The MT4 allodeterminant is borne on an HLA-DS molecule on DR5 cells

Human HLA-DR molecules have been shown to be structurally homologous to the murine I-E subregion molecules by amino acid sequence analysis. Recent studies have demonstrated the isolation of an I-A subregion-homologous molecule (HLA-DS) from human B-cell lines with the rabbit antiserum RbO3, made against a marmoset I-A-like Ia molecule. Previous work from our laboratory has demonstrated that the DR5 homozygous lymphoblastoid cell line Swei expresses at least two different Ia alpha chains and four different Ia beta chains, which associate to form four distinct human Ia molecules, alpha 1 beta 2, alpha 1 beta 3, alpha 2 beta 1, and alpha 2 beta 4, and that the alpha 2 beta 1 molecule bears the allodeterminants MB3 and MT4. To determine whether the MT4-bearing alpha 2 beta 1 molecule was an HLA-DS molecule, the alpha 2 beta 1 molecule reactive with an anti-MT4 alloserum was compared with the Ia molecule reactive with the rabbit xenoantiserum RbO3 by two-dimensional gel electrophoresis and sequential immunoprecipitation. The alpha 2 chain and the RbO3-reactive alpha chain yielded essentially similar spot patterns. The beta 1-chain spot pattern was a subset of the RbO3-reactive beta-chain spot pattern. Sequential immunoprecipitation indicated that RbO3 removed all molecules reactive with MGH88B. These results indicate that on DR5 cells the allosera-reactive alpha 2 beta 1 molecule, which bears MT4, is an HLA-DS molecule.