An accelerated form of nephrotoxic serum nephritis in the rat was examined. The experimental model consisted of preimmunization of the rat with rabbit IgG 5 days before injection of subnephrotoxic doses of rabbit anti-rat kidney serum. The immunized rats developed proteinuria during the first 24 h, increasing by 48-96 h. The early 24-h proteinuria correlated with a neutrophilic infiltration of glomeruli and with deposition of rat Ig and C. The 48- to 96-h proteinuria was associated with a glomerular infiltration by mononuclear cells and proliferation of intrinsic glomerular cells. Many of the mononuclear cells were morphologically identical to monocytes and macrophages. [3H]thymidine labeling experiments indicated that the mononuclear cells originated from dividing precursors localized outside the kidney. Preimmunized rats given systemic irradiation (the kidney being protected by a shield) showed loss of the mononuclear cell infiltrate and absence of 48- to 96-h proteinuria. We conclude that mononuclear phagocytes can infiltrate the kidney in immunological glomerular disease and might contribute to the functional abnormalities.

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