An αβ T‐cell receptor (αβTCR)/hemagglutinin (HA) peptide/human leukocyte antigen (HLA)‐DR1 complex was stabilized by flexibly linking the HA peptide with the human HA1.7 αβTCR, to increase the local concentration of the interacting proteins once the peptide has been loaded onto the major histocompatibility complex (MHC) molecule. The structure of the complex, determined by X‐ray crystallography, has a binding mode similar to that of the human B7 αβTCR on a pMHCI molecule. Twelve of the 15 MHC residues contacted are at the same positions observed earlier in class I MHC/peptide/TCR complexes. One contact, to an MHC loop outside the peptide‐binding site, is conserved and specific to pMHCII complexes. TCR gene usage in the response to HA/HLA‐DR appears to conserve charged interactions between three lysines of the peptide and acidic residues on the TCR.

The α/β T cell receptor (TCR) HA1.7 specific for the hemagglutinin (HA) antigen peptide from influenza A virus is HLA-DR1 restricted but cross-reactive for the HA peptide presented by the allo-major histocompatibility complex (MHC) class II molecule HLA-DR4. We report here the structure of the HA1.7/DR4/HA complex, determined by X-ray crystallography at a resolution of 2.4 Å. The overall structure of this complex is very similar to the previously reported structure of the HA1.7/DR1/HA complex. Amino acid sequence differences between DR1 and DR4, which are located deep in the peptide binding groove and out of reach for direct contact by the TCR, are able to indirectly influence the antigenicity of the pMHC surface by changing the conformation of HA peptide residues at position P5 and P6. Although TCR HA1.7 is cross-reactive for HA presented by DR1 and DR4 and tolerates these conformational differences, other HA-specific TCRs are sensitive to these changes. We also find a dependence of the width of the MHC class II peptide-binding groove on the sequence of the bound peptide by comparing the HA1.7/DR4/HA complex with the structure of DR4 presenting a collagen peptide. This structural study of TCR cross-reactivity emphasizes how MHC sequence differences can affect TCR binding indirectly by moving peptide atoms.

and lum, loaded onto MHC molecules, and presented on the Howard Hughes Medical Institute cell surface to the surveillance system of circulating Cambridge, Massachusetts 02138 CTL. TCR recognition of such peptide/MHCI complexes usually triggers a cytotoxic response killing the virally infected or otherwise abnormal cell. On class II MHC