B cell phenotype and function in the synovium of ACPA+ and ACPA- rheumatoid arthritis patients.
Achilleas Floudas, Candice Low, Monika Biniecka, Douglas J. Veale, Ursula Fearon
Trinity Biomedical Sciences Institute, Trinity College
Rheumatoid arthritis (RA) is a chronic inflammatory autoimmune disease of unknown and complex aetiology with severe detrimental effects for the patient’s quality of life. While rheumatoid factors (RF) and anti-citrulinated protein antibodies (ACPA) have been used extensively for the diagnosis of RA, no clear mechanism of action towards disease pathogenesis and progression has been identified. Importantly, both seropositive and seronegative RA patients experience significant improvement in disease severity following B cell depletion. Therefore, we hypothesized that B cells have a central role auto-antibody independent role in ACPA+ and ACPA- RA.
Phenotypical and functional characterization of B and T cell populations in the peripheral blood and synovium of ACPA+, ACPA- and arthralgia patients.
Flow cytometric analysis of T and B cell populations in the periphery and synovium of ACPA+, ACPA- and arthralgia patients. B cell invasion into the RA synovium as well as activation and function of sorted B cells, cultured and stimulated in vitro under normoxic (21% O2) and hypoxic (1% O2) conditions were examined.
Significant reduction in CD27+ memory and specifically, switched memory B cells, was observed between healthy subjects and APCA+ RA patients. The aforementioned decrease in memory B cells is potentially a result of increased susceptibility to FAS induced apoptosis. B cell invasion of the synovial tissue is strongly mediated by CXCR3. Despite however a marked accumulation of switched and double negative (DN) memory B cells in the synovium, no differences in synovial B cell subpopulation composition between ACPA+ and ACPA- RA patients was observed. Interestingly, sorted B cells from healthy subjects showed increased sensitivity to in vitro stimulation with increased expression of CD80 and CD86 when cultured under hypoxic conditions, while co-culture with RA patient synovial fibroblasts didn’t not enhance this effect.
The CXCR3 mediated accumulation of memory B cells in both ACPA+ and ACPA- RA, underlines a common, antibody independent, contribution of B cells in synovial inflammation and an opportunity for therapeutic intervention. While B cell activation under hypoxic conditions and increased CD80/CD86 expression is potentially an important mediator for the emergence of auto-reactive T cells and disease progression in RA.