IL-10-producing NK cells exacerbate sublethal Streptococcus pneumoniae infection in the lung

Streptococcus pneumoniae epidemiology, pathogenesis and control

Infections caused by Streptococcus pneumoniae (also known as pneumococci) pose a threat to human health. Pneumococcal infections are the most common cause of milder respiratory tract infections, such as otitis and sinusitis, and of more severe diseases, including pneumonia (with or without septicaemia) and meningitis. The introduction of pneumococcal conjugate vaccines in the childhood vaccination programme in many countries has led to a notable decrease of severe invasive pneumococcal disease in vaccinated children. However, infections caused by non-vaccine types have concurrently increased, causing invasive pneumococcal disease in unvaccinated populations (such as older adults), which has hampered the effect of these vaccines. Moreover, emerging antibiotic resistance is threatening effective therapy. Thus, new approaches are needed for the treatment and prevention of pneumococcal infections, and recent advances in the field may pave the way for new strategies. Recently, several important findings have been gained regarding pneumococcal epidemiology, genomics and the effect of the introduction of pneumococcal conjugate vaccines and of the COVID-19 pandemic. Moreover, elucidative pathogenesis studies have shown that the interactions between pneumococcal virulence factors and host receptors may be exploited for new therapies, and new vaccine candidates have been suggested. In this Review, we summarize some recent findings from clinical disease to basic pathogenesis studies that may be of importance for future control strategies.

Leukocyte kinetics and bacterial clearance during Streptococcus pneumoniae pneumonia and contributions of ICAM-1

Streptococcus pneumoniae is a leading cause of community-acquired pneumonia. Intercellular adhesion molecule-1 (ICAM-1) is an adhesion molecule that is highly expressed on the pulmonary capillary endothelium, alveolar epithelium, and other cell types within the lung. ICAM-1 plays important roles in leukocyte adhesion, migration, and motility. To determine the contributions of ICAM-1 to bacterial clearance and leukocyte kinetics during pneumonia, mice were inoculated with S. pneumoniae and evaluated 1, 4, and 7 days later. Our results show that Icam1-/- mice have a greater number of viable bacteria within the lung at each time point. The impaired clearance observed in Icam1-/- mice was not due to an impediment in leukocyte recruitment. In fact, Icam1-/- mice had a greater number of neutrophils and recruited inflammatory macrophages in the lung tissue and the alveoli/airways on day 7. In contrast, fewer alveolar macrophages were present in the bronchoalveolar lavage (BAL) of Icam1-/- mice. The loss of body weight and the concentrations of inflammatory mediators in the BAL were also significantly greater in Icam1-/- mice. Mechanistic studies to understand the defect in clearance show that neutrophils and macrophage subpopulations had no defect in phagocytosis or acidification of phagosomes. RNA sequencing reveals many differences in gene expression but no suggestion of a defect in phagocytosis or killing. Thus, ICAM-1 is necessary for the clearance of S. pneumoniae and for the resolution of pneumonia but is not required for the recruitment of neutrophils or inflammatory macrophages into the pneumonic lung parenchyma or the alveoli/airways during S. pneumoniae-induced pneumonia.NEW & NOTEWORTHY Streptococcus pneumoniae is the leading cause of community-acquired pneumonia. Our study examined ICAM-1, an adhesion molecule that is expressed on most cell types and plays important roles in leukocyte adhesion, migration, and motility. The data demonstrate that ICAM-1 is necessary for the clearance of S. pneumoniae and for the resolution of pneumonia but is not required for the recruitment of neutrophils or inflammatory macrophages into the pneumonic lung parenchyma or the alveoli/airways.

