Primary inflammatory reaction in synovial fluid and tissue in rabbit immobilization osteoarthritis. The kinetics and composition of the primary cellular inflammatory process were studied in the synovial fluid (SF) and synovial tissue (ST) compartments of a rabbit knee immobilization osteoarthritis model. Immobilization induced rapid migration of neutrophils (59% +/- 26% of all cells) into SF in three days, which was accompanied by nonspecific esterase-positive monocytes (71% +/- 8% of all mononuclear cells). This finding suggests that non-specific inflammation mediated by phagocytic leukocytes predominates the cellular response in the SF compartment. In contrast, morphometric analysis of ST proper showed an inflammatory mononuclear cell response, the intensity of which diminished over time during the study period from Day 3 (416 +/- 59 cells per 0.049 mm2 ST tissue) through Day 10 (305 +/- 32 cells) to Day 35 (174 +/- 36 cells). A dotlike T-pattern alpha-naphthyl acetate esterase (ANAE) was found in the T-cell-dependent areas of secondary lymphatic tissue in the spleen, enabling immunocytologic ANAE marker studies. The ST response in situ was predominated by tissue macrophage, though infiltrates rich in T lymphocytes were present in the immediate sublining stroma. There was a significant correlation between the intensity of the SF cell response (total recovery) and the percentage of neutrophils, but there was no correlation between the intensity of the ST response and the proportion of T lymphocytes. These T-cell accumulations together with the local proliferation of fibroblastlike lining cells and stromal fibroblasts suggest that the primary inflammatory cell response is not caused by either wear and tear or mechanically by cartilage fragments. 