Luiz Roberto Sardinha

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.

Contribution of NK, NK T, γδ T, and αβ T Cells to the Gamma Interferon Response Required for Liver Protection against Trypanosoma cruzi

ABSTRACT In the present work, we show that intracellular Trypanosoma cruzi is rarely found in the livers of acutely infected mice, but inflammation is commonly observed. The presence of numerous intrahepatic amastigotes in infected gamma interferon (IFN-γ)-deficient mice corroborates the notion that the liver is protected by an efficient local immunity. The contribution of different cell populations was suggested by data showing that CD4- and CD8-deficient mice were able to restrain liver parasite growth. Therefore, we have characterized the liver-infiltrating lymphocytes and determined the sources of IFN-γ during acute T. cruzi infection. We observed that natural killer (NK) cells increased by day 7, while T and B cells increased by day 14. Among CD3+ cells, CD4+, CD8+, and CD4− CD8− cell populations were greatly expanded. A large fraction of CD3+ cells were positive for PanNK, a β1 integrin expressed by NK and NK T cells. However, these lymphocytes were not classic NK T cells because they did not express NK1.1 and showed no preferential usage of Vβ8. Otherwise, liver NK T (CD3+ NK1.1+) cells were not increased in acutely infected mice. The majority of PanNK+ CD4+ and PanNK+ CD8+ cells expressed T-cell receptor αβ (TCRαβ), whereas PanNK+ CD4− CD8− cells were positive for TCRγδ. In fact, γδ T cells showed the most remarkable increase (40- to 100-fold) among liver lymphocytes. Most importantly, intracellular analysis revealed high levels of IFN-γ production at day 7 by NK cells and at day 14 by CD4+, CD8+, and CD4− CD8− TCRγδ+ cells. We concluded that NK cells are a precocious source of IFN-γ in the livers of acutely infected mice, and, as the disease progresses, conventional CD4+ and CD8+ T cells and γδ T cells, but not classic NK-T cells, may provide the IFN-γ required for liver protection against T. cruzi.

Pathology Affects Different Organs in Two Mouse Strains Chronically Infected by a Trypanosoma cruzi Clone: a Model for Genetic Studies of Chagas' Disease

ABSTRACT Chagas’ disease is a chronic infection caused by Trypanosoma cruzi and represents an important public health burden in Latin America. Frequently the disease evolves undetectable for decades, while in a significant fraction of the affected individuals it culminates in death by heart failure. Here, we describe a novel murine model of the chronic infection with T. cruzi using a stable clone isolated from a human patient (Sylvio X10/4). The infection in the C3H/HePAS mouse strain progresses chronically and is mainly characterized by intense cardiac inflammatory lesions that recapitulate the chronic cardiac pathology observed in the human disease. Moderate striated muscle lesions are also present in C3H/HePAS mice. Viable parasites are detected and recovered from the chronic heart lesions of C3H/HePAS mice, supporting the current notion that development of heart pathology in Chagas’ disease is related to parasite persistence in the inflamed tissue. By contrast, in infected A/J mice, chronic inflammatory lesions are targeted to the liver and the skeletal muscle, while pathology and parasites are undetectable in the heart. The phenotypic analysis of F1 (A/J × C3H/HePAS) and F2 (A/J × C3H/HePAS) mice suggests that the genetic predisposition to develop the inflammatory lesions caused by T. cruzi (Sylvio X10/4 clone) is heterogeneous because the heart and liver pathology segregate in the F2 generation. These findings raise the hypothesis that the pathology heterogeneity observed in humans with Chagas’ disease (absence and presence of cardiac or digestive chronic lesions) may be attributable to host genetic factors.

