In vivo treatment with interferon-gamma during early pregnancy in mice induces strong expression of major histocompatibility complex class I and II molecules in uterus and decidua but not in extra-embryonic tissues.

Allopregnant (NFR/N [Swiss-derived] H-2q females x 57/Bl H-2b males) and syngeneically pregnant (NFR/N x NFR/N) mice were subjected to daily injections (10(5) U/mouse/day, from Day 5.5 of gestation) of recombinant rat or mouse interferon-gamma (IFNg) in order to investigate its ability to induce extra-embryonic major histocompatibility complex (MHC) expression and antipaternal immune reactions if administered during the first part of the gestation period. In addition, a limited number of IFNg-treated embryo-transferred NFR/N mice carrying C57/B1 embryos (representing a complete allogenic pregnancy) were investigated. Mouse and rat IFNg caused the same type of histological and physiological changes, and most of the experiments were performed by using rat IFNg. Several IFNg-treated mice (irrespective of type of mating) showed a drop in weight and a high rate of resorptions at Day 12.5 of gestation. This interference with pregnancy appeared not to be caused by immunological reactions against the feto-placental unit (no leukocyte infiltration and no significant effect on serum levels of antipaternal antibodies in preimmunized allopregnant IFNg-treated mice). Immunohistochemical stainings of cryosectioned tissues at Day 9.5 of pregnancy revealed that IFNg treatment caused a strong induction of MHC class I and class II expression on most cells in the uterus and on several cells in the maternal decidua, while there was a complete absence of detectable MHC class I and class II expression in the extra-embryonic tissues. Characteristic for a Day 12.5 placenta of an IFNg-treated mouse (including embryo-transferred mice) was a strongly MHC class II-induced maternal decidua and a completely MHC class II-negative fetal placenta. The pattern of IFN-induced MHC class I expression was similar to that of class II, with the exception of class I expression on scattered cells within the basal zone. Thus, the present study provides immunohistological evidence that IFNg administered in vivo during the first part of gestation is not capable of inducing MHC expression on murine extra-embryonic cells despite an extremely high expression of MHC molecules on decidual cells in intimate contact with extra-embryonic tissues. It is likely that the resistance to IFNg-mediated induction of MHC expression on extra-embryonic cells is of basic importance for the protection of mammalian semi-allogeneic fetuses.