Missing self-recognition of Ocil/Clr-b by inhibitory NKR-P1 natural killer cell receptors

MCAM+CD161- Th17 Subset Expressing CD83 Enhances Tc17 Response in Psoriasis

Recent studies have highlighted the pathogenic roles of IL-17-producing CD8+ T cells (T-cytotoxic 17 [Tc17]) in psoriasis. However, the underlying mechanisms of Tc17 induction remain unclear. In this study, we focused on the pathogenic subsets of Th17 and their mechanism of promotion of Tc17 responses. We determined that the pathogenic Th17-enriched fraction expressed melanoma cell adhesion molecule (MCAM) and CCR6, but not CD161, because this subset produced IL-17A abundantly and the presence of these cells in the peripheral blood of patients has been correlated with the severity of psoriasis. Intriguingly, the serial analysis of gene expression revealed that CCR6+MCAM+CD161-CD4+ T cells displayed the gene profile for adaptive immune responses, including CD83, which is an activator for CD8+ T cells. Coculture assay with or without intercellular contact between CD4+ and CD8+ T cells showed that CCR6+MCAM+CD161-CD4+ T cells induced the proliferation of CD8+ T cells in a CD83-dependent manner. However, the production of IL-17A by CD8+ T cells required exogenous IL-17A, suggesting that intercellular contact via CD83 and the production of IL-17A from activated CD4+ T cells elicit Tc17 responses. Intriguingly, the CD83 expression was enhanced in the presence of IL-15, and CD83+ cells stimulated with IL-1β, IL-23, IL-15, and IL-15Rα did not express FOXP3. Furthermore, CCR6+MCAM+CD161-CD4+ T cells expressing CD83 were increased in the peripheral blood of patients, and the CD83+ Th17-type cells accumulated in the lesional skin of psoriasis. In conclusion, pathogenic MCAM+CD161- Th17 cells may be involved in the Tc17 responses via IL-17A and CD83 in psoriasis.

The inhibitory NKR-P1B receptor regulates NK cell-mediated mammary tumor immunosurveillance in mice

Natural killer (NK) cells are an important component of anti-cancer immunity, and their activity is regulated by an array of activating and inhibitory receptors. In mice, the inhibitory NKR-P1B receptor is expressed in NK cells and recognizes the C-type lectin-related protein-b (Clr-b) ligand. NKR-P1B:Clr-b interactions represent a 'missing-self' recognition system to monitor cellular levels of Clr-b on healthy and diseased cells. Here, we report an important role for NKR-P1B:Clr-b interactions in tumor immunosurveillance in MMTV-PyVT mice, which develop spontaneous mammary tumors. MMTV-PyVT mice on NKR-P1B-deficient genetic background developed mammary tumors earlier than on wild-type (WT) background. A greater proportion of tumor-infiltrating NK cells downregulate expression of the transcription factor Eomesodermin (EOMES) in NKR-P1B-deficient mice compared to WT mice. Tumor-infiltrating NK cells also downregulated CD49b expression but gain CD49a expression and exhibit effector functions, such as granzyme B upregulation and proliferation in mammary tumors. However, unlike the EOMES⁺ NK cells, the EOMES^‒ NK cell subset is unable to respond to further in vitro stimulation and exhibits phenotypic alterations associated with immune dysfunction. These alterations included increased expression of PD-1, LAG-3, and TIGIT and decreased expression of NKp46, Ly49C/I, CD11b, and KLRG-1. Furthermore, tumor-infiltrating NKR-P1B-deficient NK cells exhibited an elevated dysfunctional immune phenotype compared to WT NK cells. These findings demonstrate that the NKR-P1B receptor plays an important role in mammary tumor surveillance by regulating anti-cancer immune responses and functional homeostasis in NK cells.

