T cell activation induces human osteoclast formation via receptor activator of nuclear factor kappa B ligand-dependent and -independent mechanisms

In unstimulated conditions, osteoclast (OC) formation is regulated by strom al cell production of the key osteoclastogenic factors receptor activator o f nuclear factor kappaB ligand (RANKL) and macrophage colony-stimulating fa ctor (M-CSF). However, the mechanisms of accelerated osteoclastogenesis and bone loss characteristic of inflammatory conditions are poorly understood but appear to involve T cells. In addition, the mechanism by which OCs aris e spontaneously in cultures of peripheral blood mononuclear cells in the ab sence of stromal cells or added cytokines remains unclear. Using a stromal cell free human osteoclast generating system, we investigated the ability o f activated T cells to support osteoclastogenesis. We show that when activa ted by phytohemagglutinin-P (PHA), T cells (both CD4(+) and CD8(+)) stimula te human OC formation in vitro. Although both soluble M-CSF and RANKL were detected in activated T cell supernatants, the presence of M-CSF was not es sential for macrophage survival or RANKL-dependent osteoclast formation, su ggesting that other soluble T cell-derived factors were capable of substitu ting for this cytokine. We also found that saturating concentrations of ost eoprotegerin (OPG) failed to neutralize 30% of the observed OC formation an d that T cell conditioned medium (CM) could superinduce osteoclastogenesis in cultures of purified monocytes maximally stimulated by RANKL and M-CSF. Together, these data suggest that activated T cells support osteoclastogene sis via RANKL-dependent and -independent mechanisms. Although not relevant for T cell-induced osteoclastogenesis, secretion of soluble M-CSF is a prev iously undescribed property of activated T cells.