Human trophoblast contains an intracellular protein reactive with an antibody against CD133 - A novel marker for trophoblast

CD133 is a protein expressed on the cell membrane of a subfraction of haema topoietic stem and progenitor cells, as well as on some epithelial cells. P reviously available antibodies against CD133 recognized only the glycosylat ed protein, localized to membrane protrusions or microvilli. Due to this, i mmature intracellular stages of the CD133 protein could not be visualized u sing these antibodies. We describe reactivity of a commercially available a ntibody against CD133, called AC133-2, with an intracellular protein in tro phoblast. Both villous and extravillous cytotrophoblast, as well as syncyti otrophoblast were stained by AC133-2 in cryostat sections of first trimeste r and term placenta. Villous stroma was not stained. AC133-2 reactivity was seen in methanol-fixed primary trophoblast cells and trophoblast-derived c ell lines, and was coexpressed with cytokeratin-7. CD133 messenger RNA,vas present in trophoblast and trophoblast-derived cell lines, but also in cell s not displaying any reactivity with CD133 antibodies. AC133-2 recognized a 55-60 kDa protein on Western blots of cell extracts including trophoblast. The exact nature of this protein is not vet understood. However, AC133-2 i s applicable as a positive marker for the characterization of all subtypes of trophoblast and for trophoblast cell lines. (C) 2001 Harcourt Publishers Ltd.