Hobit expression by a subset of human liver-resident CD56bright Natural Killer cells

Liver type 1 innate lymphoid cells undergo apoptosis in murine models of macrophage activation syndrome and are dispensable for disease

Macrophage activation syndrome (MAS) exemplifies a severe cytokine storm disorder with liver inflammation. In the liver, classical natural killer (cNK) cells and liver-resident type 1 innate lymphoid cells (ILC1s) dominate the ILC population. Thus far, research has primarily focused on the corresponding role of cNK cells. Considering the liver inflammation and cytokine storm in MAS, liver-resident ILC1s represent an interesting population to explore due to their rapid cytokine production upon environmental triggers. By utilizing a Toll-like receptor (TLR)9- and TLR3:4-triggered MAS model, we showed that ILC1s highly produce IFN-γ and TNF-α. However, activated ILC1s undergo apoptosis and are strongly reduced in numbers, while cNK cells resist inflammation-induced apoptosis. Signs of mitochondrial stress suggest that this ILC1 apoptosis may be driven by inflammation-induced mitochondrial impairment. To study whether early induction of highly cytokine-producing ILC1s influences MAS development, we used Hobit KO mice due to their paucity of liver ILC1s but unaffected cNK cell numbers. Nevertheless, neither the severity of MAS features nor the total inflammatory cytokine levels were affected in these Hobit KO mice, indicating that ILC1s are dispensable for MAS pathogenesis. Collectively, our data demonstrate that ILC1s undergo apoptosis during TLR-triggering and are dispensable for MAS pathogenesis.

Decidual leukocytes respond to African lineage Zika virus infection with mild anti-inflammatory changes during acute infection in rhesus macaques

Zika virus (ZIKV) can be vertically transmitted during pregnancy resulting in a range of adverse pregnancy outcomes. The decidua is commonly found to be infected by ZIKV, yet the acute immune response to infection remains understudied in vivo. We hypothesized that in vivo African-lineage ZIKV infection induces a pro-inflammatory response in the decidua. To test this hypothesis, we evaluated the decidua in pregnant rhesus macaques within the first two weeks following infection with an African-lineage ZIKV and compared our findings to gestationally aged-matched controls. Decidual leukocytes were phenotypically evaluated using spectral flow cytometry, and cytokines and chemokines were measured in tissue homogenates from the decidua, placenta, and fetal membranes. The results of this study did not support our hypothesis. Although ZIKV RNA was detected in the decidual tissue samples from all ZIKV infected dams, phenotypic changes in decidual leukocytes and differences in cytokine profiles suggest that the decidua undergoes mild anti-inflammatory changes in response to that infection. Our findings emphasize the immunological state of the gravid uterus as a relatively immune privileged site that prioritizes tolerance of the fetus over mounting a pro-inflammatory response to clear infection.

The co-inhibitory receptor TIGIT regulates NK cell function and is upregulated in human intrahepatic CD56bright NK cells

The crosstalk between NK cells and their surrounding environment is enabled through activating and inhibitory receptors, which tightly control NK cell activity. The co-inhibitory receptor TIGIT decreases NK cell cytotoxicity and is involved in NK cell exhaustion, but has also been associated with liver regeneration, highlighting that the contribution of human intrahepatic CD56bright NK cells in regulating tissue homeostasis remains incompletely understood. A targeted single-cell mRNA analysis revealed distinct transcriptional differences between matched human peripheral blood and intrahepatic CD56bright NK cells. Multiparameter flow cytometry identified a cluster of intrahepatic NK cells with overlapping high expression of CD56, CD69, CXCR6, TIGIT and CD96. Intrahepatic CD56bright NK cells also expressed significantly higher protein surface levels of TIGIT, and significantly lower levels of DNAM-1 compared to matched peripheral blood CD56bright NK cells. TIGIT+ CD56bright NK cells showed diminished degranulation and TNF-α production following stimulation. Co-incubation of peripheral blood CD56bright NK cells with human hepatoma cells or primary human hepatocyte organoids resulted in migration of NK cells into hepatocyte organoids and upregulation of TIGIT and downregulation of DNAM-1 expression, in line with the phenotype of intrahepatic CD56bright NK cells. Intrahepatic CD56bright NK cells represent a transcriptionally, phenotypically, and functionally distinct population of NK cells that expresses higher levels of TIGIT and lower levels of DNAM-1 than matched peripheral blood CD56bright NK cells. Increased expression of inhibitory receptors by NK cells within the liver environment can contribute to tissue homeostasis and reduction of liver inflammation.

