Human natural killer cell receptor functions and their implication in diseases

Human Natural Killer Receptors, Co-Receptors, and Their Ligands

In the last 20 years, the study of human natural killer (NK) cells has moved from the first molecular characterizations of very few receptor molecules to the identification of a plethora of receptors displaying surprisingly divergent functions. We have contributed to the description of inhibitory receptors and their signaling pathways, important in fine regulation in many cell types, but unknown until their discovery in the NK cells. Inhibitory function is central to regulating NK-mediated cytolysis, with different molecular structures evolving during speciation to assure its persistence. More recently, it has become possible to characterize the NK triggering receptors mediating natural cytotoxicity, unveiling the existence of a network of cellular interactions between effectors of both natural and adaptive immunity. This unit reviews the contemporary history of molecular studies of receptors and ligands involved in NK cell function, characterizing the ligands of the triggering receptor and the mechanisms for finely regulating their expression in pathogen-infected or tumor cells. © 2018 by John Wiley & Sons, Inc.

CD8+ NK cells are predominant in chimpanzees, characterized by high NCR expression and cytokine production, and preserved in chronic HIV-1 infection

HIV-1 infection in humans results in an early and progressive NK cell dysfunction and an accumulation of an "anergic" CD56- CD16+ NK subset, which is characterised by low natural cytotoxicity receptor expression and low cytokine producing capacity. In contrast to humans, chimpanzee NK cells do not display a distinguishable CD56(bright) and CD56(dim) subset but, as shown here, could be subdivided into functionally different CD8+ and CD8- subsets. The CD8+ NK cells expressed significantly higher levels of triggering receptors including NKp46 and, upon in vitro activation, produced more IFN-gamma, TNF-alpha and CD107 than their CD8- counterparts. In addition, chimpanzee CD8- NK cells had relatively high levels of HLA-DR expression, suggestive of an activated state. Killing inhibitory receptors were expressed only at low levels; however, upon in vitro stimulation, they were up-regulated in CD8+ but not in CD8- NK cells and were functionally capable of inhibiting NKp30-triggered killing. In contrast to HIV-1-infected humans, infected chimpanzees maintained their dominant CD8+ NK cell population, with high expression of natural cytotoxicity receptors.

Human natural killer receptors, co-receptors, and their ligands

In the last 20 years, the study of human natural killer (NK) cells has moved from the first molecular characterizations of very few receptor molecules to the identification of a plethora of receptors displaying surprisingly divergent functions. Our laboratory has contributed to the description of inhibitory receptors and their signaling pathways, important in fine regulation in many cell types, but unknown until their discovery in the NK cells. Inhibitory function is central to regulating NK-mediated cytolysis, with different molecular structures evolving during speciation to assure its persistence. Only in the last ten years has it become possible to characterize the NK triggering receptors mediating natural cytotoxicity, leading to an appreciation of the existence of a cellular interaction network between effectors of both natural and adaptive immunity. This report reviews the contemporary history of molecular studies of receptors and ligands involved in NK cell function, characterizing the ligands of the triggering receptor and the mechanisms for finely regulating their expression in pathogen-infected or tumor cells.

Natural killer cell receptors

Natural killer (NK) cells are an important arm of the innate immune response that are directly involved in the recognition and lysis of virus-infected and tumor cells. Such function is under the control of a complex array of germline-encoded receptors able to deliver either inhibitory or activating signals. The majority of inhibitory receptors expressed by NK cells are major histocompatibility complex (MHC) class I-specific and display clonal and stochastic distribution on the cell surface. Thus, a given NK cell expresses at least one self class I inhibitory receptor. Under normal conditions, the strength of inhibitory signals delivered by multiple interactions always overrides the activating signals, resulting in NK cell self-tolerance. Under certain pathological conditions, such as viral infections or tumor transformation, the delicate balance of inhibition versus activation is broken, resulting in downregulation or loss of MHC class I expression. In general, the degree of inhibition induced by class I-specific receptors is proportional to the amount of these molecules on the cell surface. Thus, in transformed cells, this inhibition can be overridden by the triggering signal cascades, leading to cell activation. The majority of triggering receptors expressed by NK cells belong to the multichain immune recognition receptor (MIRR) family and use separate signal-transducing polypeptides similar to those used by other immune receptors such as the T-cell antigen receptor, the B-cell antigen receptor and other receptors expressed by myeloid cells. Inhibitory receptors are not members of the MIRR family but they are relevant for a better understanding the exquisite equilibrium and regulatory crosstalk between positive and negative signals.

Differential NKp30 Inducibility in Chimpanzee NK Cells and Conserved NK Cell Phenotype and Function in Long-Term HIV-1-Infected Animals

HIV-1 infection in chimpanzees, the closest human relative, rarely leads to disease progression. NK cells contribute to the shaping of adaptive immune responses in humans and show perturbed phenotype and function during HIV-1 infection. In this study, we provide full phenotypic, molecular, and functional characterization for triggering molecules (NKp46, NKp30 NKp80, and NKG2D) on Pan troglodytes NK cells. We demonstrate that, in this AIDS-resistant species, relevant differences to human NK cells involve NKp80 and particularly NKp30, which is primarily involved in NK-dendritic cell interactions. Resting peripheral chimpanzee NK cells have low or absent NKp30 molecule expression due to posttranscriptional regulation and increase its levels upon in vitro activation. Following long-standing HIV-1 infection, peripheral NK cells in chimpanzees have conserved triggering receptor expression and display moderate phenotypic and functional decreases only once activated and cultured in vitro. These data suggest that one of the keys to successful lentivirus control may reside in part in a different regulation of NK cell-triggering receptor expression.