The CD16 and CD32b Fc-gamma receptors regulate antibody-mediated responses in mouse natural killer cells

Natural killer (NK) cells are innate lymphocytes capable of mediating immune responses without prior sensitization. NK cells express Fc-gamma receptors (FcγRs) that engage the Fc region of IgG. Studies investigating the role of FcγRs on mouse NK cells have been limited due to lack specific reagents. In this study, we characterize the expression and biological consequences of activating mouse NK cells through their FcγRs. We demonstrate that most NK cells express the activating CD16 receptor, and a subset of NK cells also expresses the inhibitory CD32b receptor. Critically, these FcγRs are functional on mouse NK cells and can modulate antibody-mediated responses. We also characterized mice with conditional knockout alleles of Fcgr3 (CD16) or Fcgr2b (CD32b) in the NK and innate lymphoid cell (ILC) lineage. NK cells in these mice did not reveal any developmental defects and were responsive to cross-linking activating NK receptors, cytokine stimulation, and killing of YAC-1 targets. Importantly, CD16-deficient NK cells failed to induce antibody-directed cellular cytotoxicity of antibody-coated B-cell lymphomas in in vitro assays. In addition, we demonstrate the important role of CD16 on NK cells using an in vivo model of cancer immunotherapy using anti-CD20 antibody treatment of B-cell lymphomas.

The diverse roles of C-type lectin-like receptors in immunity

Our understanding of the C-type lectin-like receptors (CTLRs) and their functions in immunity have continued to expand from their initial roles in pathogen recognition. There are now clear examples of CTLRs acting as scavenger receptors, sensors of cell death and cell transformation, and regulators of immune responses and homeostasis. This range of function reflects an extensive diversity in the expression and signaling activity between individual CTLR members of otherwise highly conserved families. Adding to this diversity is the constant discovery of new receptor binding capabilities and receptor-ligand interactions, distinct cellular expression profiles, and receptor structures and signaling mechanisms which have expanded the defining roles of CTLRs in immunity. The natural killer cell receptors exemplify this functional diversity with growing evidence of their activity in other immune populations and tissues. Here, we broadly review select families of CTLRs encoded in the natural killer cell gene complex (NKC) highlighting key receptors that demonstrate the complex multifunctional capabilities of these proteins. We focus on recent evidence from research on the NKRP1 family of CTLRs and their interaction with the related C-type lectin (CLEC) ligands which together exhibit essential immune functions beyond their defined activity in natural killer (NK) cells. The ever-expanding evidence for the requirement of CTLR in numerous biological processes emphasizes the need to better understand the functional potential of these receptor families in immune defense and pathological conditions.

Alveolar macrophage metabolic programming via a C-type lectin receptor protects against lipo-toxicity and cell death

Alveolar macrophages (AM) hold lung homeostasis intact. In addition to the defense against inhaled pathogens and deleterious inflammation, AM also maintain pulmonary surfactant homeostasis, a vital lung function that prevents pulmonary alveolar proteinosis. Signals transmitted between AM and pneumocytes of the pulmonary niche coordinate these specialized functions. However, the mechanisms that guide the metabolic homeostasis of AM remain largely elusive. We show that the NK cell-associated receptor, NKR-P1B, is expressed by AM and is essential for metabolic programming. Nkrp1b^-/- mice are vulnerable to pneumococcal infection due to an age-dependent collapse in the number of AM and the formation of lipid-laden AM. The AM of Nkrp1b^-/- mice show increased uptake but defective metabolism of surfactant lipids. We identify a physical relay between AM and alveolar type-II pneumocytes that is dependent on pneumocyte Clr-g expression. These findings implicate the NKR-P1B:Clr-g signaling axis in AM-pneumocyte communication as being important for maintaining metabolism in AM.

Clr-f expression regulates kidney immune and metabolic homeostasis

The C-type lectin-related protein, Clr-f, encoded by Clec2h in the mouse NK gene complex (NKC), is a member of a family of immune regulatory lectins that guide immune responses at distinct tissues of the body. Clr-f is highly expressed in the kidney; however, its activity in this organ is unknown. To assess the requirement for Clr-f in kidney health and function, we generated a Clr-f-deficient mouse (Clr-f^−/−) by targeted deletions in the Clec2h gene. Mice lacking Clr-f exhibited glomerular and tubular lesions, immunoglobulin and C3 complement protein renal deposits, and significant abdominal and ectopic lipid accumulation. Whole kidney transcriptional profile analysis of Clr-f^−/− mice at 7, 13, and 24 weeks of age revealed a dynamic dysregulation in lipid metabolic processes, stress responses, and inflammatory mediators. Examination of the immune contribution to the pathologies of Clr-f^−/− mouse kidneys identified elevated IL-12 and IFNγ in cells of the tubulointerstitium, and an infiltrating population of neutrophils and T and B lymphocytes. The presence of these insults in a Rag1^−/−Clr-f^−/− background reveals that Clr-f^−/− mice are susceptible to a T and B lymphocyte-independent renal pathogenesis. Our data reveal a role for Clr-f in the maintenance of kidney immune and metabolic homeostasis.