IFN-γ, But Not Nitric Oxide or Specific IgG, is Essential for the In vivo Control of Low-virulence Sylvio X10/4 Trypanosoma cruzi Parasites

Highly virulent strains of Trypanosoma cruzi are frequently used as murine models of Chagas’ disease. However, these strains do not fully represent the spectrum of parasites involved in the human infection. In this paper, we analysed parasitaemia, mortality, tissue pathology and parasite‐specific IgG serum levels in immune‐deficient mice infected with Sylvio X10/4 parasites, a T. cruzi derived from a chagasic patient that yields very low parasitaemias and in C3H/HePAS mice induces a chronic cardiopathy resembling the human disease. IFN‐γ was identified as a crucial element for parasite control as its absence determined a drastic increase in parasitaemia, tissue parasitism, leukocyte infiltrates at the heart and striated muscles and mortality. The lack of IFN‐γ or IL‐12p40, a molecule shared by IL‐12 and IL‐23, also resulted in spinal cord lesions and a progressive paralysis syndrome. Whereas IgG2a was the main Ig isotype in infected C57BL/6 mice, IL‐12p40‐KO mice produced IgG2a and IgG1 and IFN‐γ‐KO mice produced only IgG1. The IFN‐γ‐protective effect was not essentially mediated by nitric oxide (NO), inasmuch as infected iNOS‐KO mice showed no parasitaemia and low tissue damage. Mice deficient in CD4+ or CD8+ T cells showed an intermediate phenotype with increased mortality and tissue pathology but no parasitaemia. Interestingly, CD28‐KO mice were unable to produce anti‐T. cruzi IgG antibodies but presented moderate tissue pathology and managed to control the infection. Thus, differently from infections with high virulence parasites, neither IgG, NO nor CD28‐mediated signalling are essential for the non‐sterile control of Sylvio X10/4 parasites.

Role of CD28 in Polyclonal and Specific T and B Cell Responses Required for Protection against Blood Stage Malaria

The role of B7/CD28 costimulatory pathway in the polyclonal and specific lymphocyte activation induced by blood stages of Plasmodium chabaudi AS was investigated in CD28 gene knockout (CD28−/−) and C57BL/6 (CD28+/+) mice. Analysis of the spleen during the acute infection revealed a similar increase in T and B cell populations in both groups of mice. Moreover, CD28−/− mice were able to develop a polyclonal IgM response to P. chabaudi. On the contrary, the polyclonal IgG2a response was markedly reduced in the absence of CD28. Production of IFN-γ; up-regulation of CD69, CD40L, CD95 (Fas), and CD95L (Fas ligand); and induction of apoptosis were also affected by the lack of CD28. Interestingly, the ability to control the first parasitemia peak was not compromised in acutely infected CD28−/− mice, but CD28−/− mice failed to eradicate the parasites that persisted in the blood for >3 mo after infection. In addition, drug-cured CD28−/− mice were unable to generate memory T cells, develop an anamnesic IgG response, or eliminate the parasites from a secondary challenge. The incapacity of CD28−/− mice to acquire a full protective immunity to P. chabaudi correlated with an impaired production of specific IgG2a. Moreover, reinfected CD28−/− mice were protected by the adoptive transfer of serum from reinfected CD28+/+ mice containing specific IgG2a. Our results demonstrate that the polyclonal lymphocyte response is only partially affected by the absence of CD28, but this coreceptor is essential to generate specific T and B cell responses required for complete protection against P. chabaudi malaria.

Regulatory T Cells Accumulate in the Lung Allergic Inflammation and Efficiently Suppress T-Cell Proliferation but Not Th2 Cytokine Production

Foxp3+CD25+CD4+ regulatory T cells are vital for peripheral tolerance and control of tissue inflammation. In this study, we characterized the phenotype and monitored the migration and activity of regulatory T cells present in the airways of allergic or tolerant mice after allergen challenge. To induce lung allergic inflammation, mice were sensitized twice with ovalbumin/aluminum hydroxide gel and challenged twice with intranasal ovalbumin. Tolerance was induced by oral administration of ovalbumin for 5 consecutive days prior to OVA sensitization and challenge. We detected regulatory T cells (Foxp3+CD25+CD4+ T cells) in the airways of allergic and tolerant mice; however, the number of regulatory T cells was more than 40-fold higher in allergic mice than in tolerant mice. Lung regulatory T cells expressed an effector/memory phenotype (CCR4highCD62LlowCD44highCD54highCD69+) that distinguished them from naive regulatory T cells (CCR4intCD62LhighCD44intCD54intCD69−). These regulatory T cells efficiently suppressed pulmonary T-cell proliferation but not Th2 cytokine production.