Single-cell transcriptomics in bone marrow delineates CD56dimGranzymeK+ subset as intermediate stage in NK cell differentiation

Human natural killer (NK) cells in lymphoid tissues can be categorized into three subsets: CD56^brightCD16⁺, CD56^dimCD16⁺ and CD69⁺CXCR6⁺ lymphoid tissue-resident (lt)NK cells. How the three subsets are functionally and developmentally related is currently unknown. Therefore, we performed single-cell RNA sequencing combined with oligonucleotide-conjugated antibodies against CD56, CXCR6, CD117 and CD34 on fresh bone marrow NK cells. A minor CD56^dimGzmK⁺ subset was identified that shared features with CD56^bright and CD56^dimGzmK^- NK cells based on transcriptome, phenotype (NKG2A^highCD16^lowKLRG1^highTIGIT^high) and functional analysis in bone marrow and blood, supportive for an intermediate subset. Pseudotime analysis positioned CD56^bright, CD56^dimGzmK⁺ and CD56^dimGzmK^- cells in one differentiation trajectory, while ltNK cells were developmentally separated. Integrative analysis with bone marrow cells from the Human Cell Atlas did not demonstrate a developmental connection between CD34⁺ progenitor and NK cells, suggesting absence of early NK cell stages in bone marrow. In conclusion, single-cell transcriptomics provide new insights on development and differentiation of human NK cells.

Characteristic Genes and Immune Infiltration Analysis for Acute Rejection after Kidney Transplantation

Background

Renal transplantation can significantly improve the survival rate and quality of life of patients with end-stage renal disease, but the probability of acute rejection (AR) in adult renal transplant recipients is still approximately 12.2%. Machine learning (ML) is superior to traditional statistical methods in various clinical scenarios. However, the current AR model is constructed only through simple difference analysis or a single queue, which cannot guarantee the accuracy of prediction. Therefore, this study identified and validated new gene sets that contribute to the early prediction of AR and the prognosis prediction of patients after renal transplantation by constructing a more accurate AR gene signature through ML technology.

Methods

Based on the Gene Expression Omnibus (GEO) database and multiple bioinformatic analyses, we identified differentially expressed genes (DEGs) and built a gene signature via LASSO regression and SVM analysis. Immune cell infiltration and immunocyte association analyses were also conducted. Furthermore, we investigated the relationship between AR genes and graft survival status.

Results

Twenty-four DEGs were identified. A 5 gene signature (CPA6, EFNA1, HBM, THEM5, and ZNF683) were obtained by LASSO analysis and SVM analysis, which had a satisfied ability to differentiate AR and NAR in the training cohort, internal validation cohort and external validation cohort. Additionally, ZNF683 was associated with graft survival.

Conclusion

A 5 gene signature, particularly ZNF683, provided insight into a precise therapeutic schedule and clinical applications for AR patients.

Intestinal CD8+ tissue-resident memory T cells: From generation to function

Tissue-resident memory T cells (Trm), and particularly the CD8⁺ subset, have been shown to play a pivotal role in protection against infections and tumors. Studies in animal models and human tissues have highlighted that, while a core functional program is shared by Trm at all anatomical sites, distinct tissues imprint unique features through specific molecular cues. The intestinal tissue is often the target of pathogens for local proliferation and penetration into the host systemic circulation, as well as a prominent site of tumorigenesis. Therefore, promoting the formation of Trm at this location is an appealing therapeutic option. The various segments composing the gastrointestinal tract present distinctive histological and functional characteristics, which may reflect on the imprinting of unique functional features in the respective Trm populations. What these features are, and whether they can effectively be harnessed to promote local and systemic immunity, is still under investigation. Here, we review how Trm are generated and maintained in distinct intestinal niches, analyzing the required molecular signals and the models utilized to uncover them. We also discuss evidence for a protective role of Trm against infectious agents and tumors. Finally, we integrate the knowledge obtained from animal models with that gathered from human studies.

