Obinutuzumab-mediated high-affinity ligation of FcγRIIIA/CD16 primes NK cells for IFNγ production

SHIP1 and its role for innate immune regulation-Novel targets for immunotherapy

Phosphoinositide-3-kinase/AKT (PI3K/AKT) signaling plays key roles in the regulation of cellular activity in both health and disease. In immune cells, this PI3K/AKT pathway is critically regulated by the phosphoinositide phosphatase SHIP1, which has been reported to modulate the function of most immune subsets. In this review, we summarize our current knowledge of SHIP1 with a focus on innate immune cells, where we reflect on the most pertinent aspects described in the current literature. We also present several small-molecule agonists and antagonists of SHIP1 developed over the last two decades, which have led to improved outcomes in several preclinical models of disease. We outline these promising findings and put them in relation to human diseases with unmet medical needs, where we discuss the most attractive targets for immune therapies based on SHIP1 modulation.

Impact of SARS-CoV-2 vaccination on FcγRIIIA/CD16 dynamics in Natural Killer cells: relevance for antibody-dependent functions

Introduction

Natural Killer (NK) cells contribute to the protective effects of vaccine-induced antibodies thanks to the low affinity receptor for IgG, FcγRIIIA/CD16, whose aggregation leads to the killing of infected cells and IFNγ release, through which they potentiate adaptive immune responses.

Methods

Forty-seven healthy young individuals undergoing either homologous (ChAdOx1-S/ChAdOx1-S) or heterologous (ChAdOx1-S/BNT162B2) SARS-CoV-2 vaccination settings were recruited. Peripheral blood samples were collected immediately prior to vaccination and 8 weeks after the booster dose. The phenotypic and functional profile of NK cells was evaluated by flow cytometry at both time points. Serum samples were tested to evaluate circulating anti-Spike IgG levels and cytomegalovirus serostatus. CD16 F158V polymorphism was assessed by sequencing analysis.

Results

The downregulation of CD16 and the selective impairment of antibody-dependent cytotoxicity and IFNγ production in CD56dim NK population, persisting 8 weeks after boosting, were observed in heterologous, but not in homologous SARS-CoV-2 vaccination scheme. While the magnitude of CD16-dependent functions of the global CD56dim pool correlated with receptor levels before and after vaccination, the responsivity of NKG2C+ subset, that displays amplified size and functionality in HCMV+ individuals, resulted intrinsically insensitive to CD16 levels. Individual CD16 responsiveness was also affected by CD16F158V polymorphism; F/F low affinity individuals, characterized by reduced CD16 levels and functions independently of vaccination, did not show post-vaccinal functional impairment with respect to intermediate and high affinity ones, despite a comparable CD16 downregulation. Further, CD16 high affinity ligation conditions by means of afucosylated mAb overcame vaccine-induced and genotype-dependent functional defects. Finally, the preservation of CD16 expression directly correlated with anti-Spike IgG titer, hinting that the individual magnitude of receptor-dependent functions may contribute to the amplification of the vaccinal response.

Conclusion

This study demonstrates a durable downmodulation of CD16 levels and Ab-dependent NK functions after SARS-CoV-2 heterologous vaccination, and highlights the impact of genetic and environmental host-related factors in modulating NK cell susceptibility to post-vaccinal Fc-dependent functional impairment.

Principles and current clinical landscape of NK cell engaging bispecific antibody against cancer

Monoclonal antibody-based targeted therapies have greatly improved treatment options for patients by binding to the innate immune system. However, the long-term efficacy of such antibodies is limited by mechanisms of drug resistance. Over the last 50 years, with advances in protein engineering technology, more and more bispecific antibody (bsAb) platforms have been engineered to meet diverse clinical needs. Bispecific NK cell engagers (BiKEs) or tri-specific NK cell engagers (TriKEs) allow for direct targeting of immune cells to tumors, and therefore resistance and serious adverse effects are greatly reduced. Many preclinical and clinical trials are currently underway, depicting the promise of antibody-based natural killer cell engager therapeutics. In this review, we compile worldwide efforts to explore the involvement of NK cells in bispecific antibodies. With a particular emphasis on lessons learned, we focus on preclinical and clinical studies in malignancies and discuss the reasons for the limited success of NK-cell engagers against solid tumors, offering plausible new ideas for curing some advanced cancers shortly.

SUMOylation and related post-translational modifications in natural killer cell anti-cancer responses

SUMOylation is a reversible modification that involves the covalent attachment of small ubiquitin-like modifier (SUMO) to target proteins, leading to changes in their localization, function, stability, and interactor profile. SUMOylation and additional related post-translational modifications have emerged as important modulators of various biological processes, including regulation of genomic stability and immune responses. Natural killer (NK) cells are innate immune cells that play a critical role in host defense against viral infections and tumors. NK cells can recognize and kill infected or transformed cells without prior sensitization, and their activity is tightly regulated by a balance of activating and inhibitory receptors. Expression of NK cell receptors as well as of their specific ligands on target cells is finely regulated during malignant transformation through the integration of different mechanisms including ubiquitin- and ubiquitin-like post-translational modifications. Our review summarizes the role of SUMOylation and other related pathways in the biology of NK cells with a special emphasis on the regulation of their response against cancer. The development of novel selective inhibitors as useful tools to potentiate NK-cell mediated killing of tumor cells is also briefly discussed.

Weaponizing natural killer cells for solid cancer immunotherapy

Enhancing natural killer (NK) cell-based innate immunity has become a promising strategy for immunotherapy against hard-to-cure solid cancers. Monoclonal antibody (mAb) therapy has been used to activate NK-cell-mediated antibody-dependent cellular cytotoxicity (ADCC) towards solid cancers. Cancer cells, however, can subvert immunosurveillance using multiple immunosuppressive mechanisms, which may hamper NK cell ADCC. Mechanisms to safely enhance ADCC by NK cells, such as utilizing temporary inhibition of receptor endocytosis to increase antibody presentation from target to effector cells can now be used to enhance NK-cell-mediated ADCC against solid tumors. This review summarizes and discusses the recent advances in the field and highlights current and potential future use of immunotherapies to maximize the therapeutic efficacy of innate anticancer immunity.

Natural Killer Cells in Chronic Lymphocytic Leukemia: Functional Impairment and Therapeutic Potential

Immunotherapy approaches have advanced rapidly in recent years. While the greatest therapeutic advances so far have been achieved with T cell therapies such as immune checkpoint blockade and CAR-T, recent advances in NK cell therapy have highlighted the therapeutic potential of these cells. Chronic lymphocytic leukemia (CLL), the most prevalent form of leukemia in Western countries, is a very immunosuppressive disease but still shows significant potential as a target of immunotherapy, including NK-based therapies. In addition to their antileukemia potential, NK cells are important immune effectors in the response to infections, which represent a major clinical concern for CLL patients. Here, we review the interactions between NK cells and CLL, describing functional changes and mechanisms of CLL-induced NK suppression, interactions with current therapeutic options, and the potential for therapeutic benefit using NK cell therapies.

