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Infection and Immunity, June 2002, p. 2812-2819, Vol. 70, No. 6
0019-9567/02/$04.00+0     DOI: 10.1128/IAI.70.6.2812-2819.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.

Fc-Dependent and Fc-Independent Opsonization of Cryptococcus neoformans by Anticapsular Monoclonal Antibodies: Importance of Epitope Specificity

Dale Netski,{dagger} and Thomas R. Kozel*

Department of Microbiology, University of Nevada School of Medicine, Reno, Nevada 89557

Received 29 October 2001/ Returned for modification 21 November 2001/ Accepted 28 February 2002

Monoclonal antibodies (MAbs) reactive with glucuronoxylomannan (GXM), the major capsular polysaccharide of the yeast Cryptococcus neoformans, produce distinct capsular reactions when viewed by differential interference contrast microscopy. These reactions depend on the epitope specificity of the antibody. Opsonic activities of immunoglobulin G1 (IgG1) MAbs that produce patterns termed rim and puffy were examined. Rim-pattern MAbs are reactive with an epitope shared by GXM serotypes A, B, C, and D. Puffy-pattern MAbs are reactive only with serotypes A and D. In phagocytosis assays, using serotype A cells and resident murine peritoneal macrophages, rim-pattern MAbs were markedly more opsonic than puffy-pattern MAbs. F(ab')2 fragments of rim-pattern MAbs were synergistic with heat-labile factors in normal human serum for opsonization of the yeast. F(ab')2 fragments of puffy-pattern MAbs were also synergistic with normal serum in opsonization but at a much lower level than fragments of rim-pattern MAbs. Normal serum alone was not opsonic. F(ab')2 fragments of rim-pattern MAbs, but not puffy-pattern MAbs, stimulated phagocytosis of encapsulated cryptococci in the absence of serum. This serum-independent opsonic action of F(ab')2 fragments was abrogated by pretreatment of macrophages with purified GXM, suggesting the involvement of a phagocyte GXM receptor. The results indicate that (i) there are multiple mechanisms by which anticapsular IgG MAbs facilitate phagocytosis of encapsulated cryptococci, (ii) some anti-GXM antibodies are opsonic in an Fc-independent manner, and (iii) opsonic activity correlates with the capsular reaction and occurs in an epitope-specific manner.

* Corresponding author. Mailing address: Department of Microbiology/320, School of Medicine, University of Nevada, Reno, NV 89557. Phone: (775) 784-4124. Fax: (775) 327-2332. E-mail: trkozel{at}med.unr.edu.

Editor: S. H. E. Kaufmann

{dagger} Present address: Department of Medicine, Johns Hopkins University School of Medicine, Baltimore, MD 21205.

Infection and Immunity, June 2002, p. 2812-2819, Vol. 70, No. 6
0019-9567/02/$04.00+0     DOI: 10.1128/IAI.70.6.2812-2819.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.



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