The Functional Diversity of Tissue-Resident Natural Killer Cells Against Infection

For decades, studies of natural killer (NK) cells have focused on those found in peripheral blood (PBNK cells) as the prototype for NK cell biology. Only recently have researchers begun to explore the diversity of tissue-resident NK (tr-NK) cells. While tr-NK cells were initially identified from mice parabiosis and intravascular staining experiments, they can also be identified by tissue retention markers such as CD69, CD103, and others. More importantly, tr-NK cells have distinct functions compared to PBNK cells. Within the liver, there are diverse subsets of tr-NK cells expressing different combinations of tissue-retention markers and transcription factors, the clinical relevance of which are still unclear. Functionally, liver tr-NK are primed with immediate responsiveness to infection and equipped with regulatory mechanisms to prevent liver damage. When decidual NK (dNK) cells were first discovered, they were mainly characterized by their reduced cytotoxicity and functions related to placental development. Recent studies, however, revealed different mechanisms by which dNK cells prevent uterine infections. The lungs are one of the most highly exposed sites for infection due to their role in oxygen exchange. Upon influenza infection, lung tr-NK cells can degranulate and produce more inflammatory cytokines than PBNK cells. Less understood are gut tr-NK cells which were recently characterized in infants and adults for their functional differences. In this mini-review, we aim to provide a brief overview of the most recent discoveries on how several tr-NK cells are implicated in the immune response against infection. This article is protected by copyright. All rights reserved.

The tumour microenvironment shapes innate lymphoid cells in patients with hepatocellular carcinoma

OBJECTIVE

Hepatocellular carcinoma (HCC) represents a typical inflammation-associated cancer. Tissue resident innate lymphoid cells (ILCs) have been suggested to control tumour surveillance. Here, we studied how the local cytokine milieu controls ILCs in HCC.

DESIGN

We performed bulk RNA sequencing of HCC tissue as well as flow cytometry and single-cell RNA sequencing of enriched ILCs from non-tumour liver, margin and tumour core derived from 48 patients with HCC. Simultaneous measurement of protein and RNA expression at the single-cell level (AbSeq) identified precise signatures of ILC subgroups. In vitro culturing of ILCs was used to validate findings from in silico analysis. Analysis of RNA-sequencing data from large HCC cohorts allowed stratification and survival analysis based on transcriptomic signatures.

RESULTS

RNA sequencing of tumour, non-tumour and margin identified tumour-dependent gradients, which were associated with poor survival and control of ILC plasticity. Single-cell RNA sequencing and flow cytometry of ILCs from HCC livers identified natural killer (NK)-like cells in the non-tumour tissue, losing their cytotoxic profile as they transitioned into tumour ILC1 and NK-like-ILC3 cells. Tumour ILC composition was mediated by cytokine gradients that directed ILC plasticity towards activated tumour ILC2s. This was liver-specific and not seen in ILCs from peripheral blood mononuclear cells. Patients with high ILC2/ILC1 ratio expressed interleukin-33 in the tumour that promoted ILC2 generation, which was associated with better survival.

CONCLUSION

Our results suggest that the tumour cytokine milieu controls ILC composition and HCC outcome. Specific changes of cytokines modify ILC composition in the tumour by inducing plasticity and alter ILC function.

Crosstalk between NK cells and hepatic stellate cells in liver fibrosis (Review)

Liver fibrosis is a common pathological process of chronic liver diseases, including viral hepatitis and alcoholic liver disease, and ultimately progresses to irreversible cirrhosis and cancer. Hepatic stellate cells (HSCs) are activated to produce amounts of collagens in response to liver injury, thus triggering the initiation and progression of fibrogenesis. Natural killer (NK) cells serve as the essential component of hepatic innate immunity and are considered to alleviate fibrosis by killing activated HSCs. Current antifibrotic interventions have improved fibrosis, but fail to halt its progression in the advanced stage. Clarifying the interaction between NK cells and HSCs will provide clues to the pathogenesis and potential therapies for advanced liver fibrosis.

Tissue-Resident Immune Cells in Humans

Tissue-resident immune cells span both myeloid and lymphoid cell lineages, have been found in multiple human tissues, and play integral roles at all stages of the immune response, from maintaining homeostasis to responding to infectious challenges to resolution of inflammation to tissue repair. In humans, studying immune cells and responses in tissues is challenging, although recent advances in sampling and high-dimensional profiling have provided new insights into the ontogeny, maintenance, and functional role of tissue-resident immune cells. Each tissue contains a specific complement of resident immune cells. Moreover, resident immune cells for each lineage share core properties, along with tissue-specific adaptations. Here we propose a five-point checklist for defining resident immune cell types in humans and describe the currently known features of resident immune cells, their mechanisms of development, and their putative functional roles within various human organs. We also consider these aspects of resident immune cells in the context of future studies and therapeutics.

