Sera from patients with recent streptococcal infection or non-suppurative sequelae exhibit with variable frequency a precipitin reaction in agar gel with a partially purified streptococcal antigen which has been shown previously to be immunologically related to human heart tissue. This precipitin could be absorbed from sera with normal human heart tissue homogenates but not with homogenates of other organs. Demonstration of this cross-reaction by heart absorption was found dependent both upon the serologic properties of individual sera and the nature or state of purification of the streptococcal product employed as test antigen. Evidence was obtained of a close association of heart-related and non-heart-related antigenic determinants in partially purified preparations of the streptococcal antigen by both gel diffusion and immunoelectrophoresis. On immunoelectrophoretic analysis, cross-reactive antigen exhibited a more rapid mobility toward the anode than M protein. It was destroyed by digestion with trypsin, pepsin, and chymotrypsin. Based on specific absorption tests with a Type 5 and Type 19 strain, the antigen was localized to cell walls and to a lesser extent to cell membranes of these strains.

Precipitating activity related to cross-reactive antibody was localized to the immunoglobulin zone in immunoelectrophoresis. Reactive sera showed diminution or loss of serological activity following heat inactivation at 56°C or after prolonged storage at 4°C. Sera containing cross-reactive precipitating antibody exhibited an immunofluorescent reaction with sarcolemma of cardiac myofibers, which was inhibited by streptococcal cross-reactive antigen. By this inhibition test, the immunofluorescent reaction related to cross-reactive antibody could be distinguished from that due to other heart-reactive factors.

Antibody to streptococcal cross-reactive antigen defined by precipitation-absorption tests was observed in 24 per cent of patients with recent history of uncomplicated streptococcal infection and in the majority of patients with acute rheumatic fever, rheumatic heart disease, or acute glomerulonephritis. It was observed rarely in patients with non-streptococcal related disease. These data provide evidence that induction of cross-reactive autoantibody to heart in certain individuals is associated with streptococcal infection.

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