Galectins in Chagas Disease: A Missing Link Between Trypanosoma cruzi Infection, Inflammation, and Tissue Damage.

Trypanosoma cruzi, the protozoan parasite causative agent of Chagas disease, affects about seven million people worldwide, representing a major global public health concern with relevant socioeconomic consequences, particularly in developing countries. In this review, we discuss the multiple roles of galectins, a family of ß-galactoside-binding proteins, in modulating both T. cruzi infection and immunoregulation. Specifically, we focus on galectin-driven circuits that link parasite invasion and inflammation and reprogram innate and adaptive immune responses. Understanding the dynamics of galectins and their ß-galactoside-specific ligands during the pathogenesis of T. cruzi infection and elucidating their roles in immunoregulation, inflammation, and tissue damage offer new rational opportunities for treating this devastating neglected disease.

TNFR1 Signaling Contributes to T Cell Anergy During Staphylococcus aureus Sepsis.

Early research on sepsis has focused on the initial hyper-inflammatory, cytokine mediated phase of the disorder whereas the events that govern the concomitant and subsequent anti-inflammatory compensatory response are not completely understood. In this context, the putative participation of TNFR1-mediated signaling in the immunosuppressive phase of Staphylococcus aureus sepsis has not been elucidated. The aim of this study was to determine the role of TNFR1 in directing the immune dysfunction during S. aureus sepsis and the potential contribution of MDSC to this process. Using a model of sepsis of peritoneal origin and tnfr1-/- mice, we demonstrated that during staphylococcal sepsis CD4+ T cell anergy is significantly dependent on TNFR1 expression and that signaling through this receptor has an impact on bacterial clearance in the spleen. MDSC played a major role in the generation of anergic CD4+ T cells and their accumulation in the spleen during S. aureus sepsis correlated with IL-6 induction. Although TNFR1 signaling was not required for MDSC accumulation and expansion in the spleen, it determined the in vivo expression of Arginase 1 and iNOS, enzymes known to participate in the suppressive function of this population. Moreover, our data indicate that TNFR1-mediated IL-10 production may modulate MDSC function during staphylococcal sepsis. Taken together these results indicate that TNFR1 plays a critical role on T cell dysfunction during S. aureus sepsis by regulating immunomodulatory mediators in MDSC. The role of TNFR1-mediated signaling during the immunosuppressive phase of staphylococcal sepsis should be considered when designing novel alternative therapeutic approaches.

Dual role of monocyte-derived dendritic cells in Trypanosoma cruzi infection.

Pathogens can cause inflammation when inoculated into the skin. The vector-transmitted protozoan parasite Trypanosoma cruzi induces poor cellular-infiltration and disseminates, causing high mortality in the experimental model. Here, we characterized the inflammatory foci at the parasite inoculation site and secondary lymphoid organs using a murine model. While no macrophages and few neutrophils and monocytes (Mo) were recruited into the skin, T. cruzi infection elicited the mobilization of Ly6C+ Mo to draining lymph nodes and spleen. Over time, this population became enriched in CD11b+ Ly6C+ CD11c+ MHCII+ CD86+ cells resembling inflammatory dendritic cells (DCs). Adoptive transfer of Ly6C+ Mo purified from the bone marrow of CD11c-GFP transgenic mice confirmed the monocytic origin of Ly6C+ DCs found in the spleen of infected animals. Isolated Mo-derived cells not only produced TNF-α and nitric oxide, but also IL-10 and displayed a poor capacity to induce lymphoproliferation. Ablation of Mo-derived cells by 5-fluorouracil confirmed their dual role during infection, limiting the parasite load by inducible nitric oxide synthase-related mechanisms and negatively affecting the development of anti-parasite T-cell response. This study demonstrated that consistent with their antagonistic properties, these cells not only control the parasite spreading but also its persistence in the host.

Trypanosoma cruzi Infection Imparts a Regulatory Program in Dendritic Cells and T Cells via Galectin-1-Dependent Mechanisms.

Galectin-1 (Gal-1), an endogenous glycan-binding protein, is widely distributed at sites of inflammation and microbial invasion. Despite considerable progress regarding the immunoregulatory activity of this lectin, the role of endogenous Gal-1 during acute parasite infections is uncertain. In this study, we show that Gal-1 functions as a negative regulator to limit host-protective immunity following intradermal infection with Trypanosoma cruzi. Concomitant with the upregulation of immune inhibitory mediators, including IL-10, TGF-ß1, IDO, and programmed death ligand 2, T. cruzi infection induced an early increase of Gal-1 expression in vivo. Compared to their wild-type (WT) counterpart, Gal-1-deficient (Lgals1(-/-)) mice exhibited reduced mortality and lower parasite load in muscle tissue. Resistance of Lgals1(-/-) mice to T. cruzi infection was associated with a failure in the activation of Gal-1-driven tolerogenic circuits, otherwise orchestrated by WT dendritic cells, leading to secondary dysfunction in the induction of CD4(+)CD25(+)Foxp3(+) regulatory T cells. This effect was accompanied by an increased number of CD8(+) T cells and higher frequency of IFN-γ-producing CD4(+) T cells in muscle tissues and draining lymph nodes as well as reduced parasite burden in heart and hindlimb skeletal muscle. Moreover, dendritic cells lacking Gal-1 interrupted the Gal-1-mediated tolerogenic circuit and reinforced T cell-dependent anti-parasite immunity when adoptively transferred into WT mice. Thus, endogenous Gal-1 may influence T. cruzi infection by fueling tolerogenic circuits that hinder anti-parasite immunity.

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.

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.

Central role of extracellular signal-regulated kinase and Toll-like receptor 4 in IL-10 production in regulatory dendritic cells induced by Trypanosoma cruzi.

Several Trypanosoma cruzi molecules that stimulate macrophages activity were described as Toll-like receptor 2 (TLR2) ligands. Besides, the models of dendritic cells (DC) are poorly characterised. We have previously demonstrated that live-trypomastigotes (Tp) plus lipopolysaccharide (LPS) induce DC with tolerogenic properties that produce high levels of interleukin (IL)-10 and an impaired capacity to induce lymphoproliferation. Here, we show that the regulatory phenotype was observed with heat-killed trypomastigotes (Tphk) stimulation, ruling out DC infection. T. cruzi induced a particular DC activation state increasing LPS-activation of extracellular regulated kinase (ERK) 1/2 and signal transducer and activator of transcription (STAT) 3. Inhibition of ERK down-regulated IL-10 production and restored DC stimulatory capacity, showing the importance of this pathway in the DC modulation. A recent work shows that signalling via TLR4 and TLR2 induces a synergism in anti-inflammatory cytokine production in murine DC. Upon TLR2 and TLR4 stimulation using Pam(3)Cys or LPS and Tphk in DC from TLR2 knock out (KO) or TLR4-mutant mice, we showed that high levels of IL-10 were independent of TLR2 but associated with TLR4 and NF-kappaB signallization. Although sialic acid has been described as a molecule responsible of DC inhibition, we determine that it is not associated with T. cruzi-IL-10 modulatory response. In conclusion, all these findings demonstrate a key role of ERK and TLR4 in association with NF-kappaB in IL-10 modulation induced by T. cruzi and suggest that this regulatory effect involves parasite-DC interactions not described yet.