Natural Killer Cells in Hepatic Ischemia-Reperfusion Injury

Ischemia-reperfusion injury can be divided into two phases, including insufficient supply of oxygen and nutrients in the first stage and then organ injury caused by immune inflammation after blood flow recovery. Hepatic ischemia-reperfusion is an important cause of liver injury post-surgery, consisting of partial hepatectomy and liver transplantation, and a central driver of graft dysfunction, which greatly leads to complications and mortality after liver transplantation. Natural killer (NK) cells are the lymphocyte population mainly involved in innate immune response in the human liver. In addition to their well-known role in anti-virus and anti-tumor defense, NK cells are also considered to regulate the pathogenesis of liver ischemia-reperfusion injury under the support of more and more evidence recently. The infiltration of NK cells into the liver exacerbates the hepatic ischemia-reperfusion injury, which could be significantly alleviated after depletion of NK cells. Interestingly, NK cells may contribute to both liver graft rejection and tolerance according to their origins. In this article, we discussed the development of liver NK cells, their role in ischemia-reperfusion injury, and strategies of inhibiting NK cell activation in order to provide potential possibilities for translation application in future clinical practice.

Human NK cells, their receptors and function

NK cells are cytotoxic components of innate lymphoid cells (ILC) that provide a first line of defense against viral infections and contribute to control tumor growth and metastasis. Their function is finely regulated by an array of HLA-specific and non-HLA-specific inhibitory and activating receptors which allow to discriminate between healthy and altered cells. Human NK cells gained a major attention in recent years because of the important progresses in understanding their biology and of some promising data in tumor therapy. In this review, we will outline well-established issues of human NK cells and discuss some of the open questions, debates, and recent advances regarding their origin, differentiation, and tissue distribution. Newly defined NK cell specializations, including the impact of inhibitory checkpoints on their function, their crosstalk with other cell types, and the remarkable adaptive features acquired in response to certain virus infections will also be discussed.

Discipline in Stages: Regulating CD8+ Resident Memory T Cells

Resident memory CD8⁺ T (T_RM) cells are a lymphocyte lineage distinct from circulating memory CD8⁺ T cells. T_RM lodge within peripheral tissues and secondary lymphoid organs where they provide rapid, local protection from pathogens and control tumor growth. However, dysregulation of CD8⁺ T_RM formation and/or activation may contribute to the pathogenesis of autoimmune diseases. Intrinsic mechanisms, including transcriptional networks and inhibitory checkpoint receptors control T_RM differentiation and response. Additionally, extrinsic stimuli such as cytokines, cognate antigen, fatty acids, and damage signals regulate T_RM formation, maintenance, and expansion. In this review, we will summarize knowledge of CD8⁺ T_RM generation and highlight mechanisms that regulate the persistence and responses of heterogeneous T_RM populations in different tissues and distinct microenvironments.

Natural Killer Cell Integrins and Their Functions in Tissue Residency

Integrins are transmembrane receptors associated with adhesion and migration and are often highly differentially expressed receptors amongst natural killer cell subsets in microenvironments. Tissue resident natural killer cells are frequently defined by their differential integrin expression compared to other NK cell subsets, and integrins can further localize tissue resident NK cells to tissue microenvironments. As such, integrins play important roles in both the phenotypic and functional identity of NK cell subsets. Here we review the expression of integrin subtypes on NK cells and NK cell subsets with the goal of better understanding how integrin selection can dictate tissue residency and mediate function from the nanoscale to the tissue environment.

Role of Natural Killer Cells in Uveal Melanoma

Simple Summary

Metastatic Uveal Melanoma (MUM) is a lethal malignancy with no durable treatment available to date. A vast majority of patients with MUM present with liver metastasis. The liver harbors metastatic disease with an apparent lack of a cytotoxic T cell response. It is becoming evident that MUM is not an immunologically silent malignancy and the investigation of non-T cell anti-tumor immunity is warranted. In this review, we highlight the relevance of Natural Killer (NK) cells in the biology and treatment of MUM. Potent anti-NK cell immunosuppression employed by uveal melanoma alludes to its vulnerability to NK cell cytotoxicity. On the contrary, micro-metastasis in the liver survive for several years within close vicinity of a plethora of circulating and liver-resident NK cells. This review provides unique perspectives into the potential role of NK cells in control or progression of uveal melanoma.

