Retinoic acid signalling in thymocytes regulates T cell development

The Vitamin A derivative retinoic acid (RA) has emerged as an important regulator of peripheral T cell responses. However, whether there is endogenous retinoic acid receptor (RAR) signaling in developing thymocytes and the potential impact of such signals in thymocyte development remains unclear. Here, using a RA sensitive reporter mouse model, we demonstrate that endogenous RAR responses are induced in CD69 + CD4 + CD8 lo and CD69 + CD4 + CD8 + thymocytes undergoing positive selection and lineage commitment, and continue to be present in both CD4 + and CD8 + single positive (SP) cells, with RA signaling further enhanced in recently generated CD69 + CD4 + SP cells. To address the potential biological significance of RA signaling in developing thymocytes, we evaluated T cell development in CD4Cre-dnRAR mice, where RA signaling is blocked in thymocytes from the CD4+CD8+ double positive (DP) stage onwards due to expression of a dominant-negative form of RAR. Interestingly, these mice displayed a marked reduction in all thymocyte subsets, with the exception of CD8 + SP cells but including ETP and DN2-4 subsets, suggesting that blocked RA signaling in DP thymocytes and their progeny indirectly impacts on thymocyte precursor entry and/or survival. Furthermore, CD4Cre-dnRAR mice showed a 4-fold reduction in CD4 + /CD8 + SP ratio that was mainly due to enhanced accumulation of mature CD8 + SP cells, indicating that RA signaling may be directly involved in regulating thymic retention and/or post-selection expansion of this cell subset. Collectively, our data suggest a direct role for RA signaling in regulating thymocyte homeostasis and T cell development.