| HLA-E has a very specialized role in cell recognition by [[natural killer cells]] (NK cells).<ref name="pmid9486650">{{cite journal | vauthors = Braud VM, Allan DS, O'Callaghan CA, Söderström K, D'Andrea A, Ogg GS, Lazetic S, Young NT, Bell JI, Phillips JH, Lanier LL, McMichael AJ | title = HLA-E binds to natural killer cell receptors CD94/NKG2A, B and C | journal = Nature | volume = 391 | issue = 6669 | pages = 795–9 | date = Feb 1998 | pmid = 9486650 | doi = 10.1038/35869 }}</ref> HLA-E binds a restricted subset of peptides derived from [[signal peptides]] of classical MHC class I molecules, namely HLA-A, B, C, G.<ref name="pmid9174606">{{cite journal | vauthors = Braud V, Jones EY, McMichael A | title = The human major histocompatibility complex class Ib molecule HLA-E binds signal sequence-derived peptides with primary anchor residues at positions 2 and 9 | journal = European Journal of Immunology | volume = 27 | issue = 5 | pages = 1164–9 | date = May 1997 | pmid = 9174606 | doi = 10.1002/eji.1830270517 }}</ref> These peptides are released from the membrane of the [[endoplasmic reticulum]] (ER) by the [[signal peptide peptidase]] and trimmed by the cytosolic [[proteasome]].<ref name="pmid11714810">{{cite journal | vauthors = Lemberg MK, Bland FA, Weihofen A, Braud VM, Martoglio B | title = Intramembrane proteolysis of signal peptides: an essential step in the generation of HLA-E epitopes | journal = Journal of Immunology | volume = 167 | issue = 11 | pages = 6441–6 | date = Dec 2001 | pmid = 11714810 | doi = 10.4049/jimmunol.167.11.6441 }}</ref><ref name="pmid12821659">{{cite journal | vauthors = Bland FA, Lemberg MK, McMichael AJ, Martoglio B, Braud VM | title = Requirement of the proteasome for the trimming of signal peptide-derived epitopes presented by the nonclassical major histocompatibility complex class I molecule HLA-E | journal = The Journal of Biological Chemistry | volume = 278 | issue = 36 | pages = 33747–52 | date = Sep 2003 | pmid = 12821659 | doi = 10.1074/jbc.M305593200 }}</ref> Upon transport into the ER lumen by the [[transporter associated with antigen processing]] (TAP), these peptides bind to a peptide binding groove on the HLA-E molecule.<ref name="pmid9427624">{{cite journal | vauthors = Braud VM, Allan DS, Wilson D, McMichael AJ | title = TAP- and tapasin-dependent HLA-E surface expression correlates with the binding of an MHC class I leader peptide | journal = Current Biology | volume = 8 | issue = 1 | pages = 1–10 | date = Jan 1998 | pmid = 9427624 | doi = 10.1016/S0960-9822(98)70014-4 }}</ref> This allows HLA-E to assemble correctly and to be expressed on the [[cell membrane|cell surface]]. NK cells recognize the HLA-E+peptide complex using the heterodimeric inhibitory receptor [[CD94]]/[[NKG2]]A/B/C.<ref name="pmid9486650"/> When CD94/NKG2A or CD94/NKG2B is engaged, it produces an inhibitory effect on the cytotoxic activity of the NK cell to prevent cell lysis. However, binding of HLA-E to CD94/NKG2C results in NK cell activation. This interaction has been shown to trigger expansion of NK cell subsets in antiviral responses.<ref>{{cite journal | vauthors = Rölle A, Pollmann J, Ewen EM, Le VT, Halenius A, Hengel H, Cerwenka A | title = IL-12-producing monocytes and HLA-E control HCMV-driven NKG2C+ NK cell expansion | journal = The Journal of Clinical Investigation | volume = 124 | issue = 12 | pages = 5305–16 | date = Dec 2014 | pmid = 25384219 | doi = 10.1172/JCI77440 | pmc=4348979}}</ref> | | HLA-E has a very specialized role in cell recognition by [[natural killer cells]] (NK cells).