A hidden antigenic determinant on membrane-bound human placental alkaline phosphatase

Two sets of monoclonal antibodies (mAbs) specific for human placental alkaline phosphatase (PLAP) were compared. One set of four mAbs was generated against solubilized and purified PLAP; the other set of seven mAbs was generated against the malignant cell line Hela TCRC-1 in which PLAP is an ectopically synthesized membrane-bound enzyme. Double immunodiffusion and competitive enzyme-linked immunosorbent assays were used to examine the relative spatial arrangement of the antigenic determinants to which each of the eleven mAbs binds. Significant differences in immunoreactivity of the antibodies were demonstrated. The mAbs to the solubilized and purified enzyme bound in either of two regions of the molecule. By contrast, all of the mAbs to PLAP as presented on the tumor cell surface bound in only one of these two regions. One of the major antigenic determinants on the solubilized enzyme is apparently unavailable for recognition by immunoreactive cells during immunization with whole cells. Furthermore, when mAbs are generated to this region using purified PLAP as the immunogen, they do not recognize membrane-bound PLAP. The 'hidden' determinant can be exposed in vitro after partial solubilization using butanol to extract the enzyme from HeLa TCRC-1 cells and subsequent treatment with 0.5% Nonidet P-40 detergent. The results of this study have implications for the potential use of mAbs in studies of other cell surface antigens and in tumor immunolocalization and drug targeting.