Host-parasite dynamics in Chagas disease from systemic to hyper-local scales.

Trypanosoma cruzi is a remarkably versatile parasite. It can parasitize almost any nucleated cell type and naturally infects hundreds of mammal species across much of the Americas. In humans, it is the cause of Chagas disease, a set of mainly chronic conditions predominantly affecting the heart and gastrointestinal tract, which can progress to become life threatening. Yet around two thirds of infected people are long-term asymptomatic carriers. Clinical outcomes depend on many factors, but the central determinant is the nature of the host-parasite interactions that play out over the years of chronic infection in diverse tissue environments. In this review, we aim to integrate recent developments in the understanding of the spatial and temporal dynamics of T. cruzi infections with established and emerging concepts in host immune responses in the corresponding phases and tissues.

B-cell memory in malaria: Myths and realities.

B-cell and antibody responses to Plasmodium spp., the parasite that causes malaria, are critical for control of parasitemia and associated immunopathology. Antibodies also provide protection to reinfection. Long-lasting B-cell memory has been shown to occur in response to Plasmodium spp. in experimental model infections, and in human malaria. However, there are reports that antibody responses to several malaria antigens in young children living with malaria are not similarly long-lived, suggesting a dysfunction in the maintenance of circulating antibodies. Some studies attribute this to the expansion of atypical memory B cells (AMB), which express multiple inhibitory receptors and activation markers, and are hyporesponsive to B-cell receptor (BCR) restimulation in vitro. AMB are also expanded in other chronic infections such as tuberculosis, hepatitis B and C, and HIV, as well as in autoimmunity and old age, highlighting the importance of understanding their role in immunity. Whether AMB are dysfunctional remains controversial, as there are also studies in other infections showing that AMB can produce isotype-switched antibodies and in mouse can contribute to protection against infection. In light of these controversies, we review the most recent literature on either side of the debate and challenge some of the currently held views regarding B-cell responses to Plasmodium infections.

Publisher Correction: c-Maf controls immune responses by regulating disease-specific gene networks and repressing IL-2 in CD4+ T cells.

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Plasmodium-specific atypical memory B cells are short-lived activated B cells.

A subset of atypical memory B cells accumulates in malaria and several infections, autoimmune disorders and aging in both humans and mice. It has been suggested these cells are exhausted long-lived memory B cells, and their accumulation may contribute to poor acquisition of long-lasting immunity to certain chronic infections, such as malaria and HIV. Here, we generated an immunoglobulin heavy chain knock-in mouse with a BCR that recognizes MSP1 of the rodent malaria parasite, Plasmodium chabaudi. In combination with a mosquito-initiated P. chabaudi infection, we show that Plasmodium-specific atypical memory B cells are short-lived and disappear upon natural resolution of chronic infection. These cells show features of activation, proliferation, DNA replication, and plasmablasts. Our data demonstrate that Plasmodium-specific atypical memory B cells are not a subset of long-lived memory B cells, but rather short-lived activated cells, and part of a physiologic ongoing B-cell response.

Distinct monocyte subset phenotypes in patients with different clinical forms of chronic Chagas disease and seronegative dilated cardiomyopathy.

BACKGROUND: Chronic infection with Trypanosoma cruzi leads to a constant stimulation of the host immune system. Monocytes, which are recruited in response to inflammatory signals, are divided into classical CD14hiCD16-, non-classical CD14loCD16+ and intermediate CD14hiCD16+ subsets. In this study, we evaluated the frequencies of monocyte subsets in the different clinical stages of chronic Chagas disease in comparison with the monocyte profile of seronegative heart failure subjects and seronegative healthy controls. The effect of the anti-parasite drug therapy benznidazole on monocyte subsets was also explored. METHODOLOGY/PRINCIPAL FINDINGS: The frequencies of the different monocyte subsets and their phenotypes were measured by flow cytometry. Trypanosoma cruzi-specific antibodies were quantified by conventional serological tests. T. cruzi-infected subjects with mild or no signs of cardiac disease and patients suffering from dilated cardiomyopathy unrelated to T. cruzi infection showed increased levels of non-classical CD14loCD16+ monocytes compared with healthy controls. In contrast, the monocyte profile in T. cruzi-infected subjects with severe cardiomyopathy was skewed towards the classical and intermediate subsets. After benznidazole treatment, non-classical monocytes CD14loCD16+ decreased while classical monocytes CD14hiCD16-increased. CONCLUSIONS/SIGNIFICANCE: The different clinical stages of chronic Chagas disease display distinct monocyte profiles that are restored after anti-parasite drug therapy. T. cruzi-infected subjects with severe cardiac disease displayed a profile of monocytes subsets suggestive of a more pronounced inflammatory environment compared with subjects suffering from heart failure not related to T. cruzi infection, supporting that parasite persistence might also alter cell components of the innate immune system.

c-Maf controls immune responses by regulating disease-specific gene networks and repressing IL-2 in CD4+ T cells.

