Inhibitory receptor signaling destabilizes immunological synapse formation in primary NK cells

Human natural killer cells: Form, function, and development

Human natural killer (NK) cells are innate lymphoid cells that mediate important effector functions in the control of viral infection and malignancy. Their ability to distinguish "self" from "nonself" and lyse virally infected and tumorigenic cells through germline-encoded receptors makes them important players in maintaining human health and a powerful tool for immunotherapeutic applications and fighting disease. This review introduces our current understanding of NK cell biology, including key facets of NK cell differentiation and the acquisition and execution of NK cell effector function. Further, it addresses the clinical relevance of NK cells in both primary immunodeficiency and immunotherapy. It is intended to provide an up-to-date and comprehensive overview of this important and interesting innate immune effector cell subset.

Peptide: MHC-based DNA vaccination strategy to activate natural killer cells by targeting killer cell immunoglobulin-like receptors

BACKGROUND

Natural killer (NK) cells are increasingly being recognized as agents for cancer immunotherapy. The killer cell immunoglobulin-like receptors (KIRs) are expressed by NK cells and are immunogenetic determinants of the outcome of cancer. In particular, KIR2DS2 is associated with protective responses to several cancers and also direct recognition of cancer targets in vitro. Due to the high homology between activating and inhibitory KIR genes to date, it has been challenging to target individual KIR for therapeutic benefit.

METHODS

A novel KIR2DS2-targeting therapeutic peptide:MHC DNA vaccine was designed and used to immunize mice transgenic for KIR genes (KIR-Tg). NK cells were isolated from the livers and spleens of vaccinated mice and then analyzed for activation by flow cytometry, RNA profiling and cytotoxicity assays. In vivo assays of NK cell function using a syngeneic cancer model (B16 melanoma) and an adoptive transfer model for human hepatocellular carcinoma (Huh7) were performed.

RESULTS

Injecting KIR-Tg mice with the vaccine construct activated NK cells in both liver and spleens of mice, with preferential activation of KIR2DS2-positive NK cells. KIR-specific activation was most marked on the CD11b+CD27+ mature subset of NK cells. RNA profiling indicated that the DNA vaccine upregulated genes associated with cellular metabolism and downregulated genes related to histone H3 methylation, which are associated with immune cell maturation and NK cell function. Vaccination led to canonical and cross-reactive peptide:MHC-specific NK cell responses. In vivo, DNA vaccination led to enhanced antitumor responses against B16F10 melanoma cells and also enhanced responses against a tumor model expressing the KIR2DS2 ligand HLA-C*0102.

CONCLUSION

We show the feasibility of a peptide-based KIR-targeting vaccine strategy to activate NK cells and hence generate functional antitumor responses. This approach does not require detailed knowledge of the tumor peptidomes nor HLA matching with the patient. It therefore offers a novel opportunity for targeting NK cells for cancer immunotherapy.

Sialic Acids and Their Influence on Human NK Cell Function

Sialic acids are sugars with a nine-carbon backbone, present on the surface of all cells in humans, including immune cells and their target cells, with various functions. Natural Killer (NK) cells are cells of the innate immune system, capable of killing virus-infected and tumor cells. Sialic acids can influence the interaction of NK cells with potential targets in several ways. Different NK cell receptors can bind sialic acids, leading to NK cell inhibition or activation. Moreover, NK cells have sialic acids on their surface, which can regulate receptor abundance and activity. This review is focused on how sialic acids on NK cells and their target cells are involved in NK cell function.

Advanced Materials and Devices for the Regulation and Study of NK Cells

Natural Killer (NK) cells are innate lymphocytes that contribute to immune protection by cytosis, cytokine secretion, and regulation of adaptive responses of T cells. NK cells distinguish between healthy and ill cells, and generate a cytotoxic response, being cumulatively regulated by environmental signals delivered through their diverse receptors. Recent advances in biomaterials and device engineering paved the way to numerous artificial microenvironments for cells, which produce synthetic signals identical or similar to those provided by the physiological environment. In this paper, we review recent advances in materials and devices for artificial signaling, which have been applied to regulate NK cells, and systematically study the role of these signals in NK cell function.

