The glucuronoxylomannan of Cryptococcus neoformans serotype A is a type 2 T-cell-independent antigen

The humoral immune response of inbred mice to immunization with the glucuronoxylomannan (GXM) of Cryptococcus neoformans was investigated both serologically and in plaque-forming cells (PFCs). The T-helper-cell-independent quality of the GXM was demonstrated by using BALB/c nu/nu mice. Primary and secondary dose responses to three antigenic forms of GXM, (i) the native antigen, (ii) a GXM-bovine serum albumin protein conjugate, and (iii) a cryptococcal whole-cell vaccine, revealed a lack of isotype class switching and anamnestic responses. Both the levels of complement-fixing anti-GXM antibody in serum and the PFC responses in the athymic mice showed no significant differences from those in the wild-type controls. However, T cells are involved in the suppression of the primary response to GXM. When BALB/cBy mice were given rabbit anti-mouse thymocyte serum along with 0.5 microgram of GXM, both antibody levels in serum and PFC responses were significantly increased over those of control mice that received GXM and normal rabbit serum. In addition, T cells were also shown to enhance the primary immune response to GXM. BALB/cBy mice were given GXM and anti-mouse thymocyte serum on day 1. On day 2, the experimental group was given anti-mouse thymocyte serum and the control group was given saline. On day 5, comparison of the PFC responses and anti-GXM antibody titers of the two groups revealed a significant increase in the immune response of the control over the experimental group. The type 2 T-cell-independent quality of GXM was also demonstrated in CBA/cHN xid mice. These mice lack the Lyb+ subset of B cells and are unable to respond to type 2 T-independent antigens but respond normally to type 1 T-independent antigens. Type III pneumococcal polysaccharide, a type 2 T-independent antigen, was used as a negative control, and trinitrophenyl-lipopolysaccharide, a type 1 T-independent antigen, was used as a positive control. The CBA/cHN xid mice failed to respond to either type III pneumococcal polysaccharide or GXM but did not respond to immunization with trinitrophenyl-lipopolysaccharide. BALB/cBy mice responded normally to all three antigens.