Cell surface expression of class II MHC proteins bound by a single peptide

On normal cells, the peptide-binding grooves of class II MHC proteins contain a wide spectrum of peptides. For some purposes, however, it would be helpful to have cells bearing class II proteins engaged by only one peptide species. In an attempt to make such cells we constructed a gene for a MHC class II β-chain, IAβ^b, covalently linked to a peptide, Eα 52-68, which is known to bind to the peptide-binding groove of IA^b. This gene, together with the gene for IAα^b, was transfected into B lymphoma cells and fibroblasts. The IA^b-Eα complex was expressed on the surfaces of these cells where it could be recognized by a mAb and T cells specific for IA^b plus Eα 52-68. Most of the peptide on fibroblasts remained covalently attached to the IA^b β-chain, but the covalent linker and/or peptide were degraded to some extent on B lymphoma cells. Nearly all of the IA^b expressed by transfected fibroblasts was occupied by the Eα peptide. Of 16 IA^b- reactive T cell hybridomas, only 3 could respond to the IA^b-Eα complex on fibroblasts, confirming the idea that recognition of MHC may often involve recognition of the peptides bound to the MHC as well.