Role of Fc Core Fucosylation in the Effector Function of IgG1 Antibodies

The presence of fucose on IgG1 Asn-297 N-linked glycan is the modification of the human IgG1 Fc structure with the most significant impact on FcɣRIII affinity. It also significantly enhances the efficacy of antibody dependent cellular cytotoxicity (ADCC) by natural killer (NK) cells in vitro, induced by IgG1 therapeutic monoclonal antibodies (mAbs). The effect of afucosylation on ADCC or antibody dependent phagocytosis (ADCP) mediated by macrophages or polymorphonuclear neutrophils (PMN) is less clear. Evidence for enhanced efficacy of afucosylated therapeutic mAbs in vivo has also been reported. This has led to the development of several therapeutic antibodies with low Fc core fucose to treat cancer and inflammatory diseases, seven of which have already been approved for clinical use. More recently, the regulation of IgG Fc core fucosylation has been shown to take place naturally during the B-cell immune response: A decrease in α-1,6 fucose has been observed in polyclonal, antigen-specific IgG1 antibodies which are generated during alloimmunization of pregnant women by fetal erythrocyte or platelet antigens and following infection by some enveloped viruses and parasites. Low IgG1 Fc core fucose on antigen-specific polyclonal IgG1 has been linked to disease severity in several cases, such as SARS-CoV 2 and Dengue virus infection and during alloimmunization, highlighting the in vivo significance of this phenomenon. This review aims to summarize the current knowledge about human IgG1 Fc core fucosylation and its regulation and function in vivo, in the context of both therapeutic antibodies and the natural immune response. The parallels in these two areas are informative about the mechanisms and in vivo effects of Fc core fucosylation, and may allow to further exploit the desired properties of this modification in different clinical contexts.

(Auto)Antibody Responses Shape Memory NK Cell Pool Size and Composition

In vivo establishment and long-term persistence of a heterogeneous memory or an adaptive NK cell pool represents a functional adaptation to human cytomegalovirus (HCMV) infection in humans. Memory NK cells are commonly identified by lack of the FcεRIγ signalling chain, variably associated to the preferential but not completely overlapping expression of the HLA-E receptor NKG2C and CD57 maturation marker. Although characterized by selective hyperresponsiveness to IgG stimulation, the impact of the CD16/antibody interaction in regulating the establishment/maintenance and size, and in determining the relative abundance of this population, is still under investigation. Memory NK cell subset ex vivo profile and in vitro responsiveness to CD16 stimulation was evaluated in HCMV⁺ healthy donors and in patients affected by immune thrombocytopenia (ITP), an antibody-mediated autoimmune disease. We identified the FcεRIγ⁻ NKG2C⁺CD57⁺ memory NK cell subset, whose abundance is uniquely associated with anti-HCMV antibody levels in healthy seropositive donors, and which is significantly expanded in ITP patients. This fully mature memory subset robustly and selectively expands in vitro in response to mAb-opsonized targets or ITP-derived platelets and displays superior CD16-dependent IFNγ production. Our work identifies opsonizing antibodies as a host-dependent factor that shapes HCMV-driven memory NK cell compartment. We first demonstrate that chronic exposure to auto-antibodies contributes to the establishment/expansion of a highly specialized and unique memory NK cell subset with distinct CD16-dependent functional capabilities. We also identify the specific contribution of the lack of FcεRIγ chain in conferring to NKG2C⁺CD57⁺ memory cells a higher responsivity to CD16 engagement.

High Density of CD16+ Tumor-Infiltrating Immune Cells in Recurrent Ovarian Cancer Is Associated with Enhanced Responsiveness to Chemotherapy and Prolonged Overall Survival

Simple Summary

The late—and in most cases at an advanced stage—diagnosis of patients with ovarian cancer (OC) and the high recurrence rate make this malignant disease the most lethal among gynecological cancers. With a mortality-to-incidence ratio of 0.74, OC is a tumor with the fifth most frequent progression after esophageal cancer, liver cancer, pancreatic cancer, and brain tumors. The updated FIGO staging system is the gold standard in the clinic and includes surgical, radiologic, and pathologic elements to describe the extent of OC. This system is used to describe tumor extent, plan further therapy, and predict prognosis. However, it is consistently observed that patients with identical stages and treatments have a completely different outcome in terms of survival and recurrence. This fact indicates that this classification alone is not sufficient for the prognosis of OC in the vast majority of cases. Over the last two decades, many studies have demonstrated the critical role of the tumor microenvironment in tumorigenesis, progression, prognosis, and response to chemotherapy. In the current study, we investigate the role of CD16 expression in OC.

Abstract

Background: Ovarian cancer (OC) is the most aggressive and fatal malignancy of the female reproductive system. Debulking surgery with adjuvant chemotherapy represents the standard treatment, but recurrence rates are particularly high. Over the past decades, the association between the immune system and cancer progression has been extensively investigated. However, the interaction between chemotherapy and cancer immune infiltration is still unclear. In this study, we examined the prognostic role of CD16 expression in OC, as related to the effectiveness of standard adjuvant chemotherapy treatment. Methods: We analyzed the infiltration by immune cells expressing CD16, a well-characterized natural killer (NK) and myeloid cell marker, in a tissue microarray (TMA) of 47 patient specimens of primary OCs and their matching recurrences by immunohistochemistry (IHC). We analyzed our data first in the whole cohort, then in the primary tumors, and finally in recurrences. We focused on recurrence-free survival (RFS), overall survival (OS), and chemosensitivity. Chemosensitivity was defined as RFS of more than 6 months. Results: There was no significant correlation between CD16 expression and prognosis in primary carcinomas. However, interestingly, a high density of CD16-expressing tumor-infiltrating immune cells (TICs) in recurrent carcinoma was associated with better RFS (p = 0.008) and OS (p = 0.029). Moreover, high CD16 cell density in recurrent ovarian carcinoma showed a significant association with chemosensitivity (p = 0.034). Univariate Cox regression analysis revealed that the high expression of CD16+ TIC in recurrent cancer biopsies is significantly associated with an increased RFS (HR = 0.49; 95% CI 0.24–0.99; p = 0.047) and OS (HR = 0.28; 95% CI 0.10–0.77; p = 0.013). However, this was not independent of known prognostic factors such as age, FIGO stage, resection status, and the number of chemotherapy cycles. Conclusions: The high density of CD16-expressing TICs in recurrent ovarian cancer is associated with a better RFS and OS, thereby suggesting a previously unsuspected interaction between standard OC chemotherapy and immune cell infiltration.

