Inflammatory osteoclastogenesis can be induced by GM-CSF and activated under TNF immunity

In inflammatory arthritis such as RA, osteoclastic activity is severely enhanced. GM-CSF was reportedly elevated in synovial fluid, but is a strong inhibitor of osteoclastogenesis; here lies a contradiction. Our objective was to examine what type of osteoclasts generate and resorb bone with resistance to GM-CSF in an inflammatory joint. Monocyte-derived cells generated in GM-CSF were morphologically and immunophenotypically different from both the conventional DC and macrophage. They could differentiate into osteoclasts in the presence of RANKL + M-CSF, acquiring a stronger osteoclastic activity under TNF treatment. Furthermore, their differentiation was not inhibited by GM-CSF, while monocyte-derived osteoclast differentiation was completely inhibited. The resorption was suppressed by GM-CSF, and the existence of another osteoclastic pathway has been suggested. Our findings indicate another type of osteoclast exists in inflammatory arthritis.