Modulation of I-A and I-E expression in macrophages by T-suppressor cells induced in Cryptococcus neoformans infected rats

Glucuronoxylomannan of Cryptococcus neoformans exacerbates in vitro yeast cell growth by interleukin 10-dependent inhibition of CD4+ T lymphocyte responses

Glucuronoxylomannan (GXM), the major capsular polysaccharide of Cryptococcus neoformans, is the most important virulence factor of this fungus. We analyzed the molecular events related to protective immune responses against a non-encapsulated strain of C. neoformans, mediated by murine splenic CD4(+) T lymphocytes in vitro, and the impact of GXM addition upon these events. Both the lymphoproliferation of CD4(+) T cells and the control of fungus growth were dependent on B7 co-stimulation. Addition of GXM did not modify CD4(+) T cell proliferation, but exacerbated infection in cultures obtained from normal and infected hosts. GXM enhanced the secretion of IL-10 and IL-4, while it reduced the production of pro-inflammatory cytokines TNF-alpha and IFN-gamma. The blockade of IL-10 activity with neutralizing antibodies increased TNF-alpha production and reduced yeast cell growth. The findings suggest that GXM exacerbates infection by down-regulating cell-mediated protective immune response and that IL-10 is implicated in yeast evasion.

The capsular polysaccharides of Cryptococcus neoformans activate normal CD4(+) T cells in a dominant Th2 pattern

Capsular components of Cryptococcus neoformans induce several deleterious effects on T cells. However, it is unknown how the capsular components act on these lymphocytes. The present study characterized cellular and molecular events involved in immunoregulation of splenic CD4(+) T cells by C. neoformans capsular polysaccharides (CPSs). The results showed that CPSs induce proliferation of normal splenic CD4(+) T cells, but not of normal CD8(+) T or B lymphocytes. Such proliferation depended on physical contact between CPSs and viable splenic adherent cells (SAC) and CD40 ligand-induced intracellular signal transduction. The absence of lymphoproliferation after fixation of SAC with paraformaldehyde has discarded the hypothesis of a superantigen-like activation. The evaluation of a cytokine pattern produced by the responding CD4(+) T lymphocytes revealed that CPSs induce a dominant Th2 pattern, with high levels of IL-4 and IL-10 production and undetectable inflammatory cytokines, such as TNF-alpha and IFN-gamma. Blockade of CD40 ligand by relevant mAb down-regulated the CPS-induced anti-inflammatory cytokine production and abolished the enhancement of fungus growth in cocultures of SAC and CD4(+) T lymphocytes. Our findings suggest that CPSs induce proliferation and differentiation of normal CD4(+) T cells into a Th2 phenotype, which could favor parasite growth and thus important deleterious effects to the host.

Immunosuppression in experimental cryptococcosis: variation of splenic and thymic populations and expression of class II major histocompatibility complex gene products

Previous studies from our laboratory have shown that infection with Cryptococcus neoformans can trigger the production of a series of suppressor cells that specifically inhibit the cell-mediated immune response to a nonrelated antigen, the human serum albumin (HSA). In the present study, we determined the variation of thymus and spleen cell populations in rats infected with C. neoformans and immunized with HSA-CFA at the time when suppressor activity was demonstrated. At 21 days postinfection, the number and the percentage of CD4+CD8+ cells were significantly increased in the thymus together with a minor imbalance in other thymocytes subsets. The study of two class II molecules encoded within the major histocompatibility complex, IA and IE, showed that the total number of class II IA-positive cells was increased in the glands of animals infected when compared to the glands of animals only immunized, while the corresponding percentages were lower than those in control rats. On the contrary a significant increase in both the number and the percentage of IE phenotype was observed in the thymus of infected rats, compared to the animals that were only immunized and used as a control. The IE/IA phenotype ratio within each group was increased in rats injected with the fungus. The study of spleen populations revealed an increase in CD4+ and CD8+ cells and a decrease in the B cells. The IE antigen was increased in the spleen of infected animals. The IA molecule expression showed no difference between the infected animals and the control groups. The IE/IA phenotypes ratio was mildly increased in the spleen of infected rats. These findings reveal that the cryptococcal infection can render an important imbalance in the thymus and spleen T-cell compartment together with a significant increase in the expression of the IE molecule at the time when the suppressor activity was demonstrated in this model.