910A 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 PI3KδD910A and PI3Kδi-treated mice. To test the role of Abs, we administered serum from WT mice to PI3KδD910A mice and found that lethality was prevented by postinfection serum. Interestingly, serum from naive WT mice provided partial protection to PI3KδD910A 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.

Role of the inhibitor of serine peptidase 2 (ISP2) of Trypanosoma brucei rhodesiense in parasite virulence and modulation of the inflammatory responses of the host.

Trypanosoma brucei rhodesiense is one of the causative agents of Human African Trypanosomiasis (HAT), known as sleeping sickness. The parasite invades the central nervous system and causes severe encephalitis that is fatal if left untreated. We have previously identified ecotin-like inhibitors of serine peptidases, named ISPs, in trypanosomatid parasitic protozoa. Here, we investigated the role of ISP2 in bloodstream form T. b. rhodesiense. We generated gene-deficient mutants lacking ISP2 (Δisp2), which displayed a growth profile in vitro similar to that of wild-type (WT) parasites. C57BL/6 mice infected with Δisp2 displayed lower blood parasitemia, a delayed hind leg pathological phenotype and survived longer. The immune response was examined at two time-points that corresponded with two peaks of parasitemia. At 4 days, the spleens of Δisp2-infected mice had a greater percentage of NOS2+ myeloid cells, IFN-γ+-NK cells and increased TNF-α compared to those infected with WT and parasites re-expressing ISP2 (Δisp2:ISP2). By 13 days the increased NOS2+ population was sustained in Δisp2-infected mice, along with increased percentages of monocyte-derived dendritic cells, as well as CD19+ B lymphocytes, and CD8+ and CD4+ T lymphocytes. Taken together, these findings indicate that ISP2 contributes to T. b. rhodesiense virulence in mice and attenuates the inflammatory response during early infection.

Dermal bacterial LPS-stimulation reduces susceptibility to intradermal Trypanosoma brucei infection.

Infections with Trypanosoma brucei sp. are established after the injection of metacyclic trypomastigotes into the skin dermis by the tsetse fly vector. The parasites then gain access to the local lymphatic vessels to infect the local draining lymph nodes and disseminate systemically via the bloodstream. Macrophages are considered to play an important role in host protection during the early stage of systemic trypanosome infections. Macrophages are abundant in the skin dermis, but relatively little is known of their impact on susceptibility to intradermal (ID) trypanosome infections. We show that although dermal injection of colony stimulating factor 1 (CSF1) increased the local abundance of macrophages in the skin, this did not affect susceptibility to ID T. brucei infection. However, bacterial LPS-stimulation in the dermis prior to ID trypanosome infection significantly reduced disease susceptibility. In vitro assays showed that LPS-stimulated macrophage-like RAW264.7 cells had enhanced cytotoxicity towards T. brucei, implying that dermal LPS-treatment may similarly enhance the ability of dermal macrophages to eliminate ID injected T. brucei parasites in the skin. A thorough understanding of the factors that reduce susceptibility to ID injected T. brucei infections may lead to the development of novel strategies to help reduce the transmission of African trypanosomes.

An invariant Trypanosoma vivax vaccine antigen induces protective immunity.

Trypanosomes are protozoan parasites that cause infectious diseases, including African trypanosomiasis (sleeping sickness) in humans and nagana in economically important livestock1,2. An effective vaccine against trypanosomes would be an important control tool, but the parasite has evolved sophisticated immunoprotective mechanisms-including antigenic variation3-that present an apparently insurmountable barrier to vaccination. Here we show, using a systematic genome-led vaccinology approach and a mouse model of Trypanosoma vivax infection4, that protective invariant subunit vaccine antigens can be identified. Vaccination with a single recombinant protein comprising the extracellular region of a conserved cell-surface protein that is localized to the flagellum membrane (which we term 'invariant flagellum antigen from T. vivax') induced long-lasting protection. Immunity was passively transferred with immune serum, and recombinant monoclonal antibodies to this protein could induce sterile protection and revealed several mechanisms of antibody-mediated immunity, including a major role for complement. Our discovery identifies a vaccine candidate for an important parasitic disease that has constrained socioeconomic development in countries in sub-Saharan Africa5, and provides evidence that highly protective vaccines against trypanosome infections can be achieved.

Mice immunization with Trypanosoma brucei gambiense translationally controlled tumor protein modulates immunoglobulin and cytokine production, as well as parasitaemia and mice survival after challenge with the parasite.

