Parasite Ag Research Articles

Expression of Inhibitory Receptors TIGIT, TIM-3, and LAG-3 on CD4+ T Cells from Patients with Different Clinical Forms of Chronic Chagas Disease.

T cells are central to the adaptive immune response against Trypanosoma cruzi infection. In chronic Chagas disease (CCD), circulating parasite-specific memory T cells show reduced functionality and increased expression of inhibitory receptors as a result of persistent antigenic stimulation. This phenotype has been linked to progression of cardiac pathology, whereas the presence of polyfunctional T cells shows association with therapeutic success. In this study, we demonstrate that T. cruzi-specific human CD4+ T cells can be identified by their expression of OX40 and CD25 upon in vitro stimulation. We characterized the expression of the inhibitory receptors T cell immunoreceptor with Ig and ITIM domains (TIGIT), T cell Ig and mucin-domain containing-3 (TIM-3), and lymphocyte activation gene 3 (LAG-3) in CD4+ T cells from CCD patients with and without cardiac alterations. Our results show that, independently of their clinical stage, CCD patients present an increased frequency of CD4+ T cells expressing TIGIT in comparison with non-T. cruzi-infected donors. Exposure to parasite Ags increases the expression of TIM-3 in CD4+ T cells from CCD patients, especially in those with cardiac compromise. Upregulation of LAG-3 was also detected in CCD individuals without cardiac manifestations, predominantly within the subpopulation of cells that did not become activated upon stimulation. Further differences were found between groups in the coexpression of these receptors. Blockade of each individual receptor did not affect activation or the production of IFN-γ and IL-10 by CD4+ T cells in response to parasite Ags. Our results suggest a role for TIGIT, TIM-3, and LAG-3 in the modulation of inflammatory phenomena thought to ultimately lead to tissue damage and cardiac pathology.

Allergic Sensitization Underlies Hyperreactive Antigen-Specific CD4+ T Cell Responses in Coincident Filarial Infection.

Among the various hypotheses put forward to explain the modulatory influence of helminth infection on allergic effector responses in humans, the IL-10-induced suppression of Th2-associated responses has been the leading candidate. To explore this helminth/allergy interaction more fully, parasite- and allergen-specific CD4(+) T cell responses in 12 subjects with filarial infections, and coincident allergic sensitization (filarial [Fil](+)allergy [A](+)) were compared with the responses to three appropriate control groups (Fil(-)A(-) [n = 13], Fil(-)A(+) [n = 12], Fil(+)A(-) [n = 11]). The most important findings revealed that Fil(+)A(+) had marked (p < 0.0001 for all cytokines) increases in parasite Ag-driven Th2 (IL-4, IL-5, IL-13), Th9 (IL-9), and the regulatory (IL-10) cytokines when compared with Fil(+)A(-) Moreover, using multiparameter flow cytometry, filarial parasite Ag induced a marked increase in not only the frequency of CD4(+) T cells producing IL-4, IL-5, IL-2, and TNF-α in Fil(+)A(+) when compared with Fil(+)A(-) patients, but also in the frequencies of polyfunctional Th2-like (CD4(+)IL-4(+)IL-5(+) and CD4(+)IL-2(+)IL-4(+)IL-5(+)TNF-α(+)) cells. The Th2-associated responses seen in the Fil(+)A(+) group were correlated with serum IgE levels (p < 0.01, r = 0.5165 for IL-4; p < 0.001, r = 0.5544 for IL-5; and p < 0.001, r = 0.4901 for IL-13) and levels of circulating eosinophils (p < 0.0116, r = 0.5656) and their degranulation/activation products (major basic protein [p < 0.001, r = 0.7353] and eosinophil-derived neurotoxin [p < 0.01, r = 0.7059]). CD4(+) responses to allergen were not different (to a large extent) among the groups. Taken together, our data suggest that allergic sensitization coincident with filarial infection drives parasite Ag-specific T cell hyperresponsiveness, which is characterized largely by an augmented Th2-dominated immune response.