PD-1 blocks lytic granule polarization with concomitant impairment of integrin outside-in signaling in the natural killer cell immunological synapse

BACKGROUND

The inhibitory receptor programmed cell death protein 1 (PD-1) is upregulated on a variety of immune cells, including natural killer (NK) cells, during chronic viral infection and tumorigenesis. Blockade of PD-1 or its ligands produces durable clinical responses with tolerable side effects in patients with a broad spectrum of cancers. However, the underlying molecular mechanisms of how PD-1 regulates NK cell function remain poorly characterized.

OBJECTIVE

We sought to determine the effect of PD-1 signaling on NK cells.

METHODS

PD-1 was overexpressed in CD16-KHYG-1 (a human NK cell line with both antibody-dependent cellular cytotoxicity through CD16 and natural cytotoxicity through NKG2D) cells and stimulated by exposing the cells to NK-sensitive target cells expressing programmed death ligand 1 (PD-L1).

RESULTS

PD-1 engagement by PD-L1 specifically blocked NK cell-mediated cytotoxicity without interfering with the conjugation between NK cells and target cells. Further examination showed that PD-1 signaling blocked lytic granule polarization in NK cells, which was accompanied by failure of integrin-linked kinase, a key molecule in the integrin outside-in signaling pathway, to accumulate in the immunological synapse after NK-target cell conjugation.

CONCLUSION

Our results suggest that NK cell cytotoxicity is inhibited by PD-1 engagement, which blocks lytic granule polarization to the NK cell immunological synapse with concomitant impairment of integrin outside-in signaling. This study provides novel mechanistic insights into how PD-1 inhibition disrupts NK cell function.

Actin retrograde flow controls natural killer cell response by regulating the conformation state of SHP-1

Natural killer (NK) cells are a powerful weapon against viral infections and tumor growth. Although the actin-myosin (actomyosin) cytoskeleton is crucial for a variety of cellular processes, the role of mechanotransduction, the conversion of actomyosin mechanical forces into signaling cascades, was never explored in NK cells. Here, we demonstrate that actomyosin retrograde flow (ARF) controls the immune response of primary human NK cells through a novel interaction between β-actin and the SH2-domain-containing protein tyrosine phosphatase-1 (SHP-1), converting its conformation state, and thereby regulating NK cell cytotoxicity. Our results identify ARF as a master regulator of the NK cell immune response. Since actin dynamics occur in multiple cellular processes, this mechanism might also regulate the activity of SHP-1 in additional cellular systems.

The Planar Lipid Bilayer System Serves as a Reductionist Approach for Studying NK Cell Immunological Synapses and Their Functions

The immunological synapse (IS) is the junction between an immune cell (e.g., a T or NK cell) and another cell (e.g., an antigen-presenting cell (APC), or a tumor cell). The formation of the IS is crucial for cell-mediated immunity, and as such, an understanding of both the composition of the IS and the factors that drive its formation are essential for understanding how and when NK cells eliminate susceptible target cells. The supported lipid bilayer (SLB) system is a highly effective tool for directly studying the IS. SLBs confer three main advantages: (1) they allow for synapse formation on a level horizontal surface, allowing for direct visualization of the IS under high resolution imaging systems, (2) they mimic the surface of a target cell by providing a fluid mosaic into which surface proteins can be embedded while permitting free motion in two dimensions, which is important for studying the dynamics of synapse formation, and (3) they allow investigators to determine the exact composition of the bilayer, thus in turn allowing them to answer very specific questions about the IS. It is our hope that this chapter will furnish readers with an awareness of the applications of the SLB system for studying the IS in NK cells, and also of a basic knowledge of how to use this system for themselves.