Harnessing CD16-Mediated NK Cell Functions to Enhance Therapeutic Efficacy of Tumor-Targeting mAbs

Simple Summary

Natural Killer (NK) cells play a major role in cancer immunotherapy based on tumor-targeting mAbs. NK cell-mediated tumor cell killing and cytokine secretion are powerfully stimulated upon interaction with IgG-opsonized tumor cells, through the aggregation of FcγRIIIA/CD16 IgG receptor. Advances in basic and translational NK cell biology have led to the development of strategies that, by improving mAb-dependent antitumor responses, may overcome the current limitations of antibody therapy attributable to tolerance, immunosuppressive microenvironment, and genotypic factors. This review provides an overview of the immunotherapeutic strategies being pursued to improve the efficacy of mAb-induced NK antitumor activity. The exploitation of antibody combinations, antibody-based molecules, used alone or combined with adoptive NK cell therapy, will be uncovered. Within the landscape of NK cell heterogeneity, we stress the role of memory NK cells as promising effectors in the next generation of immunotherapy with the aim to obtain long-lasting tumor control.

Abstract

Natural killer (NK) cells hold a pivotal role in tumor-targeting monoclonal antibody (mAb)-based activity due to the expression of CD16, the low-affinity receptor for IgG. Indeed, beyond exerting cytotoxic function, activated NK cells also produce an array of cytokines and chemokines, through which they interface with and potentiate adaptive immune responses. Thus, CD16-activated NK cells can concur to mAb-dependent “vaccinal effect”, i.e., the development of antigen-specific responses, which may be highly relevant in maintaining long-term protection of treated patients. On this basis, the review will focus on strategies aimed at potentiating NK cell-mediated antitumor functions in tumor-targeting mAb-based regimens, represented by (a) mAb manipulation strategies, aimed at augmenting recruitment and efficacy of NK cells, such as Fc-engineering, and the design of bi- or trispecific NK cell engagers and (b) the possible exploitation of memory NK cells, whose distinctive characteristics (enhanced responsiveness to CD16 engagement, longevity, and intrinsic resistance to the immunosuppressive microenvironment) may maximize therapeutic mAb antitumor efficacy.

Dendritic Cell Tumor Vaccination via Fc Gamma Receptor Targeting: Lessons Learned from Pre-Clinical and Translational Studies

Despite significant recent improvements in the field of immunotherapy, cancer remains a heavy burden on patients and healthcare systems. In recent years, immunotherapies have led to remarkable strides in treating certain cancers. However, despite the success of checkpoint inhibitors and the advent of cellular therapies, novel strategies need to be explored to (1) improve treatment in patients where these approaches fail and (2) make such treatments widely and financially accessible. Vaccines based on tumor antigens (Ag) have emerged as an innovative strategy with the potential to address these areas. Here, we review the fundamental aspects relevant for the development of cancer vaccines and the critical role of dendritic cells (DCs) in this process. We first offer a general overview of DC biology and routes of Ag presentation eliciting effective T cell-mediated immune responses. We then present new therapeutic avenues specifically targeting Fc gamma receptors (FcγR) as a means to deliver antigen selectively to DCs and its effects on T-cell activation. We present an overview of the mechanistic aspects of FcγR-mediated DC targeting, as well as potential tumor vaccination strategies based on preclinical and translational studies. In particular, we highlight recent developments in the field of recombinant immune complex-like large molecules and their potential for DC-mediated tumor vaccination in the clinic. These findings go beyond cancer research and may be of relevance for other disease areas that could benefit from FcγR-targeted antigen delivery, such as autoimmunity and infectious diseases.

Antibody Afucosylation Augments CD16-Mediated Serial Killing and IFNγ Secretion by Human Natural Killer Cells

One mechanism by which monoclonal antibodies (mAb) help treat cancer or autoimmune disease is through triggering antibody-dependent cellular cytotoxicity (ADCC) via CD16 on Natural Killer (NK) cells. Afucosylation is known to increase the affinity of mAbs for CD16 on NK cells and here, we set out to assess how mAb afucosylation affects the dynamics of NK cell interactions, receptor expression and effector functions. An IgG1 version of a clinically important anti-CD20 mAb was compared to its afucosylated counterpart (anti-CD20-AF). Opsonization of CD20-expressing target cells, 721.221 or Daudi, with anti-CD20-AF increased NK cell cytotoxicity and IFNγ secretion, compared to anti-CD20. The afucosylated mAb also caused a more rapid and greater loss of CD16 from NK cell surfaces. Loss of CD16 has recently been shown to be important for NK cell detachment and sequential engagement of multiple target cells. Here, live-cell time-lapse microscopy of individual cell-cell interactions in an aqueous environment and a three-dimensional matrix, revealed that anti-CD20-AF induced more rapid killing of opsonized target cells. In addition, NK cells detached more quickly from target cells opsonized with anti-CD20-AF compared to anti-CD20, which increased engagement of multiple targets and enabled a greater proportion of NK cells to perform serial killing. Inhibition of CD16 shedding with TAPI-0 led to reduced detachment and serial killing. Thus, disassembly of the immune synapse caused by loss of cell surface CD16 is a factor determining the efficiency of ADCC and antibody afucosylation alters the dynamics of intercellular interactions to boost serial killing.

Engineering Anti-Tumor Monoclonal Antibodies and Fc Receptors to Enhance ADCC by Human NK Cells

Tumor-targeting monoclonal antibodies (mAbs) are the most widely used and characterized immunotherapy for hematologic and solid tumors. The significance of this therapy is their direct and indirect effects on tumor cells, facilitated by the antibody's antigen-binding fragment (Fab) and fragment crystallizable region (Fc region), respectively. The Fab can modulate the function of cell surface markers on tumor cells in an agonistic or antagonistic manner, whereas the Fc region can be recognized by an Fc receptor (FcR) on leukocytes through which various effector functions, including antibody-dependent cell-mediated cytotoxicity (ADCC), can be elicited. This process is a key cytolytic mechanism of natural killer (NK) cells. These innate lymphocytes in the human body recognize tumor-bound antibodies exclusively by the IgG Fc receptor CD16A (FcγRIIIA). Two allelic versions of CD16A bind IgG with either lower or higher affinity. Cancer patients homozygous for the higher affinity allele of CD16A have been reported to respond significantly better to mAb therapies for various malignancies. These studies revealed that mAb therapy efficacy positively correlates with higher affinity binding to CD16A. Approaches to enhance tumor antigen targeting by NK cells by modifying the Fc portion of antibodies or the FcR on NK cells are the focus of this review.

