A method has been described for the detection of streptococcal antigens in tissues using the indirect immunofluorescent technique. This method has been applied to the histologic distribution in the mouse of M protein of types 1, 5, 12, and 19. Histologic localization of these M proteins was similar, and their rates of disappearance from the tissues were comparable.

The major sites of deposition were the endocardium and adjacent subendocardium of the heart, alveolar walls of the lung, glomerular tufts of the kidney, and reticulo-endothelial cells of liver, spleen, lymph nodes, and adrenal gland. M protein was distributed in considerably lesser concentration in capillary endothelium and connective tissue sites in myocardium, kidney, skin, and gastrointestinal tract. Traces were also present in adrenal cortical cells. It was observed only rarely in cell nuclei.

After injection of 0.5 mg. M protein fraction, the concentration of antigen diminished to undetectable levels in all organ sites by 4 days, except in the renal glomerulus, where traces were visible at 8 days. In mice injected with streptococcal culture intraperitoneally, M protein was detected at sites of focal abscesses in liver and spleen, and on the serous surfaces of these organs.

The histologic distribution of M protein is compared with that described previously for pneumococcal polysaccharide and animal protein. Differences in the extent of distribution and in the characteristics of antigen deposition are pointed out.

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