Fighting trypanosomiasis with an anti-trypanosome vaccine is ineffective, the parasite being protected by a Variable Surface Glycoprotein (VSG) whose structure is modified at each peak of parasitaemia, which allows it to escape the host's immune defenses. However, the host immunization against an essential factor for the survival of the parasite or the expression of its pathogenicity could achieve the same objective. Here we present the results of mouse immunization against the Translationally Controlled Tumor Protein (TCTP), a protein present in the Trypanosoma brucei gambiense (Tbg) secretome, the parasite responsible for human trypanosomiasis. Mice immunization was followed by infection with Tbg parasites. The production of IgG, IgG1 and IgG2a begun after the second TCTP injection and was dose-dependant, the maximum level of anti-TCTP antibodies remained stable up to 4 days post-infection and then decreased. Regarding cytokines (IL-2, 4, 6, 10, INFγ, TNFα), the most striking result was their total suppression after immunization with the highest TCTP dose. Compared to the control group, the immunized mice displayed a reduced first peak of parasitaemia, a 100% increase in the time to onset of the second peak, and an increased time of mice survival. The effect of immunization was only transient but demonstrated the likely important role that TCTP plays in host-parasite interactions and that some key parasite proteins could reduce infection impact.

A Critical Blimp-1-Dependent IL-10 Regulatory Pathway in T Cells Protects From a Lethal Pro-inflammatory Cytokine Storm During Acute Experimental Trypanosoma brucei Infection.

In many infectious diseases, the immune response operates as a double-edged sword. While required for protective immunity, infection-induced inflammation can be detrimental if it is not properly controlled, causing collateral body damage and potentially leading to death. It is in this context that the potent anti-inflammatory cytokine interleukin-10 (IL-10) is required to dampen the pro-inflammatory immune response that hallmarks trypanosomosis. Effective control of this infection requires not just the action of antibodies specific for the parasite's variable surface glycoprotein (VSG) coat antigens, but also a pro-inflammatory immune response mediated mainly by IFNγ, TNF, and NO. However, strict control of inflammation is mandatory, as IL-10-deficient mice succumb from an unrestrained cytokine storm within 10 days of a Trypanosome brucei infection. The relevant cellular source of IL-10 and the associated molecular mechanisms implicated in its trypanosomosis associated production are poorly understood. Using an IL-10 reporter mouse strain (Vert-X), we demonstrate here that NK cells, CD8+ T cells and CD4+ T cells as well as B cells and plasma cells constitute potential cellular sources of IL-10 within the spleen and liver during acute infection. The IL-10 wave follows peak pro-inflammatory cytokine production, which accompanied the control of peak parasitemia. Similar results were observed following conventional experimental needle infection and physiological infections via T. brucei-infected tsetse flies. Our results show that conditional T cell-specific ablation of the IL-10 regulating Prdm1 gene (encoding for the Blimp-1 transcription factor), leads to an uncontrolled trypanosome-induced pro-inflammatory syndrome like the one observed in infected IL-10-deficient mice. This result indicates that the biological role of IL-10-derived from non-T cells, including NK cells, is of minor importance when considering host survival. The cytokine IL-27 that is also considered to be an IL-10 regulator, did not affect IL-10 production during infection. Together, these data suggest that T. brucei activates a Blimp-1-dependent IL-10 regulatory pathway in T cells that acts as a critical anti-inflammatory rheostat, mandatory for host survival during the acute phase of parasitemia.

Influence of the Draining Lymph Nodes and Organized Lymphoid Tissue Microarchitecture on Susceptibility to Intradermal Trypanosoma brucei Infection.

Infection of the mammalian host with African trypanosomes begins when the tsetse fly vector injects the parasites into the skin dermis during blood feeding. After injection into the skin, trypanosomes first accumulate in the draining lymph node before disseminating systemically. Whether this early accumulation within the draining lymph node is important for the trypanosomes to establish infection was not known. Lymphotoxin-ß-deficient mice (LTß-/- mice) lack most secondary lymphoid tissues, but retain the spleen and mesenteric lymph nodes. These mice were used to test the hypothesis that the establishment of infection after intradermal (ID) T. brucei infection would be impeded in the absence of the skin draining lymph nodes. However, LTß-/- mice revealed greater susceptibility to ID T. brucei infection than wild-type mice, indicating that the early accumulation of the trypanosomes in the draining lymph nodes was not essential to establish systemic infection. Although LTß-/- mice were able to control the first parasitemia wave as effectively as wild-type mice, they were unable to control subsequent parasitemia waves. LTß-/- mice also lack organized B cell follicles and germinal centers within their remaining secondary lymphoid tissues. As a consequence, LTß-/- mice have impaired immunoglobulin (Ig) isotype class-switching responses. When the disturbed microarchitecture of the B cell follicles in the spleens of LTß-/- mice was restored by reconstitution with wild-type bone marrow, their susceptibility to ID T. brucei infection was similar to that of wild-type control mice. This effect coincided with the ability to produce significant serum levels of Ig isotype class-switched parasite-specific antibodies. Thus, our data suggest that organized splenic microarchitecture and the production of parasite-specific Ig isotype class-switched antibodies are essential for the control of ID African trypanosome infections.

To the Skin and Beyond: The Immune Response to African Trypanosomes as They Enter and Exit the Vertebrate Host.

