Alteration of migration and maturation of dendritic cells and T-cell depletion in the course of experimental Trypanosoma cruzi infection

In the Acute Phase of Trypanosoma cruzi Infection, Liver Lymphoid and Myeloid Cells Display an Ambiguous Phenotype Combining Pro- and Anti-Inflammatory Markers

Multiple cell populations, cellular biochemical pathways, and the autonomic nervous system contribute to maintaining the immunological tolerance in the liver. This tolerance is coherent because the organ is exposed to high levels of bacterial pathogen-associated molecular pattern (PAMP) molecules from the intestinal microbiota, such as lipopolysaccharide endotoxin (LPS). In the case of Trypanosoma cruzi infection, although there is a dramatic acute immune response in the liver, we observed intrahepatic cell populations combining pro- and anti-inflammatory markers. There was loss of fully mature Kupffer cells and an increase in other myeloid cells, which are likely to include monocytes. Among dendritic cells (DCs), the cDC1 population expanded relative to the others, and these cells lost both some macrophage markers (F4/80) and immunosuppressive cytokines (IL-10, TGF-β1). In parallel, a massive T cell response occured with loss of naïve cells and increase in several post-activation subsets. However, these activated T cells expressed both markers programmed cell death protein (PD-1) and cytokines consistent with immunosuppressive function (IL-10, TGF-β1). NK and NK-T cells broadly followed the pattern of T cell activation, while TCR-γδ cells appeared to be bystanders. While no data were obtained concerning IL-2, several cell populations also synthesized IFN-γ and TNF-α, which has been linked to host defense but also to tissue injury. It therefore appears that T. cruzi exerts control over liver immunity, causing T cell activation via cDC1 but subverting multiple populations of T cells into immunosuppressive pathways. In this way, T. cruzi engages a mechanism of hepatic T cell tolerance that is familiar from liver allograft tolerance, in which activation and proliferation are followed by T cell inactivation.

Modulatory Effect of Trypanosoma cruzi Infective Stages in Different Dendritic Cell Populations in vitro

Trypanosoma cruzi is a protozoan parasite that infects at least 7 million persons in the world (OMS, 2019). In endemic areas, infection normally occurs by vectorial transmission; however, outside, it normally happens by blood and includes congenital transmission. The persistence of T. cruzi during infection suggests the presence of immune evasion mechanisms and the modulation of the anti-parasite response to a profile incapable of eradicating the parasite. Dendritic cells (DCs) are a heterogeneous population of antigen-presenting cells (APCs) that patrol tissues with a key role in mediating the interface between the innate and adaptive immune response. Previous results from our lab and other groups have demonstrated that T. cruzi modulates the functional properties of DCs, in vitro and in vivo. During vectorial transmission, metacyclic (m) trypomastigotes (Tps) eliminated along with the insect feces reach the mucous membranes or injured skin. When transmission occurs by the hematic route, the parasite stage involved in the infection is the circulating or blood (b) Tp. Here, we studied in vitro the effect of both infective mTp and bTp in two different populations of DCs, bone marrow-derived DCs (BMDCs) and XS106, a cell line derived from epidermal DCs. Results demonstrated that the interaction of both Tps imparts a different effect in the functionality of these two populations of DCs, suggesting that the stage of T. cruzi and DC maturation status could define the immune response from the beginning of the ingress of the parasite, conditioning the course of the infection.

Splenic Innate B1 B Cell Plasmablasts Produce Sustained Granulocyte-Macrophage Colony-Stimulating Factor and Interleukin-3 Cytokines during Murine Malaria Infections

