Infection induces tissue-resident memory NK cells that safeguard tissue health

Generation of an Inhibitory NK Cell Subset by TGF-β1/IL-15 Polarization

NK cells have been shown to exhibit inflammatory and immunoregulatory functions in a variety of healthy and diseased settings. In the context of chronic viral infection and cancer, distinct NK cell populations that inhibit adaptive immune responses have been observed. To understand how these cells arise and further characterize their immunosuppressive role, we examined in vitro conditions that could polarize human NK cells into an inhibitory subset. TGF-β1 has been shown to induce regulatory T cells in vitro and in vivo; we therefore investigated if TGF-β1 could also induce immunosuppressive NK-like cells. First, we found that TGF-β1/IL-15, but not IL-15 alone, induced CD103+CD49a+ NK-like cells from peripheral blood NK cells, which expressed markers previously associated with inhibitory CD56+ innate lymphoid cells, including high expression of GITR and CD101. Moreover, supernatant from ascites collected from patients with ovarian carcinoma also induced CD103+CD49a+ NK-like cells in vitro in a TGF-β-dependent manner. Interestingly, TGF-β1/IL-15-induced CD103+CD56+ NK-like cells suppressed autologous CD4+ T cells in vitro by reducing absolute number, proliferation, and expression of activation marker CD25. Collectively, these findings provide new insight into how NK cells may acquire an inhibitory phenotype in TGF-β1-rich environments.

Innate and adaptive effector immune drivers of cytomegalovirus disease in lung transplantation: a double-edged sword

Up to 90% of the global population has been infected with cytomegalovirus (CMV), a herpesvirus that remains latent for the lifetime of the host and drives immune dysregulation. CMV is a critical risk factor for poor outcomes after solid organ transplant, though lung transplant recipients (LTR) carry the highest risk of CMV infection, and CMV-associated comorbidities compared to recipients of other solid organ transplants. Despite potent antivirals, CMV remains a significant driver of chronic lung allograft dysfunction (CLAD), re-transplantation, and death. Moreover, the extended utilization of CMV antiviral prophylaxis is not without adverse effects, often necessitating treatment discontinuation. Thus, there is a critical need to understand the immune response to CMV after lung transplantation. This review identifies key elements of each arm of the CMV immune response and highlights implications for lung allograft tolerance and injury. Specific attention is paid to cellular subsets of adaptive and innate immune cells that are important in the lung during CMV infection and reactivation. The concept of heterologous immune responses is reviewed in depth, including how they form and how they may drive tissue- and allograft-specific immunity. Other important objectives of this review are to detail the emerging role of NK cells in CMV-related outcomes, in addition to discussing perturbations in CMV immune function stemming from pre-existing lung disease. Finally, this review identifies potential mechanisms whereby CMV-directed treatments may alter the cellular immune response within the allograft.

No time to die: Epigenetic regulation of natural killer cell survival

NK cells are short-lived innate lymphocytes that can mediate antigen-independent responses to infection and cancer. However, studies from the past two decades have shown that NK cells can acquire transcriptional and epigenetic modifications during inflammation that result in increased survival and lifespan. These findings blur the lines between the innate and adaptive arms of the immune system, and suggest that the homeostatic mechanisms that govern the persistence of innate immune cells are malleable. Indeed, recent studies have shown that NK cells undergo continuous and strictly regulated adaptations controlling their survival during development, tissue residency, and following inflammation. In this review, we summarize our current understanding of the critical factors regulating NK cell survival throughout their lifespan, with a specific emphasis on the epigenetic modifications that regulate the survival of NK cells in various contexts. A precise understanding of the molecular mechanisms that govern NK cell survival will be important to enhance therapies for cancer and infectious diseases.