Profiling of donor-specific immune effector signatures in response to rituximab in a human whole blood loop assay using blood from CLL patients

Rituximab is widely used in the treatment of haematological malignancies, including chronic lymphocytic leukaemia (CLL), the most common leukaemia in adults. However, some patients, especially those with high tumour burden, develop cytokine release syndrome (CRS). It is likely that more patients will develop therapy-linked CRS in the future due to the implementation of other immunotherapies, such as CAR T-cell, for many malignancies. Current methods for CRS risk assessment are limited, hence there is a need to develop new methods. To better recapitulate an in vivo setting, we implemented a unique human whole blood "loop" system to study patient-specific immune responses to rituximab in blood derived from CLL patients. Upon rituximab infusion, both complement-dependent cytotoxicity (CDC) and antibody-dependent cellular cytotoxicity (ADCC) profiles were evident in CLL patient blood, coincident with CLL cell depletion. Whereas B cell depletion is induced in healthy persons in the blood loop, only patients display B cell depletion coupled with CRS. With the exception of one donor who lacked NK cells, all other five patients displayed variable B cell depletion along with CRS profile. Additionally, inhibition of CDC or ADCC via either inhibitors or antibody Fc modification resulted in skewing of the immune killing mechanism consistent with published literature. Herein we have shown that the human whole blood loop model can be applied using blood from a specific indication to build a disease-specific CRS and immune activation profiling ex vivo system. Other therapeutic antibodies used for other indications may benefit from antibody characterization in a similar setting.

Importance and Monitoring of Therapeutic Immunoglobulin G Glycosylation

The complex diantennary-type oligosaccharides at Asn297 residues of the IgG heavy chains have a profound impact on the safety and efficacy of therapeutic IgG monoclonal antibodies (mAbs). Fc glycosylation of a mAb is an established critical quality attribute (CQA), and its oligosaccharide profile is required to be thoroughly characterized by state-of-the-art analytical methods. The Fc oligosaccharides are highly heterogeneous, and the differentially glycosylated species (glycoforms) of IgG express unique biological activities. Glycoengineering is a promising approach for the production of selected mAb glycoforms with improved effector functions, and non- and low-fucosylated mAbs exhibiting enhanced antibody-dependent cellular cytotoxicity activity have been approved or are under clinical evaluation for treatment of cancers, autoimmune/chronic inflammatory diseases, and infection. Recently, the chemoenzymatic glycoengineering method that allows for the transfer of structurally defined oligosaccharides to Asn-linked GlcNAc residues with glycosynthase has been developed for remodeling of IgG-Fc oligosaccharides with high efficiency and flexibility. Additionally, various glycoengineering methods have been developed that utilize the Fc oligosaccharides of IgG as reaction handles to conjugate cytotoxic agents by "click chemistry", providing new routes to the design of antibody-drug conjugates (ADCs) with tightly controlled drug-antibody ratios (DARs) and homogeneity. This review focuses on current understanding of the biological relevance of individual IgG glycoforms and advances in the development of next-generation antibody therapeutics with improved efficacy and safety through glycoengineering.

Reprogramming the Constant Region of Immunoglobulin G Subclasses for Enhanced Therapeutic Potency against Cancer

The constant region of immunoglobulin (Ig) G antibodies is responsible for their effector immune mechanism and prolongs serum half-life, while the fragment variable (Fv) region is responsible for cellular or tissue targeting. Therefore, antibody engineering for cancer therapeutics focuses on both functional efficacy of the constant region and tissue- or cell-specificity of the Fv region. In the functional aspect of therapeutic purposes, antibody engineers in both academia and industry have capitalized on the constant region of different IgG subclasses and engineered the constant region to enhance therapeutic efficacy against cancer, leading to a number of successes for cancer patients in clinical settings. In this article, we review IgG subclasses for cancer therapeutics, including (i) IgG1, (ii) IgG2, 3, and 4, (iii) recent findings on Fc receptor functions, and (iv) future directions of reprogramming the constant region of IgG to maximize the efficacy of antibody drug molecules in cancer patients.

Bone Marrow Stromal Cell-Derived IL-8 Upregulates PVR Expression on Multiple Myeloma Cells via NF-kB Transcription Factor

Bone marrow stromal cells (BMSCs) strongly contribute to multiple myeloma (MM) progression, promoting the survival and growth of malignant plasma cells (PCs). However, the possible impact of these cells on the immune-mediated recognition of MM cells remains largely unknown. DNAM-1 activating receptor plays a prominent role in NK cell anti-MM response engaging the ligands poliovirus receptor (PVR) and nectin-2 on malignant PCs. Here, we analysed the role of MM patient-derived BMSCs in the regulation of PVR expression. We found that BMSCs enhance PVR surface expression on MM cells and promote their NK cell-mediated recognition. PVR upregulation occurs at transcriptional level and involves NF-kB transcription factor activation by BMSC-derived soluble factors. Indeed, overexpression of a dominant-negative mutant of IKBα blocked PVR upregulation. IL-8 plays a prominent role in these mechanisms since blockade of CXCR1/2 receptors as well as depletion of the cytokine via RNA interference prevents the enhancement of PVR expression by BMSC-derived conditioned medium. Interestingly, IL-8 is associated with stromal microvesicles which are also required for PVR upregulation via CXCR1/CXCR2 signaling activation. Our findings identify BMSCs as regulators of NK cell anti-MM response and contribute to define novel molecular pathways involved in the regulation of PVR expression in cancer cells.

CD16 pre-ligation by defucosylated tumor-targeting mAb sensitizes human NK cells to γc cytokine stimulation via PI3K/mTOR axis

Obinutuzumab is a glycoengineered tumor-targeting anti-CD20 mAb with a modified crystallizable fragment (Fc) domain designed to increase the affinity for the FcγRIIIA/CD16 receptor, which was recently approved for clinical use in CLL and follicular lymphoma. Here we extend our previous observation that, in human NK cells, the sustained CD16 ligation by obinutuzumab-opsonized targets leads to a markedly enhanced IFN-γ production upon a subsequent cytokine re-stimulation. The increased IFN-γ competence in response to IL-2 or IL-15 is attributable to post-transcriptional regulation, as it does not correlate with the upregulation of IFN-γ mRNA levels. Different from the reference molecule rituximab, we observe that the stimulation with obinutuzumab promotes the upregulation of microRNA (miR)-155 expression. A similar trend was also observed in NK cells from untreated CLL patients stimulated with obinutuzumab-opsonized autologous leukemia. miR-155 upregulation associates with reduced levels of SHIP-1 inositol phosphatase, which acts in constraining PI3K-dependent signals, by virtue of its ability to mediate phosphatidylinositol 3,4,5-trisphosphate (PIP3) de-phosphorylation. Downstream of PI3K, the phosphorylation status of mammalian target of rapamycin (mTOR) effector molecule, S6, results in amplified response to IL-2 or IL-15 stimulation in obinutuzumab-experienced cells. Importantly, NK cell treatment with the PI3K or mTOR inhibitors, idelalisib and rapamycin, respectively, prevents the enhanced cytokine responsiveness, thus, highlighting the relevance of the PI3K/mTOR axis in CD16-dependent priming. The enhanced IFN-γ competence may be envisaged to potentiate the immunoregulatory role of NK cells in a therapeutic setting.