<ref name="pmid9486650">{{cite journal | vauthors = Braud VM, Allan DS, O'Callaghan CA, Söderström K, D'Andrea A, Ogg GS, Lazetic S, Young NT, Bell JI, Phillips JH, Lanier LL, McMichael AJ | title = HLA-E binds to natural killer cell receptors CD94/NKG2A, B and C | journal = Nature | volume = 391 | issue = 6669 | pages = 795–9 | date = Feb 1998 | pmid = 9486650 | doi = 10.1038/35869 }}</ref> HLA-E binds a restricted subset of peptides derived from [[signal peptides]] of classical MHC class I molecules, namely HLA-A, B, C, G.<ref name="pmid9174606">{{cite journal | vauthors = Braud V, Jones EY, McMichael A | title = The human major histocompatibility complex class Ib molecule HLA-E binds signal sequence-derived peptides with primary anchor residues at positions 2 and 9 | journal = European Journal of Immunology | volume = 27 | issue = 5 | pages = 1164–9 | date = May 1997 | pmid = 9174606 | doi = 10.1002/eji.1830270517 }}</ref> These peptides are released from the membrane of the [[endoplasmic reticulum]] (ER) by the [[signal peptide peptidase]] and trimmed by the cytosolic [[proteasome]].<ref name="pmid11714810">{{cite journal | vauthors = Lemberg MK, Bland FA, Weihofen A, Braud VM, Martoglio B | title = Intramembrane proteolysis of signal peptides: an essential step in the generation of HLA-E epitopes | journal = Journal of Immunology | volume = 167 | issue = 11 | pages = 6441–6 | date = Dec 2001 | pmid = 11714810 | doi = 10.4049/jimmunol.167.11.6441 }}</ref><ref name="pmid12821659">{{cite journal | vauthors = Bland FA, Lemberg MK, McMichael AJ, Martoglio B, Braud VM | title = Requirement of the proteasome for the trimming of signal peptide-derived epitopes presented by the nonclassical major histocompatibility complex class I molecule HLA-E | journal = The Journal of Biological Chemistry | volume = 278 | issue = 36 | pages = 33747–52 | date = Sep 2003 | pmid = 12821659 | doi = 10.1074/jbc.M305593200 }}</ref> Upon transport into the ER lumen by the [[transporter associated with antigen processing]] (TAP), these peptides bind to a peptide binding groove on the HLA-E molecule.<ref name="pmid9427624">{{cite journal | vauthors = Braud VM, Allan DS, Wilson D, McMichael AJ | title = TAP- and tapasin-dependent HLA-E surface expression correlates with the binding of an MHC class I leader peptide | journal = Current Biology | volume = 8 | issue = 1 | pages = 1–10 | date = Jan 1998 | pmid = 9427624 | doi = 10.1016/S0960-9822(98)70014-4 }}</ref> This allows HLA-E to assemble correctly and to be expressed on the [[cell membrane|cell surface]]. NK cells recognize the HLA-E+peptide complex using the heterodimeric receptor [[CD94]]/[[NKG2]]A/B/C.<ref name="pmid9486650"/> When CD94/NKG2A or CD94/NKG2B is engaged, it produces an inhibitory effect on the cytotoxic activity of the NK cell to prevent cell lysis. However, binding of HLA-E to CD94/NKG2C results in NK cell activation. This interaction has been shown to trigger expansion of NK cell subsets in antiviral responses.<ref>{{cite journal | vauthors = Rölle A, Pollmann J, Ewen EM, Le VT, Halenius A, Hengel H, Cerwenka A | title = IL-12-producing monocytes and HLA-E control HCMV-driven NKG2C+ NK cell expansion | journal = The Journal of Clinical Investigation | volume = 124 | issue = 12 | pages = 5305–16 | date = Dec 2014 | pmid = 25384219 | doi = 10.1172/JCI77440 | pmc=4348979}}</ref> |