The P3N within the immunodominant influenza virus-specific D b NP 366 epitope is a partially solvent exposed, and non-prominent for TCR binding, residue that is predominantly buried within the MHC cleft [21, 23] . The NPN3A mutation leads to both decreased recruitment of CD8 + T cells and a narrowed clonotype selection profile within Vb8.3 and Vb9 regions. Structurally, the mutation leads to a loss of a number of contacts between the NPN3A peptide and the MHC-I molecule, including a contact with the gate-keeper residue at position 155, and unaltered stability of the H-2D b -NPN3A complex. The fact that the NPN3A mutation affects contacts with the MHC-I at His155, known to play an important role in TCR-pMHC structures in general, is likely to indirectly compromise the TCR recognition. By loosing the bond with the Asn3, His155 may gain more flexibility and thus be inappropriately placed for the subsequent TCR ligation onto the NPN3A peptide. Alternatively, it is also possible that a small part of the solvent-exposed head group of the Asn3 residue in the wt NP 366 peptide might, to some extent, be directly interacting with the TCR following ligation. Surprisingly, despite the loss of several contacts between NPN3A peptide and H-2D b , the stability of the peptide-MHC-I complex remains constant for both NP 366 and NPN3A. This suggests that the Asn3 as a secondary anchor does not play an important role in stabilizing the peptide within the MHC-I. The structural basis for the diminished recruitment of D b NPN3Aspecific CD8 + T cells is thus likely to rest in the way that the partially-solvent exposed residue contacts MHC-I and modifies TCR ligation.