An extracellular product of group A streptococci which induces lymphocyte blastogenesis has been purified to homogeneity by DEAE-cellulose and CM-cellulose chromatography. The protein, termed streptococcal blastogen A, has a mol wt of approximately or equal to 17,500 and is inactivated by protease treatment and by heating at 100 degrees C. The purified blastogen gave rise to multiple protein bands on nondenaturing polyacrylamide gel electrophoresis, only two of which possessed blastogenic activity. Treatment of the protein with dithiothreitol before electrophoresis resulted in the apparent conversion of the multiple forms to a single active species. Blastogen A differs in electrophoretic mobility from the streptococcal pyrogenic exotoxins and its lymphocyte stimulating activity is not inhibited by rabbit antisera to the exotoxins. An enzyme immunoassay has been developed to measure human antibodies against blastogen A. A selection of sera with varying levels of anti-DNase B contained antiblastogen A-IgG.

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