Peptic Digestion of Streptococcal M Protein I. Effect of Digestion at Suboptimal pH upon the Biological and Immunochemical Properties of Purified M Protein Extracts

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Abstract
Purified streptococcal M protein vaccines often produce non-type-specific immunotoxic reactions in the skin or the blood of humans. In an attempt to free the type-specific M antigen (TSM) of such non-type-specific (NTSM) immunotoxic properties, purified M protein preparations were subjected to brief periods (5 to 10 min) of enzymatic digestion in dilute solutions of pepsin (20 μg/ml) at pH 5.0. Such peptic digestion abolished the ability of M protein to aggregate platelets in human platelet-rich plasma and to precipitate plasma fibrinogen. It greatly reduced (up to 16-fold) the capacity of M protein to react with the NTSM complement-fixing antibody that is normally present in human serum. In contrast, it had no effect upon the reactivity of the type-specific M antigen; the pepsin-treated M protein retained its ability to inhibit type-specific streptococcal opsonization by homologous M antibody, and, moreover, retained its ability to elicit type-specific opsonic antibody responses in rabbits.