Boosting therapeutic potency of antibodies by taming Fc domain functions

Monoclonal antibodies (mAbs) are one of the most widely used drug platforms for infectious diseases or cancer therapeutics because they selectively target pathogens, infectious cells, cancerous cells, and even immune cells. In this way, they mediate the elimination of target molecules and cells with fewer side effects than other therapeutic modalities. In particular, cancer therapeutic mAbs can recognize cell-surface proteins on target cells and then kill the targeted cells by multiple mechanisms that are dependent upon a fragment crystallizable (Fc) domain interacting with effector Fc gamma receptors, including antibody-dependent cell-mediated cytotoxicity and antibody-dependent cell-mediated phagocytosis. Extensive engineering efforts have been made toward tuning Fc functions by either reinforcing (e.g. for targeted therapy) or disabling (e.g. for immune checkpoint blockade therapy) effector functions and prolonging the serum half-lives of antibodies, as necessary. In this report, we review Fc engineering efforts to improve therapeutic potency, and propose future antibody engineering directions that can fulfill unmet medical needs.

Fine-tuning the function of monoclonal antibodies (mAbs) holds promise for developing new therapeutic agents. Antibodies bind to pathogens or cancer cells, flagging them with Fc (fragment crystallizable) domain for destruction by the immune system. mAbs attached only to specific target cells enable lower side effect than other conventional drugs. Sang Taek Jung at Korea University and Tae Hyun Kang at Kookmin University, both in Seoul, reviewed recent developments in engineering therapeutic potency of mAbs. They report that mAbs can be engineered to activate effective immune cell types to treat a particular disease. Engineering can also increase mAbs’ persistence in the blood, enabling less frequent administration. Antibodies engineered to bind to two different antigens at once can also improve therapeutic efficacy. Applying these techniques could help developing new treatments against cancer, and infectious and autoimmune diseases.

Fine tuning of the DNAM-1/TIGIT/ligand axis in mucosal T cells and its dysregulation in pediatric inflammatory bowel diseases (IBD)

De-regulated T-cell activation and functions are pivotal in the orchestration of immune-mediated tissue damage in IBD. We investigated the role of DNAM-1 (co-activating)/TIGIT (co-inhibitory)/ligand axis in the regulation of T-cell functions and its involvement in IBD pathogenesis. We show that DNAM-1 and TIGIT display a peculiar expression pattern on gut mucosa T-cell populations, in a microenvironment where their shared ligands (PVR and Nectin-2) are physiologically present. Moreover, DNAM-1 family receptor/ligand system is perturbed in IBD lesions, in a disease activity-dependent manner. The expression profile of CCR6 and CD103 mucosa addressins suggests that microenvironment-associated factors, rather than skewed recruitment of circulating T-cell populations, play a more relevant role in supporting the establishment of DNAM-1 and TIGIT expression pattern in mucosal T-cell populations, and may explain its alteration in IBD. Although both co-receptors mark functionally competent T cells, DNAM-1 and TIGIT segregate on T cells endowed with different proliferative potential. Moreover, their opposing role in regulating T-cell proliferation exquisitely depends on ligand availability. All together, our data propose a role for DNAM-1 and TIGIT in regulating mucosal T-cell activation and immune homeostasis, and highlight the involvement of an imbalance of this system in IBD.

Post-translational Mechanisms Regulating NK Cell Activating Receptors and Their Ligands in Cancer: Potential Targets for Therapeutic Intervention

Efficient clearance of transformed cells by Natural Killer (NK) cells is regulated by several activating receptors, including NKG2D, NCRs, and DNAM-1. Expression of these receptors as well as their specific “induced self” ligands is finely regulated during malignant transformation through the integration of different mechanisms acting on transcriptional, post-transcriptional, and post-translational levels. Among post-translational mechanisms, the release of activating ligands in the extracellular milieu through protease-mediated cleavage or by extracellular vesicle secretion represents some relevant cancer immune escape processes. Moreover, covalent modifications including ubiquitination and SUMOylation also contribute to negative regulation of NKG2D and DNAM-1 ligand surface expression resulting either in ligand intracellular retention and/or ligand degradation. All these mechanisms greatly impact on NK cell mediated recognition and killing of cancer cells and may be targeted to potentiate NK cell surveillance against tumors. Our mini review summarizes the main post-translational mechanisms regulating the expression of activating receptors and their ligands with particular emphasis on the contribution of ligand shedding and of ubiquitin and ubiquitin-like modifications in reducing target cell susceptibility to NK cell-mediated killing. Strategies aimed at inhibiting shedding of activating ligands and their modifications in order to preserve ligand expression on cancer cells will be also discussed.

NK Cell-Based Immunotherapy for Hematological Malignancies

Natural killer (NK) lymphocytes are an integral component of the innate immune system and represent important effector cells in cancer immunotherapy, particularly in the control of hematological malignancies. Refined knowledge of NK cellular and molecular biology has fueled the interest in NK cell-based antitumor therapies, and recent efforts have been made to exploit the high potential of these cells in clinical practice. Infusion of high numbers of mature NK cells through the novel graft manipulation based on the selective depletion of T cells and CD19+ B cells has resulted into an improved outcome in children with acute leukemia given human leucocyte antigen (HLA)-haploidentical hematopoietic transplantation. Likewise, adoptive transfer of purified third-party NK cells showed promising results in patients with myeloid malignancies. Strategies based on the use of cytokines or monoclonal antibodies able to induce and optimize NK cell activation, persistence, and expansion also represent a novel field of investigation with remarkable perspectives of favorably impacting on outcome of patients with hematological neoplasia. In addition, preliminary results suggest that engineering of mature NK cells through chimeric antigen receptor (CAR) constructs deserve further investigation, with the goal of obtaining an "off-the-shelf" NK cell bank that may serve many different recipients for granting an efficient antileukemia activity.

Prognostic Impact of Natural Killer Cell Count in Follicular Lymphoma and Diffuse Large B-cell Lymphoma Patients Treated with Immunochemotherapy

PURPOSE

Natural killer (NK) cells are key effector cells for anti-CD20 monoclonal antibodies (mAb), such as obinutuzumab and rituximab. We assessed whether low pretreatment NK-cell count (NKCC) in peripheral blood or tumor tissue was associated with worse outcome in patients receiving antibody-based therapy.

PATIENTS AND METHODS

Baseline peripheral blood NKCC was assessed by flow cytometry (CD3^-CD56⁺ and/or CD16⁺ cells) in 1,064 of 1,202 patients with follicular lymphoma treated with obinutuzumab or rituximab plus chemotherapy in the phase III GALLIUM trial (NCT01332968) and 1,287 of 1,418 patients with diffuse large B-cell lymphoma (DLBCL) treated with obinutuzumab or rituximab plus cyclophosphamide, doxorubicin, vincristine, and prednisone (G-CHOP or R-CHOP) in the phase III GOYA trial (NCT01287741). The prognostic value of tumor NK-cell gene expression, as assessed by whole-transcriptome gene expression using TruSeq RNA sequencing, was also analyzed. The association of baseline variables, such as treatment arm, was evaluated using multivariate Cox regression models using a stepwise approach.

