Philippe Krebs

Inflammation and skeletal homeostasis are closely intertwined. Inflammatory diseases are associated with local and systemic bone loss, and post-menopausal osteoporosis is linked to low-level chronic inflammation. Phosphoinositide-3-kinase signalling is a pivotal pathway modulating immune responses and controlling skeletal health. Mice deficient in Src homology 2–containing inositol phosphatase 1 (SHIP1), a negative regulator of the phosphoinositide-3-kinase pathway, develop systemic inflammation associated with low body weight, reduced bone mass, and changes in bone microarchitecture. To elucidate the specific role of the immune system in skeletal development, a genetic approach was used to characterise the contribution of SHIP1-controlled systemic inflammation to SHIP1-dependent osteoclastogenesis. Lymphocyte deletion entirely rescued the skeletal phenotype in Rag2 −/− /Il2rg −/− /SHIP1 −/− mice. Rag2 −/− /Il2rg −/− /SHIP1 −/− osteoclasts, however, displayed an intermediate transcriptomic signature between control and Rag2 +/+ /Il2rg +/+ /SHIP1 −/− osteoclasts while exhibiting aberrant in vitro development and functions similar to Rag2 +/+ /Il2rg +/+ /SHIP1 −/− osteoclasts. These data establish a cell-intrinsic role for SHIP1 in osteoclasts, with inflammation as the key driver of the skeletal phenotype in SHIP1-deficient mice. Our findings demonstrate the central role of the immune system in steering physiological skeletal development.