POLYSACCHARIDE CAPSULE-MEDIATED RESISTANCE TO OPSONOPHAGOCYTOSIS IN KLEBSIELLA PNEUMONIAS

The polysaccharide capsule of Klebsiella pneumoniae is an important vi rulence factor that confers resistance to phagocytosis. The treatment of encapsulated bacteria with salicylate to inhibit capsule expression was found to enhance the phagocytosis of encapsulated bacteria by hum an neutrophils only in the presence of cell surface-specific antibodie s. Both type-specific rabbit antisera and anticapsular human hyperimmu ne globulin were employed as opsonins. Salicylate significantly enhanc ed phagocytosis with homologous, but not heterologous, whole-cell anti sera. Antisera, diluted 1:40, no longer opsonized fully encapsulated b acteria but promoted the uptake of multiple salicylate-treated bacteri a in >90% of neutrophils. Salicylate (0.25 to 1.0 mM) also enhanced op sonization with globulin against homologous bacteria. Higher salicylat e levels (1 to 2.5 mM) enhanced the opsonization of heterologous serot ypes with human globulin. The nature of antibody attachment to encapsu lated bacteria was determined by immunofluorescence. Even after the ad dition of purified capsular polysaccharide to prevent phagocytosis, K- specific antibodies attached in large amounts to bacteria. K-specific antibodies reacted with antigens throughout the capsule and showed a p redilection for a denser inner layer of the capsule, indicating that m any of the K-specific antibodies may be masked underneath the capsule surface. K-specific antibodies can also be rendered nonfunctional by s oluble, cell-free capsular antigen. In culture, large quantities of so luble capsular polysaccharide extrude from bacteria after overnight gr owth. The reduction in capsule expression caused by salicylate largely affected the soluble, cell-free fraction. Purified capsular polysacch aride was shown to retard the opsonophagocytosis of salicylate-treated bacteria in a concentration-dependent manner. However, extensive wash ing of encapsulated bacteria to remove loosely attached capsular mater ial did not significantly enhance opsonophagocytosis. In conclusion, c ell-free capsule and cell-associated capsule are antiphagocytic; both act to neutralize K-specific antibodies by binding or concealment. Sal icylate-mediated inhibition of capsule expression, particularly of the cell-free fraction, improved K-specific opsonization dramatically.