Tissue-resident memory NK cells: Homing in on local effectors and regulators

Natural killer (NK) cells are the prototype innate effector lymphocyte population that plays an important role in controlling viral infections and tumors. Studies demonstrating that NK cells form long-lived memory populations, akin to those generated by adaptive immune cells, prompted a revaluation of the potential functions of NK cells. Recent data demonstrating that NK cells are recruited from the circulation into tissues where they form long-lived memory-like populations further emphasize that NK cells have properties that mirror those of adaptive immune cells. NK cells that localize in non-lymphoid tissues are heterogeneous, and there is a growing appreciation that immune responses occurring within tissues are subject to tissue-specific regulation. Here we discuss both the immune effector and immunoregulatory functions of NK cells, with a particular emphasis on the role of NK cells within non-lymphoid tissues and how the tissue microenvironment shapes NK cell-dependent outcomes.

Group 1 ILCs: Heterogeneity, plasticity, and transcriptional regulation

Group 1 innate lymphoid cells (ILCs), comprising ILC1s and natural killer cells (NK cells), belong to a large family of developmentally related innate lymphoid cells that lack rearranged antigen-specific receptors. NK cells and ILC1s both require the transcription factor T-bet for lineage commitment but additionally rely on Eomes and Hobit, respectively, for their development and effector maturation programs. Both ILC1s and NK cells are essential for rapid responses against infections and mediate cancer immunity through production of effector cytokines and cytotoxicity mediators. ILC1s are enriched in tissues and hence generally considered tissue resident cells whereas NK cells are often considered circulatory. Despite being deemed different cell types, ILC1s and NK cells share many common features both phenotypically and functionally. Recent studies employing single cell RNA sequencing (scRNA-seq) technology have exposed previously unappreciated heterogeneity in group 1 ILCs and further broaden our understanding of these cells. Findings from these studies imply that ILC1s in different tissues and organs share a common signature but exhibit some unique characteristics, possibly stemming from tissue imprinting. Also, data from recent fate mapping studies employing Hobit, RORγt, and polychromic reporter mice have greatly advanced our understanding of the developmental and effector maturation programs of these cells. In this review, we aim to outline the fundamental traits of mouse group 1 ILCs and explore recent discoveries related to their developmental programs, phenotypic heterogeneity, plasticity, and transcriptional regulation.

Single-cell clonal tracing of glandular and circulating T cells identifies a population of CD9+ CD8+ T cells in primary Sjogren's syndrome

Primary Sjogren's syndrome (pSS) is a complex chronic autoimmune disease in which local tissue damage in exocrine glands is combined with broader systemic involvement across the body in tissues including the skin. These combined manifestations negatively impact patient health and quality of life. While studies have previously reported differences in immune cell composition in the peripheral blood of pSS patients relative to healthy control subjects, a detailed immune cell landscape of the damaged exocrine glands of these patients remains lacking. Through single-cell transcriptomics and repertoire sequencing of immune cells in paired peripheral blood samples and salivary gland biopsies, we present here a preliminary picture of adaptive immune response in pSS. We characterize a number of points of divergence between circulating and glandular immune responses that have been hitherto underappreciated, and identify a novel population of CD8+ CD9+ cells with tissue-residential properties that are highly enriched in the salivary glands of pSS patients. Through comparative analyses with other sequencing data, we also observe a potential connection between these cells and the tissue-resident memory cells found in cutaneous vasculitis lesions. Together, these results indicate a potential role for CD8+ CD9+ cells in mediating glandular and systemic effects associated with pSS and other autoimmune disorders.

NKp44/HLA-DP-dependent regulation of CD8 effector T cells by NK cells

Although natural killer (NK) cells are recognized for their modulation of immune responses, the mechanisms by which human NK cells mediate immune regulation are unclear. Here, we report that expression of human leukocyte antigen (HLA)-DP, a ligand for the activating NK cell receptor NKp44, is significantly upregulated on CD8+ effector T cells, in particular in human cytomegalovirus (HCMV)+ individuals. HLA-DP+ CD8+ T cells expressing NKp44-binding HLA-DP antigens activate NKp44+ NK cells, while HLA-DP+ CD8+ T cells not expressing NKp44-binding HLA-DP antigens do not. In line with this, frequencies of HLA-DP+ CD8+ T cells are increased in individuals not encoding for NKp44-binding HLA-DP haplotypes, and contain hyper-expanded CD8+ T cell clones, compared to individuals expressing NKp44-binding HLA-DP molecules. These findings identify a molecular interaction facilitating the HLA-DP haplotype-specific editing of HLA-DP+ CD8+ T cell effector populations by NKp44+ NK cells and preventing the generation of hyper-expanded T cell clones, which have been suggested to have increased potential for autoimmunity.