Induction of protective immunity against experimental Eimeria tenella infection using serum exosomes

Avian coccidiosis is caused by Eimeria, a unicellular, apicomplexan protist which primarily infects intestinal epithelia resulting in nutrient malabsorption and reduced growth of commercial poultry. Vaccination of chickens with exosomes isolated from antigen presenting cells containing parasite antigens (Ags) represents a promising alternative strategy to control avian coccidiosis, but is restricted in its commercial application due to limitations on production scale-up for mass immunization programs. Here, we report the biochemical and physiologic characteristics of exosomes derived from serum of Eimeria tenella-infected chickens and their feasibility for inducing protective immunity to experimental coccidiosis. Exosomes isolated from the serum of E. tenella-infected chickens contained a subset of protein Ags found in the intact parasite. Serum-derived exosomes containing these E. tenella Ags localized to the intestine and spleen following intramuscular injection into naïve chickens. In vitro ELISPOT assays revealed increased numbers of IL-2-, IL-4-, IL-6-, and IFN-γ-secreting cells in the intestine and spleen of exosome-administered chickens, compared with vehicle controls. Pre-immunization of chickens with serum exosomes from E. tenella-infected chickens increased both body weight gain and feed conversion efficiency, and reduced both fecal parasite shedding and gut lesion scores following parasite infection, compared with vehicle controls. Finally, immunization with CD80+ serum exosomes stimulated greater numbers of cytokine-producing cells, and higher levels of protective immunity to E. tenella infection, compared with CD80− exosomes. These results suggest the possibility of producing an effective, parasite-free vaccine against avian coccidiosis under field conditions using serum-derived CD80+ exosomes containing parasite Ags.

SHARED ANTIGENS BETWEEN FASCIOLA GIGANTICA AND ITS INTERMEDIATE HOST, LYMNAEA NATALENSIS

The presence of common antigens (Ag) between Fasciola gigantica and Lymnaea natalensis was demonstrated. Two different groups of mice were immunized with: crude somatic extract of L. natalensis and redia of F. gigantica. The mice's polyclonal antibodies (Ab) were evaluated by ELISA and immunoblotting. Both extracts were immunogenic in mice according to these two techniques, when evaluated against the homologous Ag. From 11 to 13 bands were identified in L. natalensis and from 9 to10 bands were recorded in redia Ag by the autologous sera. Cross-reactions between F. gigantica and L.natalensis were found by ELISA and immunoblotting. Anti-F. gigantica Ab recognized 10 components in snail crude extract. Likewise, immune sera against snail extract identified eleven bands in redia Ag. This is the first work that demonstrates the shared Ag between redia of F. gigantica and L. natalensis. From this finding it may be concluded that L. natalensis constitutes a rich source of Ag for serodiagnostic and prophylacticstudies for F. gigantica. The snail antigen (SAg) is easy to obtain, with a high recovery of proteins and low cost, compared to other parasite Ag.

Marginal Zone B Cells Regulate Antigen-Specific T Cell Responses during Infection

Marginal zone B cells (MZB) participate in the early immune response to several pathogens. In this study, we show that in μMT mice infected with Leishmania donovani, CD8 T cells displayed a greater cytotoxic potential and generated more effector memory cells compared with infected wild type mice. The frequency of parasite-specific, IFN-γ(+) CD4 T cells was also increased in μMT mice. B cells were able to capture parasites, which was associated with upregulation of surface IgM and MyD88-dependent IL-10 production. Moreover, MZB presented parasite Ags to CD4 T cells in vitro. Depletion of MZB also enhanced T cell responses and led to a decrease in the parasite burden but did not alter the generation of effector memory T cells. Thus, MZB appear to suppress protective T cell responses during the early stages of L. donovani infection.

