Antibodies to capsular polysaccharides are not protective against experimental Staphylococcus aureus endocarditis

Abstract

The protective efficacy of antibodies to the Staphylococcus aureus capsular polysaccharide was examined in a rat model of catheter-induced endocarditis. Capsular antibodies were induced either by active immunization with killed S. aureus or by passive immunization with hyperimmune rabbit antiserum to S. aureus. Control rats were injected with phosphate-buffered saline or passively immunized with normal rabbit serum or rabbit antiserum to a nonencapsulated strain. Animals with indwelling catheters were challenged intravenously with 5 x 104 to 4 x 106 CFU of the homologous S. aureus strain (capsular serotype 5 strain Reynolds or serotype 1 strain SA1 mucoid). Both immunized and control rats developed S. aureus endocarditis. The numbers of S. aureus cells recovered from the blood and aortic valve vegetations of immunized rats were similar to those control rats, indicating thai capsule- specific antibodies were not protective. To determine whether the presence of an indwelling catheter interfered with antibody-mediated protection against S. aureus endocarditis, catheters were removed 2 h after insertion in additional groups of rats. An inoculum of 108 CFU of strain Reynolds was needed to provoke endocarditis in rats catheterized for 2 h, compared with 5 x 104 CFU for rats with indwelling catheters. Passively transferred capsular antibodies were not protective since both immunized and nonimmunized animals developed endocarditis, and quantitative cultures of brood and valvular vegetations revealed no differences between immunized and control animals. The findings of this study indicate that antibodies to the capsular polysaccharide are nol protective in the rat model of experimental S. aureus endocarditis.