Atrophied thymus creates tTreg repertoire holes diminishing an antigen-specific population in the periphery

Purpose: We have shown that there is an increased ratio of total thymic T regulatory (tTreg) cells to thymic T conventional (tTcon) cells generated by the atrophied thymus related to aging. This observation, coupled with the accumulation of peripheral Treg (pTreg) cells with age, poses the difficult question of why Tregs in the periphery of the elderly are unable to suppress amplified self-reactivity-induced inflammaging. Methods: We utilized a chimeric mouse model with immune system reconstitution in which lethally irradiated rat insulin promotor-driven (RIP) mOVA host mice received mixed OT-II TCR transgenic and wild-type bone marrow, each expressing distinct congenic identifiers (CD45.1 vs CD45.2). In this system, OVA serves as a mock self-antigen, which is expressed mainly in the pancreas of the host mice. Further, our mOVA host mice carried a FoxN1-floxed gene, for induction of conditional FoxN1 knock-out, resulting in thymic atrophy analogous to age-related thymic atrophy. Results: We observed that chimeric mice with induced thymic atrophy exhibited significantly decreased OVA-specific (OT-II) Tregs, but not total (pan) Tregs, in the spleen and pancreas. The specific Treg cells of mice with thymic atrophy were also more instable, showing relatively decreased FoxP3 expression, and had greater plasticity, showing an increased Th1-like (IFNγ+FoxP3+) Treg phenotype. Conclusions: Overall, our preliminary results suggest that thymic atrophy creates changes in the antigen-specific repertoire during tTreg generation resulting in potential “holes” that may contribute to inflammaging in the elderly and negatively impact Treg cell-mediated regulation in the aged immune system.