Natural killer cells and cancer: regulation by the killer cell Ig-like receptors (KIR)

Structure/function of human killer cell immunoglobulin-like receptors: lessons from polymorphisms, evolution, crystal structures and mutations

Stimulation or tolerance of natural killer (NK) cells is achieved through a cross-talk of signals derived from cell surface activating and inhibitory receptors. Killer cell immunoglobulin-like receptors (KIR) are a family of highly polymorphic activating and inhibitory receptors that serve as key regulators of human NK cell function. Distinct structural domains in different KIR family members determine function by providing docking sites for ligands or signalling proteins. Here, we review a growing body of literature that has identified important structural elements on KIR that contribute to function through studies of engineered mutants, natural polymorphic sequence variants, crystal structure data and the conservation of protein sequences throughout primate evolution. Extensive natural polymorphism is associated with both human KIR and their ligands, MHC class I (HLA-A, -B and -C) molecules, and numerous studies have demonstrated associations between inheritance of certain combinations of KIR and HLA genes and susceptibility to several diseases, including viral infections, autoimmune disorders and cancers. In addition, certain KIR/HLA combinations can influence pregnancy and the outcome of haematopoietic stem cell transplantation. In view of the significant regulatory influences of KIR on immune function and human health, it is essential to fully understand the impacts of these polymorphic sequence variations on ligand recognition, expression and function of the receptor.

Coevolution of killer cell Ig-like receptors with HLA-C to become the major variable regulators of human NK cells

Interactions between HLA class I and killer cell Ig-like receptors (KIRs) diversify human NK cell responses. Dominant KIR ligands are the C1 and C2 epitopes of MHC-C, a young locus restricted to humans and great apes. C1- and C1-specific KIRs evolved first, being present in orangutan and functionally like their human counterparts. Orangutans lack C2 and C2-specific KIRs, but have a unique C1+C2-specific KIR that binds equally to C1 and C2. A receptor with this specificity likely provided the mechanism by which C2-KIR interaction evolved from C1-KIR while avoiding a nonfunctional intermediate, that is, either orphan receptor or ligand. Orangutan inhibitory MHC-C-reactive KIRs pair with activating receptors of identical avidity and specificity, contrasting with the selective attenuation of human activating KIRs. The orangutan C1-specific KIR reacts or cross-reacts with all four polymorphic epitopes (C1, C2, Bw4, and A3/11) recognized by human KIRs, revealing their structural commonality. Saturation mutagenesis at specificity-determining position 44 demonstrates that KIRs are inherently restricted to binding just these four epitopes, either individually or in combination. This restriction frees most HLA-A and HLA-B variants to be dedicated TCR ligands, not subject to conflicting pressures from the NK cell and T cell arms of the immune response.

The stage dependent changes in NK cell activity and the expression of activating and inhibitory NK cell receptors in melanoma patients

BACKGROUND

As NK cell antitumor activity is regulated by the balance between numerous activating and inhibitory receptors the aim of this study was to analyze the changes in these receptors expression as well as the differences in their association with NK cell activity in melanoma patients in different clinical stages.

MATERIALS AND METHODS

Forty-three melanoma patients in the three different clinical stages and 17 healthy controls were analyzed for NK cell activity by the standard (51)Cr radioactive assay, as well as for the percentage and absolute number of CD3-CD56+ NK cells and their subsets and the expression of degranulation marker CD107a, activating NKG2D and CD161, and inhibitory KIR, CD158a, and CD158b receptors on CD3-CD56+ NK cells by flow cytometry. IFN-γ and TNF-α PBL production were evaluated by enzyme-linked immunosorbent assay (ELISA).

RESULTS

We show that contrary to nonmetastatic, metastatic melanoma patients have significantly impaired NK cell activity, lower CD107a expression, as well as decreased production of IFN-γ and TNF-α. Furthermore, these melanoma patients have a significant decrease in the expression of activating NKG2D receptor, which positively correlates with NK cytotoxicity, and a significant increase in the expression of inhibitory CD158b on CD3-CD56+ NK cells compared with healthy controls.

CONCLUSIONS

In this study, we show that metastatic melanoma patients have significant changes in NK cell activity and the expression of activating as well as inhibitory NK cell receptors. These results indicate that only in this patient group, the blocking of inhibitory or enhancing of activating NK cell receptor expression may promote NK cell-mediated cytolysis.