CD8+CD161+ T-Cells: Cytotoxic Memory Cells With High Therapeutic Potential

NK1.1 and its human homolog CD161 are expressed on NK cells, subsets of CD4⁺ and CD8⁺ T cells, and NKT cells. While the expression of NK1.1 is thought to be inhibitory to NK cell function, it is reported to play both costimulatory and coinhibitory roles in T-cells. CD161 has been extensively studied and characterized on subsets of T-cells that are MR1-restricted, IL-17 producing CD4⁺ (T_H17 MAIT cells) and CD8⁺ T cells (Tc17 cells). Non-MAIT, MR1-independent CD161-expressing T-cells also exist and are characterized as generally effector memory cells with a stem cell like phenotype. Gene expression analysis of this enigmatic subset indicates a significant enhancement in the expression of cytotoxic granzyme molecules and innate like stress receptors in CD8⁺NK1.1⁺/CD8⁺CD161⁺ cells in comparison to CD8⁺ cells that do not express NK1.1 or CD161. First identified and studied in the context of viral infection, the role of CD8⁺CD161⁺ T-cells, especially in the context of tumor immunology, is still poorly understood. In this review, the functional characteristics of the CD161-expressing CD8⁺ T cell subset with respect to gene expression profile, cytotoxicity, and tissue homing properties are discussed, and application of this subset to immune responses against infectious disease and cancer is considered.

Protective Vaccination Reshapes Hepatic Response to Blood-Stage Malaria of Genes Preferentially Expressed by NK Cells

The role of natural killer (NK) cells in the liver as first-line post infectionem (p.i.) effectors against blood-stage malaria and their responsiveness to protective vaccination is poorly understood. Here, we investigate the effect of vaccination on NK cell-associated genes induced in the liver by blood-stage malaria of Plasmodium chabaudi. Female Balb/c mice were vaccinated at weeks 3 and 1 before being infected with 10⁶P. chabaudi-parasitized erythrocytes. Genes preferentially expressed by NK cells were investigated in livers of vaccination-protected and non-protected mice on days 0, 1, 4, 8, and 11 p.i. using microarrays, qRT-PCR, and chromosome landscape analysis. Blood-stage malaria induces expression of specific genes in the liver at different phases of infection, i.e., Itga1 in expanding liver-resident NK (lrNK) cells, Itga2 in immigrating conventional NK (cNK) cells; Eomes and Tbx21 encoding transcription factors; Ncr1, Tnfsf10, Prf1, Gzma, Gzmb, Gzmc, Gzmm, and Gzmk encoding cytolytic effectors; natural killer gene complex (NKC)-localized genes encoding the NK cell receptors KLRG1, KLRK1, KLRAs1, 2, 5, 7, KLRD1, KLRC1, KLRC3, as well as the three receptors KLRB1A, KLRB1C, KLRB1F and their potential ligands CLEC2D and CLEC2I. Vaccination enhances this malaria-induced expression of genes, but impairs Gzmm expression, accelerates decline of Tnfsf10 and Clec2d expression, whereas it accelerates increased expression of Clec2i, taking a very similar time course as that of genes encoding plasma membrane proteins of erythroblasts, whose malaria-induced extramedullary generation in the liver is known to be accelerated by vaccination. Collectively, vaccination reshapes the response of the liver NK cell compartment to blood-stage malaria. Particularly, the malaria-induced expansion of lrNK cells peaking on day 4 p.i. is highly significantly (p < 0.0001) reduced by enhanced immigration of peripheral cNK cells, and KLRB1F:CLEC2I interactions between NK cells and erythroid cells facilitate extramedullary erythroblastosis in the liver, thus critically contributing to vaccination-induced survival of otherwise lethal blood-stage malaria of P. chabaudi.

