BIVALENCY IS REQUIRED FOR ANTICAPSULAR MONOCLONAL-ANTIBODIES TO OPTIMALLY SUPPRESS ACTIVATION OF THE ALTERNATIVE COMPLEMENT PATHWAY BY THE CRYPTOCOCCUS-NEOFORMANS CAPSULE

Encapsulated cells of Cryptococcus neoformans are potent activators of the alternative complement pathway. Previous studies found that monoc lonal antibodies (MAbs) specific for the major capsular polysaccharide , termed glucuronoxylomannan (GXM), can markedly suppress the ability of the capsule to accumulate C3 from normal human serum via the altern ative pathway. The present study examined the abilities of F(ab)(2) an d Fab fragments of three MAbs (MAbs 439, 3C2, and 471) to mediate the suppressive effect. The results showed that F(ab)(2) fragments of all three MAbs suppressed activation and binding of C3 via the alternative pathway in a manner similar to that of intact antibodies. In contrast , Fab fragments of MAb 439 and MAI, 3C2 showed no suppressive activity , and Fab fragments of MAb 471 were markedly reduced in suppressive ac tivity. Indeed, there was an earlier accumulation of C3 on encapsulate d cryptococci in the presence of the Fab fragments. Study of subclass switch families of MAb 439 and MAb 471 found that MAbs of an immunoglo bulin G(IgG) subclass with increased flexibility in the hinge region ( IgG2b) had less suppressive activity than MAbs of IgG subclasses with less flexibility (IgG1 or IgG2a). Taken together, these results indica te that cross-linking of the capsular matrix is an essential component in suppression of the alternative complement pathway by anti-GXM MAbs .