GB24, a mouse monoclonal antibody, recognizes a trophoblast-leukocyte cross-reactive antigen (TLX), which is likely identical to the membrane cofactor protein (MCP), a complement regulatory protein. GB24 reacts also with a human acrosomal sperm antigen (Fénichel et al.: J Reprod Fertil 87:699-706, 1989). By immunofluorescence or immunoperoxidase, testicular, epididymal, and ejaculated spermatozoa were found to be positive after fixation by acetone. Motile, suspended spermatozoa became positive only through conditions known to induce acrosome reaction (A23187, follicular fluid, contact with oocytes). Ultrastructural studies with immunogold staining localized this protein on the inner acrosome membrane and in the acrosomal content. By SDS-polyacrylamide gel electrophoresis, GB24 immunoprecipitated a unique protein of 48 kDa from capacitated and A23187-induced spermatozoa under reducing conditions. No cross-reactivity was found with mouse, boar, or ram spermatozoa. Localization of this human sperm antigen recognized by GB24 and its similarity with the TLX-MCP family antigens would suggest a possible role of this molecule during fertilization in sperm-egg binding or immune protection.

GB24 is a mouse monoclonal antibody (IgG1) raised against human term placental microvilli. This antibody displayed pan-trophoblast reactivity pattern. In addition, GB24 recognized normal peripheral leukocytes and transformed cell lines (Daudi, HL-60, Jurkat, AV3, BeWo, HT-29) by using membrane immunofluorescence and flow cytometry. By SDS-polyacrylamide gel electrophoresis, this antibody immunoprecipitated two proteins of 62 kilodaltons (kDa) and 75 kDa from placental microvilli. Isoelectrofocusing analysis revealed that these two bands exhibited different isoelectric points: 4.7 for the 62 kDa protein, 4.6 and 4.4 for the two spots corresponding to the 75 kDa protein. Microvilli from different placentae revealed substantial differences regarding the intensity of the labeling of these two bands, suggesting that the antigens recognized by GB24 are probably allotypic.

This paper describes a new Mab BC-1 which is directed at a cell surface antigen expressed by extravillous cytotrophoblast and not by villous cytotrophoblast, so it is useful for distinguishing between the two populations. The antigen recognised by BC-1 is trypsin-resistant, which allows it to be used for flow cytometric analysis of living trophoblast isolated by enzymic disaggregation. However, BC-1 is not trophoblast-specific but cross-reacts with some other tissues, in particular endothelial cells and peripheral blood monocytes. The nature of this antigen has not been established.

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