The transcription factor c-Maf induces the anti-inflammatory cytokine IL-10 in CD4+ T cells in vitro. However, the global effects of c-Maf on diverse immune responses in vivo are unknown. Here we found that c-Maf regulated IL-10 production in CD4+ T cells in disease models involving the TH1 subset of helper T cells (malaria), TH2 cells (allergy) and TH17 cells (autoimmunity) in vivo. Although mice with c-Maf deficiency targeted to T cells showed greater pathology in TH1 and TH2 responses, TH17 cell-mediated pathology was reduced in this context, with an accompanying decrease in TH17 cells and increase in Foxp3+ regulatory T cells. Bivariate genomic footprinting elucidated the c-Maf transcription-factor network, including enhanced activity of NFAT; this led to the identification and validation of c-Maf as a negative regulator of IL-2. The decreased expression of the gene encoding the transcription factor RORγt (Rorc) that resulted from c-Maf deficiency was dependent on IL-2, which explained the in vivo observations. Thus, c-Maf is a positive and negative regulator of the expression of cytokine-encoding genes, with context-specific effects that allow each immune response to occur in a controlled yet effective manner.

Follicular Helper T Cells are Essential for the Elimination of Plasmodium Infection.

CD4+ follicular helper T (Tfh) cells have been shown to be critical for the activation of germinal center (GC) B-cell responses. Similar to other infections, Plasmodium infection activates both GC as well as non-GC B cell responses. Here, we sought to explore whether Tfh cells and GC B cells are required to eliminate a Plasmodium infection. A CD4 T cell-targeted deletion of the gene that encodes Bcl6, the master transcription factor for the Tfh program, resulted in complete disruption of the Tfh response to Plasmodium chabaudi in C57BL/6 mice and consequent disruption of GC responses and IgG responses and the inability to eliminate the otherwise self-resolving chronic P. chabaudi infection. On the other hand, and contrary to previous observations in immunization and viral infection models, Signaling Lymphocyte Activation Molecule (SLAM)-Associated Protein (SAP)-deficient mice were able to activate Tfh cells, GC B cells, and IgG responses to the parasite. This study demonstrates the critical role for Tfh cells in controlling this systemic infection, and highlights differences in the signals required to activate GC B cell responses to this complex parasite compared with those of protein immunizations and viral infections. Therefore, these data are highly pertinent for designing malaria vaccines able to activate broadly protective B-cell responses.

Treatment Success in Trypanosoma cruzi Infection Is Predicted by Early Changes in Serially Monitored Parasite-Specific T and B Cell Responses.

BACKGROUND: Chagas disease is the highest impact parasitic disease in Latin America. We have proposed that changes in Trypanosoma cruzi-specific immune responses might serve as surrogate indicators of treatment success. Herein, we addressed in a long-term follow-up study whether cure achieved after treatment can be predicted by changes in non-conventional indexes of anti-parasite serological and T cell activities. METHODOLOGY/PRINCIPAL FINDINGS: T. cruzi-specific T cell responses, as measured by interferon-γ ELISPOT and T. cruzi-specific antibodies assessed by ELISA, hemagglutination and immunofluorescence tests as well as by a multiplex assay incorporating 14 recombinant T. cruzi proteins were measured in 33 patients at 48-150 months post-benznidazole treatment. Cure - as assessed by conventional serological tests - was associated with an early decline in T. cruzi-specific IFN-γ-producing T cells and in antibody titers measured by the multiplex serological assay. Changes in the functional status and potential of T. cruzi-specific T cells, indicative of reduced antigen stimulation, provided further evidence of parasitological cure following benznidazole treatment. Patients showing a significant reduction in T. cruzi-specific antibodies had higher pre-therapy levels of T. cruzi-specific IFN-γ- producing T cells compared to those with unaltered humoral responses post-treatment. CONCLUSIONS/SIGNIFICANCE: Monitoring of appropriate immunological responses can provide earlier and robust measures of treatment success in T. cruzi infection.

Perturbed T cell IL-7 receptor signaling in chronic Chagas disease.

We have previously demonstrated that immune responses in subjects with chronic Trypanosoma cruzi infection display features common to other persistent infections with signs of T cell exhaustion. Alterations in cytokine receptor signal transduction have emerged as one of the cell-intrinsic mechanisms of T cell exhaustion. In this study, we performed an analysis of the expression of IL-7R components (CD127 and CD132) on CD4(+) and CD8(+) T cells and evaluated IL-7-dependent signaling events in patients at different clinical stages of chronic chagasic heart disease. Subjects with no signs of cardiac disease showed a decrease in CD127(+)CD132(+) cells and a reciprocal gain of CD127(-)CD132(+) in CD8(+) and CD4(+) T cells compared with either patients exhibiting heart enlargement or uninfected controls. T. cruzi infection, in vitro, was able to stimulate the downregulation of CD127 and the upregulation of CD132 on T cells. IL-7-induced phosphorylation of STAT5 as well as Bcl-2 and CD25 expression were lower in T. cruzi-infected subjects compared with uninfected controls. The serum levels of IL-7 were also increased in chronic chagasic patients. The present study highlights perturbed IL-7/IL-7R T cell signaling through STAT5 as a potential mechanism of T cell exhaustion in chronic T. cruzi infection.