The physiopathology of malaria, one of the most deadly human parasitic diseases worldwide, is complex, as it is a systemic disease involving multiple parasitic stages and hosts and leads to the activation of numerous immune cells and release of inflammatory mediators. While some cytokines increased in the blood of patients infected with Plasmodium falciparum have been extensively studied, others, such as granulocyte-macrophage colony-stimulating factor (GM-CSF) and interleukin-3 (IL-3), have not received much attention. GM-CSF and IL-3 belong to the β common (βc/CD131) chain family of cytokines, which exhibit pleiotropic functions, including the regulation of myeloid cell growth, differentiation, and activation. GM-CSF can be secreted by multiple cell types, whereas IL-3 is mostly restricted to T cells, yet innate response activator (IRA) B cells, a subset of innate B1 B cells, also produce significant amounts of these cytokines during bacterial sepsis via Toll-like receptor 4 (TLR4)/MyD88 sensing of lipopolysaccharides. Herein, using murine models of malaria, we report a sustained production of GM-CSF and IL-3 from IgM+ and IgM-/IgG+ CD138+ Blimp-1+ innate B1b B cell plasmablasts. IgM+ B1b B cells include IRA-like and non-IRA B cells and express higher levels of both cytokines than do their IgG+ counterparts. Interestingly, as infection progresses, the relative proportion of IgM+ B1 B cells decreases while that of IgG+ plasmablasts increases, correlating with potential isotype switching of GM-CSF- and IL-3-producing IgM+ B1 B cells. GM-CSF/IL-3+ B1 B cells originate in the spleen of infected mice and are partially dependent on type I and type II interferon signaling to produce both cytokines. These data reveal that GM-CSF and IL-3 are produced during malaria infections, initially from IgM+ and then from IgG+ B1b B cell plasmablasts, which may represent important emergency cellular sources of these cytokines. These results further highlight the phenotypic heterogeneity of innate B1 B cell subsets and of their possible fates in a relevant murine model of parasitic infection in vivo.

Trypanosoma cruzi Mexican Strains Differentially Modulate Surface Markers and Cytokine Production in Bone Marrow-Derived Dendritic Cells from C57BL/6 and BALB/c Mice

Dendritic cells (DCs) are a type of antigen-presenting cells that play an important role in the immune response against Trypanosoma cruzi, the causative agent of Chagas disease. In vitro and in vivo studies have shown that the modulation of these cells by this parasite can directly affect the innate and acquired immune response of the host in order to facilitate its biological cycle and the spreading of the species. Many studies show the mechanisms by which T. cruzi modulates DCs, but the interaction of these cells with the Mexican strains of T. cruzi such as Ninoa and INC5 has not yet been properly investigated. Here, we evaluated whether Ninoa and INC5 strains evaded the immunity of their hosts by modulating the biology and function of murine DCs. The CL-Brener strain was used as the reference strain. Herein, it was demonstrated that Ninoa was more infective toward bone marrow-derived dendritic cells (BMDCs) than INC5 and CL-Brener strains in both BMDCs of BALB/c and C57BL/6 mice. Mexican strains of T. cruzi induced different cytokine patterns. In BMDCs obtained from BALB/c mice, Ninoa strain led to the reduction in IL-6 and increased IL-10 production, while in C57BL/6 mice Ninoa strain considerably increased the productions of TNF-α and IL-10. Also, Ninoa and INC5 differentially modulated BMDC expressions of MHC-II, TLR2, and TLR4 in both BALB/c and C57BL/6 mice compared to Brazilian strain CL-Brener. These results indicate that T. cruzi Mexican strains differentially infect and modulate MHC-II, toll-like receptors, and cytokine production in DCs obtained from C57BL/6 and BALB/c mice, suggesting that these strains have developed particular modulatory strategies to disrupt DCs and, consequently, the host immune responses.

Langerin+CD8+ Dendritic Cells in the Splenic Marginal Zone: Not So Marginal After All

Dendritic cells (DC) fulfill an essential sentinel function within the immune system, acting at the interface of innate and adaptive immunity. The DC family, both in mouse and man, shows high functional heterogeneity in order to orchestrate immune responses toward the immense variety of pathogens and other immunological threats. In this review, we focus on the Langerin⁺CD8⁺ DC subpopulation in the spleen. Langerin⁺CD8⁺ DC exhibit a high ability to take up apoptotic/dying cells, and therefore they are essential to prime and shape CD8⁺ T cell responses. Next to the induction of immunity toward blood-borne pathogens, i.e., viruses, these DC are important for the regulation of tolerance toward cell-associated self-antigens. The ontogeny and differentiation pathways of CD8⁺CD103⁺ DC should be further explored to better understand the immunological role of these cells as a prerequisite of their therapeutic application.