Induction of protective immunity against Eimeria tenella infection using antigen-loaded dendritic cells (DC) and DC-derived exosomes

Current methods for sustainable control of avian coccidiosis, whether by prophylactic medication or parasite vaccination, are suboptimal. In this study, we describe an alternative immunization strategy against Eimeria tenella infection using parasite antigen (Ag)-loaded dendritic cells (DCs), or their derived exosomes, in the absence of free Ag. CD45 + intestinal DCs were isolated from E. tenella-infected chickens and loaded ex vivo with an extract of sporozoites as parasite Ag. Extracellular vesicles purified from the Ag-pulsed DCs expressed surface proteins associated with DC-derived exosomes, including major histocompatibility complex proteins (MHC I and MHC II), CD80, flotillin, and heat shock protein (HSP70). Following intramuscular immunization of chickens with Ag-pulsed DCs or Ag-pulsed DC-derived exosomes, Ag-containing cells were observed diffusely localized in the lymphoid tissue and concentrated in germinal centers of caecal tonsils, and restricted to germinal centers (GC) in the spleen. Chickens immunized with pulsed DCs or exosomes exhibited (a) higher numbers of caecal tonsil and spleen cells expressing IgG and/or IgA antibodies that were reactive with E. tenella Ag, (b) greater numbers of IL-2-, IL-16-, and IFN-γ-producing cells, and (c) higher E. tenella Ag-driven cell proliferation, compared with chickens immunized with Ag in the absence of DCs or exosomes. Chickens immunized with Ag-pulsed DCs or Ag-pulsed DC-derived exosomes and subsequently given a live E. tenella challenge infection at 10 d post-immunization displayed (a) increased body weight gains, (b) decreased feed conversion ratios, (c) reduced fecal oocyst shedding, (d) diminished intestinal lesions, and (e) lower mortality, compared with animals given Ag alone. This is the first demonstration of Ag-specific protective immunity against avian coccidiosis using parasite Ag-loaded DCs or DC-derived exosomes.

Basophils Amplify Type 2 Immune Responses, but Do Not Serve a Protective Role, during Chronic Infection of Mice with the Filarial Nematode Litomosoides sigmodontis

Chronic helminth infections induce a type 2 immune response characterized by eosinophilia, high levels of IgE, and increased T cell production of type 2 cytokines. Because basophils have been shown to be substantial contributors of IL-4 in helminth infections, and because basophils are capable of inducing Th2 differentiation of CD4(+) T cells and IgE isotype switching in B cells, we hypothesized that basophils function to amplify type 2 immune responses in chronic helminth infection. To test this, we evaluated basophil function using the Litomosoides sigmodontis filaria model of chronic helminth infection in BALB/c mice. Time-course studies showed that eosinophilia, parasite Ag-specific CD4(+) T cell production of IL-4 and IL-5 and basophil activation and IL-4 production in response to parasite Ag all peak late (6-8 wk) in the course of L. sigmodontis infection, after parasite-specific IgE has become detectable. Mixed-gender and single-sex worm implantation experiments demonstrated that the relatively late peak of these responses was not dependent on the appearance of circulating microfilariae, but may be due to initial low levels of parasite Ag load and/or habitation of the developing worms in the pleural space. Depletion of basophils throughout the course of L. sigmodontis infection caused significant decreases in total and parasite-specific IgE, eosinophilia, and parasite Ag-driven CD4(+) T cell proliferation and IL-4 production, but did not alter total worm numbers. These results demonstrate that basophils amplify type 2 immune responses, but do not serve a protective role, in chronic infection of mice with the filarial nematode L. sigmodontis.