Disruption of IL-21 signaling affects T cell-B cell interactions and abrogates protective humoral immunity to malaria.

Interleukin-21 signaling is important for germinal center B-cell responses, isotype switching and generation of memory B cells. However, a role for IL-21 in antibody-mediated protection against pathogens has not been demonstrated. Here we show that IL-21 is produced by T follicular helper cells and co-expressed with IFN-γ during an erythrocytic-stage malaria infection of Plasmodium chabaudi in mice. Mice deficient either in IL-21 or the IL-21 receptor fail to resolve the chronic phase of P. chabaudi infection and P. yoelii infection resulting in sustained high parasitemias, and are not immune to re-infection. This is associated with abrogated P. chabaudi-specific IgG responses, including memory B cells. Mixed bone marrow chimeric mice, with T cells carrying a targeted disruption of the Il21 gene, or B cells with a targeted disruption of the Il21r gene, demonstrate that IL-21 from T cells signaling through the IL-21 receptor on B cells is necessary to control chronic P. chabaudi infection. Our data uncover a mechanism by which CD4+ T cells and B cells control parasitemia during chronic erythrocytic-stage malaria through a single gene, Il21, and demonstrate the importance of this cytokine in the control of pathogens by humoral immune responses. These data are highly pertinent for designing malaria vaccines requiring long-lasting protective B-cell responses.

CD4 T-cell subsets in malaria: TH1/TH2 revisited.

CD4(+) T-cells have been shown to play a central role in immune control of infection with Plasmodium parasites. At the erythrocytic stage of infection, IFN-γ production by CD4(+) T-cells and CD4(+) T-cell help for the B-cell response are required for control and elimination of infected red blood cells. CD4(+) T-cells are also important for controlling Plasmodium pre-erythrocytic stages through the activation of parasite-specific CD8(+) T-cells. However, excessive inflammatory responses triggered by the infection have been shown to drive pathology. Early classical experiments demonstrated a biphasic CD4(+) T-cell response against erythrocytic stages in mice, in which T helper (Th)1 and antibody-helper CD4(+) T-cells appear sequentially during a primary infection. While IFN-γ-producing Th1 cells do play a role in controlling acute infections, and they contribute to acute erythrocytic-stage pathology, it became apparent that a classical Th2 response producing IL-4 is not a critical feature of the CD4(+) T-cell response during the chronic phase of infection. Rather, effective CD4(+) T-cell help for B-cells, which can occur in the absence of IL-4, is required to control chronic parasitemia. IL-10, important to counterbalance inflammation and associated with protection from inflammatory-mediated severe malaria in both humans and experimental models, was originally considered be produced by CD4(+) Th2 cells during infection. We review the interpretations of CD4(+) T-cell responses during Plasmodium infection, proposed under the original Th1/Th2 paradigm, in light of more recent advances, including the identification of multifunctional T-cells such as Th1 cells co-expressing IFN-γ and IL-10, the identification of follicular helper T-cells (Tfh) as the predominant CD4(+) T helper subset for B-cells, and the recognition of inherent plasticity in the fates of different CD4(+) T-cells.

Allopurinol reduces antigen-specific and polyclonal activation of human T cells.

Allopurinol is the most popular commercially available xanthine oxidase inhibitor and it is widely used for treatment of symptomatic hyperuricaemia, or gout. Although, several anti-inflammatory actions of allopurinol have been demonstrated in vivo and in vitro, there have been few studies on the action of allopurinol on T cells. In the current study, we have assessed the effect of allopurinol on antigen-specific and mitogen-driven activation and cytokine production in human T cells. Allopurinol markedly decreased the frequency of IFN-γ and IL-2-producing T cells, either after polyclonal or antigen-specific stimulation with Herpes Simplex virus 1, Influenza (Flu) virus, tetanus toxoid and Trypanosoma cruzi-derived antigens. Allopurinol attenuated CD69 upregulation after CD3 and CD28 engagement and significantly reduced the levels of spontaneous and mitogen-induced intracellular reactive oxygen species in T cells. The diminished T cell activation and cytokine production in the presence of allopurinol support a direct action of allopurinol on human T cells, offering a potential pharmacological tool for the management of cell-mediated inflammatory diseases.