Isotype switching from IgG3 to IgG1 converts a nonprotective murine antibody to Cryptococcus neoformans into a protective antibody.

Passively administered mAbs to Cryptococcus neoformans capsular polysaccharide can alter the course of infection in mouse models. In preliminary studies of passive Ab efficacy, most IgM, IgA, and IgG1 mAbs were protective, but the few IgG3 mAbs tested did not confer significant protection. Because IgG3 is effective in pneumococcal infections, this phenomenon was examined more rigorously by generating an IgG1 switch variant from the non-protective IgG3 mAb 3E5 and comparing its protective efficacy in a murine model of i.v. infection by using strains of both the A and D serotypes. The 3E5 IgG3 mAb did not prolong survival or reduce organ fungal burden. Rather, the IgG3 decreased survival relative to controls. In contrast, the IgG1 switch variant of 3E5 significantly prolonged survival, reduced organ colony-forming units, and reduced serum polysaccharide Ag level in infected mice. The results establish that isotype is important for Ab efficacy against C. neoformans.