The Heavy Chain Variable Segment Gene Repertoire in Chronic Chagas′ Heart Disease

Patients chronically infected with Trypanosoma cruzi develop chronic Chagas' heart disease (cChHD). Their Ab response is suspected to be involved in the cardiac pathogenesis. Reactivity of serum Abs from these patients has been extensively studied but little is known about the diversity of the in vivo IgG repertoire. We analyzed 125 variable H chain (VH) genes and compared it to repertoires from healthy individuals, and patients with autoimmune processes and other infections. VH were from plasma cells isolated from heart tissue of three cChHD patients and from a Fab combinatorial library derived from bone marrow of another cChHD patient. The role of the parasite in shaping the Ab repertoire was assessed analyzing VH genes before and after panning against T. cruzi Ag. Among recovered VH genes, a significantly increased representation of VH4 was observed. Plasma cells at the site of cardiac infiltration showed an increased VH1 usage. CDR3 lengths were similar to the ones found in the healthy repertoire and significantly shorter than in other infections. VH derived from anti-T. cruzi Fab and plasma cells showed a higher proportion of hypermutated genes, 46.9% and 43.75%, respectively, vs 30.9% of the cChHD patient repertoire, pointing to the role of parasite Ags in the shaping of the humoral response in Chagas' disease. No histological evidence of germinal center-like structures was observed in heart tissue. In accordance, VH analysis of heart plasmocytes revealed no evidence of clonal B cell expansion, suggesting that they migrated into heart tissue from secondary lymphoid organs.

Gradual Decline in Malaria-Specific Memory T Cell Responses Leads to Failure to Maintain Long-Term Protective Immunity to Plasmodium chabaudi AS Despite Persistence of B Cell Memory and Circulating Antibody

The mechanisms responsible for the generation and maintenance of immunological memory to Plasmodium are poorly understood and the reasons why protective immunity in humans is so difficult to achieve and rapidly lost remain a matter for debate. A possible explanation for the difficulty in building up an efficient immune response against this parasite is the massive T cell apoptosis resulting from exposure to high-dose parasite Ag. To determine the immunological mechanisms required for long-term protection against P. chabaudi malaria and the consequences of high and low acute phase parasite loads for acquisition of protective immunity, we performed a detailed analysis of T and B cell compartments over a period of 200 days following untreated and drug-treated infections in female C57BL/6 mice. By comparing several immunological parameters with the capacity to control a secondary parasite challenge, we concluded that loss of full protective immunity is not determined by acute phase parasite load nor by serum levels of specific IgG2a and IgG1 Abs, but appears to be a consequence of the progressive decline in memory T cell response to parasites, which occurs similarly in untreated and drug-treated mice with time after infection. Furthermore, by analyzing adoptive transfer experiments, we confirmed the major role of CD4(+) T cells for guaranteeing long-term full protection against P. chabaudi malaria.

Plasmacytoid Dendritic Cells Are Activated by Toxoplasma gondii to Present Antigen and Produce Cytokines

Infection with the parasite Toxoplasma gondii leads to the induction of a Th1-type response dominated by IFN-gamma production and control of this pathogen. Cells of the innate immune system are essential in initiating this response both through the production of IL-12 as well as the presentation of parasite-derived Ags to MHC-restricted T cells. Although dendritic cells (DCs) have been implicated in these events, the contribution of individual DC populations remains unclear. Therefore, multiparameter flow cytometry was used to identify and characterize subsets of murine DCs during acute toxoplasmosis. This approach confirmed that infection leads to the expansion and activation of conventional DC (cDC) subsets. Unexpectedly, however, this analysis further revealed that plasmacytoid DCs are also expanded and that these cells up-regulate MHC class II and costimulatory molecules associated with their acquired ability to prime naive CD4(+) T cells. Furthermore, T. gondii-activated plasmacytoid DCs produce high levels of IL-12 and both plasmacytoid DC maturation and cytokine production are dependent on TLR11. Together these studies suggest that pDCs are a prominent DC subset involved in the initial stages of T. gondii infection, presenting parasite Ags and producing cytokines that are important for controlling infection.