Abstract

Uveal melanoma has a high mortality rate following metastasis to the liver. Despite advances in systemic immune therapy, treatment of metastatic uveal melanoma (MUM) has failed to achieve long term durable responses. Barriers to success with immune therapy include the immune regulatory nature of uveal melanoma as well as the immune tolerant environment of the liver. To adequately harness the anti-tumor potential of the immune system, non-T cell-based approaches need to be explored. Natural Killer (NK) cells possess potent ability to target tumor cells via innate and adaptive responses. In this review, we discuss evidence that highlights the role of NK cell surveillance and targeting of uveal melanoma. We also discuss the repertoire of intra-hepatic NK cells. The human liver has a vast and diverse lymphoid population and NK cells comprise 50% of the hepatic lymphocytes. Hepatic NK cells share a common niche with uveal melanoma micro-metastasis within the liver sinusoids. It is, therefore, crucial to understand and investigate the role of intra-hepatic NK cells in the control or progression of MUM.

Tissue-Resident NK Cells: Development, Maturation, and Clinical Relevance

Natural killer (NK) cells belong to type 1 innate lymphoid cells (ILC1) and are essential in killing infected or transformed cells. NK cells mediate their effector functions using non-clonotypic germ-line-encoded activation receptors. The utilization of non-polymorphic and conserved activating receptors promoted the conceptual dogma that NK cells are homogeneous with limited but focused immune functions. However, emerging studies reveal that NK cells are highly heterogeneous with divergent immune functions. A distinct combination of several activation and inhibitory receptors form a diverse array of NK cell subsets in both humans and mice. Importantly, one of the central factors that determine NK cell heterogeneity and their divergent functions is their tissue residency. Decades of studies provided strong support that NK cells develop in the bone marrow. However, evolving evidence supports the notion that NK cells also develop and differentiate in tissues. Here, we summarize the molecular basis, phenotypic signatures, and functions of tissue-resident NK cells and compare them with conventional NK cells.

Helper-like Innate Lymphoid Cells in Humans and Mice

The innate lymphoid cell (ILC) family consists of natural killer (NK) cells, helper-like lymphoid cells (ILC1s, ILC2s, and ILC3s), and lymphoid tissue inducer (LTi) cells. Helper-like ILCs are considered the innate counterpart of T-helper cells because of similarities in their cytokine output and expression of key transcription factors. ILCs provide and regulate innate immune functions before the development of adaptive immunity. They are involved in host defense against pathogens, inflammation, tissue repair, and metabolic homeostasis. However, they can also be involved in inflammatory disorders and carcinogenesis. In this review, we summarize the latest research on ILC development and plasticity in humans and mice, focusing on the pathogenic role of helper-like ILCs in inflammatory disorders, such as asthma, Crohn's disease (CD), and rheumatoid arthritis (RA).

Sense and immuno-sensibility: innate lymphoid cell niches and circuits

Tissue-resident lymphocytes that lack expression of rearranged antigen receptors and are lineage negative for classical T and B cell markers are collectively known as innate lymphoid cells (ILCs). The ILC family is remarkably heterogeneous and exhibits plasticity; however, mature ILCs can be grouped based on their steady state expression of distinct surface receptors and transcription factors as well as production of signature cytokines following activation. The study of ILC subsets in mouse and human tissues has revealed that the elicitation and magnitude of their effector functions are determined by a combination of extrinsic cues specific to the niches in which they reside. In this short review, we will summarize some recent findings related to tissue-specific signals that govern ILC responses and localization.

True Detective: Unraveling Group 1 Innate Lymphocyte Heterogeneity

Innate lymphoid cells (ILCs) consist of a heterogeneous family of lymphocytes that regulate tissue homeostasis and can contribute to pathology in mice and humans. Mammalian group 1 ILCs are defined by the production of interferon (IFN)-γ and the functional dependence on the transcription factor T-bet. While recent studies demonstrate that group 1 ILCs consist of circulating mature natural killer (NK) cells and tissue-resident ILC1s, the functional, phenotypic, and developmental properties that distinguish these two cell lineages are often confusing and difficult to dissect. In this review, we critically evaluate the current knowledge of mammalian group 1 ILC heterogeneity and propose new inclusive nomenclature to clarify the roles of ILC1s and NK cells during homeostasis and disease.

