Syncytiotrophoblast brush border proteins recognized by monoclonal antibody TRA-2-10 and rabbit anti-TLX sera.

Different subsets of placental trophoblast epithelium are directly exposed to the maternal immune system during pregnancy and consequently represent major elements in allogeneic interactions. It has been proposed that the trophoblast--lymphocyte cross-reactive (TLX) alloantigen system is involved in maternal allogeneic recognition during pregnancy. Monoclonal antibody TRA-2-10 putatively recognizes TLX antigens, but its reactivity with trophoblast and normal tissues has not been documented in detail. In this report, immunohistological investigations revealed that TRA-2-10 recognizes all subsets of trophoblast in addition to amniotic and seminal vesicle epithelia. Immunoblotting demonstrated reactivity with glycoproteins of 55,000 and 65,000 mol. mass under non-reducing conditions on various cell types. These proteins displayed tissue-specific size variations and individuals varied in the amounts expressed of the two species. On the basis of blocking and immunoprecipitation experiments, TRA-2-10 reactive antigens are recognized by rabbit anti-TLX sera and are potential TLX antigen candidates. However, TLX antigens are found in seminal plasma whilst TRA-2-10 reactive antigens are not. Both TLX and TRA-2-10 antigens appear related if not identical to membrane cofactor protein (MCP) by virtue of shared molecular characteristics and blocking of lymphocyte binding of monoclonals to MCP by polyclonal anti-TLX. Extra-embryonic membranes are thus richly endowed with a complement regulatory protein which could facilitate their roles in protection of the fetus by avoidance of harmful maternal immune response amplification.