Acid sphingomyelinase (ASM) is the rate-limiting enzyme cleaving sphingomyelin into ceramide and phosphorylcholin. CD4+ Foxp3+ regulatory T (Treg) cells depend on CD28 signaling for their survival and function, a receptor that activates the ASM. Both, basal and CD28-induced ASM activities are higher in Treg cells than in conventional CD4+ T (Tconv) cells. In ASM-deficient (Smpd1-/-) as compared to wt mice, membranes of T cells contain 7-10-fold more sphingomyelin and two- to three-fold more ceramide, and are in a state of higher order than membranes of T cells from wt mice, which may facilitate their activation. Indeed, the frequency of Treg cells among CD4+ T cells in ASM-deficient mice and their suppressive activity in vitro are increased. Moreover, in vitro stimulation of ASM-deficient T cells in the presence of TGF-β and IL-2 leads to higher numbers of induced Treg cells. Pharmacological inhibition of the ASM with a clinically used tricyclic antidepressant such as amitriptyline in mice or in tissue culture of murine or human T cells induces higher frequencies of Treg cells among CD4+ T cells within a few days. This fast alteration of the balance between T cell populations in vitro is due to the elevated cell death of Tconv cells and protection of the CD25high Treg cells by IL-2. Together, these findings suggest that ASM-inhibiting antidepressants, including a fraction of the serotonin re-uptake inhibitors (SSRIs), are moderately immunosuppressive and should be considered for the therapy of inflammatory and autoimmune disorders.
Keywords: CD4+ Foxp3+ regulatory T cells; acid sphingomyelinase; immunosuppression; selective serotonine reuptake inhibitors; tricyclic antidepressants.