Alterations in CX3CL1 Levels and Its Role in Viral Pathogenesis

CX3CL1, also named fractalkine or neurotactin, is the only known member of the CX3C chemokine family that can chemoattract several immune cells. CX3CL1 exists in both membrane-anchored and soluble forms, with each mediating distinct biological activities. CX3CL1 signals are transmitted through its unique receptor, CX3CR1, primarily expressed in the microglia of the central nervous system (CNS). In the CNS, CX3CL1 acts as a regulator of microglia activation in response to brain disorders or inflammation. Recently, there has been a growing interest in the role of CX3CL1 in regulating cell adhesion, chemotaxis, and host immune response in viral infection. Here, we provide a comprehensive review of the changes and function of CX3CL1 in various viral infections, such as human immunodeficiency virus (HIV), SARS-CoV-2, influenza virus, and cytomegalovirus (CMV) infection, to highlight the emerging roles of CX3CL1 in viral infection and associated diseases.

Clonal CD8 T Cells Accumulate in the Leptomeninges and Communicate with Microglia in Human Neurodegeneration

Murine studies have highlighted a crucial role for immune cells in the meninges in surveilling the central nervous system (CNS) and influencing neuroinflammation. However, how meningeal immunity is altered in human neurodegeneration and its effects on CNS inflammation is understudied. We performed the first single-cell analysis of the transcriptomes and T cell receptor (TCR) repertoire of 104,635 immune cells from 55 postmortem human brain and leptomeningeal tissues from donors with neurodegenerative diseases including amyotrophic lateral sclerosis, Alzheimer's disease, and Parkinson's disease. RNA and TCR sequencing from paired leptomeninges and brain allowed us to perform lineage tracing to identify the spatial trajectory of clonal T cells in the CNS and its borders. We propose that T cells activated in the brain emigrate to and establish residency in the leptomeninges where they likely contribute to impairments in lymphatic drainage and remotely to CNS inflammation by producing IFNγ and other cytokines. We identified regulatory networks local to the meninges including NK cell-mediated CD8 T cell killing which likely help to control meningeal inflammation. Collectively, these findings provide not only a foundation for future studies into brain border immune surveillance but also highlight important intercellular dynamics that could be leveraged to modulate neuroinflammation.

Control of Cell Death in Health and Disease

Apoptosis, necroptosis, and pyroptosis are genetically programmed cell death mechanisms that eliminate obsolete, damaged, infected, and self-reactive cells. Apoptosis fragments cells in a manner that limits immune cell activation, whereas the lytic death programs of necroptosis and pyroptosis release proinflammatory intracellular contents. Apoptosis fine-tunes tissue architecture during mammalian development, promotes tissue homeostasis, and is crucial for averting cancer and autoimmunity. All three cell death mechanisms are deployed to thwart the spread of pathogens. Disabling regulators of cell death signaling in mice has revealed how excessive cell death can fuel acute or chronic inflammation. Here we review strategies for modulating cell death in the context of disease. For example, BCL-2 inhibitor venetoclax, an inducer of apoptosis, is approved for the treatment of certain hematologic malignancies. By contrast, inhibition of RIPK1, NLRP3, GSDMD, or NINJ1 to limit proinflammatory cell death and/or the release of large proinflammatory molecules from dying cells may benefit patients with inflammatory diseases.

Circulating NK cells establish tissue residency upon acute infection of skin and mediate accelerated effector responses to secondary infection

Natural killer (NK) cells are present in the circulation and can also be found residing in tissues, and these populations exhibit distinct developmental requirements and are thought to differ in terms of ontogeny. Here, we investigate whether circulating conventional NK (cNK) cells can develop into long-lived tissue-resident NK (trNK) cells following acute infections. We found that viral and bacterial infections of the skin triggered the recruitment of cNK cells and their differentiation into Tcf1hiCD69hi trNK cells that share transcriptional similarity with CD56brightTCF1hi NK cells in human tissues. Skin trNK cells arose from interferon (IFN)-γ-producing effector cells and required restricted expression of the transcriptional regulator Blimp1 to optimize Tcf1-dependent trNK cell formation. Upon secondary infection, trNK cells rapidly gained effector function and mediated an accelerated NK cell response. Thus, cNK cells redistribute and permanently position at sites of previous infection via a mechanism promoting tissue residency that is distinct from Hobit-dependent developmental paths of NK cells and ILC1 seeding tissues during ontogeny.

