THE IMPACT OF 3-DIMENSIONAL STRUCTURE ON THE EXPRESSION OF PL(A) ALLOANTIGENS ON HUMAN INTEGRIN BETA(3)

We have compared the binding of affinity-purified anti-Pl(A1) IgG from seven nonrelated donors with chimeric integrin subunit beta(3) molecules expressed in the baculovirus-Spodoptera frugiperda insect cell system. beta(3) chimeras were engineered to include segments of antigenic human beta(3) sequences spliced to intervening segments of nonantigenic Xenopus beta(3) sequence. Our results clearly show that antibodies from all seven donors will bind to nondenatured molecules containing the antigenic human beta(3) Cys(26)-Cys(38) loop only when it is presented in a correct orientation that must be maintained by noncontiguous human sequences. Key downstream sequences are located within the region beta(3),288-490, flanking either side of the putative long-range disulfide at Cys(435). Although our results confirm unambiguously that the Leu/Pro polymorphism at position 33 in human beta(3) is necessary for the expression of Pl(A) epitopes, they also indicate that this polymorphic sequence alone is not sufficient. The requirement for additional human PQ sequence transcends the need to maintain a correct orientation within the Cys(26)-Cys(38) loop itself, because the murine monoclonal antibody SZ21, which recognizes the sequence beta(3)28-35 contained within the Cys(26)-Cys(38) loop, binds to all chimeras containing this loop, even if the same chimeras are not recognized by anti-Pl(A1). Our results indicate that additional noncontiguous residues encompassed by the sequence 288-490 either directly contribute to the composition of the Pl(A1) epitope or, more likely, maintain the Cys(26)-Cys(38) loop in a proper orientation with respect to the remainder of the beta(3) molecule and thereby maintain proper antigenic presentation of the sequences in that loop. (C) 1995 by The American Society of Hematology.