Introduction

            Class I MHC molecules have at least two important functions.  One of these is to present peptides derived from intracellular parasites to CD8+ T cells.  Another more recently discovered function is to regulate the activity of NK cells.  This latter is achieved through the expression on NK cells of receptors for class I molecules, the largest families of which are the KIR family in humans and the Ly49 family in mice.  In addition, both humand and mice express the CD94 family of heterodimeric class I receptors that are composed of an invariant CD94 chain linked by a disulphide bond to one of a small set of NKG2 molecules.

            CD94/NKG2 receptors recognize the non-classical class I molecule HLA-E in humans [Braud et al. Nature 391, 795, 1998)], and its homologue in mice, Qa1[Salcedo et al. Eur. J. Immunol. 28, 4356, 1998; Vance et al. J. Exp. Med. 188, 1841, 1998].  Under normal circumstances a large proportion of the surface expression of each of these molecules is controlled by the availability of a single peptide derived from the leader sequence of certain classical class I molecules.  This was first revealed by the pioneering work of Forman and colleagues which demonstrated the existence of different classes of Qa1b-specific alloreactive clones [Soloski et al. Immunol. Rev. 147, 67, 1995].  Type 1 and type 2 clones were both TAP-dependent, implying that they recognized a form of Qa1 whose expression was controlled by cytosol-derived peptides.  However, whereas type 2 clones would recognize Qa1b in all strains of mice, type 1 clones failed to recognize Qa1b in H-2k strains.  Elegant biochemical and genetic studies revealed that the strain specificity of type 1 clones was due to their inability to recognize Qa1 molecules containing peptides derived from the leader sequences of certain classical class I molecules [Aldrich et al. Cell 79, 649, 1994].  In non-H-2k strains  the class I leader sequence peptide that binds to Qa1 has the sequence AMAPRTLLL and is known as the Qdm peptide, whereas in H-2k strains it has the sequence AMVPRTLLL and is designated Qdm-k.  Peptide elution experiments demonstrated directly that Qdm and Qdm-k peptides not only associate with Qa1 molecules but are extraordinarily abundant, perhaps accounting for the majority of all Qa1-associated peptide.

            More recent studies have demonstrated that the stable assembly and surface expression of HLA-E is similarly dependent on the presence within the cell of peptides, homologous to Qdm, derived from the leader sequences of certain HLA-A, B, and C molecules.  By using a soluble tetrameric form of HLA-E associated with such peptides, Braud et al. [Nature 391, 795, 1998] were able to demonstrate that HLA-E bound to CD94/NKG2 receptors on NK cells.  Because NK cells bearing inhibitory CD94/NKG2 receptors are unable to lyse target cells that express both an appropriate “donor” class I molecule and a functional TAP complex, it is believed that an important function of CD94+ NK cells is to detect and eliminate defective or infected cells that lack proper expression of either class I molecules or TAP.

             In adults, about 50% of NK cells express CD94/NKG2 receptors that bind soluble Qa1-Qdm complexes.  Studying the interaction of these cells with target cells bearing Qa1 molecules is complicated by the fact that Qa1 receptor-expressing (Qa1R+) adult NK cells also express a variable selection of Ly49 receptors that can deliver inhibitory or activatory signals following interaction with classical class I molecules.  To avoid this problem we have taken advantage of the fact that NK cells derived from immature early thymic progenitors in vitro are deficient in the expression of Ly49 receptors [Manoussaka et al. J. Immunol. 158, 112, 1997], lack detectable receptors for classical class I molecules [Toomey et al. J. Immunol. 163, 3176, 1999], yet express CD94/NKG2 receptors for Qa1 [Toomey et al. J. Immunol. 163, 3176, 1999; Sivakumar et al. J. Immunol. 162, 6976, 1999].    Because Qa1R are acquired in a stochastic manner during the development of NK cells in vitro [Toomey et al. J. Immunol. 163, 3176, 1999], it is possible to select clones or lines that are composed predominantly of Qa1R- or Qa1R+ cells.  In this study we have used such selected NK lines to examine in detail the circumstances under which target cells can be protected from lysis by Qa1R+ NK cells.