Thymus-dependent and -independent regulation of Ia antigen expression in situ by cells in the synovium of rats with streptococcal cell wall-induced arthritis. Differences in site and intensity of expression in euthymic, athymic, and cyclosporin A-treated LEW and F344 rats.

Euthymic LEW rats, when injected with streptococcal cell walls, exhibited rapid onset development of acute exudative arthritis coincident with enhanced synovial expression of Ia antigen. By 21 d after injection, the expression of Ia was markedly increased compared with basal conditions and paralleled the severity of the later developing proliferative and erosive disease. Immunodeficient athymic and cyclosporin A-treated LEW rats developed only the early phase arthritis, which was again paralleled by synovial Ia expression. Chronic expression of high levels of Ia antigen was not observed. Histocompatible F344 rats, both athymic and euthymic, developed minimal, if any, clinically significant arthritis and did not exhibit the enhanced Ia expression demonstrated in the LEW rats. Our results indicate that enhanced synovial Ia expression parallels clinical disease severity and varies by rat strain, and that the rapid onset enhanced synovial Ia expression is thymus independent, whereas the markedly enhanced chronic phase Ia expression is thymus dependent.