Natural killer (NK) cells express many newly identified activating and inhibitory receptors that upon engagement by cognate ligands on target tumor cells regulate NK cell antitumor activity. Recently, several paired NK cell receptor families that include receptors with similar binding specificities but opposite function have been defined. The expression of most important activating receptors, natural killer group 2D (NKG2D), natural cytotoxic receptors (NCR), DNAX accessory molecule-1 (DNAM1) and activating killer cell immunoglobulin-like receptors (KAR) is often decreased, while the expression of most prominent inhibitory NK cell receptors, killer cell inhibitory immunoglobulin-like receptors (KIR) and CD94/NKG2A, may occasionally be increased in malignancies. These data indicate that impaired NK cell antitumor response results from NK cell receptor alterations induced by suppressive factors in the tumor microenvironment, including cytokines, growth factors, enzymes and metabolites, as well as by chronic NK cell receptor engagement by the tumor. The established alterations in NK cell receptor expression in cancer patients represent potential disease biomarkers and may aid in choosing therapies that upregulate activating or block inhibitory receptor function. Accumulating knowledge of NK cell biology has been helpful in creating novel therapeutic approaches that by release from tumor-influenced immunosuppression potentiate NK cell activity in cancer patients.

Topic Editor Dr. Monteiro is the co-founder of Inatherys. Topic Editor Dr. Zarrin declares no conflict of interest with regard to the Research Topic theme.

Natural Killer (NK) cells were discovered ca 1975, as the first group of lymphoid cells that were neither T cells nor B cells. Since then, the dissection of the biology of NK cells has been growing exponentially with many seminal discoveries from the identification of MHC class I-specific inhibitory receptors to the discovery of receptor-ligand pairs involved in NK cell activation and to the manipulation of NK cells in cancer. In this research topic, we asked a group of thought leaders in NK cell biology to review recent advances in their origins and biology, and their roles in cancer, infection and inflammation. Together, these 25 articles provide a timely survey of NK cells as critical immunologic components of health and disease. They will hopefully prompt further dialogue and developments in basic and translational immunology.

Natural killer (NK) cells were classically defined as being anti-viral and anti-tumoral large innate lymphocytes with primarily a cytotoxic role. Recent studies have greatly expanded the roles and functions of these NK cells to include immunoregulatory mechanisms, hematopoiesis, and tissue repair and regeneration. With the diverse array of roles these cells can have, the likelihood of unique subsets and subpopulations of NK cells that serve specific functions has increased. NK cells can express a wide repertoire of both activating and inhibitory receptors that could lead to specific identifying markers of the unique functional subsets of NK cells. One family of receptors that could aid in discerning these subsets include the inhibitory C-type lectin, Ly49, family of receptors (found in mice) or the functional analogue in humans, the killer cell immunoglobulin receptors (KIRs). Inhibitory receptors within these families have different binding affinities to specific MHC class I haplotypes, which suggest that different functional populations of NK cells can exist based on the spectrum of potential binding affinities to self. An idea was proposed concerning this affinity to self, which stated that NK cells with high binding affinity to self were the highly active and cytotoxic population of NK cells analogous to positive selection of thymocytes in the thymus. In contrast, the population of cells with low affinity to self was hyporesponsive. This hypothesis is known as either licensing, education, or arming. We hypothesize that the idea of licensing goes beyond a simple on and off switch, but rather confers unique functional characteristics to the NK cell subsets expressing these different inhibitory receptors. The licensed population is what is considered the prototypical NK cell with direct anti-viral and anti-tumor functionality and high levels of IFNγ production. In line with this cytotoxic role, it also serves as an immunosuppressive immune cell population by directly lysing activated T cells and regulating the immune system. This population of NK cells is thus the effector/suppressor (E/S) population. Contrastingly, the unlicensed NK cells play a unique role in promoting the adaptive immune response and serving as a direct link between the innate and adaptive immune system. This population of NK cells preferentially traffics to the lymph nodes and produces high levels of GM-CSF, thus aiding in dendritic cell expansion and activation of T cells. Additionally, this population is demonstrated to traffic to the sites of tissue injury and damage and produce IL-22, aiding in the tissue regenerative process after either virally related tissue injury or radiation based injury. Thus, the unlicensed population of NK cells serves as a helper/repair population. Therefore, we propose that the concept of licensing and self-recognition can be utilized to identify functionally distinct subpopulations of NK cells in a number of immunological responses and challenges.