Dendritic Nanomedicine with Boronate Bonds for Augmented Chemo-Immunotherapy via Synergistic Modulation of Tumor Immune Microenvironment

Unsatisfied tumor accumulation of chemotherapeutic drugs and a complicated immunosuppressive microenvironment diminish the immune response rate and the therapeutic effect. Surface modification of these drugs with target ligands can promote their cellular internalization, but the modified drugs may be subjected to unexpected immune recognition and clearance. Herein, a phenylboronic acid (PBA) group-shieldable dendritic nanomedicine that integrates an immunogenic cell death (ICD)-inducing agent (epirubicin, Epi) and an indoleamine 2,3-dioxgenase 1 (IDO1) inhibitor (NLG919) is reported for tumor chemo-immunotherapy. This NLG919-loaded Epi-conjugated PEGylated dendrimers bridged with boronate bonds (NLG919@Epi-DBP) maintains a stable nanostructure during circulation. Under a moderate acidic condition, the PBA group exposes to the sialic acid residue on the tumor cell membrane to enhance the internalization and penetration of NLG919@Epi-DBP. At pH 5.0, NLG919@Epi-DBP rapidly disassembles to release the incorporated Epi and NLG919. Epi triggers robust ICD of tumor cells that evokes strong immune response. In addition, inhibition of the IDO1 activity downregulates the metabolism of L-tryptophan to kynurenine, leading to a reduction in the recruitment of immunosuppressive cells and modulation of the tumor immune microenvironment. Collectively, this promising strategy has been demonstrated to evoke robust immune response as well as remodel the immunosuppressive microenvironment for an enhanced chemo-immunotherapeutic effect.

Tissue T cells in prophylactic and therapeutic vaccination responses

In this conference report, I highlight the potential to target tissue-resident T cells to enhance prophylactic and therapeutic vaccine immunity. I describe our recent findings on exploiting frontline sentinal immunosurveillance by liver-resident immunity for functional cure of hepatitis B. We showed that therapeutic vaccine-induced HBV-specific T cells are constrained by liver-resident NK cells; cytokine-activation and PD-L1 blockade of NK cells converted them into helpers able to instead boost HBV-specific T cells. Turning to tissue-resident T cells in the lung, we found this pool can include T cells able to recognise SARS-CoV-2, including cross-reactive responses present prior to the pandemic. The importance of inducing T cells with future prophylactic vaccines was underscored by their selective expansion in a subset of donors aborting SARS-CoV-2 infection without detectable antibodies.

Early antigen receptor signaling in natural killer cells alters STAT4-dependent fate decisions via epigenetic remodeling

Natural Killer (NK) cells are innate cytotoxic lymphocytes that possess features of adaptive immunity, including antigen specificity and clonal expansion. NK cells rapidly respond to cytokines released during the innate phase of viral infection and are thought to migrate from circulation into infected organs to execute their early effector functions. However, recent evidence suggests that tissue-resident NK cells are among the first responders to viral infection. In this study, we observe that antigen receptor signaling precedes substantial proinflammatory cytokine signaling in a population of NK cells during mouse cytomegalovirus infection. Early antigen receptor signals epigenetically prime NK cells for optimal expansion during the later adaptive phase of the antiviral response. Mechanistically, receptor signaling increases chromatin accessibility at STAT4-binding genomic sites within differentiating NK cells. To promote adaptive programming of NK cells during infection, activating receptor-dependent epigenetic remodeling antagonizes IL-12 driven terminal maturation, poises NK cells for proliferation via sustained CDK6 expression, and antagonizes early apoptosis of short-lived effector cells via suppression of Bim. Thus, antigen receptor signaling alters an IL-12 dependent fate decision during the innate-to-adaptive transition of antiviral NK cells.