RESULTS

In this exploratory analysis, low baseline peripheral blood NKCC was associated with shorter progression-free survival (PFS) in both follicular lymphoma [hazard ratio (HR), 1.48; 95% confidence interval (CI), 1.02-2.14; P = 0.04] and DLBCL (HR, 1.36; 95% CI, 1.01-1.83; P = 0.04), and overall survival in follicular lymphoma (HR, 2.20; 95% CI, 1.26-3.86; P = 0.0058). Low tumor NK-cell gene expression was associated with shorter PFS in G-CHOP-treated patients with DLBCL (HR, 1.95; 95% CI, 1.22-3.15; P < 0.01).

CONCLUSIONS

These findings indicate that the number of NK cells in peripheral blood may affect the outcome of patients with B-cell non-Hodgkin lymphoma receiving anti-CD20-based immunochemotherapy.

The Ubiquitin-proteasome pathway regulates Nectin2/CD112 expression and impairs NK cell recognition and killing

Nectin2 is a member of immunoglobulin-like cell adhesion molecules and plays a prominent role in the establishment of adherens and tight junctions. It is also upregulated on the surface of tumor and virus-infected cells where it functions as a ligand for the activating receptor CD226, thus contributing to cytotoxic lymphocyte-mediated recognition and killing of damaged cells. Little is currently known about the regulation of Nectin2 expression and, in particular, whether posttranscriptional and posttranslational mechanisms are involved. Here, we analyzed Nectin2 expression on a panel of human tumor cell lines and primary cultures and we found that Nectin2 is mainly expressed in cytoplasmic pools. Moreover, we demonstrated that ubiquitination of Nectin2 promotes its degradation and is responsible for protein intracellular retention. Indeed, inhibition of the ubiquitin pathway results in increased Nectin2 surface expression and enhances tumor cell susceptibility to NK cell cytotoxicity. Our results demonstrate a previously unknown mechanism of Nectin2 regulation revealing that the ubiquitin pathway represents a potential target of intervention in order to increase susceptibility to NK cell-mediated lysis.

An Engineered Human Fc variant With Exquisite Selectivity for FcγRIIIaV158 Reveals That Ligation of FcγRIIIa Mediates Potent Antibody Dependent Cellular Phagocytosis With GM-CSF-Differentiated Macrophages

IgG antibodies mediate the clearance of target cells via the engagement of Fc gamma receptors (FcγRs) on effector cells by eliciting antibody-dependent cellular cytotoxicity and phagocytosis (ADCC and ADCP, respectively). Because (i) the IgG Fc domain binds to multiple FcγRs with varying affinities; (ii) even low Fc:FcγRs affinity interactions can play a significant role when antibodies are engaged in high avidity immune complexes and (iii) most effector cells express multiple FcγRs, the clearance mechanisms that can be mediated by individual FcγR are not well-understood. Human FcγRIIIa (hFcγRIIIa; CD16a), which exists as two polymorphic variants at position 158, hFcγRIIIa_V158 and hFcγRIIIa_F158, is widely considered to only trigger ADCC, especially with natural killer (NK) cells as effectors. To evaluate the role of hFcγRIIIa ligation in myeloid-derived effector cells, and in particular on macrophages and monocytes which express multiple FcγRs, we engineered an aglycosylated engineered human Fc (hFc) variant, Fc3aV, which binds exclusively to hFcγRIIIa_V158. Antibodies formatted with the Fc3aV variant bind to the hFcγRIIIa_V158 allotype with a somewhat lower K_D than their wild type IgG1 counterparts, but not to any other hFcγR. The exceptional selectivity for hFcγRIIIa_V158 was demonstrated by SPR using increased avidity, dimerized GST-fused versions of the ectodomains of hFcγRs and from the absence of binding of large immune complex (IC) to CHO cells expressing each of the hFcγRs, including notably, the FcγRIIIa_F158 variant or the highly homologous FcγRIIIb. We show that even though monocyte-derived GM-CSF differentiated macrophages express hFcγRIIIa at substantially lower levels than the other two major activating receptors, namely hFcγRI or hFcγRIIa, Fc3aV-formatted Rituximab and Herceptin perform ADCP toward CD20- and Her2-expressing cancer cells, respectively, at a level comparable to that of the respective wild-type antibodies. We further show that hFcγRIIIa activation plays a significant role on ADCC by human peripheral monocytes. Our data highlight the utility of Fc3aV and other similarly engineered exquisitely selective, aglycosylated Fc variants toward other hFcγRs as tools for the detailed molecular understanding of hFcγR biology.

Cocultures of human colorectal tumor spheroids with immune cells reveal the therapeutic potential of MICA/B and NKG2A targeting for cancer treatment

Background

Immunotherapies still fail to benefit colorectal cancer (CRC) patients. Relevant functional assays aimed at studying these failures and the efficacy of cancer immunotherapy in human are scarce. 3D tumor cultures, called tumor organoids or spheroids, represent interesting models to study cancer treatments and could help to challenge these issues.

Methods

We analyzed heterotypic cocultures of human colon tumor-derived spheroids with immune cells to assess the infiltration, activation and function of T and NK cells toward human colorectal tumors in vitro.

Results

We showed that allogeneic T and NK cells rapidly infiltrated cell line-derived spheroids, inducing immune-mediated tumor cell apoptosis and spheroid destruction. NKG2D, a key activator of cytotoxic responses, was engaged on infiltrating cells. We thus assessed the therapeutic potential of an antibody targeting the specific ligands of NKG2D, MICA and MICB, in this system. Anti-MICA/B enhanced immune-dependent destruction of tumor spheroid by driving an increased NK cells infiltration and activation. Interestingly, tumor cells reacted to immune infiltration by upregulating HLA-E, ligand of the inhibitory receptor NKG2A expressed by CD8 and NK cells. NKG2A was increased after anti-MICA/B treatment and, accordingly, combination of anti-MICA/B and anti-NKG2A was synergistic. These observations were ultimately confirmed in a clinical relevant model of coculture between CRC patients-derived spheroids and autologous tumor-infiltrating lymphocytes.

Conclusions

Altogether, we show that tumor spheroids represent a relevant tool to study tumor-lymphocyte interactions on human tissues and revealed the antitumor potential of immunomodulatory antibodies targeting MICA/B and NKG2A.

Electronic supplementary material

The online version of this article (10.1186/s40425-019-0553-9) contains supplementary material, which is available to authorized users.

