Biochemical characterization of a P43,12 complex: Comparison with human and murine class I molecules

A monoclonal antibody designated as C21 reacting with a p43,12 complex was developed against human thymocytes. It stained predominantly the early hematopoietic cells of the lymphoid lineage and also thymocytes, peripheral B-cells and activated T- and B-cells similarly to OKT10. The heavy chain of this antigen was a glycoprotein of M, 43,000 (p43). Sequential immunoprecipitation with C21 and OKT10 antibodies indicated that they both reacted with an identical heavy-chain molecule. This observation was further documented by two-dimensional analysis. Monoclonal antibody C21 was used to probe a p43,12 complex further. Structural polymorphism of the p43 heavy chain isolated from T- and B-cells of different individuals was not detected by chymotryptic peptide mappIng, although molecules from these cell types possessed a different charge on two-dimensional gels. An unusual observation was made regarding this complex on MOLT4 cells. The light chain co-precipitated from these cells was 12,000 daltons and had a pI distinct from that of β₂-microglobulin but similar to the pI of the βt molecule. Comparison between chymotryptic peptide maps of the p43 heavy chain and those of the human and murine class I molecules such as HLA, T6, H-2K, Qa-2 and TL revealed no apparent homology. We have shown, however, that the peptide backbone of p43, as studied by both tunicamycin treatment of cells and endoglycosidase F digestion of immunoprecipitates, was identical in size to that of murine Qa-1. These results suggest that the p43 antigen may be homologous to murine Qa-1 or another class I antigen encoded in the murine TL:Qa region.