Lectin-like transcript 1 as a natural killer cell-mediated immunotherapeutic target for triple negative breast cancer and prostate cancer

Breast and prostate cancer are the leading causes of death in females and males, respectively. Triple negative breast cancer (TNBC) does not express the estrogen receptor, progesterone receptor, or human epidermal growth factor receptor 2, resulting in limited treatment options. Androgen deprivation therapy is the standard care for prostate cancer patients; however, metastasis and recurrence are seen in androgen-independent prostate cancer. Both prostate and breast cancer show higher resistance after recurrence and metastasis, which increases the difficulty of treatment. Natural killer (NK) cells play a critical role during innate immunity and tumor recognition and elimination. NK cell function is determined by a delicate balance of inhibitory signals and activation signals received through cell surface receptors. Lectin-like transcript 1 (LLT1, CLEC2D, OCIL) is a ligand of NK cell inhibitory receptor NKRP1A (CD161). Several studies have that reported higher expression of LLT1 is associated with the development of various tumors. Our studies revealed that TNBC and prostate cancer cells express higher levels of LLT1. In the presence of a monoclonal antibody against LLT1, NK cell-mediated killing of TNBC and prostate cancer cells were greatly enhanced. This review highlights the potential that using monoclonal antibodies to block LLT1 - NKRP1A interactions could be an effective immunotherapeutic approach to treat triple negative breast cancer and prostate cancer.

Immune Sensing of Cell Death through Recognition of Histone Sequences by C-Type Lectin-Receptor-2d Causes Inflammation and Tissue Injury

The immune system monitors the health of cells and is stimulated by necrosis. Here we examined the receptors and ligands driving this response. In a targeted screen of C-type lectin receptors, a Clec2d reporter responded to lysates from necrotic cells. Biochemical purification identified histones, both free and bound to nucleosomes or neutrophil extracellular traps, as Clec2d ligands. Clec2d recognized poly-basic sequences in histone tails and this recognition was sensitive to post-translational modifications of these sequences. As compared with WT mice, Clec2d^-/- mice exhibited reduced proinflammatory responses to injected histones, and less tissue damage and improved survival in a hepatotoxic injury model. In macrophages, Clec2d localized to the plasma membrane and endosomes. Histone binding to Clec2d did not stimulate kinase activation or cytokine production. Rather, histone-bound DNA stimulated endosomal Tlr9-dependent responses in a Clec2d-dependent manner. Thus, Clec2d binds to histones released upon necrotic cell death, with functional consequences to inflammation and tissue damage.

Mouse Cytomegalovirus m153 Protein Stabilizes Expression of the Inhibitory NKR-P1B Ligand Clr-b

Natural killer (NK) cells are a subset of innate lymphoid cells (ILC) capable of recognizing stressed and infected cells through multiple germ line-encoded receptor-ligand interactions. Missing-self recognition involves NK cell sensing of the loss of host-encoded inhibitory ligands on target cells, including MHC class I (MHC-I) molecules and other MHC-I-independent ligands. Mouse cytomegalovirus (MCMV) infection promotes a rapid host-mediated loss of the inhibitory NKR-P1B ligand Clr-b (encoded by Clec2d) on infected cells. Here we provide evidence that an MCMV m145 family member, m153, functions to stabilize cell surface Clr-b during MCMV infection. Ectopic expression of m153 in fibroblasts augments Clr-b cell surface levels. Moreover, infections using m153-deficient MCMV mutants (Δm144-m158 and Δm153) show an accelerated and exacerbated Clr-b downregulation. Importantly, enhanced loss of Clr-b during Δm153 mutant infection reverts to wild-type levels upon exogenous m153 complementation in fibroblasts. While the effects of m153 on Clr-b levels are independent of Clec2d transcription, imaging experiments revealed that the m153 and Clr-b proteins only minimally colocalize within the same subcellular compartments, and tagged versions of the proteins were refractory to coimmunoprecipitation under mild-detergent conditions. Surprisingly, the Δm153 mutant possesses enhanced virulence in vivo, independent of both Clr-b and NKR-P1B, suggesting that m153 potentially targets additional host factors. Nevertheless, the present data highlight a unique mechanism by which MCMV modulates NK ligand expression.IMPORTANCE Cytomegaloviruses are betaherpesviruses that in immunocompromised individuals can lead to severe pathologies. These viruses encode various gene products that serve to evade innate immune recognition. NK cells are among the first immune cells that respond to CMV infection and use germ line-encoded NK cell receptors (NKR) to distinguish healthy from virus-infected cells. One such axis that plays a critical role in NK recognition involves the inhibitory NKR-P1B receptor, which engages the host ligand Clr-b, a molecule commonly lost on stressed cells ("missing-self"). In this study, we discovered that mouse CMV utilizes the m153 glycoprotein to circumvent host-mediated Clr-b downregulation, in order to evade NK recognition. These results highlight a novel MCMV-mediated immune evasion strategy.

