Effect of thyrotropin-releasing hormone on immune functions of peripheral blood mononuclear cells

The tripeptide thyrotropin-releasing hormone (TRH) works as a hypothalamic hormone, but is found also outside the brain in intrinsic nerve fibers of the gastrointestinal tract. There is evidence that TRH modulates the activity of immunocompetent cells, although there are only very few data on TRH-mediated immune effector functions. Since we could recently show that TRH inhibits monocyte activities we were also interested in other possible TRH modulated immune functions. Peripheral blood mononuclear cells (PBMC) from ten healthy subjects were cultured for 7 days and pulsed with 0.125 and 0.250 μg/ml Pokeweed mitogen (PWM). 10^-12 to 10^-6M TRH was added simultaneously with PWM. Lymphocyte proliferation ([³H]thymidine incorporation), interferon-_γ (IFN-_γ) activity (RIA) and immunoglobulin activities (IgG, IgM, IgA; ELISA) were determined in the supernatants. We could demonstrate a TRH-dependent decrease in PWM-pulsed IgG activity with significant (α = 0.05) values at 10^-8 and 10^-10 M (-29 ± 6%/-16 ± 3% for PWM 0.125 μg/ml and -17 ± 9%/-11 ± 9% for PWM 0.250 μg/ml). This inhibitory effect could be abolished by an anti-TRH antiserum. There was no TRH effect on IgM and IgA activities, IFN-_γ activity and lymphocyte proliferation compared with the PWM stimulated values alone. The described TRH effect on the polyclonal IgG response by PBMC gives further evidence for a functional link between the immune system and the endocrine system, although its underlying mechanism is not yet clear.