The liver plays a major role in clearance and destruction of blood trypomastigotes in Trypanosoma cruzi chronically infected mice.

Intravenous challenge with Trypanosoma cruzi can be used to investigate the process and consequences of blood parasite clearance in experimental Chagas disease. One hour after intravenous challenge of chronically infected mice with 5×106 trypomastigotes, the liver constituted a major site of parasite accumulation, as revealed by PCR. Intact parasites and/or parasite remnants were visualized at this time point scattered in the liver parenchyma. Moreover, at this time, many of liver-cleared parasites were viable, as estimated by the frequency of positive cultures, which considerably diminished after 48 h. Following clearance, the number of infiltrating cells in the hepatic tissue notably increased: initially (at 24 h) as diffuse infiltrates affecting the whole parenchyma, and at 48 h, in the form of large focal infiltrates in both the parenchyma and perivascular spaces. Phenotypic characterization of liver-infiltrating cells 24 h after challenge revealed an increase in Mac1+, CD8+ and CD4+ cells, followed by natural killer (NK) cells. As evidence that liver-infiltrating CD4+ and CD8+ cells were activated, increased frequencies of CD69+CD8+, CD69+CD4+ and CD25+CD122+CD4+ cells were observed at 24 and 48 h after challenge, and of CD25−CD122+CD4+ cells at 48 h. The major role of CD4+ cells in liver protection was suggested by data showing a very high frequency of interferon (IFN)-γ-producing CD4+ cells 24 h after challenge. In contrast, liver CD8+ cells produced little IFN-γ, even though they showed an enhanced potential for secreting this cytokine, as revealed by in vitro T cell receptor (TCR) stimulation. Confirming the effectiveness of the liver immune response in blood parasite control during the chronic phase of infection, no live parasites were detected in this organ 7 days after challenge.

Susceptibility of the different developmental stages of the asexual (schizogonic) erythrocyte cycle of Plasmodium chabaudi chabaudi to hyperimmune serum, immunoglobulin (Ig)G1, IgG2a and F(ab′)2 fragments

The mechanisms by which antibodies interfere with Plasmodium growth are still under debate. Characterizing the asexual erythrocyte stages susceptible to antibodies from hyperimmune individuals is therefore a relevant contribution to vaccine research. In this study, using a virulent and synchronous murine malaria parasite, Plasmodium chabaudi chabaudi AJ, we have shown that trophozoites and circulating schizonts are not the main targets for antibodies from hyperimmune serum. In drug‐cured mice challenged with a high inoculum of ring‐infected erythrocytes, parasitemias do not decline until the moment of erythrocyte rupture, suggesting that effector mechanisms operate immediately prior to reinvasion. Confirming these findings, treatment of primary‐infected mice with hyperimmune serum inhibited the generation of new ring forms, but did not alter the numbers of schizont‐infected erythrocytes, despite the fact that these cells were recognized by immunoglobulin (Ig)G antibodies. When these mice were treated with IgG1 or IgG2a purified from hyperimmune serum, both subclasses limited reinvasion, but IgG2a showed a stronger protective activity. The fact that Fc digestion decreases but does not abrogate protection suggests that both Fc‐dependent and independent mechanisms participate in this process. Treatment with cobra venom factor did not interfere with the antibody‐mediated protection, ruling out the participation of the complement system in both lysis and phagocytosis of merozoites or infected erythrocytes. Therefore, in mice suffering from P. c. chabaudi AJ malaria, merozoite neutralization seems to be a major mechanism of protection conferred by hyperimmune serum antibodies. However, FcγR‐mediated interactions, or other mechanisms not yet defined, may also contribute to inhibit erythrocyte reinvasion.

