The serine proteinase (SP) released into the environment by most strains of S. aureus cleaves human IgG, IgM and IgA of both subclasses--IgA 1 and IgA 2. SP cleaves H chains of all immunoglobulin classes and the SC of S-IgA, the L chains are degraded partially. The SP-induced cleavage results in a large spectrum of fragments under reducing conditions within a broad range of Mr (approx. 41,000 to less than 12,400). This indicates that the enzyme does not affect the Ig molecule in the hinge region only. The degree of cleavage depends on the enzyme:substrate ratio and on the duration of incubation. The generation of small fragments is associated with the loss of antigenic determinants that results from the decreased binding of the cleaved material in the ELISA method. Partial cleavage of L chains suggests that the enzyme alters part of the molecule that is involved in antigen binding. Even if the ability of antigen binding remains preserved after cleaving Ig with SP, the antibody function is disturbed by splitting off the Fc region or by its degradation into small fragments. SP has to be considered as one of the virulence factors of S. aureus that may protect bacteria against the defence mechanisms of the host.

Laboratory of Special Medical Microbiology and Immunology 1st Medical Faculty Charles University Studnickova Prague Czechoslovakia

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