NK cells need a license to kill

The issue of how tolerance to self is achieved in natural killer (NK) cells is a complex one. Now, Wayne Yokoyama's group have shown that NK cells are 'licensed' for functional competence by interactions between an inhibitory NK-cell receptor and a self MHC class I molecule.

The authors used a target-cell-free system to assess the function of individual NK cells by crosslinking the activating receptor NK1.1. Large numbers of NK cells from wild-type mice produced cytoplasmic interferon-γ (IFN-γ), but few NK cells from mice deficient in MHC class I molecules could do so. This NK-cell-intrinsic defect indicated that interaction with host MHC class I molecules is important for the functional maturation of NK cells, a process that the authors term licensing.

By investigating the role of various LY49 molecules at the surface of NK cells from hosts with different MHC haplotypes, the authors found that, paradoxically, the ability of NK cells to produce cytokines and lyse target cells depends on the expression of self-MHC-class-I-specific, inhibitory NK-cell receptors belonging to the LY49 family. To examine this in more detail, the authors generated a transgenic mouse using a single-chain trimer consisting of a peptide, β₂-microglobulin and the H2-K^b heavy chain, which binds only LY49C. These mice were backcrossed to gene-knockout mice to produce mice that did not express any MHC class I molecules except H2-K^b. In these mice, the expression of H2-K^b alone was sufficient to license LY49C⁺ NK cells for IFN-γ production, whereas LY49A⁺ NK cells were not licensed by H2-K^b.

Next, the authors used a bicistronic vector containing green fluorescent protein (GFP) and LY49A — which binds a different MHC class I molecule than does LY49C — to transduce haematopoietic stem cells. GFP⁺ NK cells produced twofold more IFN-γ than GFP⁻ NK cells, showing that transduced LY49A expression conferred a positive effect on NK-cell function in hosts that express its cognate MHC class I molecule. Expression of LY49A molecules lacking a cytoplasmic domain or lacking the immunoreceptor tyrosine-based inhibitory motif (ITIM) was unable to increase NK-cell activity, showing that LY49A can directly confer the licensing signal on NK cells through the ITIM. However, examination of NK cells from mice lacking SHP1 (SRC-homology-2-domain-containing protein tyrosine phosphatase 1), which binds the ITIM that mediates inhibitory signalling, showed that licensing is distinct from the ITIM-mediated inhibition by SHP1 in effector responses.

So, this study defines a mechanism for NK-cell tolerance: NK cells that express a self-MHC-class-I-specific inhibitory receptor become licensed for functional competence, whereas those lacking such a receptor do not become licensed and are functionally inert. Licensing resembles the positive selection of T cells in the thymus but is distinct in that it requires the engagement of a self-MHC-specific, germline-encoded receptor that has inhibitory function in the effector response, indicating that other germline-encoded, ITIM-containing receptors might have roles in the development of other leukocytes.