Adhesion in NK cell function

EP3 enhances adhesion and cytotoxicity of NK cells toward hepatic stellate cells in a murine liver fibrosis model

Natural killer (NK) cells exhibit antifibrotic properties in liver fibrosis (LF) by suppressing activated hepatic stellate cell (HSC) populations. Prostaglandin E2 (PGE2) plays a dual role in innate and adaptive immunity. Here, we found that E-prostanoid 3 receptor (EP3) was markedly downregulated in NK cells from liver fibrosis mice and patients with liver cirrhosis. NK cell-specific deletion of EP3 aggravated hepatic fibrogenesis in mouse models of LF. Loss of EP3 selectively reduced the cytotoxicity of the CD27+CD11b+ double positive (DP) NK subset against activated HSCs. Mechanistically, deletion of EP3 impaired the adhesion and cytotoxicity of DP NK cells toward HSCs through modulation of Itga4-VCAM1 binding. EP3 upregulated Itga4 expression in NK cells through promoting Spic nuclear translocation via PKC-mediated phosphorylation of Spic at T191. Activation of EP3 by sulprostone alleviated CCL4-induced liver fibrosis in mice. Thus, EP3 is required for adhesion and cytotoxicity of NK cells toward HSCs and may serve as a therapeutic target for the management of LF.

Myosin 1g and 1f: A Prospective Analysis in NK Cell Functions

NK cells are contained in the ILC1 group; they are recognized for their antiviral and antitumor cytotoxic capacity; NK cells also participate in other immune response processes through cytokines secretion. However, the mechanisms that regulate these functions are poorly understood since NK cells are not as abundant as other lymphocytes, which has made them difficult to study. Using public databases, we identified that NK cells express mRNA encoding class I myosins, among which Myosin 1g and Myosin 1f are prominent. Therefore, this mini-review aims to generate a model of the probable participation of Myosin 1g and 1f in NK cells, based on information reported about the function of these myosins in other leukocytes.

Mechanosensation and Mechanotransduction in Natural Killer Cells

Natural killer (NK) cells are a main subset of innate lymphocytes that contribute to host immune protection against viruses and tumors by mediating target cell killing and secreting a wide array of cytokines. Their functions are finely regulated by a balance between activating and inhibitory receptors and involve also adhesive interactions. Mechanotransduction is the process in which physical forces sensed by mechanosensors are translated into chemical signaling. Herein, we report findings on the involvement of this mechanism that is mainly mediated by actin cytoskeleton, in the regulation of NK cell adhesion, migration, tissue infiltration and functions. Actin represents the structural basis for NK cell immunological synapse (NKIS) and polarization of secretory apparatus. NK-target cell interaction involves the formation of both uropods and membrane nanotubes that allow target cell interaction over long distances. Actin retrograde flow (ARF) regulates NK cell signaling and controls the equilibrium between activation versus inhibition. Activating NKIS is associated with rapid lamellipodial ARF, whereas lower centripetal actin flow is present during inhibitory NKIS where β actin can associate with the tyrosine phosphatase SHP-1. Overall, a better knowledge of mechanotransduction might represent a future challenge: Realization of nanomaterials tailored for NK cells, would be important to translate in vitro studies in in vivo new immunotherapeutic approaches.

A Developed NK-92MI Cell Line with Siglec-7neg Phenotype Exhibits High and Sustainable Cytotoxicity against Leukemia Cells

Altered sialic acid processing that leads to upregulation of cell surface sialylation is recognized as a key change in malignant tissue glycosylation. This cancer-associated hypersialylation directly impacts the signaling interactions between tumor cells and their surrounding microenvironment, especially the interactions mediated by immune cell surface sialic acid-binding immunoglobulin-like lectins (Siglecs) to relay inhibitory signals for cytotoxicity. First, we obtained a Siglec-7^neg NK-92MI cell line, NK-92MI-S7N, by separating a group of Siglec-7^neg cell population from an eight-month-long-term NK-92MI in vitro culture by fluorescence-activated cell sorting (FACS). The effect of Siglec-7 loss on NK-92MI-S7N cells was characterized by the cell morphology, proliferation, and cytotoxic activity via FACS, MTS assay, cytotoxic assay, and natural killer (NK) degranulation assay. We found the expression levels of Siglec-7 in NK-92MI were negatively correlated with NK cytotoxicity against leukemia cells. This NK-92MI-S7N cell not only shared very similar phenotypes with its parental cells but also possessed a high and sustainable killing activity. Furthermore, this Siglec-7^neg NK line was unexpectedly capable of eliminating a NK-92MI-resistant leukemia cell, THP-1, through enhancing the effector-target interaction. In this study, a NK cell line with high and sustainable cytotoxicity was established and this cell may provide a potential application in NK-based treatment for leukemia patients.

