Concomitant but Not Causal Association Between Surface Charge and Inhibition of Phagocytosis by Cryptococcal Polysaccharide

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Abstract
The mechanism by which capsular polysaccharides inhibit phagocytosis is not clearly understood. The association between a negative surface charge and inhibition of phagocytosis by the capsular polysaccharide C. neoformans was investigated. A 2-polymer aqueous-phase system containing phosphate ions was used to assess surface charge. Opsonins such as normal bovine serum and normal human immunoglobulin G reduced the surface charge on non-encapsulated cryptococci and simultaneously enhanced phagocytosis. These same opsonins had no effect on phagocytosis or surface charge of encapsulated cryptococci. F(ab'')2 fragments of normal human immunoglobulin G neither enhanced phagocytosis nor altered the surface charge of non-encapsulated cryptococci. Addition of purified cryptococcal polysaccharide to non-encapsulated cells inhibited phagocytosis of the yeast and induced a strong negative charge at the yeast surface. Chemical modification to reduce the surface charge of either purified cryptococcal polysaccharide or intact encapsulated cryptococci produced a small loss of phagocytosis-inhibiting activity but all treated polysaccharide preparations retained a significant ability to inhibit phagocytosis of the yeast. These results indicated that the association between surface charge and inhibition of phagocytosis was largely circumstantial and presence of a negative surface charge could not account for the powerful antiphagocytic action of cryptococcal polysaccharide.