Rabbits were immunized with bovine serum albumin and later challenged intraperitoneally with the same antigen. Polyarthritis, characterized by synovial lining layer hyperplasia and leukocyte infiltration, was conditional on animals having excess free serum antigen in the presence of circulating immune complexes (IC). Nonimmune controls did not develop polyarthritis following antigen challenge. Data are presented that strongly suggest that synovitis was triggered by deposition of IC in the synovia. During the chronic phase of polyarthritis the synovium was clear of antigen but there was marked retention within joint fibrocartilage, suggesting that antigen, immobilized locally, may maintain the chronic synovitis.
Clinical relevance: There are several animal models in which experimental arthritis has been induced by antigenic challenge. The resemblance of the lesions to those of human rheumatoid arthritis has provided the rationale for studying the immune events associated with the onset of acute and maintenance of chronic phases of experimental arthritis.