CD38 modulates cytokine secretion by NK cells through the Sirt1/NF-κB pathway, suppressing immune surveillance in colorectal cancer

Tregs and M2-type macrophages are essential for immune surveillance. CD38 + NK cells are involved in immunoregulation by modulating cytokine secretion. This study investigated how CD38 + NKs affect Tregs and macrophages in colorectal cancer (CRC). Higher proportions of CD38 + NKs and Tregs were detected in bloods and tumor tissues of CRC patients than that in the samples from healthy controls (HCs). Compared with CD38 + NKs from HCs, the NK cells from CRC promoted the differentiation of Tregs from CD4 + T cells, and secreted increased levels of IL-10, TGF-β and TNF-α and decreased levels of IFN-γ. CD38 + NKs from CRC expressed higher levels of CD38, NF-κB and acetyl-NF-κB and lower levels of Sirt1. When CRC CD38 + NK cells were treated with anti-CD38 monoclonal antibody, the above trends were reversed. CRC CD38 + NKs with treatment of NF-κB inhibitor also showed opposite effects on cytokine secretion and CD4 + T-cell differentiation. After treatment with a Sirt1 activator, NF-κB signaling was inhibited in these CD38 + NKs, whereas treatment with a Sirt1 inhibitor activated NF-κB signaling. The supernatants of CRC CD38 + NK culture promoted M0 macrophage polarization to M2-type. We suggest that CD38 modulates cytokine secretion by NK cells through Sirt1/NF-κB signaling pathway, thereby suppressing immune surveillance in tumorigenesis.

NK cells contribute to the resolution of experimental malaria-associated acute respiratory distress syndrome after antimalarial treatment

In both humans and mice, natural killer (NK) cells are important lymphocytes of the innate immune system. They are often considered pro-inflammatory effector cells but may also have a regulatory or pro-resolving function by switching their cytokine profile towards the production of anti-inflammatory cytokines, including interleukin-10 (IL-10) and transforming growth factor-β, and by killing pro-inflammatory immune cells. Here, the role of NK cells in the resolution of malaria lung pathology was studied. Malaria complications, such as malaria-associated acute respiratory distress syndrome (MA-ARDS), are often lethal despite the rapid and efficient killing of Plasmodium parasites with antimalarial drugs. Hence, studying the resolution and healing mechanisms involved in the recovery from these complications could be useful to develop adjunctive treatments. Treatment of Plasmodium berghei NK65-infected C57BL/6 mice with a combination of artesunate and chloroquine starting at the appearance of symptoms was used as a model to study the resolution of MA-ARDS. The role of NK cells was studied using anti-NK1.1 depletion antibodies and NK cell-deficient mice. Using both methods, NK cells were found to be dispensable in the development of MA-ARDS, as shown previously. In contrast, NK cells were crucial in the initiation of resolution upon antimalarial treatment, as survival was significantly decreased in the absence of NK cells. Considerably increased IL-10 expression by NK cells suggested an anti-inflammatory and pro-resolving phenotype. Despite the increase in Il10 expression in the NK cells, inhibition of the IL-10/IL-10R axis using anti-IL10R antibodies had no effect on the resolution for MA-ARDS, suggesting that the pro-resolving effect of NK cells cannot solely be attributed to their IL-10 production. In conclusion, NK cells contribute to the resolution of experimental MA-ARDS.

Bioinformatics characteristics and expression analysis of IL-8 and IL-10 in largemouth bass (Micropterus salmoides) upon Nocardia seriolae infection

IL-8 and IL-10 are crucial inflammatory cytokines that participate in defending host cells against infections. To demonstrate the function of the two interleukin genes in largemouth bass (Micropterus salmoides), we initially cloned and identified the cDNA sequences of il-8 and il-10 in largemouth bass, referred to as Msil-8 and Msil-10, respectively. The open reading frame (ORF) of Msil-8 was 324 bp in length, encoding 107 amino acids, while the ORF of Msil-10 consisted of 726 bp and encoded 241 amino acids. Furthermore, the functional domains of the SCY domain in MsIL-8 and the IL-10 family signature motif in MsIL-10 were highly conserved across vertebrates. Additionally, both MsIL-8 and MsIL-10 showed close relationships with M. dolomieu. Constitutive expression of Msil-8 and Msil-10 was observed in various tissues, with the highest level found in the head kidney. Subsequently, largemouth bass were infected with Nocardia seriolae via intraperitoneal injection to gain a further understanding of the function of these two genes. Bacterial loads were initially detected in the foregut, followed by the midgut, hindgut, and liver. The mRNA expression of Msil-8 was significantly down-regulated after infection, especially at 2 days post-infection (DPI), with a similar expression to Msil-10. In contrast, the expression of Msil-8 and Msil-10 was significantly upregulated in the foregut at 14 DPI. Taken together, these results reveal that the function of IL-8 and IL-10 was likely hindered by N. seriolae, which promoted bacterial proliferation and intercellular diffusion.

