PROBLEM: Immunologic recognition of pregnancy is important for normal gestation. Successful pregnancy is characterized by a Th2 dominance, whereas there is a Th1 dominance in failed pregnancies. We assume that a signal given by the fertilized ovum induces a Th2 shift, necessary for a normal outcome. In vitro fertilization provides a tool for testing this possibility. METHOD OF STUDY: Phytohemagglutinin-stimulated lymphocytes were incubated for 48 hr in the presence of culture media from in vitro fertilized eggs, as well as in follicular fluid (FF) and control supernatants. Total RNA was isolated from the lymphocytes by the guanidine-isothiocyanate method and interleukin (IL)-10 mRNA expression was detected by reverse transcription-polymerase chain reaction. RESULTS: Ten percent of the activated lymphocytes incubated with FF expressed IL-10 mRNA, whereas 88% of the lymphocytes activated with supernatants of sperm + oocytes gave a positive signal. Significantly (P < 0.05) fewer (50%) lymphocytes stimulated in the presence of control supernatants also expressed mRNA for IL-10. In supernatants of activated lymphocytes incubated with the culture medium of spermia + oocytes, the concentration of IL-10 was significantly higher than in the lymphocytes incubated with FF. CONCLUSION: These data suggest that the presence of the fertilized ovum induces a Th2 shift, which enables pregnancy to proceed to term.

The immunological effects of progesterone are mediated by a 34-kDa protein named the progesterone-induced blocking factor (PIBF). PIBF induces increased production of Th2-type cytokines; thus, it might stimulate antibody synthesis by B cells. There is a population of antibodies which, owing to the presence of a mannose-rich oligosaccharide residue on one of the Fab arms of the molecule, possess an asymmetric structure. Due to the asymmetric structure these molecules have no effector functions; however, they might act as blocking antibodies. This study was aimed at investigating the effect of progesterone-dependent immunomodulation on antibody production by B cells, with special emphasis on the synthesis of asymmetric nonprecipitating antibodies. The ratio of asymmetric IgG was significantly higher in supernatants of hybridoma cells cultured in the presence of PIBF than in those cultured in the absence of PIBF. Lymphocytes from healthy pregnant women produce significantly more PIBF than those of women with pathological pregnancies. The present studies revealed a positive relationship between asymmetric antibody content of the sera and PIBF expression on lymphocytes. Blocking of progesterone receptors by RU 486 or neutralizing endogenous PIBF activity by specific antibody significantly reduced the production of asymmetric antibodies in pregnant mice. Effector function of conventional and asymmetric antibodies was compared in a TNFα neutralization assay. Purified asymmetric anti-TNFα antibodies did not neutralize the cytotoxic effect of TNFα on L929 murine fibroblast target cells, whereas conventional anti-TNFα antibodies in the same concentration significantly (P< 0.001) reduced cytotoxicity. Our data suggest that PIBF induces increased production of asymmetric antibodies. These antibodies fail to exhibit effector functions and by blocking fetally derived antigens might contribute to protection of the fetus.

PROBLEM: This study was aimed at investigating the involvement of an altered cytokine pattern in the immunomodulatory and anti‐abortive effects of a progesterone‐induced immunomodulatory protein (PIBF).METHOD: PIBF expression on lymphocytes of healthy pregnant women and from women at risk for premature pregnancy termination was determined. In sera of the same women TNFα was quantified by a bioassay using L929 cells. NK activity was determined by a single cell cytotoxicity assay. Cytokine production of the lymphocytes or murine spleen cells was measured by ELISA or detected by immunocytochemistry. In pregnant mice endogenous PIBF activity was neutralized by anti‐PIBF IgG.RESULTS: Sera of women at risk for premature pregnancy termination contained significantly higher concentrations of TNFα than those from healthy pregnant women and PIBF expression on the lymphocytes was inversely related to serum concentration of TNFα. Increased NK activity of lymphocytes after neutralization of endogenous PIBF activity is corrected by anti‐IL2 treatment and PIBF inhibits IL12 expression on activated lymphocytes. PIBF increases IL‐10 production by activated spleen cells. In pregnant mice, neutralization of endogenous PIBF activity by specific antibody results in increased resorption rate and reduced splenic IL‐10 production.CONCLUSIONS: Our data allow the assumption that via blocking IL‐12 production PIBF inhibits NK activation with a concomitant reduction of TNFα levels. Disturbances in this system might lead to the expression of the known synergistic effect of IL‐12 and TNFα, resulting in a Th1 type cytokine dominance and pregnancy termination.