Peripheral Blood B cell Subsets and BAFF/APRIL Levels and their Receptors are Disturbed in Rheumatoid Arthritis but not in Ankylosing Spondylitis

Background: To evaluate the distribution of circulating B cell subsets and their expression of BAFF/APRIL receptors (BAFF-R, TACI and BCMA) as well as circulating levels of BAFF and APRIL in patients with rheumatoid arthritis (RA) or ankylosing spondylitis (AS) compared to healthy controls (HC).Methods: 59 patients with RA, 61 patients with AS and 61 HC were evaluated. All patients were receiving traditional treatments and had not received prior biological treatment. Peripheral blood B cell subsets were assessed using multicolor flow cytometry using CD27, CD38 and IgD staining. Expression of BAFF-R, TACI and BCMA was analyzed on each cell subset.Results: Distribution of peripheral B cells subsets was disturbed in RA compared to HC, with a decreased proportion of naive and transitional B cells (p<0.005), whereas B cell subsets were comparable between AS and HC. Circulating BAFF did not differ between the three groups, while the ratio of BAFF/B cell number was significantly higher in RA compared to HC (p<0.001). Circulating APRIL levels were increased in RA compared to HC (p<0.001). Circulating BAFF and APRIL, and BAFF/B cell ratio did not differ between AS and HC. We also observed increased expression of BCMA, but not BAFF-R in RA, on both naive and memory B cell subsets (post germinal center) (p<0.005), whereas TACI expression was decreased on memory B cells (p=0.001). The expression of BAFF/APRIL receptors did not differ between AS and HC.Conclusion: Disturbances in B cell homeostasis in RA may promote B cell survival and deregulation, favoring the emergence of autoimmune B cells. Conversely, B cell homeostasis is not disrupted in AS.