Canine Macrophage DH82 Cell Line As a Model to Study Susceptibility to Trypanosoma cruzi Infection

Trypanosoma cruzi is an obligatory intracellular protozoan parasite, and it is the etiological agent of Chagas' disease that is endemic in the Americas. In addition to humans, a wide spectrum of mammals can be infected by T. cruzi, including dogs. Dogs develop acute and chronic disease, similar to human infection. T. cruzi can infect almost all cell types and after cell invasion, the metacyclics trypomastigotes localize in the cytoplasm, where they transform into amastigotes, the replicative form of T. cruzi in mammals. After amastigote multiplication and differentiation, parasites lyse host cells and spread through the body by blood circulation. In this work, we evaluated the in vitro ability of T. cruzi to infect a canine macrophage cell line DH82 compared with RAW264.7, a murine tissue culture macrophage. Our results have shown that the T. cruzi is able to infect, replicate and differentiate in DH82 cell line. We observed that following treatment with LPS and IFN-γ DH82 cells were more resistant to infection and that resistance was not related reactive oxygen species production in our system. In this study, we also found that DH82 cells became more susceptible to T. cruzi infection when cocultured with apoptotic cells. The analysis of cytokine production has showed elevated levels of the TGF-β, IL-10, and TNF-α produced by T. cruzi-infected canine macrophages. Additionally, we demonstrated a reduced expression of the MHC class II and CD80 by infected DH82 cell line.

Dendritic Cells: A Double-Edged Sword in Immune Responses during Chagas Disease

Dendritic cells (DCs) are the most important member of the antigen presenting cells group due to their ability to recognize antigen at the infection site and their high specialized antigen internalization capacity. These cells have central role in connecting the innate and adaptive immune responses against Trypanosoma cruzi, the causative agent of Chagas disease. These first line defense cells modulate host immune response depending on type, maturation level, cytokine milieu and DC receptor involved in the interactions with T. cruzi, influencing the development of the disease clinic forms. Here, we present a review of DCs-T. cruzi interactions both in human and murine models, pointing out the parasite ability to manipulate DCs activity for the purpose of evading innate immune response and assuring its own survival and persistence.

Escaping Deleterious Immune Response in Their Hosts: Lessons from Trypanosomatids

The Trypanosomatidae family includes the genera Trypanosoma and Leishmania, protozoan parasites displaying complex digenetic life cycles requiring a vertebrate host and an insect vector. Trypanosoma brucei gambiense, Trypanosoma cruzi, and Leishmania spp. are important human pathogens causing human African trypanosomiasis (HAT or sleeping sickness), Chagas' disease, and various clinical forms of Leishmaniasis, respectively. They are transmitted to humans by tsetse flies, triatomine bugs, or sandflies, and affect millions of people worldwide. In humans, extracellular African trypanosomes (T. brucei) evade the hosts' immune defenses, allowing their transmission to the next host, via the tsetse vector. By contrast, T. cruzi and Leishmania sp. have developed a complex intracellular lifestyle, also preventing several mechanisms to circumvent the host's immune response. This review seeks to set out the immune evasion strategies developed by the different trypanosomatids resulting from parasite-host interactions and will focus on: clinical and epidemiological importance of diseases; life cycles: parasites-hosts-vectors; innate immunity: key steps for trypanosomatids in invading hosts; deregulation of antigen-presenting cells; disruption of efficient specific immunity; and the immune responses used for parasite proliferation.

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

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

Effect of treatment with cyclophosphamide in low doses upon the onset of delayed type hypersensitivity in mice chronically infected with Trypanosoma cruzi: involvement of heart interstitial dendritic cells