Production of recombinant soluble dimeric C-type lectin-like receptors of rat natural killer cells

Working at the border between innate and adaptive immunity, natural killer (NK) cells play a key role in the immune system by protecting healthy cells and by eliminating malignantly transformed, stressed or virally infected cells. NK cell recognition of a target cell is mediated by a receptor “zipper” consisting of various activating and inhibitory receptors, including C-type lectin-like receptors. Among this major group of receptors, two of the largest rodent receptor families are the NKR-P1 and the Clr receptor families. Although these families have been shown to encode receptor-ligand pairs involved in MHC-independent self-nonself discrimination and are a target for immune evasion by tumour cells and viruses, structural mechanisms of their mutual recognition remain less well characterized. Therefore, we developed a non-viral eukaryotic expression system based on transient transfection of suspension-adapted human embryonic kidney 293 cells to produce soluble native disulphide dimers of NK cell C-type lectin-like receptor ectodomains. The expression system was optimized using green fluorescent protein and secreted alkaline phosphatase, easily quantifiable markers of recombinant protein production. We describe an application of this approach to the recombinant protein production and characterization of native rat NKR-P1B and Clr-11 proteins suitable for further structural and functional studies.

Oligomeric Architecture of Mouse Activating Nkrp1 Receptors on Living Cells

Mouse activating Nkrp1 proteins are commonly described as type II transmembrane receptors with disulfide-linked homodimeric structure. Their function and the manner in which Nkrp1 proteins of mouse strain (C57BL/6) oligomerize are still poorly understood. To assess the oligomerization state of Nkrp1 proteins, mouse activating EGFP-Nkrp1s were expressed in mammalian lymphoid cells and their oligomerization evaluated by Förster resonance energy transfer (FRET). Alternatively, Nkrp1s oligomers were detected by Western blotting to specify the ratio between monomeric and dimeric forms. We also performed structural characterization of recombinant ectodomains of activating Nkrp1 receptors. Nkrp1 isoforms c1, c2 and f were expressed prevalently as homodimers, whereas the Nkrp1a displays larger proportion of monomers on the cell surface. Cysteine-to-serine mutants revealed the importance of all stalk cysteines for protein dimerization in living cells with a major influence of cysteine at position 74 in two Nkrp1 protein isoforms. Our results represent a new insight into the oligomerization of Nkrp1 receptors on lymphoid cells, which will help to determine their function.

Recognition of host Clr-b by the inhibitory NKR-P1B receptor provides a basis for missing-self recognition

The interaction between natural killer (NK) cell inhibitory receptors and their cognate ligands constitutes a key mechanism by which healthy tissues are protected from NK cell-mediated lysis. However, self-ligand recognition remains poorly understood within the prototypical NKR-P1 receptor family. Here we report the structure of the inhibitory NKR-P1B receptor bound to its cognate host ligand, Clr-b. NKR-P1B and Clr-b interact via a head-to-head docking mode through an interface that includes a large array of polar interactions. NKR-P1B:Clr-b recognition is extremely sensitive to mutations at the heterodimeric interface, with most mutations severely impacting both Clr-b binding and NKR-P1B receptor function to implicate a low affinity interaction. Within the structure, two NKR-P1B:Clr-b complexes are cross-linked by a non-classic NKR-P1B homodimer, and the disruption of homodimer formation abrogates Clr-b recognition. These data provide an insight into a fundamental missing-self recognition system and suggest an avidity-based mechanism underpins NKR-P1B receptor function.