CCR5 drives NK cell-associated airway damage in pulmonary ischemia-reperfusion injury

Primary graft dysfunction (PGD) limits clinical benefit after lung transplantation, a life-prolonging therapy for patients with end-stage disease. PGD is the clinical syndrome resulting from pulmonary ischemia-reperfusion injury (IRI), driven by innate immune inflammation. We recently demonstrated a key role for NK cells in the airways of mouse models and human tissue samples of IRI. Here, we used 2 mouse models paired with human lung transplant samples to investigate the mechanisms whereby NK cells migrate to the airways to mediate lung injury. We demonstrate that chemokine receptor ligand transcripts and proteins are increased in mouse and human disease. CCR5 ligand transcripts were correlated with NK cell gene signatures independently of NK cell CCR5 ligand secretion. NK cells expressing CCR5 were increased in the lung and airways during IRI and had increased markers of tissue residency and maturation. Allosteric CCR5 drug blockade reduced the migration of NK cells to the site of injury. CCR5 blockade also blunted quantitative measures of experimental IRI. Additionally, in human lung transplant bronchoalveolar lavage samples, we found that CCR5 ligand was associated with increased patient morbidity and that the CCR5 receptor was increased in expression on human NK cells following PGD. These data support a potential mechanism for NK cell migration during lung injury and identify a plausible preventative treatment for PGD.

Cardinal features of immune memory in innate lymphocytes

The ability of vertebrates to 'remember' previous infections had once been attributed exclusively to adaptive immunity. We now appreciate that innate lymphocytes also possess memory properties akin to those of adaptive immune cells. In this Review, we draw parallels from T cell biology to explore the key features of immune memory in innate lymphocytes, including quantity, quality, and location. We discuss the signals that trigger clonal or clonal-like expansion in innate lymphocytes, and highlight recent studies that shed light on the complex cellular and molecular crosstalk between metabolism, epigenetics, and transcription responsible for differentiating innate lymphocyte responses towards a memory fate. Additionally, we explore emerging evidence that activated innate lymphocytes relocate and establish themselves in specific peripheral tissues during infection, which may facilitate an accelerated response program akin to those of tissue-resident memory T cells.

PD-1 Mediated Regulation of Unique Activated CD8 + T Cells by NK Cells in the Submandibular Gland

The increasing utilization of anti-PD-1 immune checkpoint blockade (ICB) has led to the emergence of immune-related adverse events (irAEs), including sicca syndrome. Interestingly, we found that the submandibular gland (SMG) of PD-1 deficient mice harbors a large population of CD8 + T cells, reminiscing ICB induced sicca. This phenotype was also observed in the SMG of both NK cell-depleted C57BL/6 animals and NK cell-deficient animals. Mechanistically, using mice conditionally deficient for PD-L1 in the NK cell lineage, we discovered that NK cells regulate CD8 + T cell homeostasis via the PD-1/PD-L1 axis in this organ. Importantly, single-cell RNA sequencing of PD-1 deficient SMG CD8 + T cells reveals a unique transcriptional profile consistent with TCR activation. These cells have limited TCR diversity and phenotypically overlap with GzmK + CD8 + T autoimmune cells identified in primary Sjögren's syndrome patients. These insights into NK cell immunoregulation in the SMG, and the consequences of disrupted CD8 + T cell homeostasis, provide opportunities for preventing the development of irAEs.

Highlights

Elevated CD8 + T cells in the submandibular gland (SMG) of PD-1 deficient mice parallel sicca-like irAEs seen in ICB patients. In addition to their previously described hyporesponsive phenotype, NK cells in the SMG regulate CD8 + T cell homeostasis through the PD-L1/PD-1 axis. PD-1 deficient SMG CD8 + T cells display unique transcriptional profiles associated with proinflammatory functions, TCR activation, interferon stimulation, and exhaustion. Oligoclonal expansion and similarities in TCR sequences indicate T cell activation and a preference for recognizing specific antigens.