Deposition of circulating antigen--antibody complexes in the gastrointestinal tract of rabbits with chronic serum sickness.

The possible role of circulating immune complexes (IC) in the production gastrointestinal lesions was studied in rabbits with chronic serum sickness (CSS) induced by multiple daily injections of bovine serum albumin (BSA). All rabbits generating a marked antibody response developed IC glomerulonephritis. In approximately 50% of these rabbits granular deposits of BSA, rabbit IgG, and C3 were also found in the gastrointestinal tract. The immune deposits in the gastrointestinal tract were mainly present in the vessel walls, close to the intestinal glands and the surface epithelium, and between the smooth muscle cells. This was accompanied by slight to moderate edema of the mucosa and the submucosa and mild infiltration of inflammatory cells. Electron-densedeposits were found in a pattern corresponding to that observed for BSA, rabbit IgG, and C3. Degranulated neutrophils, basophils, and mast cells were noticed in the interstitium. The presence in the same areas of granular deposits of BSA, IgG, and C3, corresponding to electron-dense deposits, suggests that the deposits contain BSA-anti-BSA complexes. These findings show that in rabbits with CSS circulating IC may localize and induce injury in the gastrointestinal tract.

Experimental immune complex disease of the lung. The pathogenesis of a laboratory model resembling certain human interstitial lung diseases.

Membranous and/or proliferative pneumonitis, similar in certain features to human interstitial pneumonitis, developed in rabbits making hyperactive antibody response to daily injections of bovine serum albumin (BSA) administered in multiple large doses sufficient to maintain the state of relative antigen-antibody equivalence. The pulmonary lesions were associated with deposition in alveolar capillary walls and interstitium of antigen, host globulin and complement, presumably in immune complexes. In some rabbits chronic interstitial pneumonitis, characterized by thickening of alveolar capillary walls, interstitial fibrosis and deposition of fibrinogen, was observed. The production of immune complex pneumonitis seems to depend on the degree of the antibody response because rabbits developing chronic serum sickness with low doses of BSA, rabbits with acute serum sickness as well as nonresponders showed no pulmonary alterations. This observation is comparable to that described by Dixon in his studies on experimental immune complex glomerulonephritis. It is conceivable that the pulmonary pathology shown here is produced by formation of larger amounts of complexes which may persist longer at critical levels in the circulation than in rabbits immunized with a single daily injection of BSA. In conclusion this study suggests: first, that experimental chronic serum sickness can be used as a model, not only for glomerulonephritis, but also for experimental systemic disease, comparable to human systemic diseases produced by circulating antigen-antibody complexes; and second, that the pathogenesis proposed here offers an alternative to using antilung basement membrane pneumonitis for the experimental approach to the study of human lung immunopathology.