IL-15 Complex-Induced IL-10 Enhances Plasmodium-specific CD4+ T Follicular Helper Differentiation and Antibody Production

Malaria, which results from infection with Plasmodium parasites, remains a major public health problem. Although humans do not develop long-lived, sterilizing immunity, protection against symptomatic disease develops after repeated exposure to Plasmodium parasites and correlates with the acquisition of humoral immunity. Despite the established role Abs play in protection from malaria disease, dysregulated inflammation is thought to contribute to the suboptimal immune response to Plasmodium infection. Plasmodium berghei ANKA (PbA) infection results in a fatal severe malaria disease in mice. We previously demonstrated that treatment of mice with IL-15 complex (IL-15C; IL-15 bound to an IL-15Rα-Fc fusion protein) induces IL-10 expression in NK cells, which protects mice from PbA-induced death. Using a novel MHC class II tetramer to identify PbA-specific CD4+ T cells, in this study we demonstrate that IL-15C treatment enhances T follicular helper (Tfh) differentiation and modulates cytokine production by CD4+ T cells. Moreover, genetic deletion of NK cell-derived IL-10 or IL-10R expression on T cells prevents IL-15C-induced Tfh differentiation. Additionally, IL-15C treatment results in increased anti-PbA IgG Ab levels and improves survival following reinfection. Overall, these data demonstrate that IL-15C treatment, via its induction of IL-10 from NK cells, modulates the dysregulated inflammation during Plasmodium infection to promote Tfh differentiation and Ab generation, correlating with improved survival from reinfection. These findings will facilitate improved control of malaria infection and protection from disease by informing therapeutic strategies and vaccine design.

Neutrophil responsiveness to IL-10 impairs clearance of Streptococcus pneumoniae from the lungs

The early immune response to bacterial pneumonia requires a careful balance between pathogen clearance and tissue damage. The anti-inflammatory cytokine interleukin (IL)-10 is critical for restraining otherwise lethal pulmonary inflammation. However, pathogen-induced IL-10 is associated with bacterial persistence in the lungs. In this study, we used mice with myeloid cell specific deletion of IL-10R to investigate the cellular targets of IL-10 immune suppression during infection with Streptococcus pneumoniae, the most common bacterial cause of pneumonia. Our findings suggest that IL-10 restricts the neutrophil response to S. pneumoniae, as neutrophil recruitment to the lungs was elevated in myeloid IL-10 receptor (IL-10R)-deficient mice and neutrophils in the lungs of these mice were more effective at killing S. pneumoniae. Improved killing of S. pneumoniae was associated with increased production of reactive oxygen species and serine protease activity in IL-10R-deficient neutrophils. Similarly, IL-10 suppressed the ability of human neutrophils to kill S. pneumoniae. Burdens of S. pneumoniae were lower in myeloid IL-10R-deficient mice compared with wild-type mice, and adoptive transfer of IL-10R-deficient neutrophils into wild-type mice significantly improved pathogen clearance. Despite the potential for neutrophils to contribute to tissue damage, lung pathology scores were similar between genotypes. This contrasts with total IL-10 deficiency, which is associated with increased immunopathology during S. pneumoniae infection. Together, these findings identify neutrophils as a critical target of S. pneumoniae-induced immune suppression and highlight myeloid IL-10R abrogation as a mechanism to selectively reduce pathogen burdens without exacerbating pulmonary damage.