Acute infection with Trypanosoma cruzi results in intense myocarditis, which progresses to a chronic, asymptomatic indeterminate form. The evolution toward this chronic cardiac form occurs in approximately 30% of all cases of T. cruzi infection. Suppression of delayed type hypersensitivity (DTH) has been proposed as a potential explanation of the indeterminate form. We investigated the effect of cyclophosphamide (CYCL) treatment on the regulatory mechanism of DTH and the participation of heart interstitial dendritic cells (IDCs) in this process using BALB/c mice chronically infected with T. cruzi. One group was treated with CYCL (20 mg/kg body weight) for one month. A DTH skin test was performed by intradermal injection of T. cruzi antigen (3 mg/mL) in the hind-footpad and measured the skin thickness after 24 h, 48 h and 72 h. The skin test revealed increased thickness in antigen-injected footpads, which was more evident in the mice treated with CYCL than in those mice that did not receive treatment. The thickened regions were characterised by perivascular infiltrates and areas of necrosis. Intense lesions of the myocardium were present in three/16 cases and included large areas of necrosis. Morphometric evaluation of lymphocytes showed a predominance of TCD8 cells. Heart IDCs were immunolabelled with specific antibodies (CD11b and CD11c) and T. cruzi antigens were detected using a specific anti-T. cruzi antibody. Identification of T. cruzi antigens, sequestered in these cells using specific anti-T. cruzi antibodies was done, showing a significant increase in the number of these cells in treated mice. These results indicate that IDCs participate in the regulatory mechanisms of DTH response to T. cruzi infection.

Impairment in natural killer cells editing of immature dendritic cells by infection with a virulent Trypanosoma cruzi population

Early interactions between natural killer (NK) and dendritic cells (DC) shape the immune response at the frontier of innate and adaptive immunity. Activated NK cells participate in maturation or deletion of DCs that remain immature. We previously demonstrated that infection with a high virulence (HV) population of the protozoan parasite Trypanosoma cruzi downmodulates DC maturation and T-cell activation capacity. Here, we evaluated the role of NK cells in regulating the maturation level of DCs. Shortly after infection with HV T. cruzi, DCs in poor maturation status begin to accumulate in mouse spleen. Although infection induces NK cell cytotoxicity and cytokine production, NK cells from mice infected with HV T. cruzi exhibit reduced ability to lyse and fail to induce maturation of bone marrow-derived immature DCs (iDCs). NK-mediated lysis of iDCs is restored by in vitro blockade of the IL-10 receptor during NK-DC interaction or when NK cells are obtained from T. cruzi-infected IL-10 knockout mice. These results suggest that infection with a virulent T. cruzi strain alters NK cell-mediated regulation of the adaptive immune response induced by DCs. This regulatory circuit where IL-10 appears to participate might lead to parasite persistence but can also limit the induction of a vigorous tissue-damaging T-cell response.

Different parasite inocula determine the modulation of the immune response and outcome of experimental Trypanosoma cruzi infection

During infection, the host response develops effector mechanisms to combat the parasite. However, this response can become uncontrolled or regulated by mechanisms that modulate the inflammatory reaction. The number of parasites that infects the host, such as trypomastigotes in Chagas disease, may also influence immune activation and disease pathology. We evaluated the inflammation and immune regulation that follows Trypanosoma cruzi infection with low (300), intermediate (3000) or high (30000) parasite loads. Our results showed that the load of parasite inoculum influenced disease outcome: the higher the number of parasites in the inoculum, the lower were the survival rates. There was a strong association between parasitism and inflammatory infiltrate in the heart and the parasite inoculum determined cytokine interplay in this tissue, as shown by increased interferon-γ, tumour necrosis factor-α, interleukin-17 (IL-17) and IL-23 in the 300 and 30000 inoculum groups, higher IL-4 and IL-10 in the intermediate-inoculum mice, and elevated IL-6 production in the heart of mice in the 3000 and 30000 groups. The number of T cells and antigen-presenting cells was augmented in the infected groups, especially for the splenic CD4(+) CD25(+) regulatory T cells expressing CD45RB(low) , GITR, PD-1 and FoxP3 in the group with the highest inoculum. Interestingly, these mice also presented an apparent decrease in CD4(+) CD25(+) FoxP3(+) cells in the cardiac infiltrate, in contrast to the intermediate inoculum group, which showed elevated numbers of these regulatory leucocytes in the heart. Finally, our results demonstrated that parasite load during T. cruzi infection is linked to the response pattern that will result in parasite/inflammation control or tissue damage.

