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Infect Immun, April 1998, p. 1547-1553, Vol. 66, No. 4
0019-9567/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.

Bivalency Is Required for Anticapsular Monoclonal Antibodies To Optimally Suppress Activation of the Alternative Complement Pathway by the Cryptococcus neoformans Capsule

Thomas R. Kozel,* Randall S. MacGill, and Kevin K. Wall

Department of Microbiology and the Cell and Molecular Biology Program, School of Medicine, University of Nevada, Reno, Nevada 89557

Received 12 September 1997/Returned for modification 27 October 1997/Accepted 9 January 1998

Encapsulated cells of Cryptococcus neoformans are potent activators of the alternative complement pathway. Previous studies found that monoclonal 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 alternative pathway. The present study examined the abilities of F(ab)2 and 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 MAb 3C2 showed no suppressive activity, and Fab fragments of MAb 471 were markedly reduced in suppressive activity. Indeed, there was an earlier accumulation of C3 on encapsulated cryptococci in the presence of the Fab fragments. Study of subclass switch families of MAb 439 and MAb 471 found that MAbs of an immunoglobulin 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 indicate that cross-linking of the capsular matrix is an essential component in suppression of the alternative complement pathway by anti-GXM MAbs.

* Corresponding author. Mailing address: Department of Microbiology/320, School of Medicine, University of Nevada, Reno, NV 89557. Phone: (702) 784-6161. Fax: (702) 784-1620. E-mail: trkozel{at}med.unr.edu.




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