Cryptococcal capsular polysaccharide-induced modulation of murine immune responses

Cryptococcus neoformans, an opportunistic fungal pathogen, often causes serious and life-threatening infections in immunocompromised hosts as well as in normal individuals. In the present study, purified cryptococcal capsular polysaccharide antigen was examined for its effect on several parameters of immune response and its ability to induce immune response to itself. Injection of the antigen into mice resulted in a dose-related specific antibody response which was detected at the individual antibody plaque-forming-cell level by a hemolytic assay in gel. Relatively low doses of cryptococci induced a maximal response, whereas higher doses resulted in a markedly depressed response. The antibody response to the cryptococcal capsular polysaccharide antigen appeared to be T cell independent and regulated by suppressor T cells, since mice injected with antilymphocyte serum or antithymocyte serum showed specific antibody responses to the antigen that were higher than those of untreated mice. It also markedly affected the in vitro mixed-lymphocyte reaction when added to cultures of mouse spleen cells being challenged in vitro with mitomycin C-treated allogeneic cells. The lower doses stimulated the response, whereas higher doses suppressed it. The macrophage response to yeast cells but not opsonized sheep erythrocytes was also modulated by the cryptococcal antigen.