Identification of regulatory T cells during experimental Leishmania infantum infection.

Leishmania infantum is the causative agent of zoonotic visceral leishmaniasis (ZVL), a disease frequently characterized by specific impairment of cell-mediated immune responses and uncontrolled parasitization. Regulatory T cells (Treg) have been shown to be involved in the direct induction of immunosuppression of effector immune response during chronic Leishmania infections. The present study aims to investigate the possible involvement of Treg cells during L. infantum infection. Results indicate that CD4(+)CD25(+) regulatory T cells are present in L. infantum-infected BALB/c mice and exhibit phenotypic and functional characteristics of Treg. The presence of high levels of Foxp3 gene expression and surface expression of alpha(E)beta(7) integrin (CD103) suggest a predisposition for Treg retention within sites of L. infantum infection, as is the case of the spleen and draining lymph nodes, consequently influencing local immune response. Th1 and Th2 effector immune responses seem inadequate, due to Treg expansion. Foxp3 expressing CD4(+)CD25(+) T cells are capable of producing TGF-beta and may contribute to immunosuppression and better control of parasite-mediated-immunopathology during infection. Surprisingly, IL-10 producing-CD4(+)CD25(-)Foxp3(-) T cells were also identified as an additional source of IL-10 and may represent a type 1 regulatory T (Tr1) cell subset that is being induced by L. infantum parasites. These findings suggest that distinct regulatory T cells develop in response to L. infantum and may play a possible role in promoting parasite persistence and the establishment of chronic infection.

Intra-specific variability of virulence in Leishmania infantum zymodeme MON-1 strains.

This study aims to characterize the intra-specific variability of virulence in Leishmania infantum zymodeme MON-1 strains isolated from dogs and immunocompetent and immunosuppressed patients through the evaluation of growth pattern, infective ability and immunopathogenicity. Two of the strains, classified as the most virulent, presented higher levels of macrophage infection, increased promastigote replication in culture medium and as well as amastigote multiplication within macrophages. These strains caused the most pathogenic infection inducing splenomegalia and maximum parasite loads in spleen and liver of BALB/c mice. The other strains exhibited either low virulence, with reduced infective capability and low replication levels, or an intermediate virulent phenotype showing mixed features similar to low and high virulent phenotypes. A correlation between the infectivity, growth dynamics and pathogenicity of each strain and the humoral and cellular immune response was demonstrated. Strains with accentuated virulent phenotype induced higher levels of anti-Leishmania IgG1 antibodies and TGF-beta but reduced production of IFN-gamma. Virulence phenotype seems to be a characteristic of each strain regardless of the host (dog or human) from which it was firstly isolated.

H-2 complex influences cytokine gene expression in Leishmania infantum-infected macrophages.

This work aims to study the influence of H-2 locus in the control of Leishmania infantum infection by evaluating whether cytokine responses by host macrophages of different H-2 haplotype are differentially regulated, either induced or actively impaired during parasite growth and replication. This study shows that macrophages of "non-cure" phenotype (H-2(d)) are more susceptible to infection with virulent L. infantum promastigotes. Virulent parasites lead to impaired IL-12 and inhibited TNF-alpha expression. The degree of parasite virulence is an important contributing factor to differences detected in cytokine expression. Virulent parasites also induced TGF-beta, a deactivating cytokine that is known to suppress Th-1 type responses, thus allowing the parasite to subvert antimicrobial activity and increase its chances of survival. Depending on specific host haplotype, cells differentially respond to infection since TNF-alpha expression is inhibited and TGF-beta is enhanced by macrophages of "non-cure" phenotype, thus perhaps determining their degree of susceptibility in this strain of mice.

Dynamics of CD62L/CD45RB CD4+ and CD8+ lymphocyte subsets in hepatic and splenic tissues during murine visceral leishmaniasis.

This study aimed to characterise, for the first time, the dynamics of CD4+ and CD8+ lymphocyte CD62L/CD45RB subsets, during visceral leishmaniasis. Memory/activated status of hepatic and splenic T cells was compared in mice strains with "cure" and "non-cure" phenotypes to Leishmania infantum infection. In both mice strains, a correlation between the dynamics of the memory CD4+ and CD8+ T cells (CD62Llow/CD45RBlow) subsets in the liver and the pre-activated phenotype of lymphocytes (CD62Llow/CD45RBhigh) from the spleen was detected suggesting that this organ is the source of Leishmania-specific T lymphocytes that migrate to the liver, where parasite replication is highly active. In the liver, these pre-activated cells become effector T lymphocytes, however, a strong regulation of CD8+ T cell effector function was observed, probably preventing hepatic tissue damage. Comparing mice strains with "cure" and "non-cure" phenotype, an imbalance between "protective" CD45RBhigh and "pathogenic" CD45RBlow CD4+ subsets in B10.D2/n animals might be involved in the evolution of a non-healing infection.

Hepatic cellular immune responses in mice with "cure" and "non-cure" phenotype to Leishmania infantum infection: importance of CD8+ T cells and TGF-beta production.

The objective of this study was to analyse hepatic cellular immune response of mice with "cure" and "non-cure" phenotypes to Leishmania infantum infection. During infection establishment, elevated TGF-beta levels and absence of a Th1 response may have contributed to parasite multiplication and to similar hepatic parasitic loads. Later in infection, an increase in the number and activation levels of CD8+ cells was observed simultaneously with parasite elimination, but only significant in "cure" strain. During this recovering phase, "non-cure" animals showed low Th2 cytokine levels, while TGF-beta production was higher than in "cure" mice. These results point out to a role for CD8+ T cells in liver acquired immune response and to TGF-beta regulation of "cure" and "non-cure" phenotype to L. infantum infection.