MIF synergizes with Trypanosoma cruzi antigens to promote efficient dendritic cell maturation and IL-12 production via p38 MAPK

Macrophage migration inhibitory factor (MIF) has been found to be involved in host resistance to several parasitic infections. To determine the mechanisms of the MIF-dependent responses to Trypanosoma cruzi, we investigated host resistance in MIF-/- mice (on the BALB/c background) during an intraperitoneal infection. We focused on the potential involvement of MIF in dendritic cell (DC) maturation and cytokine production. Following a challenge with 5 x 10³T. cruzi parasites, wild type (WT) mice developed a strong IL-12 response and adequate maturation of the draining mesenteric lymph node DCs and were resistant to infection. In contrast, similarly infected MIF^-/- mice mounted a weak IL-12 response, displayed immature DCs in the early phases of infection and rapidly succumbed to T. cruzi infection. The lack of maturation and IL-12 production by the DCs in response to total T. cruzi antigen (TcAg) was confirmed by in vitro studies. These effects were reversed following treatment with recombinant MIF. Interestingly, TcAg-stimulated bone marrow-derived DCs from both WT and MIF^-/- mice had increased ERK1/2 MAPK phosphorylation. In contrast, p38 phosphorylation was only upregulated in WT DCs. Reconstitution of MIF to MIF^-/- DCs upregulated p38 phosphorylation. The MIF-p38 pathway affected MHC-II and CD86 expression as well as IL-12 production. These findings demonstrate that the MIF-induced early DC maturation and IL-12 production mediates resistance to T. cruzi infection, probably by activating the p38 pathway.

Evasion of immune responses by Trypanosoma cruzi, the etiological agent of Chagas disease

Infection with the protozoan parasite Trypanosoma cruzi leads to Chagas disease, which affects millions of people in Latin America. Infection with T. cruzi cannot be eliminated by the immune system. A better understanding of immune evasion mechanisms is required in order to develop more effective vaccines. During the acute phase, parasites replicate extensively and release immunomodulatory molecules that delay parasite-specific responses mediated by T cells. This immune evasion allows the parasite to spread in the host. In the chronic phase, parasite evasion relies on its replication strategy of hijacking the TGF-β signaling pathway involved in inflammation and tissue regeneration. In this article, the mechanisms of immune evasion described for T. cruzi are reviewed.

Dendritic cells devoid of IL-10 induce protective immunity against the protozoan parasite Trypanosoma cruzi

In diverse models of microbial infections, protection is improved by immunization with dendritic cells (DC) loaded with whole pathogen lysate. However, pathogens that modulate DC function as a way to evade immunity may represent a challenge for these vaccination strategies. Thus, DC must be instructed in a particular manner to circumvent this issue and drive an effective immune response. Trypanosoma cruzi or its molecules alter DC function and, as we demonstrated, this phenomenon is associated with the parasite-driven stimulation of IL-10 production by DC. Here, we show that DC from IL-10-deficient mice pulsed in vitro with trypomastigote lysate secreted increased amounts of Th1-related cytokines and stimulated higher allogeneic and antigen-specific lymphocyte responses than their wild-type counterparts. In a model of DC-based immunization, these antigen-pulsed IL-10-deficient DC conferred protection against T. cruzi infection to recipient mice. Efficient immunity was associated with enhanced antigen-specific IFN-gamma production and endogenous DC activation. We illustrate for the first time a DC-based vaccination against T. cruzi and evidence the key role of IL-10 produced by sensitizing DC in inhibiting the induction of protection. These results support the rationale for vaccination strategies that timely suppress the effect of specific cytokines secreted by antigen presenting DC.

Modulation of dendritic cell responses by parasites: a common strategy to survive

Parasitic infections are one of the most important causes of morbidity and mortality in our planet and the immune responses triggered by these organisms are critical to determine their outcome. Dendritic cells are key elements for the development of immunity against parasites; they control the responses required to eliminate these pathogens while maintaining host homeostasis. However, there is evidence showing that parasites can influence and regulate dendritic cell function in order to promote a more permissive environment for their survival. In this review we will focus on the strategies protozoan and helminth parasites have developed to interfere with dendritic cell activities as well as in the possible mechanisms involved.

