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Inducible H‐2M and constitutive H‐2O/H‐2A expression on trophoblast cells

Trophoblast cells escape maternal immune attack due to their class II negative state. This generally accepted hypothesis, however, became complicated when these antigens were shown induced by IFN‐γ, 5‐AzaC, LPS, mycobacterium and other factors, an expression correlated, as expected, to fetal abortion. Using untreated trophoblast cells as controls in the study of class II antigen regulation we found that, although the cells were not expressing surface class II molecules, they contained class II positive intracellular pools, which after short treatment with IFN could be emptied to the culture supernatants. Using immunofluorescence, ELISA, confocal microscopy, immunoaffinity purification, subcellular fractionation as well as Western blots and RT‐PCR experiments we localized, identified the class II proteins in trophoblasts and studied their mechanisms of excretion. It was shown that classical class II molecules co‐localized with the invariant chain in rab7 and rab11 positive endosomes and could be secreted from the cells via the endosomal pathway, since their extracellular detection could be inhibited by pepstatin, brefeldin and leupeptin. RT‐PCR experiments using H‐2M, H‐2O and H‐2A specific primers showed that H‐2O and H‐2A are constitutively expressed in trophoblasts, whereas H‐2M is inducible by IFN. Cyclohexamide treatment blocked secretion of class II antigens from these cells indicating that H‐2M is involved in the secretion and/or appearance to the membrane.