Impaired ATM Activation in B Cells Promotes Osteoclastogenic Cytokine Production and Bone Erosion in Patients with Rheumatoid Arthritis

Rheumatoid arthritis (RA) patients may display atypical CD21-/lo B cells in their blood but the significance of this observation remains unclear. We report here that the group of RA patients with elevated frequencies of CD21-/lo B cells shows decreased ataxia-telangiectasia mutated (ATM) expression and activation in B cells compared with other RA patients and healthy donor controls. In agreement with ATM involvement in the regulation of V(D)J recombination, RA patients with defective ATM function displayed a skewed B cell receptor (BCR) Igk repertoire, which resembled that of ataxia-telangiectasia (AT) patients. This repertoire was characterized by increased Jk1 and decreased upstream Vk gene segment usage, suggesting improper secondary recombination processes and selection. In addition, altered ATM function in B cells was associated with decreased osteoprotegerin (OPG) and increased RANKL production. These changes favor bone loss and correlated with a higher prevalence of erosive disease in RA patients with impaired ATM function. Using a humanized mouse model, we also show that ATM inhibition in vivo induces an altered Igk repertoire and RANKL production by immature B cells in the bone marrow, leading to decreased bone density. We conclude that dysregulated ATM function in B cells promotes bone erosion and the emergence of circulating CD21-/lo B cells, thereby contributing to RA pathophysiology. RNA was extracted from batch sorted CD19+CD21+CD10-CD27- naïve B cells isolated from donors using the Absolutely RNA microprep kit (Stratagene). 100-200 ng of RNA was obtained per sample, and the quality of the purified RNA was assessed by the Bioanalyzer from Agilent. Using the Ovation biotin system kit from Nugen, 30-50ng of RNA was amplified and labeled to produce cDNA. Labeled cDNA was hybridized on chips containing the whole human genome (Human Genome U133 2.0 from Affymetrix).