T helper 2 (Th2) cells regulate helminth infections, allergic disorders, tumor immunity, and pregnancy by secreting various cytokines. It is likely that there are undiscovered Th2 signaling molecules. Although steroids are known to be immunoregulators, de novo steroid production from immune cells has not been previously characterized. Here, we demonstrate production of the steroid pregnenolone by Th2 cells in vitro and in vivo in a helminth infection model. Single-cell RNA sequencing and quantitative PCR analysis suggest that pregnenolone synthesis in Th2 cells is related to immunosuppression. In support of this, we show that pregnenolone inhibits Th cell proliferation and B cell immunoglobulin class switching. We also show that steroidogenic Th2 cells inhibit Th cell proliferation in a Cyp11a1 enzyme-dependent manner. We propose pregnenolone as a “lymphosteroid,” a steroid produced by lymphocytes. We speculate that this de novo steroid production may be an intrinsic phenomenon of Th2-mediated immune responses to actively restore immune homeostasis.
Differential upregulation of the steroid biosynthetic pathway during Th2 differentiation
T helper cells produce the steroid pregnenolone in vitro and in vivo
Steroidogenic Th2 cells suppress Th cell proliferation in a Cyp11a1-dependent manner
Pregnenolone inhibits B cell immunoglobulin class switching in vitro
Immune cells signal by secreting cytokines and by direct cell-to-cell contact. Although steroids are known to be immunoregulators, immune cells are not generally thought to be steroidogenic. Here, Mahata et al. demonstrate that T helper (Th) cells synthesize and secrete the steroid pregnenolone in vitro and in vivo. Pregnenolone inhibits Th cell proliferation and B cell immunoglobulin class switching, and steroidogenic Th cells inhibit Th cell proliferation. This evidence suggests that steroidogenic Th cells contribute to immune homeostasis.