IL-15 Expression on RA Synovial Fibroblasts Promotes B Cell Survival

Introduction: The purpose of this study was to examine the role of RA Synovial Fibroblast (RASFib) IL-15 expression on B cell survival. Methods: Magnetically sorted peripheral blood memory B cells from 15 healthy subjects were cocultured with RASFib. Results: RASFib constitutively expressed membrane IL-15. Survival of isolated B cells cultured for 6 days, below 5%, was extended in coculture with RASFib to 52+/28% (p<0.001). IL-15 neutralizing agents but not isotype controls, reduced this rate to 31+/26% (p<0.05). Interestingly, rhIL-15 had no effect on isolated B cells but significantly increased their survival in coculture with RASFib. In parallel, B cell IL-15R chains were upregulated in cocultures. BAFF and VCAM-1, that are expressed on RASFib, were tested as potential candidates involved in upregulating B cell IL-15R. Culture of B cells in the presence of rhBAFF or rhVCAM-1 resulted in significantly increased survival, together with upregulation of all three IL-15R chains; in parallel, rhIL-15 potentiated the anti-apoptotic effect of BAFF and VCAM-1. Both BAFF and VCAM-1 neutralizing agents downmodulated the effect of RASFib on B cell survival and IL-15R expression. In parallel, rhIL-15 had a lower effect on the survival of B cells cocultured with RASFib in the presence of BAFF or VCAM-1 neutralizing agents. Peripheral blood B cells from 15 early RA patients demonstrated an upregulated IL-15R and increased survival in cocultures. Conclusion: IL-15 expression on RASFib significantly contributes to the anti-apoptotic effect of RASFib on B cells. IL-15 action is facilitated by BAFF and VCAM-1 expressed on RASFib, through an upregulation of IL-15R chains.