Signaling through the TLR family of molecular pattern recognition receptors has been implicated in the induction of innate and adaptive immune responses. A role for TLR signaling in the maintenance and/or regulation of Treg function has been proposed, however its functional relevance remains unclear. Here we have shown that TLR9 is highly expressed by human Treg secreting the antiinflammatory cytokine IL-10 induced following stimulation of blood and tissue CD3+ T cells in the presence of 1α,25-dihydroxyvitamin D3 (1α25VitD3), the active form of Vitamin D, with or without the glucocorticoid dexamethasone. By contrast, TLR9 was not highly expressed by naturally occurring CD4+CD25+ Treg or by Th1 and Th2 effector cells. Induction of TLR9, but not other TLRs, was IL-10 dependent and primarily regulated by 1α25VitD3 in vitro. Furthermore, ingestion of calcitriol (1α25VitD3) by human volunteers led to an increase of both IL-10 and TLR9 expression by CD3+CD4+ T cells analyzed directly ex vivo. Stimulation of 1α25VitD3-induced IL-10–secreting Treg with TLR9 agonists, CpG oligonucleotides, resulted in decreased IL-10 and IFN-γ synthesis and a concurrent loss of regulatory function, but, unexpectedly, increased IL-4 synthesis. We therefore suggest that TLR9 could be used to monitor and potentially modulate the function of 1α25VitD3-induced IL-10–secreting Treg in vivo, and that this has implications in cancer therapy and vaccine design.
Zoë Urry, Emmanuel Xystrakis, David F. Richards, Joanne McDonald, Zahid Sattar, David J. Cousins, Christopher J. Corrigan, Emma Hickman, Zarin Brown, Catherine M. Hawrylowicz