The protective efficacy of
antibodies to the Staphylococcus aureus type 5 capsular polysaccharide (CP5) was examined in a modified model of
catheter-induced
endocarditis. Rats were catheterized by surgically passing a
polyethylene catheter through the right carotid artery and aortic valve into the left ventricle. The following day, the rats were injected by the intraperitoneal (i.p.) route with
immunoglobulin G (
IgG) purified from nonimmunized rabbits or from rabbits immunized with a
conjugate vaccine composed of CP5 and CP8 linked covalently to recombinant Pseudomonas aeruginosa exotoxoid A. One day after passive immunization, the animals were challenged i.p. with one of three serotype 5 S. aureus isolates (strain Reynolds, Lowenstein, or VP) or nontypeable strain 521. Protection was evaluated by comparing quantitative cultures of blood, endocardial vegetations, and kidneys from control and immune animals. For experiments performed with S. aureus Reynolds and Lowenstein, rats given capsular
antibodies (645 microg of CP5-specific
IgG) showed a significantly (P < 0.05) lower prevalence of
endocarditis than rats injected with nonimmune
IgG. Similarly, quantitative cultures of the blood, kidneys, and aortic valve vegetations revealed that fewer S. aureus cells were recovered from rats given
capsule-specific
IgG than from rats administered nonimmune
IgG. Rats challenged with strain VP were protected with 1.145 mg of CP5-specific
IgG. Capsular
antibodies did not protect against
infection elicited by a nontypeable strain. These results demonstrate that capsular
antibodies elicited by immunization with a
polysaccharide-
protein conjugate vaccine protect experimental animals against serotype 5 S. aureus
infection in a modified model of
endocarditis.