Since their discovery NK cells have come out as potential tools to fight cancer and viruses. This finding early urged different groups to study the mechanisms governing NK cell function. The identification of the MHC-I-specific inhibitory receptors (i.e. KIRs, NKG2A and certain Ly49 molecules) allowed defining rather rapidly how NK cells could avoid self-aggression and how they could be directed towards targets that were forced, by viral infection or tumor transformation, to down-regulate MHC-I expression. In a second time, also the repertoire of surface activating receptors addressing NK cytotoxicity towards tumors and pathogens was mostly defined. In spite of the first findings, however, most recent studies may suggest that NK cells and their receptors might not have been evolved to kill tumor targets and, perhaps, they might have been only partially influenced, in their evolution, by the need of recognizing viruses. Indeed certain NK receptors known to activate NK cell cytotoxicity (NKp30, DNAM-1, NKp80) can also participate at regulatory interactions occurring between NK and myeloid cells. In addition, a peculiar NK cell subset which intensively populate decidua during the first trimester of pregnancy, through the engagement of specific receptors and the interaction with decidual DC, produce chemokines and pro-angiogenic cytokines, and induce Tregs. Thus, in this context, NK cells favor decidua vascularization and development of the (semiallogeneic) foetus in a tolerant environment. Viruses have nevertheless played an important role in shaping the NK cell receptor repertoire. Several studies have unveiled clues of the evolutionary struggle between these pathogens and NK cells. Different NK receptors, including NKp46, NKp30, NKp44, NKG2D, NKG2C, Ly49, and certain KIRs have been demonstrated to recognize virus-encoded or virus-induced ligands. The expression of TLR specifically recognizing microbial products, together with the unexpected role of KIR3DL2 in shuttling these products to TLR-containing endosomes have also been documented in NK cells. On the other side, different viral immune evasion molecules have been shown to interfere with the expression of ligands for T or NK cell activating receptors. In addition, viral infections can occur in the reproductive stage of life cycle, and may represent a serious threat for the species propagation. Thus the control of viruses, together with the maintenance of foetus during pregnancy, should represent major evolutionary forces in shaping NK-receptors. Along this line, the NK-mediated control of tumors should not be under the same evolutionary pressure, as tumors mostly appear later in the life cycle, and the recognition of tumor-encoded ligands may be less efficient (as the NK cell receptors might have not been selected for such aim). This may be the reason why, although displaying strong antitumor activity in vitro, NK cells could hardly contain tumor burden in vivo. In addition the pathogen-driven evolution of NK cell function may also favor the role of NK cells in the insurgence of immune-mediated diseases. This research topic will collect contributions that may clarify the relationships between the evolution of the NK receptors and their role in an efficient recognition of viruses and tumor cells or in immune-mediated diseases.

Recognition and killing of aberrant, infected or tumor targets by Natural Killer (NK) cells is mediated by positive signals transduced by activating receptors upon engagement of ligands on target surface. These stimulatory pathways are counterbalanced by inhibitory receptors that raise NK cell activation threshold through negative antagonist signals. While regulatory effects are necessary for physiologic control of autoimmune aggression, they may restrain the ability of NK cells to activate against disease. Overcoming this barrier to immune surveillance, multiple approaches to enhance NK-mediated responses are being investigated since two decades. Propelled by considerable advances in the understanding of NK cell biology, these studies are critical for effective translation of NK-based immunotherapy principles into the clinic. In humans, dominant inhibitory signals are transduced by Killer Immunoglobulin Like Receptors (KIR) recognizing cognate HLA class I on target cells. Conversely, KIR recognition of “missing self-HLA” - due to HLA loss or HLA/ KIR mismatch - triggers NK-mediated tumor rejection. Initially observed in murine transplant models, these antitumor effects were later found to have important implications for the clinical outcome of haplotype-mismatched stemcell transplantation. Here, donor NK subsets protect against acute myeloid leukemia (AML) relapse through missing self recognition of donor HLA-C allele groups (C1 or C2) and/or Bw4 epitope. These studies were subsequently extended by trials investigating the antileukemia effects of adoptively transferred haplotype-mismatched NK cells in non-transplant settings. Other mechanisms have been found to induce clinically relevant NK cell alloreactivity in transplantation, e.g., post-reconstitution functional reversal of anergic NK cells. More recently, activating KIR came into the spotlight for their potential ability to directly activate donor NK cells through in vivo recognition of HLA or other ligands. Novel therapeutic monoclonal antibodies (mAb) may optimize NK-mediated effects. Examples include obinutuzumab (GA101), a glyco-engineered anti-CD20 mAb with increased affinity for the FcγRIIIA receptor, enhancing antibody-dependent cellular cytotoxicity; lirilumab (IPH2102), a first-in-class NK-specific checkpoint inhibitor, blocking the interaction between the major KIR and cognate HLA-C antigens; and elotuzumab (HuLuc63), a humanized monoclonal antibody specific for SLAMF7, whose anti-myeloma therapeutic effects are partly due to direct activation of SLAMF7-expressing NK cells. In addition to conventional antibodies, NK cell-targeted bispecific (BiKEs) and trispecific (TriKEs) killer engagers have also been developed. These proteins elicit potent effector functions by binding target ligands (e.g., CD19, CD22, CD30, CD133, HLA class II, EGFR) on one arm and NK receptors on the other. An additional innovative approach to direct NK cell activity is genetic reprogramming with chimeric antigen receptors (CAR). To date, primary NK cells and the NK92 cell line have been engineered with CAR specific for antigens expressed on multiple tumors. Encouraging preclinical results warrant further development of this approach. This Research Topic welcomes contributions addressing mechanisms of NK-mediated activation in response to disease as well as past and contemporary strategies to enhance NK mediated reactivity through control of the interactions between NK receptors and their ligands.

Our understanding of the function of natural killer (NK) cells has dramatically changed in recent years. The discovery of NK receptors specific for MHC class I molecules, and the study of the role of co-stimulatory and adhesion molecules have led to an understanding of how NK cells recognize tumor and virally infected cells that have lost expression of MHC class I molecules or have altered distribution of normal cell surface molecules. Such recognition events lead to intracellular signals which can be either stimulatory or inhibitory. This book provides an insight into how NK cells develop, how they learn to distinguish altered cells from normal cells, and into their biological role in controlling infections and tumors.