Selective Treg recruitment to bone remodeling niches is required for digit tip regeneration

Adult mammals have limited capacity for tissue regeneration, where most injuries resolve through fibrotic scarring rather than functional tissue restoration1-4. Studies in regenerative vertebrate species, including amphibians, teleost fish, reptiles and mammals, have established that the innate immune system plays instructive roles in regeneration5-11, yet the role of adaptive immune cells and how the immune response distinguishes regenerative from non-regenerative injuries, remain poorly understood. The mouse digit tip provides a rare mammalian model of complete multi-tissue regeneration where distal amputation through the terminal phalanx (P3) triggers complete multi-tissue regrowth, whereas a more proximal amputation of the same bone results in fibrotic scarring12-16. Using an intravital multiphoton imaging approach capable of longitudinally tracking bone remodeling and immune cells in live mice17, we find that regulatory T cells (Tregs) are selectively recruited to regenerating but not scarring digit tips. Tregs localize first to sites of osteoclast-mediated bone resorption and persist at the bone surface when an expanding stromal progenitor pool, known as the blastema, initiates digit regrowth. Acute depletion of Tregs impairs bone resorption and subsequent bone regrowth. Mice lacking T and B cells or CD4+ and CD8+ T cells show similar bone remodeling defects, suggesting a dominant role for Tregs within the adaptive immune compartment in promoting mammalian digit tip regeneration. Treg depletion impairs regeneration through an IL-10-independent mechanism, pointing to a non-canonical effector program. Lastly, pharmacological blockade of the chemokine receptor CXCR4 reduces Treg recruitment to the bone compartment, diminishes bone-associated macrophage accumulation, and attenuates bone degradation in regenerative amputations. Together, these findings identify Tregs as essential regulators of bone remodeling during mammalian digit tip regeneration.