| Following activation, CD8+ T cells express numerous natural killer cell receptors (NKR) that are capable of augmenting or diminishing cytotoxic T cell function. One such receptor is CD94/NKG2A, whose ligand is HLA-E (Qa-1b in mice). Until recently, CD94/NKG2A+, CD8+ T cells were studied within the context of chronic human disease. Results from these studies suggested that chronically stimulated CD8+ T cells expressed CD94/NKG2A to protect the host from potential immunopathology. Using normal human donors and acute murine infection models, we show that CD94/NKG2A is expressed on CD8+ T cells following infection and that receptor expression is not correlated with loss of CTL function. We also describe the binding motif of HLA-E, defining the amino acid requirements for peptide binding and subsequent interaction with CD94/NKG2A.; HLA-E is the primary ligand for CD94/NKG2A inhibitory receptors expressed on NK cell and T cells. HLA-E preferentially assembles with a homologous set of peptides derived from the leader sequence of class Ia molecules, but its capacity to bind and present other peptides remains to be fully explored. The peptide-binding motif of HLA-E was investigated by folding HLA-E in vitro in the presence of peptide libraries derived from a nonameric leader peptide sequence randomized at individual anchor positions. A high degree of selectivity was observed at four of five total anchor positions, with preference for amino acids present in HLA-E-binding peptides from class Ia leader sequences. Selectivity was also observed at the non-anchor P5 position, with preference for positively charged amino acids, suggesting that electrostatic interactions involving the P5 side chain may facilitate assembly of HLA-E peptide complexes. The observed HLAE peptide-binding motif was strikingly similar to that previously identified for the murine class Ib molecule, Qa-1. Experiments with HLA-E tetramers bearing peptides substituted at non-anchor positions demonstrated that P5 and P8 are primary contact residues for interaction with CD94/NKG2 receptors. A conservative replacement of Arg for Lys at P5 completely abrogated binding to CD94/NKG2. Despite conservation of peptide binding specificity in HLA-E and Qa-1, cross-species tetramer staining experiments demonstrated that the interaction surfaces on CD94/NKG2 and the class Ib ligands have diverged between primates and rodents. (Abstract shortened by UMI.)... |
