A recombinant single‐chain human class II MHC molecule (HLA‐DR1) as a covalently linked heterotrimer of α chain, β chain, and antigenic peptide, with immunogenicity in vitro and reduced affinity for bacterial superantigens

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Abstract
Major histocompatibility complex (MHC) class II molecules bind to numerous peptides and display these on the cell surface for T cell recognition. In a given immune response, receptors on T cells recognize antigenic peptides that are a minor population of MHC class II‐bound peptides. To control which peptides are presented to T cells, it may be desirable to use recombinant MHC molecules with covalently bound antigenic peptides. To study T cell responses to such homogenous peptide‐MHC complexes, we engineered an HLA‐DR1 cDNA coding for influenza hemagglutinin, influenza matrix, or HIV p24 gag peptides covalently attached via a peptide spacer to the N terminus of the DR1 β chain. Co‐transfection with DR α cDNA into mouse L cells resulted in surface expression of HLA‐DR1 molecules that reacted with monoclonal antibodies (mAb) specific for correctly folded HLA‐DR epitopes. This suggested that the spacer and peptide did not alter expression or folding of the molecule. We then engineered an additional peptide spacer between the C terminus of a truncated β chain (without transmembrane or cytoplasmic domains) and the N terminus of full‐length DR α chain. Transfection of this cDNA into mouse L cells resulted in surface expression of the entire covalently linked heterotrimer of peptide, β chain, and α chain with the expected molecular mass of approximately 66 kDa. These single‐chain HLA‐DR1 molecules reacted with mAb specific for correctly folded HLA‐DR epitopes, and identified one mAb with [MHC + peptide] specificity. Affinity‐purified soluble secreted single‐chain molecules with truncated α chain moved in electrophoresis as compact class II MHC dimers. Cell surface two‐chain or single‐chain HLA‐DR1 molecules with a covalent HA peptide stimulated HLA‐DR1‐restricted HA‐specific T cells. They were immunogenic in vitro for peripheral blood mononuclear cells. The two‐chain and single‐chain HLA‐DR1 molecules with covalent HA peptide had reduced binding for the bacterial superantigens staphylococcal enterotoxin A and B and almost no binding for toxic shock syndrome toxin‐1. The unique properties of these engineered HLA‐DR1 molecules may facilitate our understanding of the complex nature of antigen recognition and aid in the development of novel vaccines with reduced superantigen binding.