Model for IL-7–mediated OC formation under conditions of inflammation.

Under inflammatory conditions, increased production of the inflammatory cytokines IL-1 and TNFα leads to production of IL-7 from bone marrow stromal cells and osteoblasts. IL-7 is also known to feedback on monocytes, inducing further IL-1 and TNFα secretion.21The effects of increased IL-7 production are 2-fold. First, increased IL-7 results in induction of B-lymphopoiesis and an increase in the number of B220⁺ pre-B cells, a population reported to be capable of acting as OC precursors.14 Second, IL-7 has direct effects on T cells, resulting in the production of the osteoclastogenic cytokines M-CSF and RANKL, as well as other unidentified osteoclastogenic factors. Thus, a combination of an increased OC precursor pool, along with the production of osteoclastogenic cytokines by T cells, leads to the enhanced OC production and increased bone resorption characteristic inflammatory processes such as rheumatoid arthritis and periodontitis.