Unique functions of splenic CD8alpha+ dendritic cells during infection with intracellular pathogens

Deciphering the prerequisites for the induction of protective cytotoxic T cell responses is essential for future development of more effective CD8(+) T cell-based vaccines against infectious diseases and cancer. Since crucial events for CD8(+) T cell priming and differentiation occur during the first contacts of naïve T cells with distinct antigen-presenting cells (APCs), the identification and therapeutic targeting of these 'master' APCs has become a major quest in the field. A decade ago, dendritic cells (DCs) were discovered as potent APCs, as they combine all major features for the initiation of T cell responses: (1) naïve DCs demonstrate high endocytic activity and scan continuously their environment in strategic positions throughout the whole body; (2) after activation (e.g. during pathogen invasion), DCs migrate into T cell zones of their draining lymphatic compartments, meanwhile processing captured antigen and maturing in order to stimulate encountered antigen-specific T cells. During the last years, different subsets of DCs that can be distinguished by specific surface marker expression and effector functions have been identified in mice. Their distinct functional capabilities have led to the concept of work-sharing; "migrating" DCs primarily transport antigens to the lymph node, where a specialized subset of "resident" DCs, defined by the expression of the CD8alphaalpha homodimer (CD8alpha(+) DCs), primes CD8(+) T cells upon antigen cross-presentation. Accordingly, CD8alpha(+) DCs have been found to prime CD8(+) T cells against different viruses as well as intracellular bacteria such as Listeria monocytogenes (L.m.). Recently, L.m. was found to survive specifically in splenic CD8alpha(+) DCs shortly after intravenous infection. Further experiments revealed a more generalized sampling activity of CD8alpha(+) DCs for blood-borne particles. These findings indicate that splenic CD8alpha(+) DCs might unite efficacious antigen-trapping with the licence to prime CD8(+) T cells. This new aspect of DC function could have evolved to guarantee a more rapid antigen-specific response against generalized infections.

Cutting edge: small molecule CD40 ligand mimetics promote control of parasitemia and enhance T cells producing IFN-gamma during experimental Trypanosoma cruzi infection

Host resistance to Trypanosoma cruzi infection depends on a type 1 response characterized by a strong production of IL-12 and IFN-gamma. Amplifying this response through CD40 triggering results in control of parasitemia. Two newly synthesized molecules (<3 kDa) mimicking trimeric CD40L (mini CD40Ls(-1) and (-2)) bind to CD40, activate murine dendritic cells, and elicit IL-12 production. Wild-type but not CD40 knockout mice exhibited a sharp decrease of parasitemia and mortality when inoculated with T. cruzi mixed with miniCD40Ls. Moreover, the immunosuppression induced by T. cruzi infection was impaired in mice treated with miniCD40Ls, as shown by proliferation of splenic lymphocytes, percentage of CD8(+) T cells, and IFN-gamma production. Mice surviving T. cruzi infection in the presence of miniCD40L(-1) were immunized against a challenge infection. Our results indicate that CD40L mimetics are effective in vivo and promote the control of T. cruzi infection by overcoming the immunosuppression usually induced by the parasites.

Thromboxane A2 is a key regulator of pathogenesis during Trypanosoma cruzi infection

Chagas' disease is caused by infection with the parasite Trypanosoma cruzi. We report that infected, but not uninfected, human endothelial cells (ECs) released thromboxane A(2) (TXA(2)). Physical chromatography and liquid chromatography-tandem mass spectrometry revealed that TXA(2) is the predominant eicosanoid present in all life stages of T. cruzi. Parasite-derived TXA(2) accounts for up to 90% of the circulating levels of TXA(2) in infected wild-type mice, and perturbs host physiology. Mice in which the gene for the TXA(2) receptor (TP) has been deleted, exhibited higher mortality and more severe cardiac pathology and parasitism (fourfold) than WT mice after infection. Conversely, deletion of the TXA(2) synthase gene had no effect on survival or disease severity. TP expression on somatic cells, but not cells involved in either acquired or innate immunity, was the primary determinant of disease progression. The higher intracellular parasitism observed in TP-null ECs was ablated upon restoration of TP expression. We conclude that the host response to parasite-derived TXA(2) in T. cruzi infection is possibly an important determinant of mortality and parasitism. A deeper understanding of the role of TXA(2) may result in novel therapeutic targets for a disease with limited treatment options.

