A monomoprhic
monoclonal antibody (LA45 antibody) reactive with "a new activation-induced surface structure on human T lymphocytes" (
LA45 antigen) that resembled free class I heavy chains has recently been described (Schnabl, E., H. Stockinger, O. Majdic, H. Gaugitsch, I.J.D. Lindley, D. Maurer, A. Hajek-Rosenmayr, and W. Knapp. 1990. J. Exp. Med. 171:1431). This antibody was used to clone a class I-like heavy chain (LA45 gene) from the HUT 102
tumor cell, which paradoxically did not give rise to the
LA45 antigen on transfection into monkey COS cells. We show here that the LA45 gene is HLA-Aw66.2, a previously uncharacterized allele of the
HLA-A locus. The previously determined LA45 sequence differs from that of HLA-Aw66.2, from HUT 102, and the CR-B B cell line derived from the same individual as HUT 102 by substitution of
tryptophan for
serine at position 4 in the alpha 1 domain. Transfection of HLA-Aw66.2, and of a mutant of this gene with
serine 4 substituted for
tryptophan, into a human B cell line (C1R) both resulted in expression of the LA45
epitope. Furthermore, we find expression of the LA45
epitope on Epstein Barr virus-transformed B cell lines as well as
lectin-activated T cells, but not on long-term T cell lines or unstimulated peripheral blood T cells. The specificity of the LA45 antibody is polymorphic and the presence of the LA45
epitope is precisely correlated with the sequence
arginine,
asparagine (RN) at residues 62 and 63 of the helix of the alpha 1 domain. The LA45
epitope is broadly distributed, being associated with half the alleles of both
HLA-A and -B loci but none of the
HLA-C locus. All the results are consistent with the presence of pools of free
HLA-A and -B heavy chains at the surfaces of certain cell types but not others. Such molecules are probably responsible for the HLA-associated class I
alloantigens of
lectin-activated T cells. We hypothesize the free heavy chains result from dissociation of
beta 2-microglobulin from subpopulations of empty
HLA-A,B molecules, or molecules with weakly bound
peptides, that vary in size depending on cellular activation and
peptide supply.