Soluble interleukin-6 (IL-6) receptor in the sera of pregnant women forms a complex with IL-6 and augments human chorionic gonadotropin production by normal human trophoblasts through binding to the IL-6 signal transducer.

To study the role of soluble interleukin-6 receptor (sIL-6R) during pregnancy, sIL-6R levels in the sera of pregnant women in the first, second, and third trimesters were determined and found to remain unchanged during pregnancy, but were significantly higher than those in nonpregnant women in the follicular, ovulatory, and luteal phases of the menstrual cycle (P < 0.001). IL-6 levels, however, in the sera of pregnant women at all trimesters showed no difference from those in nonpregnant women at any stage of the menstrual cycle. Recombinant sIL-6R (rsIL-6R) augmented hCG production by rIL-6-stimulated trophoblasts dose dependently, but failed to enhance hCG production by unstimulated trophoblasts. rIL-6- and rsIL-6R-induced hCG production was significantly blocked by anti-IL-6R antibody, PM1; antisignal transducing glycoprotein 130 (gp130) antibody, GPX7; or a tyrosine kinase inhibitor, genistein. Thus, sIL-6R in serum from pregnant women forms a complex with placental and decidual IL-6 in a manner similar to trophoblast membrane-bound IL-6R. These two discrete types of IL-6R and IL-6 complex might act cooperatively by binding to gp130 and subsequently evoking tyrosine kinase activity in the trophoblasts to produce hCG in vivo.