We report the existence of an extracellular staphylococcal product, designated staphylococcal decomplementation antigen (DA), that causes rapid consumption of early-reacting complement components up to and including C5 in human serum. Complement activation occurs as a consequence of immune complex formation between DA and specific human immunoglobulin G antibodies and proceeds primarily via the classical pathway. The terminal components C7, C8, and C9 are not consumed during the process. Levels of DA production do not correlate with the expression of classical pathogenic factors, such as coagulase, clumping factor, protein A, or alpha-toxin. DA is a nondialyzable macromolecule eluting in a molecular-weight region of 70,000 to 120,000 on Sephacryl S-300 and displaying an apparent sedimentation coefficient of 3 to 4 S on sucrose density gradients. The molecule is remarkably stable and resists destruction upon boiling for 30 min or by treatment with pronase, lysostaphin, DNase, or RNase. We anticipate that DA protects staphylococci from complement attack through induction of abortive, complement-consuming reactions in the fluid phase.
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