Invariant and Noninvariant Natural Killer T Cells Exert Opposite Regulatory Functions on the Immune Response during Murine Schistosomiasis

CD1d-restricted natural killer T (NKT) cells represent a heterogeneous population of innate memory immune cells expressing both NK and T-cell markers distributed into two major subsets, i.e., invariant NKT (iNKT) cells, which express exclusively an invariant T-cell receptor (TCR) alpha chain (Valpha14Jalpha18 in mice), and non-iNKT cells, which express more diverse TCRs. NKT cells quickly produce Th1- and/or Th2-type cytokines following stimulation with glycolipid antigen (Ag) and, through this property, play potent immunoregulatory roles in autoimmune diseases, cancer, and infection. No study has addressed the role of NKT cells in metazoan parasite infections so far. We show that during murine schistosomiasis, the apparent frequency of both iNKT cells and non-iNKT cells decreased in the spleen as early as 3 weeks postinfection (p.i.) and that both populations expressed a greater amount of the activation marker CD69 at 6 weeks p.i., suggesting an activated phenotype. Two different NKT-cell-deficient mouse models, namely, TCR Jalpha18-/- (exclusively deficient in iNKT cells) and CD1d-/- (deficient in both iNKT and non-iNKT cells) mice, were used to explore the implication of these subsets in infection. We show that whereas both iNKT and non-iNKT cells do not have a major impact on the immune response during the early phase (1 and 4 weeks) of infection, they exert important, although opposite, effects on the immune response during the acute phase of the disease (7 and 12 weeks), after schistosome egg production. Indeed, iNKT cells contribute to Th1 cell differentiation whereas non-iNKT cells might be mostly implicated in Th2 cell differentiation in response to parasite Ag. Our findings suggest, for the first time, that helminths activate both iNKT and non-iNKT cells in vivo, enabling them to differentially influence the Th1/Th2 balance of the immune response.

Carriage of DRB1*13 Is Associated with Increased Posttreatment IgE Levels against Schistosoma mansoni Antigens and Lower Long-Term Reinfection Levels

Praziquantel treatment for Schistosoma mansoni infection enhances Th2 responsiveness against parasite Ags, but also increases the variance in Ab isotype levels. This effect may arise partly from genetic heterogeneity. In this study, associations between HLA polymorphisms at three loci (HLA-DQB1, HLA-DQA1, and HLA-DRB1) and posttreatment Ig responses to S. mansoni Ags were assessed in 199 individuals aged 7-50 years from Uganda. Blood samples were assayed for IgG1, IgG4, and IgE levels against soluble worm Ag (SWA), soluble egg Ag, tegument Ag, and a recombinant tegumental Ag (rSm 22.6) 7 wk after treatment. Multivariate ANOVA analysis initially revealed associations between carriage of DRB1*13 and increased levels of IgG1, IgG4, and IgE against SWA, tegument Ag, and rSM22.6. Subsequent analysis of covariance, which controlled for correlations between isotype levels and also included pretreatment IL-4, IL-5, and IL-13 responsiveness against SWA as covariates, revealed an independent association only between DRB1*13 and a factor score summarizing IgE levels to worm-derived Ags, which was strongest in adults. A post hoc age- and sex-stratified analysis revealed lower reinfection intensities at 1 year, 22 mo, and 6 years after the first round of treatment among carriers of DRB1*13. These results indicate that genetic background has a prominent influence on the posttreatment Th2 immune response to S. mansoni Ags, as well as a downstream association with long-term reinfection levels.