CD8α+ Dendritic Cells Are Required for Efficient Entry of Listeria monocytogenes into the Spleen

In addition to their bridging function between innate and adaptive immunity, dendritic cells (DCs) may also contribute to primary resistance against infection. Here we analyzed the role of DCs during infection with Listeria monocytogenes by performing systemic in vivo depletion of these cells. We showed that CD8alpha(+) DCs were crucial for L. monocytogenes spreading and proliferation in the spleen. Efficient and rapid uptake of L. monocytogenes by CD8alpha(+) DCs required the small GTPase Rac1 and is a general characteristic of this DC subpopulation in filtering particles out of the blood. Thus, CD8alpha(+) DCs appear to play an important role for efficient bacterial entry into the spleen, which is of relevance for subsequent immune responses.

Loss of dendritic cell migration and impaired resistance to Leishmania donovani infection in mice deficient in CCL19 and CCL21

The encounter between APC and T cells is crucial for initiating immune responses to infectious microorganisms. In the spleen, interaction between dendritic cells (DC) and T cells occurs in the periarteriolar lymphoid sheath (PALS) into which DC and T cells migrate from the marginal zone (MZ) along chemokine gradients. However, the importance of DC migration from the MZ into the PALS for immune responses and host resistance to microbial infection has not yet been elucidated. In this study, we report that following Leishmania donovani infection of mice, the migration of splenic DC is regulated by the CCR7 ligands CCL19/CCL21. DC in plt/plt mutant mice that lack these chemokines are less activated and produce less IL-12, compared with those in wild-type mice. Similar findings are seen when mice are treated with pertussis toxin, which blocks chemokine signaling in vivo. plt/plt mice had increased susceptibility to L. donovani infection compared with wild-type mice, as determined by spleen and liver parasite burden. Analysis of splenic cytokine profiles at day 14 postinfection demonstrated that IFN-gamma and IL-4 mRNA accumulation was comparable in wild-type and plt/plt mice. In contrast, accumulation of mRNA for IL-10 was elevated in plt/plt mice. In addition, plt/plt mice mounted a delayed hepatic granulomatous response and fewer effector T cells migrated into the liver. Taken together, we conclude that DC migration from the MZ to the PALS is necessary for full activation of DC and the optimal induction of protective immunity against L. donovani.

Transfection of Trypanosoma cruzi with host CD40 ligand results in improved control of parasite infection

We have previously shown that infection by Trypanosoma cruzi, a parasitic protozoan, is reduced by injection of CD40 ligand (CD40L)-transfected 3T3 fibroblasts (D. Chaussabel, F. Jacobs, J. de Jonge, M. de Veerman, Y. Carlier, K. Thielemans, M. Goldman, and B. Vray, Infect. Immun. 67:1929-1934, 1999). This prompted us to transfect T. cruzi with the murine CD40L gene and to study the consequences of this transfection on the course of infection. For this, epimastigotes (Y strain) were electroporated with the pTEX vector alone or the pTEX-CD40L construct, and transfected cells were selected for their resistance to Geneticin G418. Then strain Y-, pTEX-, and pTEX-CD40L-transfected epimastigotes were transformed by metacyclogenesis into mammalian infective forms called Y, YpTEX, and YpTEX-CD40L trypomastigotes. Transfection of the CD40L gene and expression of the CD40L protein were assessed by reverse transcription-PCR and Western blot analysis. The three strains of parasites were infective in vitro for mouse peritoneal macrophages. When organisms were inoculated into mice, a very low level of parasitemia and no mortality were seen with the YpTEX-CD40L strain compared to the Y and YpTEX strains. Furthermore, the proliferative capacity and the secretion of gamma interferon were both preserved in spleen cells (SCs) from YpTEX-CD40L-infected mice but not with SCs from Y- and YpTEX-infected mice. These results suggest that the CD40L produced by transfected T. cruzi is involved in the modulation of an antiparasite immune response. Moreover, mice surviving YpTEX-CD40L infection resisted a challenge infection with the wild-type strain. Taken together, our data demonstrate the feasibility of generating a T. cruzi strain expressing a bioactive host costimulatory molecule that counteracts the immunodeficiency induced by the parasite during infection and enhances protective immunity against a challenge infection.