SMPD1 and major depressive disorder: In an observational prospective clinical study with patients suffering from major depression, we observed that acid sphingomyelinase-inhibiting antidepressants induced a stronger relative increase in the frequency of CD4<sup>+</sup> Foxp3<sup>+</sup> regulatory T cells in peripheral blood than acid sphingomyelinase-non- or weakly inhibiting antidepressants.