African trypanosomes are single-celled extracellular protozoan parasites transmitted by tsetse fly vectors across sub-Saharan Africa, causing serious disease in both humans and animals. Mammalian infections begin when the tsetse fly penetrates the skin in order to take a blood meal, depositing trypanosomes into the dermal layer. Similarly, onward transmission occurs when differentiated and insect pre-adapted forms are ingested by the fly during a blood meal. Between these transmission steps, trypanosomes access the systemic circulation of the vertebrate host via the skin-draining lymph nodes, disseminating into multiple tissues and organs, and establishing chronic, and long-lasting infections. However, most studies of the immunobiology of African trypanosomes have been conducted under experimental conditions that bypass the skin as a route for systemic dissemination (typically via intraperitoneal or intravenous routes). Therefore, the importance of these initial interactions between trypanosomes and the skin at the site of initial infection, and the implications for these processes in infection establishment, have largely been overlooked. Recent studies have also demonstrated active and complex interactions between the mammalian host and trypanosomes in the skin during initial infection and revealed the skin as an overlooked anatomical reservoir for transmission. This highlights the importance of this organ when investigating the biology of trypanosome infections and the associated immune responses at the initial site of infection. Here, we review the mechanisms involved in establishing African trypanosome infections and potential of the skin as a reservoir, the role of innate immune cells in the skin during initial infection, and the subsequent immune interactions as the parasites migrate from the skin. We suggest that a thorough identification of the mechanisms involved in establishing African trypanosome infections in the skin and their progression through the host is essential for the development of novel approaches to interrupt disease transmission and control these important diseases.

T. brucei infections abrogate diverse plasma cell-mediated effector B cell responses, independently of their specificity, affinity and host genetic background.

Antibody-mediated parasite killing is considered the most effective host immune response against extracellular trypanosome parasites. However, due to host-parasite co-evolution pressure, these parasites have "learned" how to hijack the host immune system via the development of immune evasion strategies. Hereby they prevent elimination and promote transmission. In the past, our group has shown that African trypanosome parasites are able to "shut down" the host B cell compartment, via the abolishment of the homeostatic B cell compartment. In line with this, we have reported that trypanosome infections result in detrimental outcomes on auto-reactive and cancer B cells. To unravel the immune mechanisms involved in these processes we adopted here a well-defined B cell vaccine model, i.e. the thymo-dependent hapten-carrier NP-CGG (4-Hydroxy-3-nitrophenylacetyl-Chicken Gamma Globulin) emulsified in Alum adjuvant. Results show that T. brucei infections abrogate the circulating titres of vaccine-induced CGG-specific as well as NP-specific IgG1+ antibodies, a hallmark of memory B cell responses in this model. This happens independently of their affinity and IFNÉ£ signalling. Next, we demonstrate that T. brucei infections also induce a decrease of anti-NP IgG3+ antibodies induced by the administration of NP coupled to Ficoll, a thymo-independent antigen. Confirming the non-specificity of the infection-associated immunopathology, this report also shows that trypanosome infections abolish vaccine-induced memory response against malaria parasite in BALB/c mice. Together, these data indicates that T. brucei infections impair every stages of B cell development, including effector plasma B cells, independently of their specificity and affinity as well as the host genetic background.

Inflammation following trypanosome infection and persistence in the skin.

Human African trypanosomes rely for their transmission on tsetse flies (Glossina sp.) that inoculate parasites into the skin during blood feeding. The absence of a protective vaccine, limited knowledge about the infection immunology, and the existence of asymptomatic carriers sustaining transmission are major outstanding challenges towards elimination. All these relate to the skin where (i) parasites persist and transmit to tsetse flies and (ii) a successful vaccination strategy should ideally be effective. Host immune processes and parasite strategies that underlie early infection and skin tropism are essential aspects to comprehend the transmission-success of trypanosomes and the failure in vaccine development. Recent insights into the early infection establishment may pave the way to novel strategies aimed at blocking transmission.

Inducible Germline IgMs Bridge Trypanosome Lytic Factor Assembly and Parasite Recognition.

Trypanosomiasis is a devastating neglected tropical disease affecting livestock and humans. Humans are susceptible to two Trypanosoma brucei subspecies but protected from other trypanosomes by circulating high-density lipoprotein (HDL) complexes called trypanosome lytic factors (TLFs) 1 and 2. TLFs contain apolipoprotein L-1 contributing to lysis and haptoglobin-related protein (HPR), which can function as a ligand for a parasite receptor. TLF2 also uniquely contains non-covalently associated immunoglobin M (IgM) antibodies, the role and origin of which remain unclear. Here, we show that these TLF2-associated IgMs interact with both HPR and alternate trypanosome surface proteins, including variant surface glycoprotein, likely facilitating complex biogenesis and TLF uptake into parasites. TLF2-IgMs are germline antibodies that, while present at basal concentrations in healthy individuals, are elicited by trypanosome infection in both murine models and human sleeping sickness patients. These data suggest that poly- and self-reactive germline antibodies such as TLF2-associated IgMs play a role in antimicrobial immunity.