Background

CD4+ FoxP3+ regulatory T cells (Treg) are an important population of suppressor T cells that have shown promise in treating experimental autoimmune disease, allograft rejection, and graft-versus-host disease. It is known that the migration of Treg is essential for their suppressive function in transplantation. We have shown that vitamin A metabolite all-trans retinoic acid (ATRA) and the mTOR inhibitor rapamycin, two compounds which improve the generation of induced Treg, have differential effects on chemokine receptor expression, migration, and therapeutic efficacy of murine induced Treg. In this study, we investigated the influence of ATRA and rapamycin on naturally-occuring Treg.

Methods

To determine the influence of ATRA and rapamycin on chemokine receptor expression on naturally-occurring Treg, freshly-isolated C57BL/6 CD4+ CD25+ Treg or CD4+ CD25- conventional T cells were cultured with a combination of ATRA, rapamycin, or TGF-Β for 2-8 days, and then analysed by flow cytometry.

Results

In contrast to induced Treg and conventional T cells which uniformly upregulated the gut-homing chemokine receptor 9 (CCR9) in the presence of ATRA, we found that ATRA upregulated CCR9 in only about half of murine naturally-occurring Treg, with delayed kinetics. Moreover, the bimodal distribution of CCR9+ versus CCR9- Treg subsets was not due to differences in the activation marker CD62L, extent of proliferation, intensity levels of FoxP3, or expression of Helios, which is a marker of naturally-occurring Treg. Furthermore, we found that the presence of rapamycin almost completely prevented the appearance of the CCR9+ fraction in naturally-occuring Treg, but only modestly interfered with ATRA-induced CCR9 upregulation in induced Treg and conventional T cells.

Conclusions

Compared to induced Treg and conventional T cells, naturally-occuring Treg upregulate the gut-homing CCR9 to a lesser extent in the presence of ATRA, and they are more susceptible to rapamycin blockade of ATRA-induced CCR9 expression. These findings suggest that naturally-occuring Treg may be disadvantaged in migrating to and ameliorating inflammation in the gut, such as in intestinal GVHD, but have an advantage in migrating to sites of inflammation other than the gut, such as in the setting of solid organ allotransplantation.

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