Progranulin-dependent repair function of Regulatory T cells drive bone fracture healing.

Local immunoinflammatory events instruct skeletal stem cells (SSCs) to repair/regenerate bone after injury, but mechanisms are incompletely understood. We hypothesized that specialized Regulatory T (Treg) cells are necessary for bone repair and interact directly with SSCs through organ-specific messages. Both in human patients with bone fracture and mouse model of bone injury, we identified a bone injury-responding Treg subpopulation with bone-repair capacity marked by CCR8. Local production of CCL1 induced a massive migration of CCR8+ Treg cells from periphery to the injury site. Depending on secretion of progranulin (PGRN), a protein encoded by the granulin (Grn) gene, CCR8+ Treg cells supported the accumulation and osteogenic differentiation of SSCs, and thereby bone repair. Mechanistically, we revealed that CCL1 enhanced expression level of basic leucine zipper ATF-like transcription factor (BATF) in CCR8+ Treg cells, which bound to Grn promoter and increased Grn translational output and then PGRN secretion. Together, our work provides a new perspective in osteoimmunology and highlights possible ways of manipulating Treg cell signaling to enhance bone repair and regeneration.