EVIDENCE FOR THE CONTINUOUS RECRUITMENT AND ACTIVATION OF T-CELLS INTO THE JOINTS OF PATIENTS WITH RHEUMATOID-ARTHRITIS

Rheumatoid arthritis (RA) synovial fluid (SF)T cells express the activ ation markers CD69, HLA-DR and very late antigen (VLA)-1, but surprisi ngly few bear interleukin-2 receptors (CD25). This unusual activation state is commonly assumed to be due to stimulation by local antigen, y et T cells activated in vitro express activation antigens in the clear ly defined sequence: CD69, CD25, HLA-DR and finally VLA-1. Two possibl e explanations for the activation state of SF cells are: first, they c omprise several subpopulations each expressing different activation an tigens or, second, activation markers are up-regulated by mechanisms o ther than antigen stimulation. To examine these hypotheses, double- an d triple-color immunofluorescence techniques were applied to four T ce ll populations: normal peripheral blood T cells activated in vitro, RA SF T cells, T cells from an in vivo model of migration [tuberculin pu rified protein derivative (PPD)-induced skin blisters] and T cells co- cultured with endothelial cells (EC). The results confirmed that in vi tro activated T cells expressed activation markers in the sequence des cribed above, with significant CD25 expression and few cells co-expres sing CD69 with HLA-DR or VLA-1. In contrast, almost half the SF T cell s were CD69(+)HLA-DR(+) but CD25(-); a significant minority were CD69( +)VLA-1(+). T cells from PPD-induced skin blisters were already HLA-DR (+) and VLA-1(+) at 24 h, although, in vitro, PPD-activated T cells up -regulated HLA-DR and VLA-1 only after 1 week, suggesting that pre-act ivated T cells were preferentially recruited into the blisters. Finall y, T cells were found to up-regulate CD69 and, to a lesser extent, HLA -DR after adhering to EC in vitro. In summary, the paradoxical activat ion state of SF T cells cannot be explained solely by single or multip le rounds of activation in situ. At least two other mechanisms, the pr eferential recruitment of pre-activated T cells and the induction of H LA-DR and especially CD69 by endothelial contact during migration, may also play a role.