Impaired Macrophage Responses May Contribute to Exacerbation of Blood-Stage Plasmodium chabaudi chabaudi Malaria in Interleukin-12-Deficient Mice

Aiming to clarify the role of endogenous interleukin-12 (IL-12) in protective immunity against blood stages of Plasmodium chabaudi chabaudi (AS), we evaluated the course of infection in IL-12p40 gene knockout (IL-12p40KO) and wild-type (WT) C57BL/6 mice, focusing (1) on the ability of T cells to develop adequate type 1 responses and (2) on the potentiality of macrophages to respond to parasites, interferon-gamma (IFN-gamma), or both. We observed that IL-12p40KO mice develop significantly higher parasitemias during the acute infection, although mice from both groups clear the parasites within a month and similarly eliminate a secondary challenge. Thus, fully protective immunity to P. c. chabaudi can be generated in the absence of IL-12. However, this cytokine may promote parasite control during the early phase of infection. The increased acute parasitemia of IL-12p40KO mice was associated with both impaired IFN-gamma and nitric oxide (NO) response by spleen cells. Because stimulation with recombinant IFN-gamma (rIFN-gamma) failed to improve the NO response in IL-12p40KO macrophages, we investigated whether these cells have an intrinsic defect. Analysis of peritoneal macrophages revealed that IL-12p40KO cells produce higher levels of transforming growth factor-beta1 (TGF-beta1) compared with WT cells and respond to infected erythrocytes or rIFN-gamma by releasing little NO. Moreover, IL-12p40KO macrophages had a severely impaired ability to internalize opsonized infected erythrocytes, suggesting that the low effector profile assumed by these cells may compromise antibody-mediated immunity. Taken together, our results support the idea that the absence of IL-12p40 not only affects IFN-gamma production but also has deep consequences in macrophage effector functions that may contribute to exacerbation of the early phase of P. c. chabaudi malaria.

Characterization of the Spleen B-Cell Compartment at the Early and Late Blood-Stage Plasmodium chabaudi Malaria

Abstract Polyclonal B‐cell activation is a feature of the early spleen cell response to blood‐stage Plasmodium chabaudi malaria. Immunity to blood‐stage malaria is guaranteed by the generation of B cells able to produce parasite‐specific antibodies mainly from the immunoglobulin (Ig)G2a isotype. In the present study, we characterized the spleen B‐cell compartment during blood‐stage P. chabaudi infection. The numbers of B220+ and B220LOW CD138+ (plasma) cells increased sharply between days 4 and 7 post‐infection (p.i.). At this time B220+ cells expressed surface (s)IgM, but nearly all B220LOW CD138+ cells showed concomitantly intracellular (i)IgM and IgG2a. Both follicular and marginal zone B cells were activated expressing high amounts of CD69. At day 40 p.i., B220LOW CD138+ cell population was still increased but, differently from acute infection, 61.1% of these cells were positive for iIgG2a while only 14.2% expressed iIgM. Moreover, at days 20 and 40 p.i., 29.2% and 13.0% of B220+ cells expressed sIgG2a, respectively. According to cell size and expression of CD80, CD86, CD11b, CD44 and CD38, B220+ sIgG2a+ cells had a phenotype characteristic of activated/memory B cells. Furthermore, 14.1% of B220+ sIgG2a+ cells at day 30 p.i. expressed a marginal zone B‐cell phenotype. Importantly, B cells from 40‐day‐infected mice were very efficient in presenting parasite antigens leading to proliferation of both CD4+ and CD8+ cells. Our results contribute for understanding the dynamics of B cells during P. chabaudi infection, underlying the mechanisms of antigen presentation and antibody production, which are essential for the acquisition of protective immunity against malaria.