The origins of colonic T_Reg cells

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It has been previously suggested that thymus-derived regulatory T (T_Reg) cells primarily recognize self antigens, whereas peripherally derived T_Reg cells respond to foreign antigens. Indeed, conversion of forkhead box P3 (FOXP3)⁻CD4⁺ T cells to FOXP3⁺ T_Reg cells is known to be favoured by the tolerogenic microenvironment of the gut, where there is an abundance of innocuous foreign antigens, including food- and microbiota-derived components. However, Cebula et al. now suggest that most colonic T_Reg cells, including T_Reg cells that recognize intestinal commensal bacteria, develop in the thymus.

The authors used transgenic mice that express a limited but diverse T cell receptor (TCR) repertoire (TCR^mini mice) to investigate the origin of colonic T_Reg cells on the basis of high-throughput TCR sequencing analyses. T cells from TCR^mini mice express a single TCR β-chain combined with a variety of TCR α-chains, and their development and function was found to be comparable to that of non-transgenic T cells.

Sorted individual FOXP3⁺ T cells and FOXP3⁻CD4⁺ T cells from the thymus, peripheral lymph nodes and intestines of TCR^mini mice were analysed in terms of the complementarity determining region 3 (CDR3) sequence of the TCR α-chain that they expressed. It was found that the most abundant TCRs did not overlap between the FOXP3⁺ and FOXP3⁻CD4⁺ T cell subsets, irrespective of the organ of origin. Moreover, 86% of the TCRs from colonic T_Reg cells were identical to 50% of the TCRs from FOXP3⁺CD4⁺ thymocytes. This suggests that in all organs that were analysed, including the intestines, peripheral conversion of FOXP3⁻CD4⁺ T cells to FOXP3⁺ T_Reg cells did not account for the majority of T_Reg cells. A similar result was observed in transgenic mice that had an even broader TCR repertoire. So, colonic T_Reg cells seem to derive predominantly from FOXP3⁺CD4⁺ thymocytes.

colonic T_Reg cells seem to derive predominantly from FOXP3⁺CD4⁺ thymocytes

But do these thymus-derived intestinal T_Reg cells respond to foreign antigens such as those derived from commensal bacteria? Treatment of TCR^mini mice with antibiotics altered the frequency of the most dominant colonic T_Reg cell TCRs, which probably reflects the antigen-specific expansion or contraction of colonic T_Reg cell populations. However, the diversity of the colonic T_Reg cell TCR repertoire was not reduced following the changes in the composition of colonic microbiota. Moreover, most colonic T_Reg cell TCRs were shared with FOXP3⁺CD4⁺ thymocytes even after treatment with antibiotics. This indicates that changes in the composition of the population of intestinal commensal bacteria did not promote substantial recruitment of peripherally derived T_Reg cells.

Finally, hybridomas that were generated from colonic T_Reg cells showed responsiveness to sterile filtrates of caecal contents and to sonicates of individual commensal bacterial species. As 90% of the TCRs of hybridomas that responded to caecal contents were expressed by FOXP3⁺CD4⁺ thymocytes, the authors conclude that thymus-derived T_Reg cells have a leading role in establishing intestinal T cell tolerance against microbiota.