The Roles of Liver-Resident Lymphocytes in Liver Diseases

Tissue-resident lymphocytes usually reside in barrier sites and are involved in innate and adaptive immunity. In recent years, many studies have shown that multiple types of lymphocytes are resident in the liver, including memory CD8⁺ T (T_RM) cells; "unconventional" T cells, such as invariant natural killer T (iNKT) cells, mucosal associated invariant T (MAIT) cells, and γδT cells; innate lymphoid cells (ILCs) such as natural killer (NK) cells and other ILCs. Although diverse types of tissue-resident lymphocytes share similar phenotypes, functional properties, and transcriptional regulation, the unique microenvironment of the liver can reshape their phenotypic and functional characteristics. Liver-resident lymphocytes serve as sentinels and perform immunosurveillance in response to infection and non-infectious insults, and are involved in the maintenance of liver homeostasis. Under the pathological conditions, distinct liver-resident lymphocytes exert protective or pathological effects in the process of various liver diseases. In this review, we highlight the unique properties of liver-resident lymphocytes, and discuss their functional characteristics in different liver diseases.

Liver-Derived TGF-β Maintains the EomeshiTbetlo Phenotype of Liver Resident Natural Killer Cells

The adult human liver hosts a complex repertoire of liver resident and transient natural killer (NK) cell populations with diverse phenotypes and functions. Liver resident NK cells are CD56^bright NK cells defined by a unique expression profile of transcription factors and cell surface markers (Eomes^hiTbet^loTIGIT⁺CD69⁺CXCR6⁺CD49e⁻). Despite extensive characterization of the phenotype of liver resident NK cells, it remains unclear how factors within the liver microenvironment induce and maintain this unique phenotype. In this study, we have explored the factors regulating the phenotype of liver resident NK cells. Isolation of healthy liver resident NK cells from donor liver perfusate and in vitro culture results in the gradual loss of the characteristic Tbet^lo phenotype, with the cells increasing Tbet expression significantly at day 7. This phenotypic loss could be halted through the dose-dependent addition of liver conditioned media (LCM), generated from the ex vivo culture of liver biopsies from healthy organ donors. TGF-β, but not IL-10, replicated the Tbet suppressive effects of LCM in both liver resident and peripheral blood NK cells. Furthermore, blocking TGF-β receptor signaling using the inhibitor SB431542, reversed the effect of LCM treatment on liver resident NK cells, causing the loss of tissue resident Eomes^hi Tbet^lo phenotype. Our findings identify liver-derived TGF-β as an important component of the liver microenvironment, which acts to regulate and maintain the phenotype of liver resident NK cells.

CD49a Expression Identifies a Subset of Intrahepatic Macrophages in Humans

Macrophages play central roles in inflammatory reactions and initiation of immune responses during infections. More than 80% of total tissue macrophages are described to be located in the liver as liver-resident macrophages, also named Kupffer cells (KCs). While studies in mice have established a central role of liver-resident KCs in regulating liver inflammation, their phenotype and function are not well-characterized in humans. Comparing paired human liver and peripheral blood samples, we observed significant differences in the distribution of macrophage (Mφ) subsets, with lower frequencies of CD14^hiCD16^lo and higher frequencies of CD14^int−hiCD16^int Mφ in human livers. Intrahepatic Mφ consisted of diverse subsets with differential expression of CD49a, a liver-residency marker previously described for human and mice NK cells, and VSIG4 and/or MARCO, two recently described human tissue Mφ markers. Furthermore, intrahepatic CD49a⁺ Mφ expressed significantly higher levels of maturation and activation markers, exhibited higher baseline levels of TNF-α, IL-12, and IL-10 production, but responded less to additional in vitro TLR stimulation. In contrast, intrahepatic CD49a⁻ Mφ were highly responsive to stimulation with TLR ligands, similar to what was observed for CD49a⁻ monocytes (MOs) in peripheral blood. Taken together, these studies identified populations of CD49a⁺, VSIG4⁺, and/or MARCO⁺ Mφ in human livers, and demonstrated that intrahepatic CD49a⁺ Mφ differed in phenotype and function from intrahepatic CD49a⁻ Mφ as well as from peripheral blood-derived monocytes.

Parallel worlds of the adaptive and innate immune cell networks

Adaptive and innate immune cells have typically been functionally and temporally segregated even though they share a number of salient features. Over the past decade, significant advances have been made in understanding the composition and diversity of both innate and adaptive cell populations. This has shed light on how cells from two distinct pathways are so highly complementary. Innate lymphoid cells (ILCs) are pivotally positioned in tissues to form a stable population akin to tissue-resident T cells that protects the body. Nevertheless, the pathway by which different lymphocytes enter tissues, terminally differentiate and are replenished to maintain populations remains incompletely understood. Recent evidence challenges our assumptions about the sedentary lifestyles of so called 'tissue-resident cells' and pushes us to consider their roles in orchestrating protection of the immune system beyond the classical models.