NKG2D is a Key Receptor for Recognition of Bladder Cancer Cells by IL-2-Activated NK Cells and BCG Promotes NK Cell Activation

Intravesical instillation of bacillus Calmette–Guérin (BCG) is used to treat superficial bladder cancer, either papillary tumors (after transurethral resection) or high-grade flat carcinomas (carcinoma in situ), reducing recurrence in about 70% of patients. Initially, BCG was proposed to work through an inflammatory response, mediated by phagocytic uptake of mycobacterial antigens and cytokine release. More recently, other immune effectors such as monocytes, natural killer (NK), and NKT cells have been suggested to play a role in this immune response. Here, we provide a comprehensive study of multiple bladder cancer cell lines as putative targets for immune cells and evaluated their recognition by NK cells in the presence and absence of BCG. We describe that different bladder cancer cells can express multiple activating and inhibitory ligands for NK cells. Recognition of bladder cancer cells depended mainly on NKG2D, with a contribution from NKp46. Surprisingly, exposure to BCG did not affect the immune phenotype of bladder cells nor increased NK cell recognition of purified IL-2-activated cell lines. However, NK cells were activated efficiently when BCG was included in mixed lymphocyte cultures, suggesting that NK activation after mycobacteria treatment requires the collaboration of various immune cells. We also analyzed the percentage of NK cells in peripheral blood of a cohort of bladder cancer patients treated with BCG. The total numbers of NK cells did not vary during treatment, indicating that a more detailed study of NK cell activation in the tumor site will be required to evaluate the response in each patient.

Balance between activating NKG2D, DNAM-1, NKp44 and NKp46 and inhibitory CD94/NKG2A receptors determine natural killer degranulation towards rheumatoid arthritis synovial fibroblasts

Rheumatoid arthritis (RA) is an autoimmune disease characterized by chronic inflammation and synovial hyperplasia leading to progressive joint destruction. Fibroblast-like synoviocytes (FLS) are central components of the aggressive, tumour-like synovial structure termed pannus, which invades the joint space and cartilage. A distinct natural killer (NK) cell subset expressing the inhibitory CD94/NKG2A receptor is present in RA synovial fluid. Little is known about possible cellular interactions between RA-FLS and NK cells. We used cultured RA-FLS and the human NK cell line Nishi, of which the latter expresses an NK receptor repertoire similar to that of NK cells in RA synovial fluid, as an in vitro model system of RA-FLS/NK cell cross-talk. We show that RA-FLS express numerous ligands for both activating and inhibitory NK cell receptors, and stimulate degranulation of Nishi cells. We found that NKG2D, DNAM-1, NKp46 and NKp44 are the key activating receptors involved in Nishi cell degranulation towards RA-FLS. Moreover, blockade of the interaction between CD94/NKG2A and its ligand HLA-E expressed on RA-FLS further enhanced Nishi cell degranulation in co-culture with RA-FLS. Using cultured RA-FLS and the human NK cell line Nishi as an in vitro model system of RA-FLS/NK cell cross-talk, our results suggest that cell-mediated cytotoxicity of RA-FLS may be one mechanism by which NK cells influence local joint inflammation in RA.

Overcoming chemoresistance of small-cell lung cancer through stepwise HER2-targeted antibody-dependent cell-mediated cytotoxicity and VEGF-targeted antiangiogenesis

Small-cell lung cancer (SCLC) easily recurs with a multidrug resistant phenotype. However, standard therapeutic strategies for relapsed SCLC remain unestablished. We found that human epidermal growth factor receptor 2 (HER2) is not only expressed in pretreated human SCLC specimens, but is also upregulated when HER2-positive SCLC cells acquire chemoresistance. Trastuzumab induced differential levels of antibody-dependent cell-mediated cytotoxicity (ADCC) to HER2-positive SCLC cells. Furthermore, as a mechanism of the differential levels of ADCC, we have revealed that coexpression of intracellular adhesion molecule (ICAM)-1 on SCLC cells is essential to facilitate and accelerate the trastuzumab-mediated ADCC. Although SN-38-resistant SCLC cells lacking ICAM-1 expression were still refractory to trastuzumab, their in vivo growth was significantly suppressed by bevacizumab treatment due to dependence on their distinctive and abundant production of vascular endothelial growth factor. Collectively, stepwise treatment with trastuzumab and bevacizumab is promising for the treatment of chemoresistant SCLC.