Regulatory Networks Induced by Live Parasites Impair Both Th1 and Th2 Pathways in Patent Lymphatic Filariasis: Implications for Parasite Persistence

Patent lymphatic filariasis is characterized by a profound down-regulation of immune responses with both parasite Ag-specific tolerance and bystander suppression. Although this down-regulation is confined to the Th1 arm of the immune system in response to parasite Ag, we hypothesized a more generalized suppression in response to live parasites. Indeed, when we examined the cytokine profile of a cohort of filaria-infected (n = 10) and uninfected (n = 10) individuals in response to live infective-stage larvae or microfilariae of Brugia malayi, we found significant impairment of both Th1 and Th2 cytokines characterized by diminished production of IFN-gamma, TNF-alpha, IL-4, IL-5, and IL-10 in infected patients. The molecular basis of this impaired Th1/Th2 response was examined, and we identified three major networks of immunoregulation and tolerance. First, impaired induction of T-bet and GATA-3 mRNA underlies the Th1/Th2 deficiency in infected individuals. Second, regulatory networks, as evidenced by significantly increased expression of Foxp3 (natural regulatory T cell marker) and regulatory effectors such as TGF-beta, CTLA-4, PD-1, ICOS, and indoleamine 2,3-dioxygenase play an important role in immunosuppression. Third, the compromise of effector T cell function is mediated by the enhanced induction of anergy-inducing factors cbl-b, c-cbl (cbl is abbreviation for Casitas B lymphoma), Itch, and Nedd4. Indeed, blocking CTLA-4 or neutralizing TGF-beta restored the ability to mount Th1/Th2 responses to live parasites and reversed the induction of anergy-inducing factors. Hence, we conclude that a profound impairment of live parasite-specific Th1 and Th2 immune responses occurs in lymphatic filariasis that is governed at the transcriptional level by a complex interplay of inhibitory mediators.

Reduced Cord Blood Immune Effector‐Cell Responsiveness Mediated by CD4+Cells Induced in Utero as a Consequence of PlacentalPlasmodium falciparumInfection

To determine mechanisms of neonatal parasite antigen (Ag)-specific immune suppression associated with placental Plasmodium falciparum infection, we isolated cord blood mononuclear cells (CBMCs) from Gabonese neonates born to mothers with differing histories of P. falciparum infection and performed ex vivo and in vitro studies to evaluate immune regulatory activity. We found increased ex vivo percentages of CD4(+)CD25(hi) and CD4(+)CD25(+)CTLA-4(+) cells and increased interleukin (IL)-10 responses to parasite Ag in vitro in CBMCs from neonates born to mothers with placental P. falciparum infection at delivery. Depleting CBMCs of CD4(+)CD25(+) cells before cell culture led to the abrogation of parasite Ag-specific IL-10 responses, to enhanced interferon- gamma responses, and to enhanced expression of CD25 on CD8(+) T cells and of major histocompatibility complex class I and II on monocytes. These data demonstrate that parasite Ag-specific CD4(+) regulatory cells are generated in utero as a consequence of placental P. falciparum infection.

CCR8 Is Expressed by Antigen-Elicited, IL-10-Producing CD4+CD25+ T Cells, Which Regulate Th2-Mediated Granuloma Formation in Mice

CCR8 was initially described as a Th2 cell-restricted receptor, but this has not been fully tested in vivo. The present study used ex vivo and in vivo approaches to examine the distribution and functional significance of CCR8 among CD4+ T cells. Populations of cytokine-secreting CD4+ T cells were generated in primed mice with Th1 or Th2 cell-mediated pulmonary granulomas, respectively elicited by i.v. challenge with either Mycobacteria bovis purified protein derivative- or Schistosoma mansoni egg Ag (SEA)-coated beads. Cytokine-producing CD4+ T cells were isolated from Ag-stimulated draining lymph node cultures by positive selection. Quantitative analysis of cytokine mRNA indicated enriched populations of IFN-gamma-, IL-4-, and IL-10-producing cells. Analysis of chemokine receptor mRNA indicated that IL-10+ cells selectively expressed CCR8 in the SEA bead-elicited type 2 response. The IL-10+CCR8+ populations were CD25+ and CD44+ but lacked enhanced Foxp3 expression. Adoptive transfer to naive recipients indicated that IL-10+ T cells alone could not transfer type 2 inflammation. Analysis of SEA bead-challenged CCR8-/- mice indicated significantly impaired IL-10 production as well as reductions in granuloma eosinophils. Adoptive transfer of CD4+CCR8+/+ T cells corrected cytokine and inflammation defects, but the granuloma eosinophil recruitment defect persisted when donor cells were depleted of IL-10+ cells. Accordingly, local IL-10 production correlated with CCR8 ligand (CCL1) expression and the appearance of CCR8+ cells in granulomatous lungs. Thus, IL-10-producing, CCR8+CD4+CD25+CD44+ T cells are generated during SEA challenge, which augment the Th2-mediated eosinophil-rich response to the parasite Ags.