NKR-P1B expression in gut-associated innate lymphoid cells is required for the control of gastrointestinal tract infections

Helper-type innate lymphoid cells (ILC) play an important role in intestinal homeostasis. Members of the NKR-P1 gene family are expressed in various innate immune cells, including natural killer (NK) cells, and their cognate Clr ligand family members are expressed in various specialized tissues, including the intestinal epithelium, where they may play an important role in mucosal-associated innate immune responses. In this study, we show that the inhibitory NKR-P1B receptor, but not the Ly49 receptor, is expressed in gut-resident NK cells, ILC, and a subset of γδT cells in a tissue-specific manner. ILC3 cells constitute the predominant cell subset expressing NKR-P1B in the gut lamina propria. The known NKR-P1B ligand Clr-b is broadly expressed in gut-associated cells of hematopoietic origin. The genetic deletion of NKR-P1B results in a higher frequency and number of ILC3 and γδT cells in the gut lamina propria. However, the function of gut-resident ILC3, NK, and γδT cells in NKR-P1B-deficient mice is impaired during gastrointestinal tract infection by Citrobacter rodentium or Salmonella typhimurium, resulting in increased systemic bacterial dissemination in NKR-P1B-deficient mice. Our findings highlight the role of the NKR-P1B:Clr-b recognition system in the modulation of intestinal innate immune cell functions.

Herpesvirus Evasion of Natural Killer Cells

Natural killer (NK) cells play an important role in the host response against viral infections and cancer development. They are able to kill virus-infected and tumor cells, and they produce different important cytokines that stimulate the antiviral and antitumor adaptive immune response, particularly interferon gamma. NK cells are of particular importance in herpesvirus infections, which is illustrated by systemic and life-threatening herpesvirus disease symptoms in patients with deficiencies in NK cell activity and by the myriad of reports describing herpesvirus NK cell evasion strategies. The latter is particularly obvious for cytomegaloviruses, but increasing evidence indicates that most, if not all, members of the herpesvirus family suppress NK cell activity to some extent. This review discusses the different NK cell evasion strategies described for herpesviruses and how this knowledge may translate to clinical applications.

The Inhibitory NKR-P1B:Clr-b Recognition Axis Facilitates Detection of Oncogenic Transformation and Cancer Immunosurveillance

Natural killer (NK) cells express receptors specific for MHC class I (MHC-I) molecules involved in "missing-self" recognition of cancer and virus-infected cells. Here we elucidate the role of MHC-I-independent NKR-P1B:Clr-b interactions in the detection of oncogenic transformation by NK cells. Ras oncogene overexpression was found to promote a real-time loss of Clr-b on mouse fibroblasts and leukemia cells, mediated in part via the Raf/MEK/ERK and PI3K pathways. Ras-driven Clr-b downregulation occurred at the level of the Clrb (Clec2d) promoter, nascent Clr-b transcripts, and cell surface Clr-b protein, in turn promoting missing-self recognition via the NKR-P1B inhibitory receptor. Both Ras- and c-Myc-mediated Clr-b loss selectively augmented cytotoxicity of oncogene-transformed leukemia cells by NKR-P1B⁺ NK cells in vitro and enhanced rejection by WT mice in vivo Interestingly, genetic ablation of either one (Clr-b^+/-) or two Clr-b alleles (Clr-b^-/-) enhanced survival of Eμ-cMyc transgenic mice in a primary lymphoma model despite preferential rejection of Clr-b^-/- hematopoietic cells previously observed following adoptive transfer into naïve wild-type mice in vivo Collectively, these findings suggest that the inhibitory NKR-P1B:Clr-b axis plays a beneficial role in innate detection of oncogenic transformation via NK-cell-mediated cancer immune surveillance, in addition to a pathologic role in the immune escape of primary lymphoma cells in Eμ-cMyc mice in vivo These results provide a model for the human NKR-P1A:LLT1 system in cancer immunosurveillance in patients with lymphoma and suggest it may represent a target for immune checkpoint therapy.Significance: A mouse model shows that an MHC-independent NK-cell recognition axis enables the detection of leukemia cells, with implications for a novel immune checkpoint therapy target in human lymphoma. Cancer Res; 78(13); 3589-603. ©2018 AACR.

