The MS remyelinating drug bexarotene (an RXR agonist) promotes induction of human Tregs and suppresses Th17 differentiation in vitro

AbstractThe retinoid X receptor (RXR) agonist bexarotene has recently been shown to promote remyelination in individuals with multiple sclerosis. Murine studies demonstrated that RXR agonists can have anti-inflammatory effects by enhancing the ability of all-trans-retinoic acid (αtRA), the primary active metabolite of vitamin A, to promote T regulatory cell (Treg) induction and reduce Th17 differentiation in vitro, following stimulation of naïve CD4 cells in the presence of TGF-β.Stimulating naïve human CD4 T cells for 7 days, in the presence of either Treg or Th17 skewing cytokines ± bexarotene (1 μM), ± other RXR agonists (9CisRA and NRX 194204), or ± αtRA (100 nM) shows that RXR agonists, including bexarotene, are capable of tipping the human Treg/Th17 axis in favour of Treg induction. Furthermore, this occurs independently of αtRA and retinoic acid receptor (RAR) signalling. Tregs induced in the presence of bexarotene express many of the canonical markers of T cell regulation and are functionally suppressive in vitro.These findings support a potential immunomodulatory role for bexarotene and highlight the possible therapeutic application of RXR agonists in autoimmune disease, with bexarotene’s pro-remyelinating effects making multiple sclerosis a particularly attractive disease target.Significance StatementThe pan-retinoid X receptor (RXR) agonist bexarotene has recently been shown to promote remyelination in patients with multiple sclerosis. Here we demonstrate that bexarotene, and other RXR agonists have immunomodulating effects, tipping the Th17/T regulatory cell (Treg) differentiation axis in favour of Treg development.These findings lend support to the idea of developing RXR agonists as treatments of autoimmune diseases, in particular multiple sclerosis.