Bone marrow derived mesenchymal stromal cells provide survival signals to B-cells in vitro – no major role for BAFF

Abstract

Background Mesenchymal stromal cells (MSCs) are a unique cell type that has strong anti-proliferative effects on co-cultured activated T and B-cells in vitro. Based on our observation of significant differences between rheumatoid arthritis (RA) and osteoarthritis (OA) bone marrow B-cell compartments, we hypothesized that RA bone marrow MSCs may contribute to the pathogenesis of RA by enhancing B-cell survival. Objectives To compare the effect of RA and OA bone marrow derived MSCs (RA-MSCs, OA-MSCs) on the survival of healthy donor purified B-cells. Methods RA-MSCs (n=7) and OA-MSCs (n=5) were isolated from patients undergoing hip replacement surgery, and cultured in vitro for 2-5 passages. Washed cells were co-cultured with CD20+ B-cells for 60 hours in 17 different co-culture experiments. Cell survival was analyzed using 7-amino-actinomycin D (7AAD) labelling and flow-cytometric analysis and compared to the survival of B-cells cultured without MSCs (n=8). Expression of B-cell activating factor (BAFF) mRNA and protein was determined by RT-PCR and flow-cytomery after labelling with BAFF-specific antibodies. Results We observed that the presence of both RA-MSCs and OA-MSCs in the cultures significantly enhanced B-cell survival (70,11±7,28% and 54,65±11,83% viable cells, respectively) as compared to controls (35,13±13,83%, p < 0,001, Kruskal-Wallis ANOVA), the effect being more prominent in RA-MSCs (p < 0,05, Tukey-Kramer test). Both RA-MSCs and OA-MSCc displayed expression of BAFF mRNA and protein. We did not observe a convincing enhancement of BAFF mRNA expression by TNF-α. Blocking BAFF signalling by specific BAFF and BAFF-R antibodies, reduced the survival of B-cells by 20%, but did not abrogate the positive effect of MSCs on B-cell survival. Conclusions MSC interaction with B-cells may provide additional stimuli for lymphocyte survival via an as yet unidentified factor and therefore contribute to the pathogenesis of RA. BAFF, though produced by MSCs, is of minor importance in this setting. Further studies to identify the molecular basis of our observation are warranted.