NK cells are innate immune lymphocytes with potent effector functions against infected and tumor cells. NK cells integrate signals received through target cell ligand-mediated engagement of activation receptors with those from inhibitory receptor engagement by MHC class I ligands expressed on targets. Absence of MHC class I on target cells often leads to NK cell activation. This phenomenon, termed “missing-self,” allows NK cells to attack and eliminate cells with aberrantly low or absent expression of MHC class I (Kärre et al., 1986
), as with transformed or virally infected cells. Hence, NK cells use inhibitory receptors to assess the surface of self-tissues for MHC class I expression, providing a line of defense against pathogens and abnormal cell growth.
MHC class I molecules are also crucial to acquisition of effector function by NK cells in vivo as NK cells from MHC class I–deficient hosts are defective in natural killing (Bix et al., 1991
; Höglund et al., 1991
) and hyporesponsive to triggering through their activation receptors (Fernandez et al., 2005
; Kim et al., 2005
). Recent data obtained in MHC-sufficient hosts support the hypothesis that cognate interaction between inhibitory receptors and self-MHC is necessary for acquisition of effector function. For example, Ly49C+
NK cells, which bind a self-MHC I ligand (H2Kb
) in the H2b
haplotype of C57BL/6 mice, display more robust production of cytokines upon stimulation than NK cells expressing only Ly49A, which has no H2b
ligand (Kim et al., 2005
). This is most evident in a C57BL/6 transgenic (Tg) mouse expressing a single-chain trimer H2Kb
) molecule, consisting of the H2Kb
heavy chain covalently linked to β2
m) and the SIINFEKL peptide from ovalbumin. In mice where SCT-Kb
is the only MHC class I molecule expressed, i.e., SCT-Kb
Tg mouse on the Kb−/−
(triple KO; TKO) background, and Ly49C is the sole NK cell receptor capable of binding the SCT-Kb
molecule, only Ly49C+
NK cells display the licensed phenotype (Kim et al., 2005
). Thus, engagement of self–MHC-specific receptor “licenses” NK cells to be functionally competent to be triggered through their activation receptors.
Although there has been debate on the meaning of the term “licensing,” most groups now agree that the engagement of inhibitory receptors by self–MHC class I results in education of NK cells to become functionally competent (Anfossi et al., 2006
; Raulet and Vance, 2006
). Moreover, such education effects have also been observed with human NK cells via self-HLA recognition by killer immunoglobulin-like receptors (KIRs), which are related by convergent evolution to murine Ly49 receptors (Kelley et al., 2005
; Anfossi et al., 2006
; Yu et al., 2007
; Kim et al., 2008
). Thus, self–MHC class I engagement by NK cell inhibitory receptors appears to be a fundamental element in acquisition of NK cell effector function.
Conventional murine NK cells are thought to develop primarily, if not completely, in the BM where they progress through a series of stages en route to full maturation (Di Santo, 2006
; Kim et al., 2002
). After this process, they leave the BM and populate the peripheral tissues. Egress from the BM is generally accepted as a marker of mature conventional NK cells, as cells isolated from peripheral lymphoid tissues demonstrate effector function upon stimulation. We previously hypothesized licensing to be a developmental process because Ly49 receptors are first expressed at an immature stage during NK cell maturation in the BM (Kim et al., 2002
). Thereafter, developing NK cells undergo constitutive proliferation that appears to be influenced by MHC class I and the relevant Ly49 receptor (Kim et al., 2005
). However, it is not known if licensing can occur outside the context of development within the BM, or whether the unlicensed phenotype is fixed.
Herein, we performed adoptive transfer of peripheral, hyporesponsive NK cells from MHC class I–deficient donors to MHC class I–sufficient hosts, resulting in the generation of functional donor NK cells. Expression of a host MHC class I ligand specific for a donor cell inhibitory NK cell receptor was necessary for the observed gain of function, which is highly suggestive of licensing as a mechanism. We conclude that the unlicensed phenotype is not fixed in apparently mature peripheral NK cells, and that licensing may not be exclusively a developmental process in the BM.