Inhibiting T-Cell-Mediated Rejection of the Porcine Meniscus Through Freeze-Thawing and Downregulating Porcine Xenoreactive Antigen Genes

Immune rejection presents a significant challenge in xenogenic meniscal transplantation. Pigs are widely regarded as an advantageous tissue source for such transplants, with porcine GGTA1, CMAH, and B4GALNT2 being among the most common xenoreactive antigen (Ag) genes. While some studies have suggested that allogeneic meniscus (AM) transplants may exhibit immunoprivileged properties, our study observed slight immunological rejection has been observed following contact between human meniscal cells (HMCs) and human peripheral blood mononuclear cells (PBMCs). Given the limited systematic research on immune responses following xenograft meniscus transplantation, we established porcine meniscus transplantation (PMT) models to comprehensively assess the immunogenicity of porcine meniscus (PM) from both innate and adaptive immune perspectives. Our investigations confirmed that PMT beneath the epidermis led to innate cell infiltration into the xenografts and T-cell activation in local lymph nodes. T-cell activation upregulated the interleukin (IL)-17 signaling pathway, disrupting collagen organization and metabolic processes, thereby hindering PM regeneration. Using freeze-thaw treatment on PM alleviated T-cell activation post-transplantation by eliminating xenogenic DNA. In vitro findings demonstrated that gene editing in porcine meniscal cells (PMCs) suppressed human T-cell activation by downregulating the expression of xenoreactive Ag genes. These results suggest that GGTA1/CMAH/B4GALNT2 knockout (KO) pigs hold significant promise for advancing the field of meniscal transplantation.

Streptococcus pneumoniae drives specific and lasting Natural Killer cell memory

NK cells are important mediators of innate immunity and play an essential role for host protection against infection, although their responses to bacteria are poorly understood. Recently NK cells were shown to display memory properties, as characterized by an epigenetic signature leading to a stronger secondary response. Although NK cell memory could be a promising mechanism to fight against infection, it has not been described upon bacterial infection. Using a mouse model, we reveal that NK cells develop specific and long-term memory following sub-lethal infection with the extracellular pathogen Streptococcus pneumoniae. Memory NK cells display intrinsic sensing and response to bacteria in vitro, in a manner that is enhanced post-bacterial infection. In addition, their transfer into naïve mice confers protection from lethal infection for at least 12 weeks. Interestingly, NK cells display enhanced cytotoxic molecule production upon secondary stimulation and their protective role is dependent on Perforin and independent of IFNγ. Thus, our study identifies a new role for NK cells during bacterial infection, opening the possibility to harness innate immune memory for therapeutic purposes.

Potential targeted therapy based on deep insight into the relationship between the pulmonary microbiota and immune regulation in lung fibrosis

Pulmonary fibrosis is an irreversible disease, and its mechanism is unclear. The lung is a vital organ connecting the respiratory tract and the outside world. The changes in lung microbiota affect the progress of lung fibrosis. The latest research showed that lung microbiota differs in healthy people, including idiopathic pulmonary fibrosis (IPF) and acute exacerbation-idiopathic pulmonary fibrosis (AE-IPF). How to regulate the lung microbiota and whether the potential regulatory mechanism can become a necessary targeted treatment of IPF are unclear. Some studies showed that immune response and lung microbiota balance and maintain lung homeostasis. However, unbalanced lung homeostasis stimulates the immune response. The subsequent biological effects are closely related to lung fibrosis. Core fucosylation (CF), a significant protein functional modification, affects the lung microbiota. CF regulates immune protein modifications by regulating key inflammatory factors and signaling pathways generated after immune response. The treatment of immune regulation, such as antibiotic treatment, vitamin D supplementation, and exosome micro-RNAs, has achieved an initial effect in clearing the inflammatory storm induced by an immune response. Based on the above, the highlight of this review is clarifying the relationship between pulmonary microbiota and immune regulation and identifying the correlation between the two, the impact on pulmonary fibrosis, and potential therapeutic targets.