Cytotoxicity of CD56(bright) NK cells towards autologous activated CD4+ T cells is mediated through NKG2D, LFA-1 and TRAIL and dampened via CD94/NKG2A

In mouse models of chronic inflammatory diseases, Natural Killer (NK) cells can play an immunoregulatory role by eliminating chronically activated leukocytes. Indirect evidence suggests that NK cells may also be immunoregulatory in humans. Two subsets of human NK cells can be phenotypically distinguished as CD16⁺CD56^dim and CD16^dim/−CD56^bright. An expansion in the CD56^bright NK cell subset has been associated with clinical responses to therapy in various autoimmune diseases, suggesting an immunoregulatory role for this subset in vivo. Here we compared the regulation of activated human CD4⁺ T cells by CD56^dim and CD56^bright autologous NK cells in vitro. Both subsets efficiently killed activated, but not resting, CD4⁺ T cells. The activating receptor NKG2D, as well as the integrin LFA-1 and the TRAIL pathway, played important roles in this process. Degranulation by NK cells towards activated CD4⁺ T cells was enhanced by IL-2, IL-15, IL-12+IL-18 and IFN-α. Interestingly, IL-7 and IL-21 stimulated degranulation by CD56^bright NK cells but not by CD56^dim NK cells. NK cell killing of activated CD4⁺ T cells was suppressed by HLA-E on CD4⁺ T cells, as blocking the interaction between HLA-E and the inhibitory CD94/NKG2A NK cell receptor enhanced NK cell degranulation. This study provides new insight into CD56^dim and CD56^bright NK cell-mediated elimination of activated autologous CD4⁺ T cells, which potentially may provide an opportunity for therapeutic treatment of chronic inflammation.

SHP-2 expression negatively regulates NK cell function

Src homology region 2-containing protein tyrosine phosphatase-2 (SHP-2) is required for full activation of Ras/ERK in many cytokine and growth factor receptor signaling pathways. In contrast, SHP-2 inhibits activation of human NK cells upon recruitment to killer cell Ig-like receptors (KIR). To determine how SHP-2 impacts NK cell activation in KIR-dependent or KIR-independent signaling pathways, we employed knockdown and overexpression strategies in NK-like cell lines and analyzed the consequences on functional responses. In response to stimulation with susceptible target cells, SHP-2-silenced NK cells had elevated cytolytic activity and IFN-gamma production, whereas cells overexpressing wild-type or gain-of-function mutants of SHP-2 exhibited dampened activities. Increased levels of SHP-2 expression over this range significantly suppressed microtubule organizing center polarization and granzyme B release in response to target cells. Interestingly, NK-target cell conjugation was only reduced by overexpressing SHP-2, but not potentiated in SHP-2-silenced cells, indicating that conjugation is not influenced by physiological levels of SHP-2 expression. KIR-dependent inhibition of cytotoxicity was unaffected by significant reductions in SHP-2 levels, presumably because KIR were still capable of recruiting the phosphatase under these limiting conditions. In contrast, the general suppressive effect of SHP-2 on cytotoxicity and cytokine release was much more sensitive to changes in cellular SHP-2 levels. In summary, our studies have identified a new, KIR-independent role for SHP-2 in dampening NK cell activation in response to tumor target cells in a concentration-dependent manner. This suppression of activation impacts microtubule organizing center-based cytoskeletal rearrangement and granule release.

