We propose to test the hypothesis that catabolic antibodies play an important innate defense role in protecting against Staphylococcus aureus infections. S. aureus is an opportunistic pathogen that colonizes the skin (primarily the anterior nasal vestibule) of approximately 20-30% of the population without causing clinical symptoms. If, however, the skin is damaged, S. aureus can gain entry into the host and infections may result in a broad-range of conditions ranging from minor skin complications to lethal infections. Of growing concern are the steadily growing numbers of community-acquired methicillin-resistant S. aureus (CA-MRSA) infections in young, otherwise healthy people. Natural antibodies represent the spontaneous repertoire of circulating immunoglobulins in healthy, unimmunized individuals that form a part of the innate immune system, and they promote the clearance of pathogenic substances from the circulation and prevent pathogen dissemination. Proteolytic antibodies are immunoglobulins endowed with a capacity to catalyze the hydrolysis of polypeptide antigenic substrates, and they hold the potential of specific and efficient catabolism of polypeptides. A single molecule of a catabolic, proteolytic antibody can degrade thousands of antigen molecules. Moreover, the peptide bond cleavage reaction generally results in functional protein inactivation. In comparison, conventional antibodies act stoichiometrically (e.g., IgG binds at most 2 antigen molecules) and the binding is reversible, with the result that active antigen molecules are released upon dissociation of immune complexes. There is growing evidence that microbial antigens can also be targeted by catabolic antibodies. gp120, a coat protein of HIV, is shown to be cleaved by specific proteolytic antibodies that recognize the superantigenic site of gp120 (superantigen, i.e., an antigen recognized by antibodies present in the preimmune repertoire without the requirement for adaptive diversification of the antibody variable domains). To date, a role for catabolic antibodies in defense against bacterial infections has not been examined. In preliminary studies, we observed the cleavage of five important proteins expressed by S. aureus, Efb, protein A, LukF, Map and ClfA by antibody preparations from human subjects. In the present proposal, we will confirm this phenomenon using sera from patients defined by their S. aureus carriage state, characterize the proteolytic specificity and rates, and determine the effects of catalysis on virulence protein function. PUBLIC HEALTH RELEVANCE: S. aureus infections are a growing problem in both community and hospital settings and antibiotic resistant strains have complicated the available treatment strategies. This proposal aims to identify a family of antibodies that also serve as enzymes (catalytic) that cleave virulence factors associated with disease. Identification and characterization of these catalytic antibodies will result in the development of new preventative and treatment strategies against S. aureus infections.