Memory NK Cell Features Exploitable in Anticancer Immunotherapy

Besides their innate ability to rapidly produce effector cytokines and kill virus-infected or transformed cells, natural killer (NK) cells display a strong capability to adapt to environmental modifications and to differentiate into long-lived, hyperfunctional populations, dubbed memory or memory-like NK cells. Despite significant progress in the field of NK cell-based immunotherapies, some factors including their short life span and the occurrence of a tumor-dependent functional exhaustion have limited their clinical efficacy so that strategies aimed at overcoming these limitations represent one of the main current challenges in the field. In this scenario, the exploitation of NK cell memory may have a considerable potential. This article summarizes recent evidence in the literature on the peculiar features that render memory NK cells an attractive tool for antitumor immunotherapy, including their long-term survival and in vivo persistence, the resistance to tumor-dependent immunosuppressive microenvironment, the amplified functional responses to IgG-opsonized tumor cells, and in vitro expansion capability. Along with highlighting these issues, we speculate that memory NK cell-based adoptive immunotherapy settings would greatly take advantage from the combination with tumor-targeting therapeutic antibodies (mAbs), as a strategy to fully unleash their clinical efficacy.

FcγRIIIa-dependent IFN-γ release in whole blood assay is predictive of therapeutic IgG1 antibodies safety

Immunomodulatory monoclonal IgG1 antibodies developed for cancer and autoimmune disease have an inherent risk of systemic release of pro-inflammatory cytokines. In vitro cytokine release assays are currently used to predict cytokine release syndrome (CRS) risk, but the validation of these preclinical tools suffers from the limited number of characterized CRS-inducing IgG1 antibodies and the poor understanding of the mechanisms regulating cytokine release. Here, we incubated human whole blood from naïve healthy volunteers with four monoclonal IgG1 antibodies with different proven or predicted capacity to elicit CRS in clinic and measured cytokine release using a multiplex assay. We found that, in contrast to anti-CD52 antibodies (Campath-1H homolog) that elicited high level of multiple inflammatory cytokines from human blood cells in vitro, other IgG1 antibodies with CRS-inducing potential consistently induced release of a single tested cytokine, interferon (IFN)-γ, with a smaller magnitude than Campath. IFN-γ expression was observed as early as 2-4 h after incubation, mediated by natural killer cells, and dependent upon tumor necrosis factor and FcγRIII. Importantly, the magnitude of the IFN-γ response elicited by IgG1 antibodies with CRS-inducing potential was determined by donor FcγRIIIa-V158F polymorphism. Overall, our results highlight the importance of FcγRIIIa-dependent IFN-γ release in preclinical cytokine release assay for the prediction of CRS risk associated with therapeutic IgG1 antibodies.

Hypersensitivity Reactions to Obinutuzumab in Cynomolgus Monkeys and Relevance to Humans

Obinutuzumab (GA101, Gazyva™, Gazyvaro®, F. Hoffmann-La Roche AG, Basel, Switzerland) is a humanized, glycoengineered type II antibody targeted against CD20. The preclinical safety evaluation required to support clinical development and marketing authorization of obinutuzumab included repeat-dose toxicity studies in cynomolgus monkeys for up to 6-month dosing with a 9-month recovery period. Results from those studies showed decreases in circulating B cells and corresponding B-cell depletion in lymphoid tissues, consistent with the desired pharmacology of obinutuzumab. Hypersensitivity reactions were noted at all doses in the 6-month study and were attributed to the foreign recognition of the drug construct in cynomolgus monkeys. Findings in monkeys were classified as acute hypersensitivity reactions that were evident immediately after dosing, such as excessive salivation, erythema, pruritus, irregular respiration, or ataxia, or chronic hypersensitivity reactions characterized by glomerulonephritis, arteritis/periarteritis, and inflammation in several tissues including serosal/adventitial inflammation. Immune complex deposits were demonstrated in tissues by immunohistochemistry, immunofluorescence, and electron microscopy. Some of, but not all, the animals that developed these reactions had detectable antidrug antibodies or circulating immune complexes accompanied by loss of drug exposure and pharmacodynamic effect. On the basis of clinical evidence to date, hypersensitivity reactions following obinutuzumab are rare, further supporting the general view that incidence and manifestation of immunogenicity in nonclinical species are generally not predictive for humans.

Tumor-Targeting Anti-CD20 Antibodies Mediate In Vitro Expansion of Memory Natural Killer Cells: Impact of CD16 Affinity Ligation Conditions and In Vivo Priming

Natural killer (NK) cells represent a pivotal player of innate anti-tumor immune responses. The impact of environmental factors in shaping the representativity of different NK cell subsets is increasingly appreciated. Human cytomegalovirus (HCMV) infection profoundly affects NK cell compartment, as documented by the presence of a CD94/NKG2C⁺FcεRIγ^- long-lived “memory” NK cell subset, endowed with enhanced CD16-dependent functional capabilities, in a fraction of HCMV-seropositive subjects. However, the requirements for memory NK cell pool establishment/maintenance and activation have not been fully characterized yet. Here, we describe the capability of anti-CD20 tumor-targeting therapeutic monoclonal antibodies (mAbs) to drive the selective in vitro expansion of memory NK cells and we show the impact of donor’ HCMV serostatus and CD16 affinity ligation conditions on this event. In vitro expanded memory NK cells maintain the phenotypic and functional signature of their freshly isolated counterpart; furthermore, our data demonstrate that CD16 affinity ligation conditions differently affect memory NK cell proliferation and functional activation, as rituximab-mediated low-affinity ligation represents a superior proliferative stimulus, while high-affinity aggregation mediated by glycoengineered obinutuzumab results in improved multifunctional responses. Our work also expands the molecular and functional characterization of memory NK cells, and investigates the possible impact of CD16 functional allelic variants on their in vivo and in vitro expansions. These results reveal new insights in Ab-driven memory NK cell responses in a therapeutic setting and may ultimately inspire new NK cell-based intervention strategies against cancer, in which the enhanced responsiveness to mAb-bound target could significantly impact therapeutic efficacy.

CD16A Activation of NK Cells Promotes NK Cell Proliferation and Memory-Like Cytotoxicity against Cancer Cells

CD16A is a potent cytotoxicity receptor on human natural killer (NK) cells, which can be exploited by therapeutic bispecific antibodies. So far, the effects of CD16A-mediated activation on NK cell effector functions beyond classical antibody-dependent cytotoxicity have remained poorly elucidated. Here, we investigated NK cell responses after exposure to therapeutic antibodies such as the tetravalent bispecific antibody AFM13 (CD30/CD16A), designed for the treatment of Hodgkin lymphoma and other CD30⁺ lymphomas. Our results reveal that CD16A engagement enhanced subsequent IL2- and IL15-driven NK cell proliferation and expansion. This effect involved the upregulation of CD25 (IL2Rα) and CD132 (γ_c) on NK cells, resulting in increased sensitivity to low-dose IL2 or to IL15. CD16A engagement initially induced NK cell cytotoxicity. The lower NK cell reactivity observed 1 day after CD16A engagement could be recovered by reculture in IL2 or IL15. After reculture in IL2 or IL15, these CD16A-experienced NK cells exerted more vigorous IFNγ production upon restimulation with tumor cells or cytokines. Importantly, after reculture, CD16A-experienced NK cells also exerted increased cytotoxicity toward different tumor targets, mainly through the activating NK cell receptor NKG2D. Our findings uncover a role for CD16A engagement in priming NK cell responses to restimulation by cytokines and tumor cells, indicative of a memory-like functionality. Our study suggests that combination of AFM13 with IL2 or IL15 may boost NK cell antitumor activity in patients by expanding tumor-reactive NK cells and enhancing NK cell reactivity, even upon repeated tumor encounters. Cancer Immunol Res; 6(5); 517-27. ©2018 AACR.