Minimal requirement for induction of natural cytotoxicity and intersection of activation signals by inhibitory receptors

Natural killer (NK) cells provide innate control of infected and neoplastic cells. Multiple receptors have been implicated in natural cytotoxicity, but their individual contribution remains unclear. Here, we studied the activation of primary, resting human NK cells by Drosophila cells expressing ligands for receptors NKG2D, DNAM-1, 2B4, CD2, and LFA-1. Each receptor was capable of inducing inside-out signals for LFA-1, promoting adhesion, but none induced degranulation. Rather, release of cytolytic granules required synergistic activation through coengagement of receptors, shown here for NKG2D and 2B4. Although engagement of NKG2D and 2B4 was not sufficient for strong target cell lysis, collective engagement of LFA-1, NKG2D, and 2B4 defined a minimal requirement for natural cytotoxicity. Remarkably, inside-out signaling induced by each one of these receptors, including LFA-1, was inhibited by receptor CD94/NKG2A binding to HLA-E. Strong inside-out signals induced by the combination of NKG2D and 2B4 or by CD16 could overcome CD94/NKG2A inhibition. In contrast, degranulation induced by these receptors was still subject to inhibition by CD94/NKG2A. These results reveal multiple layers in the activation pathway for natural cytotoxicity and that steps as distinct as inside-out signaling to LFA-1 and signals for granule release are sensitive to inhibition by CD94/NKG2A.

LFA-1 and CD2 synergize for the Erk1/2 activation in the Natural Killer (NK) cell immunological synapse

Natural killer (NK) cell recognition and formation of a conjugate with target cells, followed by intracellular signal pathway activation and degradation of cytolytic granules, are essential for NK cell cytotoxicity. In this study, NK92 cells were used to investigate synapse formation and subsequent signaling after binding to the target cell. The binding rate of the NK92-target cell was associated with NK92 cell cytotoxicity. Confocal results showed that adhesion molecules, LFA-1 (CD11a) and CD2, accumulated at the interface of the NK92-K562 contact. Ligation with K562 cells activated the Erk1/2 signal pathway of NK92 cells. The blocking of the NK-target conjugate by EDTA or anti-CD11a or/and anti-CD2 antibody decreased the phosphorylation of Erk1/2 and NK cell cytotoxicity. Inhibition of Erk1/2 phosphorylation by the chemical inhibitor U0126 suppressed the cytolytic activity of NK92 cells, but had no effect on NK-target conjugate formation. Thus, conjugate formation of the NK92-target cell was prerequisite to NK cell activation, and subsequent signal transduction was also required for NK cell cytotoxicity.

Signal transduction during natural killer cell activation

Understanding of transmembrane signaling during NK-cell activation has greatly expanded during the past few years. The discovery and characterization of novel triggering and inhibitory receptors have revealed the complexity of these processes. This unit focuses on receptor-initiated signaling pathways that modulate NK functions. Establishing the roles of different signaling pathways in NK cells is a crucial step in the design of therapeutic approaches for selective enhancement or suppression of NK-cell activation.

Human T-cell leukemia virus type 1 (HTLV-1) p12I down-modulates ICAM-1 and -2 and reduces adherence of natural killer cells, thereby protecting HTLV-1-infected primary CD4+ T cells from autologous natural killer cell-mediated cytotoxicity despite the reduction of major histocompatibility complex class I molecules on infected cells

Although natural killer (NK) cell-mediated control of viral infections is well documented, very little is known about the ability of NK cells to restrain human T-cell leukemia virus type 1 (HTLV-1) infection. In the current study we show that NK cells are unable to kill HTLV-1-infected primary CD4+ T cells. Exposure of NK cells to interleukin-2 (IL-2) resulted in only a marginal increase in their ability to kill HTLV-1-infected primary CD4+ T cells. This inability of NK cells to kill HTLV-1-infected CD4+ T cells occurred despite the down-modulation of major histocompatibility complex (MHC) class I molecules, one of the ligands for the major NK cell inhibitory receptor, by HTLV-1 p12(I) on CD4+ T cells. One reason for this diminished ability of NK cells to kill HTLV-1-infected cells was the decreased ability of NK cells to adhere to HTLV-1-infected cells because of HTLV-1 p12(I)-mediated down-modulation of intercellular adhesion molecule 1 (ICAM-1) and ICAM-2. We also found that HTLV-1-infected CD4+ T cells did not express ligands for NK cell activating receptors, NCR and NKG2D, although they did express ligands for NK cell coactivating receptors, NTB-A and 2B4. Thus, despite HTLV-1-mediated down-modulation of MHC-I molecules, HTLV-1-infected primary CD4+ T cells avoids NK cell destruction by modulating ICAM expression and shunning the expression of ligands for activating receptors.