Hepatic Natural Killer Cells: Organ-Specific Sentinels of Liver Immune Homeostasis and Physiopathology

The liver is considered a preferential tissue for NK cells residency. In humans, almost 50% of all intrahepatic lymphocytes are NK cells that are strongly imprinted in a liver-specific manner and show a broad spectrum of cellular heterogeneity. Hepatic NK (he-NK) cells play key roles in tuning liver immune response in both physiological and pathological conditions. Therefore, there is a pressing need to comprehensively characterize human he-NK cells to better understand the related mechanisms regulating their effector-functions within the dynamic balance between immune-tolerance and immune-surveillance. This is of particular relevance in the liver that is the only solid organ whose parenchyma is constantly challenged on daily basis by millions of foreign antigens drained from the gut. Therefore, the present review summarizes our current knowledge on he-NK cells in the light of the latest discoveries in the field of NK cell biology and clinical relevance.

Tissue-resident Eomes+ NK cells are the major innate lymphoid cell population in human infant intestine

Innate lymphoid cells (ILC), including natural killer (NK) cells, are implicated in host-defense and tissue-growth. However, the composition and kinetics of NK cells in the intestine during the first year of life, when infants are first broadly exposed to exogenous antigens, are still unclear. Here we show that CD103⁺ NK cells are the major ILC population in the small intestines of infants. When compared to adult intestinal NK cells, infant intestinal NK cells exhibit a robust effector phenotype, characterized by Eomes, perforin and granzyme B expression, and superior degranulation capacity. Absolute intestinal NK cell numbers decrease gradually during the first year of life, coinciding with an influx of intestinal Eomes⁺ T cells; by contrast, epithelial NKp44⁺CD69⁺ NK cells with less cytotoxic capacity persist in adults. In conclusion, NK cells are abundant in infant intestines, where they can provide effector functions while Eomes⁺ T cell responses mature.

Innate lymphoid cells (ILC), including natural killer (NK) cells, are important innate immune regulators. Here the authors show that, in human infant intestines, CD103⁺Eomes⁺ NK cells are the predominant ILC population, but are replaced gradually by Eomes⁺ T cells, while NKp44⁺ NK cells persist in adult intestines.

Human liver-derived CXCR6+ NK cells are predominantly educated through NKG2A and show reduced cytokine production

NK cells have been implicated to affect the outcome of numerous liver diseases. In particular, members of the killer-cell Ig-like receptor (KIR) family, predominantly expressed by NK cells, have been associated with the outcome of hepatitis C virus infection and clearance of hepatocellular carcinoma. Inhibitory KIRs tune NK cell function through interaction with HLA class I, a process termed education. Nevertheless, the impact of the hepatic environment on NK cell education is incompletely understood. Therefore, we investigated the composition and function of hepatic KIR-expressing NK cells. Matched PBMC and hepatic lymphocytes were isolated from 20 individuals undergoing liver surgery and subsequently phenotypically analyzed for expression of KIRs and markers for tissue residency using flow cytometry. NK cell function was determined by co-culturing NK cells with the target cell line 721.221 and subsequent assessment of CD107a, IFN-γ, and TNF-α expression. Liver-resident CXCR6⁺ /CD56^Bright NK cells lacked KIRs and were predominantly educated through NKG2A, while CXCR6^- /CD16⁺ NK cells expressed KIRs and resembled peripheral blood NK cells. Hepatic NK cells showed lower response rates compared to peripheral blood NK cells; in particular, CXCR6⁺ NK cells were hyporesponsive to stimulation with target cells. The high proportion of educated NK cells in both subsets indicates the importance of self-inhibitory receptors for the balance between maintenance of self-tolerance and functional readiness. However, the reduced functionality of hepatic NK cells may reflect the impact of the tolerogenic hepatic environment on NK cells irrespective of NK cell education.

Tissue-Resident Lymphocytes in Solid Organ Transplantation: Innocent Passengers or the Key to Organ Transplant Survival?

