Constitutively activated N-Ras stimulates LFA-1-dependent adhesion in rheumatoid arthritis synovial fluid T lymphocytes

Abstract

Synovial fluid (SF) T lymphocytes isolated from rheumatoid arthritis (RA) patients display constitutive activation of the integrin LFA-1. LFA-1-dependent T cell interactions with ICAM-bearing fibroblast-like synoviocytes and macrophages contribute to the inflammatory state of the joint via retention of T lymphocytes in the synovium and induction of inflammatory cytokines. SF components, notably TGF-β, are sufficient and required to sustain activation of LFA-1 in SF T lymphocytes, but little is known about the intracellular signalling pathways regulating LFA-1-dependent adhesion in SF T lymphocytes. We have recently found that deregulated signalling via the small GTPases Ras and Rap1 is responsible for chronic oxidative stress in SF T lymphocytes. Although activated Rap1 is a critical mediator of T cell receptor and CD31 -stimulated LFA-1 adhesion, SF T lymphocytes adhere via LFA-1 in the complete absence of Rap1 signalling. We find that both N-Ras and K-Ras isoforms of Ras are constitutively activated in SF T cells, and that transient expression of activated N-Ras, but not K-Ras or H-Ras, stimulates LFA-1-dependent adhesion in Jurkat T lymphocytes. Adhesion induced by N-Ras is insensitive to overexpression of the Rap1 negative regulatory protein RapGAP, consistent with the observation that SF T cells are highly adherant despite undetectable levels of activated Rap1. We provide pharmacological evidence indicating that N-Ras and Rap1 induce adhesion by distinct signalling mechanisms. Identification of N-Ras as a key intracellular signalling protein mediating lymphocyte retention in RA joints provides potential targets for new therapeutic strategies for RA.