Molecular evidence has been obtained for a novel monomorphic HLA class II molecule distinct from HLA-DP/DQ/DR using a panel of lymphoblastoid cells which include HLA-loss mutants. The expression of this molecule was investigated using monomorphic affinitypurified mouse monoclonal antibodies (mAbs), including one of the IgG2a subclass designated EDU-1. This antibody reacts strongly in a cell-binding radioimmunoassay with HLA-DR and -DQ loss mutants derived from a lymphoblastoid parental cell. The EDU-1 mAb also reacted with a local panel of homozygous Epstein-Barr virustransformed cell lines. The reactive molecules were further detected on allostimulated T-cell clones and various leukemic cells including those of myeloid origin which lack surface expression of HLA-DQ molecules. Thus the class II molecule described in this study corresponds to a monomorphic HLA class II determinant expressed on a variety of cells of different origin and HLA phenotypes. Moreover, this antigen structure is distinct from that of HLA-DP/DQ/DR as shown by direct immunoprecipitation, serial immunodepletion experiments, and twodimensional gel electrophoresis. The molecule could be specified by new class II genes between DP and DQ. An alternative explanation for the genetic basis of the novel molecule is the existence of isotypic associations between alpha and beta chains of various class II molecules (DP, DX, DZ, and DO) but not DR and DQ as the mutant cells tested lack the latter genes. identified and characterized (Shacl~elford et al. 1982, Trowsdale et al. 1985). All are heterodimers consisting of noncovalently coupled transmembrane alpha and beta polypeptides of approximate relative mass (Mr) 34 000 and 29 000, respectively. These have been defined using cellular (see Kissmeyer-Nielsen 1975), serological (see Bodmer et al. 1977), biochemical (Crumpton et al. 1984), and DNA cloning techniques (Korman et al. 1985, Larhammar et al. 1982, Long et al. 1983). In addition to the genes coding for the HLA-DP/-DQ/-DR products, additional loci, designated DX~ and DXยข (Okada et al. 1985), DO n (Tonnelle et al. 1985) and DZ~ (Spielman et al. 1984), have been located between the DQ and DP subregions. There are several hundred kilobases of DNA between DP and DQ which may include as yet additional undiscovered class II genes. Although mRNAs for the DO and DZ genes have been found, it is not yet clear whether these loci code for cell surface-expressed products (Trowsdale and Kelly 1985). Additional HLA class II complexity of heterodimers has been recently shown. At least two different mechanisms may increase the class II repertoire: (a) transassociations of alpha and beta chains of homologous loci in heterozygous individuals (Charron et al. 1984) and (b) interisotypic associations of DR~ and DQยข chains in lymphoblastoid cells (Lotteau et al. 1987).
The present paper reports molecular evidence for a novel monomorphic HLA class II molecule (provisionally termed DN) distinct from HLA-DP/-DQ/-DR.