Tumor immunotherapy: New aspects of natural killer cells

A group of impressive immunotherapies for cancer treatment, including immune checkpoint-blocking antibodies, gene therapy and immune cell adoptive cellular immunotherapy, have been established, providing new weapons to fight cancer. Natural killer (NK) cells are a component of the first line of defense against tumors and virus infections. Studies have shown dysfunctional NK cells in patients with cancer. Thus, restoring NK cell antitumor functionality could be a promising therapeutic strategy. NK cells that are activated and expanded ex vivo can supplement malfunctional NK cells in tumor patients. Therapeutic antibodies, chimeric antigen receptor (CAR), or bispecific proteins can all retarget NK cells precisely to tumor cells. Therapeutic antibody blockade of the immune checkpoints of NK cells has been suggested to overcome the immunosuppressive signals delivered to NK cells. Oncolytic virotherapy provokes antitumor activity of NK cells by triggering antiviral immune responses. Herein, we review the current immunotherapeutic approaches employed to restore NK cell antitumor functionality for the treatment of cancer.

Human NK Cells Downregulate Zap70 and Syk in Response to Prolonged Activation or DNA Damage

The extent of NK cell activity during the innate immune response affects downstream immune functions and, ultimately, the outcome of infectious or malignant disease. However, the mechanisms that terminate human NK cell responses have yet to be defined. When activation receptors expressed on NK cell surfaces bind to ligands on diseased cells, they initiate a signal that is propagated by a number of intracellular kinases, including Zap70 and Syk, eventually leading to NK cell activation. We assayed Zap70 and Syk content in NK cells from healthy human donors and identified a subset of NK cells with unusually low levels of these two kinases. We found that this Zap70^lowSyk^low subset consisted of NK cells expressing a range of surface markers, including CD56^hi and CD56^low NK cells. Upon in vitro stimulation with target cells, Zap70^lowSyk^low NK cells failed to produce IFN-γ and lysed target cells at one third the capacity of Zap70^hiSyk^hi NK cells. We determined two independent in vitro conditions that induce the Zap70^lowSyk^low phenotype in NK cells: continuous stimulation with activation beads and DNA damage. The expression of inhibitory receptors, including NKG2A and inhibitory killer Ig-like receptors (KIRs), was negatively correlated with the Zap70^lowSyk^low phenotype. Moreover, expression of multiple KIRs reduced the likelihood of Zap70 downregulation during continuous activation, regardless of whether NK cells had been educated through KIR-HLA interactions in vivo. Our findings show that human NK cells are able to terminate their functional activity without the aid of other immune cells through the downregulation of activation kinases.

Gradual Increase of FcγRIIIa/CD16a Expression and Shift toward IFN-γ Secretion during Differentiation of CD56dim Natural Killer Cells

Natural killer (NK) cell effector functions include cytotoxicity and secretion of cytokines such as interferon-γ (IFN-γ). The immature CD56^bright subset of human NK cells lacks expression of FcγRIIIa/CD16a, one of the low-affinity immunoglobulin G receptors, or exhibits low-density expression (CD56^brightCD16^-/dim) and produces IFN-γ in response to cytokine stimulation, whereas the mature CD56^dimCD16⁺ subset is the most cytotoxic one. A further differentiation/maturation of the latter subset according to the gradual loss of NKG2A and/or gain of KIR2DL (CD158a and CD158b) has been demonstrated and the ability to produce IFN-γ in response to activating receptor (AR) co-engagement is gradually acquired during terminal differentiation. In the course of flow cytometry analysis of CD56^dim NK cells, we noted a substantial intraindividual heterogeneity of expression of FcγRIIIa. FcγRIIIa is unique among ARs: it does not require the co-engagement of other ARs to induce substantial cytotoxicity or cytokine synthesis in CD56^dim cells. We, therefore, investigated whether individual differentiation/maturation of polyclonal CD56^dim NK cells defined by expression of NKG2A/KIR2DL is related to FcγRIIIa expression and to the heterogeneity of NK cell responses upon FcγRIIIa engagement. When we analyzed unstimulated CD56^dim cells by increasing level of FcγRIIIa expression, we found that the proportion of the more differentiated CD158a,h⁺ and/or CD158b,j⁺ cells and that of the less differentiated NKG2A⁺ cells gradually increased and decreased, respectively. FcγRIIIa engagement by using plate-bound murine anti-CD16 monoclonal antibody (mAb) or rituximab or trastuzumab (two therapeutic mAbs), resulted in donor-dependent partial segregation of IFN-γ-producing and/or degranulating CD56^dim cells. Importantly, the proportion of CD158a,h/b,j⁺ cells and that of NKG2A⁺ cells was increased and decreased, respectively, IFN-γ-producing cells, whereas these proportions were poorly modified in degranulating cells. Similar results were observed after engagement of ARs by a combination of mAbs targeting NKG2D, NKp30, NKp46, and 2B4. Thus, the gradual increase of FcγRIIIa expression is an important feature of the differentiation/maturation of CD56^dim cells and this differentiation/maturation is associated with a shift in functionality toward IFN-γ secretion observed upon both FcγRIIIa-dependent and FcγRIIIa-independent stimulation. The functional heterogeneity related to the differentiation/maturation of CD56^dim NK cells could be involved in the variability of the clinical responses observed in patients treated with therapeutic mAbs.

Regulation of NKG2D-Dependent NK Cell Functions: The Yin and the Yang of Receptor Endocytosis

Natural-killer receptor group 2, member D (NKG2D) is a well characterized natural killer (NK) cell activating receptor that recognizes several ligands poorly expressed on healthy cells but up-regulated upon stressing stimuli in the context of cancer or viral infection. Although NKG2D ligands represent danger signals that render target cells more susceptible to NK cell lysis, accumulating evidence demonstrates that persistent exposure to ligand-expressing cells causes the decrease of NKG2D surface expression leading to a functional impairment of NKG2D-dependent NK cell functions. Upon ligand binding, NKG2D is internalized from the plasma membrane and sorted to lysosomes for degradation. However, receptor endocytosis is not only a mechanism of receptor clearance from the cell surface, but is also required for the proper activation of signalling events leading to the functional program of NK cells. This review is aimed at providing a summary of current literature relevant to the molecular mechanisms leading to NKG2D down-modulation with particular emphasis given to the role of NKG2D endocytosis in both receptor degradation and signal propagation. Examples of chronic ligand-induced down-regulation of NK cell activating receptors other than NKG2D, including natural cytotoxicity receptors (NCRs), DNAX accessory molecule-1 (DNAM1) and CD16, will be also discussed.