Short-term outcomes of solid organ transplantation have improved dramatically over the past several decades; however, long-term survival has remained static over the same period, and chronic rejection remains a major cause of graft failure. The importance of donor, or "passenger," lymphocytes to the induction of tolerance to allografts was recognized in the 1990s, but their precise contribution to graft acceptance or rejection has not been elucidated. Recently, specialized populations of tissue-resident lymphocytes in nonlymphoid organs have been described. These lymphocytes include tissue-resident memory T cells, regulatory T cells, γδ T cells, invariant natural killer T cells, and innate lymphoid cells. These cells reside in commonly transplanted solid organs, including the liver, kidneys, heart, and lung; however, their contribution to graft acceptance or rejection has not been examined in detail. Similarly, it is unclear whether tissue-resident cells derived from the pool of recipient-derived lymphocytes play a specific role in transplantation biology. This review summarizes the evidence for the roles of tissue-resident lymphocytes in transplant immunology, focussing on their features, functions, and relevance for solid organ transplantation, with specific reference to liver, kidney, heart, and lung transplantation.

Molecular Mechanisms Directing Migration and Retention of Natural Killer Cells in Human Tissues

A large body of data shows that Natural Killer (NK) cells are immune effectors exerting a potent cytolytic activity against tumors and virus infected cells. The discovery and characterization of several inhibitory and activating receptors unveiled most of the mechanisms allowing NK cells to spare healthy cells while selectively attacking abnormal tissues. Nevertheless, the mechanisms ruling NK cell subset recirculation among the different compartments of human body have only lately started to be investigated. This is particularly true for pathological settings such as tumors or infected tissues but also for para-physiological condition like pregnant human uterine mucosa. It is becoming evident that the microenvironment associated to a particular clinical condition can deeply influence the migratory capabilities of NK cells. In this review we describe the main mechanisms and stimuli known to regulate the expression of chemokine receptors and other molecules involved in NK cell homing to either normal or pathological/inflamed tissues, including tumors or organs such as lung and liver. We will also discuss the role played by the chemokine/chemokine receptor axes in the orchestration of physiological events such as NK cell differentiation, lymphoid organ retention/egress and recruitment to decidua during pregnancy.

Immune function of plasmacytoid dendritic cells, natural killer cells, and their crosstalk in HBV infection

Chronic hepatitis B virus infection is a major health problem, with over 245 million chronic carriers worldwide. This persistent infection is thought to be associated with inefficient innate and adaptive immune responses. Natural killer cells (NK cells) and plasmacytoid dendritic cells (pDCs) are the major innate immune cells which respond to viral infection at the early phase and are considered major components of the antiviral immune response. In this review, we summarize recent findings regarding the role of NK cells, pDCs, and their cross-talk in HBV infection and its chronicity. Although the data regarding the biological function of pDCs and NK cells in HBV infection is still controversial, many studies show that in chronic HBV infection, the cytotoxicity of NK cells is retained, while their capacity to secrete cytokines is strongly impaired. In addition, interferon-α production by pDCs is impaired during chronic HBV infection, and the virus interferes with pDC-NK cell interaction.

Tissue-resident NK cells differ in their expression profile of the nutrient transporters Glut1, CD98 and CD71

Metabolism is a critical basis for immune cell functionality. It was recently shown that NK cell subsets from peripheral blood modulate their expression of nutrient receptors following cytokine stimulation, demonstrating that NK cells can adjust to changes in metabolic requirements. As nutrient availability in blood and tissues can significantly differ, we examined NK cells isolated from paired blood-liver and blood-spleen samples and compared expression of the nutrient transporters Glut1, CD98 and CD71. CD56^bright tissue-resident (CXCR6⁺) NK cells derived from livers and spleens expressed lower levels of Glut1 but higher levels of the amino acid transporter CD98 following stimulation than CD56^bright NK cells from peripheral blood. In line with that, CD56^dim NK cells, which constitute the main NK cell population in the peripheral blood, expressed higher levels of Glut1 and lower levels of CD98 and CD71 compared to liver CD56^bright NK cells. Our results show that NK cells from peripheral blood differ from liver- and spleen-resident NK cells in the expression profile of nutrient transporters, consistent with a cell-adaptation to the different nutritional environment in these compartments.

Peripheral Tissue Chemokines: Homeostatic Control of Immune Surveillance T Cells

Cellular immunity is governed by a complex network of migratory cues that enable appropriate immune cell responses in a timely and spatially controlled fashion. This review focuses on the chemokines and their receptors regulating the steady-state localisation of immune cells within healthy peripheral tissues. Steady-state immune cell traffic is not well understood but is thought to involve constitutive (homeostatic) chemokines. The recent discovery of tissue-resident memory T cells (T_RM cells) illustrates our need for understanding how chemokines control immune cell mobilisation and/or retention. These studies will be critical to unravel novel pathways for preserving tissue function (aging) and preventing tissue disease (vaccination).