IFN-γ and IL-10 Mediate Parasite-Specific Immune Responses of Cord Blood Cells Induced by Pregnancy-Associated Plasmodium falciparum Malaria

Available evidence suggests that immune cells from neonates born to mothers with placental Plasmodium falciparum (Pf) infection are sensitized to parasite Ag in utero but have reduced ability to generate protective Th1 responses. In this study, we detected Pf Ag-specific IFN-gamma(+) T cells in cord blood from human neonates whose mothers had received treatment for malaria or who had active placental Pf infection at delivery, with responses being significantly reduced in the latter group. Active placental malaria at delivery was also associated with reduced expression of monocyte MHC class I and II in vivo and following short term in vitro coculture with Pf Ag compared with levels seen in neonates whose mothers had received treatment during pregnancy. Given that APC activation and Th1 responses are driven in part by IFN-gamma and down-regulated by IL-10, we examined the role of these cytokines in modulating the Pf Ag-specific immune responses in cord blood samples. Exogenous recombinant human IFN-gamma and neutralizing anti-human IL-10 enhanced T cell IFN-gamma production, whereas recombinant human IFN-gamma also restored MHC class I and II expression on monocytes from cord blood mononuclear cells cocultured with Pf Ag. Accordingly, active placental malaria at delivery was associated with increased frequencies of Pf Ag-specific IL-10(+)CD4(+) T cells in cord blood mononuclear cell cultures from these neonates. Generation and maintenance of IL-10(+) T cells in utero may thus contribute to suppression of APC function and Pf Ag-induced Th1 responses in newborns born to mothers with placental malaria at delivery, which may increase susceptibility to infection later in life.

Functional Maturation of CD4+CD25+CTLA4+CD45RA+ T Regulatory Cells in Human Neonatal T Cell Responses to Environmental Antigens/Allergens

A number of laboratories have reported cord blood T cell responses to ubiquitous environmental Ags, including allergens, by proliferation and cytokine secretion. Moreover, the magnitude of these responses has been linked with risk for subsequent expression of allergy. These findings have been widely interpreted as evidence for transplacental priming and the development of fetal T memory cells against Ags present in the maternal environment. However, we present findings below that suggest that neonatal T cell responses to allergens (and other Ags) differ markedly from those occurring in later life. Notably, in contrast to allergen-responsive adult CD4(+) T cell cultures, responding neonatal T cell cultures display high levels of apoptosis. Comparable responses were observed against a range of microbial Ags and against a parasite Ag absent from the local environment, but not against autoantigen. A notable finding was the appearance in these cultures of CD4(+)CD25(+)CTLA4(+) T cells that de novo develop MLR-suppressive activity. These cells moreover expressed CD45RA and CD38, hallmarks of recent thymic emigrants. CFSE-labeling studies indicate that the CD4(+)CD25(+) cells observed at the end of the culture period were present in the day 0 starting populations, but they were not suppressive in MLR responses. Collectively, these findings suggest that a significant component of the reactivity of human neonatal CD4(+) T cells toward nominal Ag (allergen) represents a default response by recent thymic emigrants, providing an initial burst of short-lived cellular immunity in the absence of conventional T cell memory, which is limited in intensity and duration via the parallel activation of regulatory T cells.