NK cell education via nonclassical MHC and non-MHC ligands

Natural killer (NK) cell education, a process for achieving functional maturation and self-tolerance, has been previously defined by the interaction between self-major histocompatibility complex class I (MHC-I) molecules and their specific inhibitory receptors. Over the past several years, growing evidence has highlighted the important roles of nonclassical MHC-I and non-MHC-I molecules in NK cell education. Herein, we review the current knowledge of NK cell education, with a particular focus on nonclassical MHC-I- and non-MHC-I-dependent education, and compare them with the classical MHC-I-dependent education theory. In addition, we update and extend this theory by presenting the 'Confining Model', discussing cis and trans characteristics, reassessing quantity and quality control, and elucidating the redundancy of NK cell education in tumor and virus infection.

Imaging of Cell-Cell Communication in a Vertical Orientation Reveals High-Resolution Structure of Immunological Synapse and Novel PD-1 Dynamics

The immunological synapse (IS) is one of the most pivotal communication strategies in immune cells. Understanding the molecular basis of the IS provides critical information regarding how immune cells mount an effective immune response. Fluorescence microscopy provides a fundamental tool to study the IS. However, current imaging techniques for studying the IS cannot sufficiently achieve high resolution in real cell-cell conjugates. In this study, we present a new device that allows for high-resolution imaging of the IS with conventional confocal microscopy in a high-throughput manner. Combining micropits and single-cell trap arrays, we have developed a new microfluidic platform that allows visualization of the IS in vertically "stacked" cells. Using this vertical cell pairing (VCP) system, we investigated the dynamics of the inhibitory synapse mediated by an inhibitory receptor, programed death protein-1, and the cytotoxic synapse at the single-cell level. In addition to the technique innovation, we have demonstrated novel biological findings by this VCP device, including novel distribution of F-actin and cytolytic granules at the IS, programed death protein-1 microclusters at the NK IS, and kinetics of cytotoxicity. We propose that this high-throughput, cost-effective, easy-to-use VCP system, along with conventional imaging techniques, can be used to address a number of significant biological questions in a variety of disciplines.

Imaging of Cell-Cell Communication in a Vertical Orientation Reveals High-Resolution Structure of Immunological Synapse and Novel PD-1 Dynamics

The immunological synapse (IS) is one of the most pivotal communication strategies in immune cells. Understanding the molecular basis of the IS provides critical information regarding how immune cells mount an effective immune response. Fluorescence microscopy provides a fundamental tool to study the IS. However, current imaging techniques for studying the IS cannot sufficiently achieve high resolution in real cell-cell conjugates. In this study, we present a new device that allows for high-resolution imaging of the IS with conventional confocal microscopy in a high-throughput manner. Combining micropits and single-cell trap arrays, we have developed a new microfluidic platform that allows visualization of the IS in vertically "stacked" cells. Using this vertical cell pairing (VCP) system, we investigated the dynamics of the inhibitory synapse mediated by an inhibitory receptor, programed death protein-1, and the cytotoxic synapse at the single-cell level. In addition to the technique innovation, we have demonstrated novel biological findings by this VCP device, including novel distribution of F-actin and cytolytic granules at the IS, programed death protein-1 microclusters at the NK IS, and kinetics of cytotoxicity. We propose that this high-throughput, cost-effective, easy-to-use VCP system, along with conventional imaging techniques, can be used to address a number of significant biological questions in a variety of disciplines.

NK cells after transplantation: friend or foe

Natural killer (NK) cells are effector cells of the innate immune system that can lyse target cells without prior sensitization and have an important role in host defense to pathogens and transformed cells. A balance between negative and positive signals transmitted via germ line-encoded inhibitory and activating receptors controls the function of NK cells. Although the concept of "missing-self" would suggest that NK cells could target foreign allografts, the prevailing dogma has been that NK cells are not active participants in the mechanisms that culminate in the rejection of solid organ allografts. Recent studies, however, challenge this conclusion and instead implicate NK cells in contributing to both graft rejection and tolerance to an allograft. In this review, we highlight recent studies with the goal of understanding the complex NK cell interactions that impact alloimmunity.