Modulation of Immune Signaling and Metabolism Highlights Host and Fungal Transcriptional Responses in Mouse Models of Invasive Pulmonary Aspergillosis

Incidences of invasive pulmonary aspergillosis, an infection caused predominantly by Aspergillus fumigatus, have increased due to the growing number of immunocompromised individuals. While A. fumigatus is reliant upon deficiencies in the host to facilitate invasive disease, the distinct mechanisms that govern the host-pathogen interaction remain enigmatic, particularly in the context of distinct immune modulating therapies. To gain insights into these mechanisms, RNA-Seq technology was utilized to sequence RNA derived from lungs of 2 clinically relevant, but immunologically distinct murine models of IPA on days 2 and 3 post inoculation when infection is established and active disease present. Our findings identify notable differences in host gene expression between the chemotherapeutic and steroid models at the interface of immunity and metabolism. RT-qPCR verified model specific and nonspecific expression of 23 immune-associated genes. Deep sequencing facilitated identification of highly expressed fungal genes. We utilized sequence similarity and gene expression to categorize the A. fumigatus putative in vivo secretome. RT-qPCR suggests model specific gene expression for nine putative fungal secreted proteins. Our analysis identifies contrasting responses by the host and fungus from day 2 to 3 between the two models. These differences may help tailor the identification, development, and deployment of host- and/or fungal-targeted therapeutics.

CMV and natural killer cells: shaping the response to vaccination

Cytomegaloviruses (CMVs) are highly prevalent, persistent human pathogens that not only evade but also shape our immune responses. Natural killer (NK) cells play an important role in the control of CMV and CMVs have in turn developed a plethora of immunoevasion mechanisms targeting NK cells. This complex interplay can leave a long-lasting imprint on the immune system in general and affect responses toward other pathogens and vaccines. This review aims to provide an overview of NK cell biology and development, the manipulation of NK cells by CMVs and the potential impact of these evasion strategies on responses to vaccination.

A Viral Immunoevasin Controls Innate Immunity by Targeting the Prototypical Natural Killer Cell Receptor Family

Natural killer (NK) cells play a key role in innate immunity by detecting alterations in self and non-self ligands via paired NK cell receptors (NKRs). Despite identification of numerous NKR-ligand interactions, physiological ligands for the prototypical NK1.1 orphan receptor remain elusive. Here, we identify a viral ligand for the inhibitory and activating NKR-P1 (NK1.1) receptors. This murine cytomegalovirus (MCMV)-encoded protein, m12, restrains NK cell effector function by directly engaging the inhibitory NKR-P1B receptor. However, m12 also interacts with the activating NKR-P1A/C receptors to counterbalance m12 decoy function. Structural analyses reveal that m12 sequesters a large NKR-P1 surface area via a "polar claw" mechanism. Polymorphisms in, and ablation of, the viral m12 protein and host NKR-P1B/C alleles impact NK cell responses in vivo. Thus, we identify the long-sought foreign ligand for this key immunoregulatory NKR family and reveal how it controls the evolutionary balance of immune recognition during host-pathogen interplay.

Interferon-Dependent Induction of Clr-b during Mouse Cytomegalovirus Infection Protects Bystander Cells from Natural Killer Cells via NKR-P1B-Mediated Inhibition

Natural killer (NK) cells are innate lymphocytes that aid in self-nonself discrimination by recognizing cells undergoing pathological alterations. The NKR-P1B inhibitory receptor recognizes Clr-b, a self-encoded marker of cell health downregulated during viral infection. Here, we show that Clr-b loss during mouse cytomegalovirus (MCMV) infection is predicated by a loss of Clr-b (Clec2d) promoter activity and nascent transcripts, driven in part by MCMV ie3 (M122) activity. In contrast, uninfected bystander cells near MCMV-infected fibroblasts reciprocally upregulate Clr-b expression due to paracrine type-I interferon (IFN) signaling. Exposure of fibroblasts to type-I IFN augments Clec2d promoter activity and nascent Clr-b transcripts, dependent upon a cluster of IRF3/7/9 motifs located ∼200 bp upstream of the transcriptional start site. Cells deficient in type-I IFN signaling components revealed IRF9 and STAT1 as key transcription factors involved in Clr-b upregulation. In chromatin immunoprecipitation experiments, the Clec2d IRF cluster recruited STAT2 upon IFN-α exposure, confirming the involvement of ISGF3 (IRF9/STAT1/STAT2) in positively regulating the Clec2d promoter. These findings demonstrate that Clr-b is an IFN-stimulated gene on healthy bystander cells, in addition to a missing-self marker on MCMV-infected cells, and thereby enhances the dynamic range of innate self-nonself discrimination by NK cells.