Airway Prevotella promote TLR2-dependent neutrophil activation and rapid clearance of Streptococcus pneumoniae from the lung

This study investigates how specific members of the lung microbiome influence the early immune response to infection. Prevotella species are a major component of the endogenous airway microbiota. Increased abundance of Prevotella melaninogenica correlates with reduced infection with the bacterial pathogen Streptococcus pneumoniae, indicating a potentially beneficial role. Here, we show that P. melaninogenica enhances protection against S. pneumoniae, resulting in rapid pathogen clearance from the lung and improved survival in a mouse lung co-infection model. This response requires recognition of P. melaninogenica lipoproteins by toll-like receptor (TLR)2, the induction of TNFα, and neutrophils, as the loss of any of these factors abrogates Prevotella-induced protection. Improved clearance of S. pneumoniae is associated with increased serine protease-mediated killing by lung neutrophils and restraint of P. melaninogenica-induced inflammation by IL-10 in co-infected mice. Together, these findings highlight innate immune priming by airway Prevotella as an important protective feature in the respiratory tract.

How the airway microbiome protects against bacterial pneumonia remains unclear. Here, the authors identify airway bacterial species that activate the immune system to facilitate rapid clearance of the pathogen Streptococcus pneumoniae from the lung.

Corynebacterium Species Inhibit Streptococcus pneumoniae Colonization and Infection of the Mouse Airway

The stability and composition of the airway microbiome is an important determinant of respiratory health. Some airway bacteria are considered to be beneficial due to their potential to impede the acquisition and persistence of opportunistic bacterial pathogens such as Streptococcus pneumoniae. Among such organisms, the presence of Corynebacterium species correlates with reduced S. pneumoniae in both adults and children, in whom Corynebacterium abundance is predictive of S. pneumoniae infection risk. Previously, Corynebacterium accolens was shown to express a lipase which cleaves host lipids, resulting in the production of fatty acids that inhibit growth of S. pneumoniae in vitro. However, it was unclear whether this mechanism contributes to Corynebacterium-S. pneumoniae interactions in vivo. To address this question, we developed a mouse model for Corynebacterium colonization in which colonization with either C. accolens or another species, Corynebacterium amycolatum, significantly reduced S. pneumoniae acquisition in the upper airway and infection in the lung. Moreover, the lungs of co-infected mice had reduced pro-inflammatory cytokines and inflammatory myeloid cells, indicating resolution of infection-associated inflammation. The inhibitory effect of C. accolens on S. pneumoniae in vivo was mediated by lipase-dependent and independent effects, indicating that both this and other bacterial factors contribute to Corynebacterium-mediated protection in the airway. We also identified a previously uncharacterized bacterial lipase in C. amycolatum that is required for inhibition of S. pneumoniae growth in vitro. Together, these findings demonstrate the protective potential of airway Corynebacterium species and establish a new model for investigating the impact of commensal microbiota, such as Corynebacterium, on maintaining respiratory health.

Role of IL-10-Producing Natural Killer Cells in the Regulatory Mechanisms of Inflammation during Systemic Infection

Natural killer (NK) cells have the dual ability to produce pro-inflammatory (IFNγ) and anti-inflammatory (IL-10) cytokines during systemic infection, which points to their crucial role both as inflammatory effectors for infection clearance and as regulators to counterbalance inflammation to limit immune-mediated damage to the host. In particular, immunosuppressive IL-10 secretion by NK cells has been described to occur in systemic, but not local, infections as a recent immunoregulatory mechanism of inflammation that may be detrimental or beneficial, depending on the timing of release, type of disease, or the infection model. Understanding the factors that drive the production of IL-10 by NK cells and their impact during dualistic inflammatory states, such as sepsis and other non-controlled inflammatory diseases, is relevant for achieving effective therapeutic advancements. In this review, the evidence regarding the immunoregulatory role of IL-10-producing NK cells in systemic infection is summarized and discussed in detail, and the potential molecular mechanisms that drive IL-10 production by NK cells are considered.

