and Are Epigenetically Stabilized in T-Bet Tregs That Transiently Accumulate in Influenza A Virus-Infected Lungs.

During influenza A virus (IAV) infections, CD4 T cell responses within infected lungs mainly involve T helper 1 (Th1) and regulatory T cells (Tregs). Th1-mediated responses favor the co-expression of T-box transcription factor 21 (T-bet) in Foxp3 Tregs, enabling the efficient Treg control of Th1 responses in infected tissues. So far, the exact accumulation kinetics of T cell subsets in the lungs and lung-draining lymph nodes (dLN) of IAV-infected mice is incompletely understood, and the epigenetic signature of Tregs accumulating in infected lungs has not been investigated. Here, we report that the total T cell and the two-step Treg accumulation in IAV-infected lungs is transient, whereas the change in the ratio of CD4 to CD8 T cells is more durable. Within lungs, the frequency of Tregs co-expressing T-bet is steadily, yet transiently, increasing with a peak at Day 7 post-infection. Interestingly, T-bet Tregs accumulating in IAV-infected lungs displayed a strongly demethylated locus, similarly as in T-bet conventional T cells, and a fully demethylated Treg-specific demethylated region (TSDR) within the locus. In summary, our data suggest that T-bet but not T-bet Tregs are epigenetically stabilized during IAV-induced infection in the lung.