IFN-γ Facilitates Release of Class II-Loaded Intracellular Pools in Trophoblast Cells: A Novel Property Independent of Protein Synthesis

Abstract

Interferon-γ (IFN-γ) is an abortion-inducing factor, yet its effects in such a reaction are subject to various levels of regulation. The trophoblast cell line TROPHO-1 can be induced by IFN-γ to express mRNA and surface class II major histocompatibility complex (MHC) proteins after 8 and 48 h of stimulation, respectively. Untreated cells, however, show an intracellular accumulation of class II antigens earlier (6 h), indicating the existence of MHC pools in the cystosol independent of any induction. On addition of IFN-γ, immunofluorescence, subcellular fractionation, and ELISA experiments showed that class II antigen activity detected in the endosomal compartments of the cells could be measured in the culture supernatants. These soluble class II proteins, when isolated and purified using magnetic bead isolation techniques and tested in SDS-PAGE gel and Western blot experiments, had a molecular weight of 70kDa. Administration of these molecules to pregnant mice as culture supernatants increased the abortion rate and decreased maternal hematocrit levels, effects that could be immunoabsorbed by anti-I-A^d monoclonal antibodies (mAb). These results indicate that although surface class II molecules are not expressed on trophoblast cells, they accumulate in endosomal compartments and can be released from the cells on addition of IFN-γ. This new IFN-γ property, to mobilize intracellular pools of class II MHC antigens in trophoblast cells independent of de novo protein synthesis and induce their release to the extracellular matrix, is a mechanism that appears to be involved in the fetal rejection process, facilitating priming of the maternal organism against the fetal allograft.