Involvement of TIGIT in Natural Killer Cell Exhaustion and Immune Escape in Patients and Mouse Model With Liver Echinococcus multilocularis Infection

BACKGROUND AND AIMS

Alveolar echinococcosis (AE) is a lethal helminthic liver disease caused by persistent infection with Echinococcus multilocularis. Although more attention has been paid to the immunotolerance of T cells caused by E. multilocularis infection, the role of natural killer (NK) cell, a critical player in liver immunity, is seldom studied.

APPROACH AND RESULTS

Here, we observed that NK cells from the blood and closed liver tissue (CLT) of AE patients expressed a higher level of inhibitory receptor TIGIT and were functionally exhausted with a lower expression of granzyme B, perforin, interferon-gamma (IFN-γ), and TNF-α. Addition of anti-TIGIT (T-cell immunoreceptor with immunoglobulin and immunoreceptor tyrosine-based inhibitory motif domain) monoclonal antibody into AE patients' peripheral blood mononuclear cell culture significantly enhanced the synthesis of IFN-γ and TNF-α by NK cells, indicating the reversion of exhausted NK cells by TIGIT blockade. In the mouse model of E. multilocularis infection, liver and splenic TIGIT⁺ NK cells progressively increased dependent of infection dosage and timing and were less activated and less degranulated with lower cytokine secretion. Furthermore, TIGIT deficiency or blockade in vivo inhibited liver metacestode growth, reduced liver injury, and increased the level of IFN-γ produced by liver NK cells. Interestingly, NK cells from mice with persistent chronic infection expressed a higher level of TIGIT compared to self-healing mice. To look further into the mechanisms, more regulatory CD56^bright and murine CD49a⁺ NK cells with higher TIGIT expression existed in livers of AE patients and mice infected with E. multilocularis, respectively. They coexpressed higher surface programmed death ligand 1 and secreted more IL-10, two strong inducers to mediate the functional exhaustion of NK cells.

CONCLUSIONS

Our results indicate that inhibitory receptor TIGIT is involved in NK cell exhaustion and immune escape from E. multilocularis infection.

Immune responses to bacterial lung infections and their implications for vaccination

The pulmonary immune system plays a vital role in protecting the delicate structures of gaseous exchange against invasion from bacterial pathogens. With antimicrobial resistance becoming an increasing concern, finding novel strategies to develop vaccines against bacterial lung diseases remains a top priority. In order to do so, a continued expansion of our understanding of the pulmonary immune response is warranted. Whilst some aspects are well characterised, emerging paradigms such as the importance of innate cells and inducible immune structures in mediating protection provide avenues of potential to rethink our approach to vaccine development. In this review, we aim to provide a broad overview of both the innate and adaptive immune mechanisms in place to protect the pulmonary tissue from invading bacterial organisms. We use specific examples from several infection models and human studies to depict the varying functions of the pulmonary immune system that may be manipulated in future vaccine development. Particular emphasis has been placed on emerging themes that are less reviewed and underappreciated in vaccine development studies.

Characteristics of tissue-resident ILCs and their potential as therapeutic targets in mucosal and skin inflammatory diseases

Discovery of innate lymphoid cells (ILCs), which are non-T and non-B lymphocytes that have no antigen-specific receptors, changed the classical concept of the mechanism of allergy, which had been explained mainly as antigen-specific acquired immunity based on IgE and Th2 cells. The discovery led to dramatic improvement in our understanding of the mechanism of non-IgE-mediated allergic inflammation. Numerous studies conducted in the past decade have elucidated the characteristics of each ILC subset in various organs and tissues and their ontogeny. We now know that each ILC subset exhibits heterogeneity. Moreover, the functions and activating/suppressing factors of each ILC subset were found to differ among both organs and types of tissue. Therefore, in this review, we summarize our current knowledge of ILCs by focusing on the organ/tissue-specific features of each subset to understand their roles in various organs. We also discuss ILCs' involvement in human inflammatory diseases in various organs and potential therapeutic/preventive strategies that target ILCs.