Dissecting natural killer cell activation pathways through analysis of genetic mutations in human and mouse

Natural killer (NK) cell cytotoxicity is mediated by multiple germ line-encoded activating receptors that recognize specific ligands expressed by tumor cells and virally infected cells. These activating receptors are opposed by NK inhibitory receptors, which recognize major histocompatibility complex class I molecules on potential targets, raising the threshold for NK cell activation. Once an abnormal cell has been detected, NK cells are the sentinel source of cytolytic mediators, such as granzymes and perforins, as well as interferon-gamma, which can polarize the immune response to a T-helper 1 cell type. Activation signals are transmitted by adhesion-dependent pathways, immunoreceptor tyrosine-based activation motif (ITAM)-dependent pathways, DAP10 ITAM-independent pathways, and by signaling through immunoreceptor tyrosine-based switch motifs. These pathways activate downstream signaling partners to trigger NK cell cytotoxicity. Some of these downstream molecules are unique to the various pathways, and some of these molecules are shared. Because of the complexity of signals involved in NK cell-target cell interaction, the generation of mice with targeted mutations in signaling molecules involved in adhesion, activation, or inhibition is essential for a precise dissection of the mechanisms regulating NK cell effector functions. Here we review recent advances in the genetic analysis of the signaling pathways that mediate NK cell killing.

Cytolytic granule polarization and degranulation controlled by different receptors in resting NK cells

The relative contribution to cytotoxicity of each of the multiple NK cell activation receptors has been difficult to assess. Using Drosophila insect cells, which express ligands of human NK cell receptors, we show that target cell lysis by resting NK cells is controlled by different receptor signals for cytolytic granule polarization and degranulation. Intercellular adhesion molecule (ICAM)-1 on insect cells was sufficient to induce polarization of granules, but not degranulation, in resting NK cells. Conversely, engagement of the Fc receptor CD16 by rabbit IgG on insect cells induced degranulation without specific polarization. Lysis by resting NK cells occurred when polarization and degranulation were induced by the combined presence of ICAM-1 and IgG on insect cells. Engagement of receptor 2B4 by CD48 on insect cells induced weak polarization and no degranulation. However, coengagement of 2B4 and CD16 by their respective ligands resulted in granule polarization and cytotoxicity in the absence of leukocyte functional antigen-1-mediated adhesion to target cells. These data show that cytotoxicity by resting NK cells is controlled tightly by separate or cooperative signals from different receptors for granule polarization and degranulation.

NK cell activity in tuberculosis is associated with impaired CD11a and ICAM-1 expression: a regulatory role of monocytes in NK activation

Although the role of natural killer (NK) cells in mycobacterial infections is unclear, it has been postulated that they contribute to protective immunity through the production of interferon (IFN)-gamma. In this study, we evaluate the effect of interleukin (IL)-10, IL-15 and IL-18 on NK lytic activity through the expression of CD16, CD11a and CD69 molecules and the induction of IFN-gamma production in patients with tuberculosis (TB) and healthy individuals (N). Our results showed an impairment of NK lytic activity and a gradual down-regulation of costimulatory and adhesion molecules on NK cells which were dependent on the severity of the disease. NK lytic activity was increased by exogenous IL-15 and IL-18 in both TB and N, and by neutralization of endogenous IL-10 only in TB; IL-15 and IL-18 increased CD69 receptor expression, while anti-IL-10 up-regulated CD16 and CD11a expression in TB. Mycobacterium tuberculosis reduced the number of intracellular adhesion molecule (ICAM)-1(+) CD14(+) cells, but in the presence of IL-15, IL-18 and anti-IL-10 its expression was up-regulated. In cells from TB patients, the observed effects of IL-15 and IL-18 on NK function were not dependent on IL-10 modulation of the surface expression of activator/adhesion molecules. In the absence of monocytes, IL-10 activated NK cells, suggesting an indirect effect on their function. Furthermore, in TB patients the depletion of monocytes increased the production of IFN-gamma by NK cells. Therefore, monocytes from TB patients regulated the NK function involving IL-10 which, through an indirect mechanism, led to the down-regulation of costimulatory/adhesion molecules and/or IFN-gamma production.