Modeling the interactions of a peptide-major histocompatibility class I ligand with its receptors. II. Cross-reaction between a monoclonal antibody and two αβ T cell receptors

The recombinant antibody, pSAN13.4.1, has a unique T cell like specificity; it binds an Influenza Hemagglutinin octapeptide (Ha255−262) in an MHC (H-2Kk)-restricted manner, and a detailed comparison of the fine specificity of pSAN13.4.1 with the fine specificity of two Ha255−262-specific, H-2Kk-restricted T cell hybridomas has supported this contention. A three-dimensional model of pSAN13.4.1 has been derived by homology modeling techniques. Subsequently, the structure of the pSAN13.4.1 antibody in complex with the antigenic Ha-Kk ligand was derived after a flexible and automated docking of the MHC-peptide pair into the Fab combining site. Interestingly, the most energetically favored binding mode shows numerous analogies to the recently determined recognition of class I MHC-peptide complexes by αβ T cell receptors (TCRs). The pSAN13.4.1 also binds diagonally across the MHC binding groove but is more deeply anchored to the peptide-MHC (pep/MHC) ligand than TCRs, notably through numerous interactions of its heavy chain. The present model accounts well for the experimentally determined binding affinity of a set of 144 single amino acid substituted Ha analogues and the observed shared specificity between the pSAN antibody and two different T cell receptors for the Ha-Kk antigenic ligand. Analogies and differences between Fab and TCR recognition are explained by dissecting the binding role of each chain of the immune receptors as well as the contribution of all peptide amino acids.