Age-related thymic atrophy impairs development and function of an antigen-specific tTreg clone

Purpose: The atrophied thymus generates an increased ratio of polyclonal thymic Regulatory T (tTreg) cells to thymic conventional T (tTcon) cells, and peripheral Treg (pTreg) cells accumulate during aging. So, why are pTregs in the elderly unable to effectively suppress age-related inflammation ("inflammaging")? Methods: We utilized a mock self-antigen (autoimmune pancreatitis) chimeric mouse model, in which irradiated rat insulin promotor (RIP)-driven mOVA mice received mixed OT-II TCR transgenic and wild-type bone marrow. Thus, we can easily visualize the generation and activation of an antigen-specific Treg cell clone. Additionally, our mOVA mice carry a FoxN1-floxed gene for induction of conditional thymic atrophy, analogous to the aged thymus. Results: The chimeric mice with thymic atrophy exhibited significant decline in central and peripheral OVA-specific (OT-II) Tregs, but not total (pan) Tregs. Further, intrinsic Treg changes in FoxP3 expression was observed, suggestive of reduced suppressive capacity. This was confirmed via functional assays showing that OVA-specific Tregs were significantly less able to suppress antigen-specific stimulation of Teffs in vitro. Additionally, we performed TCR sequencing and observed a trend for decreased TCR diversity in tTregs in mice with thymic atrophy. Conclusion: These data indicate that although the effects of age-related thymic atrophy do not affect pan-Treg generation, certain tissue-specific Treg clones may experience abnormal thymic selection, creating holes in Treg-mediated immunoregulation. This, combined with enhanced pan-pTreg cells, may help explain inflammaging.