Intravenous injection of Type 1 streptococcal M protein into mice and rats produced lesions confined to renal glomeruli. Thrombi of eosinophilic amorphous material, seen to occlude glomerular capillaries, were shown to contain M protein and fibrinogen. Gradual regression of the morphological lesions was observed during the 3 weeks following injection. Initial abnormal proteinuria and azotemia returned to control levels by the end of the 1st week; a second rise in urinary protein excretion and urea retention was demonstrated in some rats coincident with appearance of anti-M antibodies. The mechanism of renal localization of streptococcal M protein by means of a complex with fibrinogen was suggested, which may comprise an initial phase in the pathogenesis of acute poststreptococcal glomerulonephritis.

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