Human influenza-specific adaptive NK cells mediate potent HLA-E-restricted responses against antigenically distinct influenza strains

Abstract

Abstract Novel influenza vaccine approaches to achieve cross-protection against a broad range of antigenically distinct strains are needed. Beyond their ability to eliminate infected cells without the need for prior sensitization, natural killer (NK) cells have been shown to mediate virus-specific recall responses. Notably, exposure to influenza antigens induces protective influenza-specific memory NK cells in mice, but evidence of influenza-specific memory NK cells in humans is still lacking. Using intracellular cytokine staining, we found that NK cells from 45% of adults can robustly respond to nucleoprotein (NP) from at least 2 heterosubtypic influenza strains completely independent of T cell help. To further determine if a subset of NK cells can specifically react to NP peptide pools from distinct strains, we clonally expanded individual NK cells (NKCL) and measured H1N1, H2N2 and H3N2 NP-specific killing by 33 NKCL from 6 donors. 63% of NKCL showed positive responses against the 2009 H1N1 pandemic strain (2%–57% specific lysis). Strikingly, half of H1N1 NP-specific NKCL also displayed cytotoxicity against either H2N2 or H3N2 (9%–100% specific lysis) or against both H2N2 and H3N2 strains (5%–11% specific lysis). Potent killing by NKCLs was associated with high cell surface expression of the activating NKG2C receptor, a ligand for HLA-E. Accordingly, we identified an HLA-E-restricted NP epitope that triggers activation of influenza-specific NK cells. Collectively, these data provide the first mechanistic evidence of influenza-specific memory NK cells in humans and suggest influenza-specific responses mediated by NK cells may have the potential to be harnessed to enhance influenza vaccine-induced cross-protection.