Objective. To study the CD69 activation pathway in synovial fluid (SF) T lymphocytes from patients with rheumatoid arthritis (RA).

Methods. Peripheral blood mononuclear cells (PBMC) or SF mononuclear cells (SFMC) were used in proliferation assays with anti‐CD69, anti‐CD28, anti‐CD3, phorbol myristate acetate (PMA), and/or recombinant interleukin‐2 (IL‐2). CD69+, CD69—, and resting SF T cells were also proliferated. CD25 expression and production of IL‐2 after CD69 activation were assessed by flow cytometry and in a bioassay with the IL‐2‐dependent cell line CTLL‐2.

Results. RA SFMC did not proliferate either in the presence of anti‐CD69 monoclonal antibodies alone or with concomitant PMA activation, when compared with paired or control PBMC. Similar low proliferative responses via the CD3 or CD28 pathway with PMA were observed. This defective proliferation of RA SFMC after stimulation through the CD69 molecule was explained in part by a failure to express CD25 and to produce IL‐2. SF CD69‐ T cells and resting SF T cells had higher rates of proliferation through the alternative costimulatory pathway CD28 than did SF CD69+ T cells or freshly isolated SF T cells.

Conclusion. Freshly isolated SF T cells present a profound state of hyporesponsiveness through the CD69 and CD28 costimulatory pathways. This state appears to be dependent on the activation status of SF T cells, since CD69— and resting SF T cells showed recovery of the ability to proliferate through the CD28 activation pathway.