GENDER DIFFERENCES IN IMMUNE RESPONSIVENESS TO GONADOTROPIN-RELEASING HORMONE IN MICE • 528

Abstract

We have previously demonstrated that GnRH analogs modulate the severity of disease in a murine model of systemic lupus erythematosus (SLE). Although GnRH antagonists ameliorated disease in intact and castrated male and female NZB F1 hybrid mice, GnRH agonists exacerbated disease activity in females only, regardless of ovariectomy. Because estrogens are known to increase the expression of the GnRH receptor at the level of the pituitary, and because lymphocytes are thought to express the GnRH receptor, we speculated that the increased responsiveness to GnRH observed in females might relate to increased expression of the GnRH receptor on lymphoid cells in females. We used competitive reverse transcription polymerase chain reaction (RT-PCR) to quantitate the expression of GnRH receptor mRNA in whole spleen in intact and castrated male and female NZB F1 hybrid mice after treatment with GnRH analogues and vehicle. Unexpectedly, we found that GnRH receptor expression was higher in males than in females, regardless of treatment. Using a constant amount (1.3 fg) of a competitive DNA fragment in each PCR reaction tube, we found a ratio of product:competitor of 2.76 ±.87 in males and a ratio of 1.1 ±.4 in females (p = 0.05; n = 10/group). We performed additional experiments in a normal mouse strain (DBA/2) to test immune responsiveness to GnRH in vitro. We measured thymidine uptake to quantitate proliferative responsiveness of splenic mononuclear cells to mitogen in the presence of various physiological concentrations of GnRH. Females responded with an increase in proliferative response to Concanavalin A at all concentrations of GnRH from 10 -7 M to 10 -11 M: Δ c.p.m. using 10 -9 M GnRH = 18,809 ± 2,514 compared to medium. Males responded with a decrease in proliferation at all GnRH concentrations tested:Δ c.p.m. using 10 -9 M GnRH = -13,791 ± 6705 compared to medium (p = 0.004; n=5 experiments). We conclude that gender differences in responsiveness to GnRH may contribute to the gender differences in the immune system and/or to the gender differences in the expression of autoimmune diseases.