CTLA-4 blockage increases resistance to infection with the intracellular protozoan Trypanosoma cruzi.

Recent studies have revealed an important role for CTLA-4 as a negative regulator of T cell activation. In the present study, we evaluated the importance of CTLA-4 to the immune response against the intracellular protozoan, Trypanosoma cruzi, the causative agent of Chagas' disease. We observed that the expression of CTLA-4 in spleen cells from naive mice cultured in the presence of live trypomastigote forms of T. cruzi increases over time of exposure. Furthermore, spleen cells harvested from recently infected mice showed a significant increase in the expression of CTLA-4 when compared with spleen cells from noninfected mice. Blockage of CTLA-4 in vitro and/or in vivo did not restore the lymphoproliferative response decreased during the acute phase of infection, but it resulted in a significant increase of NO production in vivo and in vitro. Moreover, the production of IFN-gamma in response to parasite Ags was significantly increased in spleen cells from anti-CTLA-4-treated infected mice when compared with the production found in cells from IgG-treated infected mice. CTLA-4 blockade in vivo also resulted in increased resistance to infection with the Y and Colombian strains of T. cruzi. Taken together these results indicate that CTLA-4 engagement is implicated in the modulation of the immune response against T. cruzi by acting in the mechanisms that control IFN-gamma and NO production during the acute phase of the infection.

Dendritic cells from malaria-infected mice are fully functional APC.

Malaria infection has long been associated with diminished T cell responses in vitro and more recently in experimental studies in vivo. Suppression of T cell-proliferative responses during malaria has been attributed to macrophages in a variety of murine and human systems. More recently, however, attention has been directed at the role of dendritic cells in this phenomenon, with several studies suggesting that maturation of dendritic cells is inhibited in vitro by the presence of malaria-infected E. In the studies reported here, we have examined the function of dendritic cells taken directly from infected mice. We found that they express high levels of costimulatory proteins and class II MHC, can activate naive T cells to produce IL-2 as efficiently as dendritic cells from uninfected mice, and support high levels of IFN-gamma production by naive T cells through an IL-12-dependent mechanism. Dendritic cells from infected mice also support higher levels of TNF-alpha production by naive T cells. These same dendritic cells present parasite Ag to a malaria-specific T cell hybridoma, a finding that demonstrates that dendritic cells participate in the generation of Ag-specific immunity during infection. Our findings challenge the contention that dendritic cell function is inhibited by malaria infection.

Effects of interleukin-4 deprivation and treatment on resistance to Trypanosoma cruzi.

Trypanosoma cruzi (Y strain)-infected interleukin-4(-/-) (IL-4(-/-)) mice of strains 129/J, BALB/c, and C57BL/6 showed no significant difference in parasitemia levels or end point mortality rates compared to wild-type (WT) mice. Higher production of gamma interferon (IFN-gamma) by parasite antigen (Ag)-stimulated splenocytes was observed only for C57BL/6 IL-4(-/-) mice. Treatment of 129/J WT mice with recombinant IL-4 (rIL-4), rIL-10, anti-IL-4, and/or anti-IL-10 monoclonal antibodies (MAbs) did not modify parasitism. However, WT mice treated with rIL-4 and rIL-10 had markedly increased parasitism and suppressed IFN-gamma synthesis by spleen cells stimulated with parasite Ag, concanavalin A, or anti-CD3. Addition of anti-IL-4 MAbs to splenocyte cultures from infected WT 129/J, BALB/c, or C57BL/6 mice failed to modify IFN-gamma synthesis levels; in contrast, IL-10 neutralization increased IFN-gamma production and addition of rIL-4 and/or rIL-10 diminished IFN-gamma synthesis. We conclude that endogenous IL-4 is not a major determinant of susceptibility to Y strain T. cruzi infection but that IL-4 can, in association with IL-10, modulate IFN-gamma production and resistance.