The human T6
antigen was studied by two
monoclonal antibodies: OKT6 and Leu-6. A third
monoclonal antibody, C56 (developed in our laboratory), was found to have similar properties to those of OKT6. On SDS-PAGE, all three
antibodies precipitated a 48,000-12,000-dalton heterodimer. Two-dimensional gel electrophoresis and chymotryptic
peptide map analysis revealed that these
antibodies precipitated in identical 48,000-dalton heavy chain which was distinguishable from the
HLA-A,B,C heavy chains. The single 12,000-dalton light chain precipitated with OKT6 antibody was shown to be distinct from
beta 2-microglobulin by its pI. The two light chains precipitated with Leu-6 antibody were resolved by charge into
beta 2-microglobulin and the more basic 12,000-dalton
peptide identical to that precipitated with OKT6. In addition to
beta 2-microglobulin, the latter component (presumably beta t) was also found in the light-chain fraction precipitated from the thymocytes with a
monoclonal antibody recognizing the framework of
HLA-A,B,C heavy chains. Using chymotryptic
peptide mapping, no polymorphism was detected among the heavy chains of the T6
antigen isolated from thymocytes of four individuals. All three
monoclonal antibodies failed to precipitate murine TL from
ASL1 leukemia cell lysates. Similarly, none of the six monoclonal and two conventional anti-TL
antibodies reacted with T6. Although a high degree of homology was found by
peptide map analysis among the TL molecules encoded by the Tlaa, Tlad and Tlae alleles, a comparison between their
peptide maps and that of T6 revealed no similarity. Despite previous suggestions that T6 is homologous to murine TL, the present biochemical studies do not support this hypothesis.