Characterization of the Anti-Inflammatory Capacity of IL-10-Producing Neutrophils in Response to Streptococcus pneumoniae Infection

Neutrophils are immune cells classically defined as pro-inflammatory effector cells. However, current accumulated evidence indicates that neutrophils have more versatile immune-modulating properties. During acute lung infection with Streptococcus pneumoniae in mice, interleukin-10 (IL-10) production is required to temper an excessive lung injury and to improve survival, yet the cellular source of IL-10 and the immunomodulatory role of neutrophils during S. pneumoniae infection remain unknown. Here we show that neutrophils are the main myeloid cells that produce IL-10 in the lungs during the first 48 h of infection. Importantly, in vitro assays with bone-marrow derived neutrophils confirmed that IL-10 can be induced by these cells by the direct recognition of pneumococcal antigens. In vivo, we identified the recruitment of two neutrophil subpopulations in the lungs following infection, which exhibited clear morphological differences and a distinctive profile of IL-10 production at 48 h post-infection. Furthermore, adoptive transfer of neutrophils from WT mice into IL-10 knockout mice (Il10^-/-) fully restored IL-10 production in the lungs and reduced lung histopathology. These results suggest that IL-10 production by neutrophils induced by S. pneumoniae limits lung injury and is important to mediate an effective immune response required for host survival.

Natural Killer Cells in Immunotherapy: Are We Nearly There?

Natural killer (NK) cells are potent anti-tumor and anti-microbial cells of our innate immune system. They are equipped with a vast array of receptors that recognize tumor cells and other pathogens. The innate immune activity of NK cells develops faster than the adaptive one performed by T cells, and studies suggest an important immunoregulatory role for each population against the other. The association, observed in acute myeloid leukemia patients receiving haploidentical killer-immunoglobulin-like-receptor-mismatched NK cells, with induction of complete remission was the determinant to begin an increasing number of clinical studies administering NK cells for the treatment of cancer patients. Unfortunately, even though transfused NK cells demonstrated safety, their observed efficacy was poor. In recent years, novel studies have emerged, combining NK cells with other immunotherapeutic agents, such as monoclonal antibodies, which might improve clinical efficacy. Moreover, genetically-modified NK cells aimed at arming NK cells with better efficacy and persistence have appeared as another option. Here, we review novel pre-clinical and clinical studies published in the last five years administering NK cells as a monotherapy and combined with other agents, and we also review chimeric antigen receptor-modified NK cells for the treatment of cancer patients. We then describe studies regarding the role of NK cells as anti-microbial effectors, as lessons that we could learn and apply in immunotherapy applications of NK cells; these studies highlight an important immunoregulatory role performed between T cells and NK cells that should be considered when designing immunotherapeutic strategies. Lastly, we highlight novel strategies that could be combined with NK cell immunotherapy to improve their targeting, activity, and persistence.

Airway Natural Killer Cells and Bacteria in Health and Disease

Natural killer (NK) cells are innate lymphoid cells at the interface between innate and adaptive immunity and mostly studied for their important roles in viral infections and malignant tumors. They can kill diseased cells and produce cytokines and chemokines, thereby shaping the adaptive immune response. Nowadays, NK cells are considered as a strong weapon for cancer immunotherapy and can for example be transduced to express tumor-specific chimeric antigen receptors or harnessed with therapeutic antibodies such as the so-called NK engagers. Whereas a large body of literature exists about the antiviral and antitumoral properties of NK cells, their potential role in bacterial infections is not that well delineated. Furthermore, NK cells are much more heterogeneous than previously thought and have tissue-characteristic features and phenotypes. This review gives an overview of airway NK cells and their position within the immunological army dressed against bacterial infections in the upper and predominantly the lower respiratory tracts. Whereas it appears that in several infections, NK cells play a non-redundant and protective role, they can likewise act as rather detrimental. The use of mouse models and the difficulty of access to human airway tissues for ethical reasons might partly explain the divergent results. However, new methods are appearing that are likely to reduce the heterogeneity between studies and to give a more coherent picture in this field.