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.