 TGF-β is also responsible for the development of TRMs, thus promoting TRM differentiation in a manner similar to Treg differentiation.Most lymphocytes express sphingosine-1-phosphate receptors (S1P1-5), which are G protein-coupled receptors located in the cell membrane that bind to the ligand sphingosine-1-phosphate (S1P).S1P is a sphingolipid metabolite that is abundant in the bloodstream and, upon binding to S1P1, promotes lymphocyte egress from lymphoid organs so they can travel to affected tissues.