Antisera prepared in rabbits against cell walls or M protein preparations of a group A type 5 strain of streptococcus have been found reactive by immunofluorescence and complement fixation with human heart tissue. The reactant in heart tissue was localized to cardiac myofibers of all heart specimens tested and in smooth muscle elements of vessel walls and endocardium of a proportion of heart specimens tested. Rheumatic and nonrheumatic hearts showed comparable reactivity. The reactant was also identified in human skeletal muscle and in heart and skeletal muscle of the rabbit.

In the streptococcus, the cross-reactive antigen was demonstrated in cell walls and in acid extracts of cell walls by immunofluorescent absorption tests. Following absorption of antisera with cell walls, antibody bound to cell walls could be eluted at pH 3.0, and shown to possess serologic reaction with heart.

Cross-reactive antigen was partially purified from acid extracts of cell walls by chromatography on DEAE- and CM-cellulose columns and reactive fractions were consistently found to contain M protein. However, the serologic activities of type-specific M protein and cross-reactive antigen could be distinguished by absorption tests.

Absorption tests were employed for screening acid extracts of 52 streptococcal strains for presence of cross-reactive antigen. Positive tests were obtained with four type 5 and two type 19 strains, and presumptively positive tests with two additional type 19 strains. Forty-four strains of various types of group A and of groups C, D and G were found negative by this procedure.

The immunologic relationship between streptococcal cell wall antigen and myofibers and smooth muscle of vessel walls is consistent with the hypothesis that bound γ-globulin observed in rheumatic hearts in these sites is derived from immune bodies.

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