The effects of B cell depletion on bone turnover in rheumatoid arthritis

Abstract

Rheumatoid arthritis (RA) is the most prevalent inflammatory joint disease. B cells have a role in both the pathogenesis of RA and the regulation of bone cell activity. Depletion of B cells by the anti-CD20 antibody rituximab (RTX) is a highly effective treatment of RA, which is now well established. However, the role of B-cells in bone turnover is controversial. The aim of this thesis was to investigate the effects of B cell depletion on bone turnover in RA. It is postulated that prolonged B cell depletion in patients with RA may have a beneficial effect on the bone loss that would otherwise be expected in active disease. Furthermore, this affect may be direct through modulation of osteoclastogenesis or indirect through attenuation of systemic inflammation and increased physical activity. Preliminary results in forty-six RA patients six months after RTX indicated that there was a significant suppression in bone resorption accompanied to a lesser degree by an increase in bone formation. However, in a second prospective cohort of forty-five RA patients treated with RTX over twelve months, bone mineral density (BMD) fell at the femur sites, but was maintained at the lumbar spine and forearm. There was a significant increase in bone formation, but no significant change in bone resorption or osteocyte markers. Additionally, the effects of RTX on bone turnover were influenced by vitamin D status, gender and menopausal state. Results of in vitro osteoclastogenesis with peripheral blood mononuclear cells (PBMCs) isolated from the blood of twelve self-reported healthy volunteers; indicated that in vitro B cell depletion via magnetic-activated cell sorting (MACS), significantly increased osteoclast formation. In contrast, PBMCs isolated from the blood of five RA patients, up to twelve months post B cell depletion with RTX, resulted in decreased osteoclast formation using the same standardised culture system. In summary, the results of the pilot study showed that B cell depletion significantly decreased bone resorption and increased bone formation in RA, possibly via a direct effect on osteoclasts and osteoblasts, respectively, or at least partially explained by the decreased inflammation and disease activity. However, this was not confirmed in the prospective study as the results were confounded by a high prevalence of vitamin D deficiency and these patients had significant falls in femur BMD and evidence of higher bone turnover. Furthermore, as there were no control groups it was difficult to establish whether depletion of B cells had in fact slowed down the expected bone loss in these patients. The results of the in vitro experiments indicated that under basal conditions i.e. in healthy subjects, the production of osteoprotegerin by B cells outweighed the production of receptor activator of nuclear factor - κb ligand (RANKL). However, in pro-inflammatory states, where B cells are activated e.g. RA, B cells produce cytokines like RANKL that stimulate osteoclastogenesis resulting in an increased production of osteoclasts. Hence B cell depletion ii in this latter situation caused a reduction in osteoclast generation. Further work is now required to investigate if subsets of pathogenic B cells i.e. not found in healthy individuals are specific to inflammatory bone erosion.