T-regulatory cell protection of progenitor cells from CD4+ T-cell-mediated cytotoxicity is essential for endogenous mouse digit-tip regeneration.

Regeneration of amputated digit tips in humans and mice relies on osteoclast-dependent bone erosion coupled with osteoblast-mediated bone replacement. Currently, little is known of the impact of lymphoid immune cells, i.e., T cells, B cells, and NK cells, on digit-tip regeneration. Using lymphoid-deficient mutant mice, we revealed lymphoid immunity as a net negative regulator of regeneration. CD8+ cells are thought to negatively regulate fracture repair; however, we showed that adoptive cell transfer (ACT) of CD8+ T cells into lymphoid-deficient hosts did not impact regeneration. In contrast, ACT of CD4+ T cells potently inhibited regeneration via osteoclast and osteoblast progenitor-cell cytotoxicity. CD4+ T-cell-mediated inhibition of regeneration was rescued by supplementation with T regulatory cells or recombinant RANKL, a mediator of osteoclast differentiation. ACT of IFN-{gamma}-deficient CD4+ T cells abolished cytotoxic activity and rescued regeneration. Future strategies protecting endogenous progenitor cells could enhance human tissue repair and autologous stem-cell therapies. One sentence summaryEndogenous progenitor cells are vulnerable to CD4+ T-cell-mediated cytotoxicity during digit-tip regeneration and require T-regulatory-cell-mediated protection from autoimmune attack. HighlightsO_LIDigit-tip regeneration is enhanced with the loss of lymphoid immunity. C_LIO_LIRegeneration requires T regulatory cells (T-regs) for maintenance of osteoclastogenesis when other lymphoid cells are present. C_LIO_LIT-regs enhance regeneration in the absence of lymphoid immunity during the anabolic phase. C_LIO_LILike thymic NK cells, CD4+ T cells and not CD8+ T cells are responsible for inhibition of regeneration. C_LIO_LIRANKL is essential to the rate-limiting catabolic phase of digit-tip regeneration. C_LIO_LIBoth T-regs and recombinant RANKL can rescue CD4+ T-cell inhibition. C_LIO_LIGenetic knockout of key cytotoxicity genes (IFN{gamma}, Prf1, and TNF) in immune-competent mice enhances regeneration. C_LIO_LICD4+ T-cell ACT induces both apoptosis and necroptosis. C_LIO_LICD4+ T-cell cytotoxicity is dependent on IFN{gamma}. C_LI