Clr-a: A Novel Immune-Related C-Type Lectin-like Molecule Exclusively Expressed by Mouse Gut Epithelium

The mouse gut epithelium represents a constitutively challenged environment keeping intestinal commensal microbiota at bay and defending against invading enteric pathogens. The complex immunoregulatory network of the epithelial barrier surveillance also involves NK gene complex (NKC)-encoded C-type lectin-like molecules such as NKG2D and Nkrp1 receptors. To our knowledge, in this study, we report the first characterization of the orphan C-type lectin-like molecule Clr-a encoded by the Clec2e gene in the mouse NKC. Screening of a panel of mouse tissues revealed that Clec2e transcripts are restricted to the gastrointestinal tract. Using Clr-a-specific mAb, we characterize Clr-a as a disulfide-linked homodimeric cell surface glycoprotein. Of note, a substantial fraction of Clr-a molecules are retained intracellularly, and analyses of Clr-a/Clr-f hybrids attribute intracellular retention to both the stalk region and parts of the cytoplasmic domain. Combining quantitative PCR analyses with immunofluorescence studies revealed exclusive expression of Clr-a by intestinal epithelial cells and crypt cells throughout the gut. Challenge with polyinosinic-polycytidylic acid results in a rapid and strong downregulation of intestinal Clr-a expression in contrast to the upregulation of Clr-f, a close relative of Clr-a, that also is specifically expressed by the intestinal epithelium and acts as a ligand of the inhibitory Nkrp1g receptor. Collectively, we characterize expression of the mouse NKC-encoded glycoprotein Clr-a as strictly associated with mouse intestinal epithelium. Downregulation upon polyinosinic-polycytidylic acid challenge and expression by crypt cells clearly distinguish Clr-a from the likewise intestinal epithelium-restricted Clr-f, pointing to a nonredundant function of these highly related C-type lectin-like molecules in the context of intestinal immunosurveillance.

Vaccinia virus Western Reserve induces rapid surface expression of a host molecule detected by the antibody 4C7 that is distinct from CLEC2D

In this study, the effect of active infection with vaccinia virus Western Reserve (VACV WR) on expression of C-type lectin domain family 2 (CLEC2D), a ligand of the human NK cell inhibitory receptor NKR-P1, was examined. As predicted, VACV infection led to a loss of CLEC2D mRNA in 221 cells, a B cell lymphoma line. Surprisingly, VACV infection of 221 cells caused a dramatic increase in cell surface staining for one CLEC2D-specific antibody, 4C7. There were no changes in other antibodies specific for CLEC2D and no indication that NK cells with NKR-P1A were inhibited, suggesting 4C7 detects a non-CLEC2D molecule following infection. The rapid increase in 4C7 signal requires virus attachment and is disrupted by UV treatment, but does not depend on new transcription or translation of either cellular or viral proteins. 4C7 does react with intracellular compartments, suggesting the molecule that is detected at the surface following infection is derived from an intracellular store. The phenomenon extends beyond lymphoid cells: it was observed in the non-human primate cell line Cos-7, but not with myxoma, a poxvirus distinct from VACV. To our knowledge, this is the first report of VACV or any poxvirus leading to rapid externalization of a host molecule. Among the VACV strains tested, the phenomenon was restricted to VACV WR and IHD-W, suggesting it has a virulence-, as opposed to a replication-related, function.

Expansion and Protection by a Virus-Specific NK Cell Subset Lacking Expression of the Inhibitory NKR-P1B Receptor during Murine Cytomegalovirus Infection

NK cells play a major role in immune defense against human and murine CMV (MCMV) infection. Although the MCMV genome encodes for MHC class I-homologous decoy ligands for inhibitory NK cell receptors to evade detection, some mouse strains have evolved activating receptors, such as Ly49H, to recognize these ligands and initiate an immune response. In this study, we demonstrate that approximately half of the Ly49H-expressing (Ly49H(+)) NK cells in the spleen and liver of C57BL/6 mice also express the inhibitory NKR-P1B receptor. During MCMV infection, the NKR-P1B(-)Ly49H(+) NK cell subset proliferates to constitute the bulk of the NK cell population. This NK cell subset also confers better protection against MCMV infection compared with the NKR-P1B(+)Ly49H(+) subset. The two populations are composed of cells that differ in their surface expression of receptors such as Ly49C/I and NKG2A/C/E, as well as developmental markers, CD27 and CD11b, and the high-affinity IL-2R (CD25) following infection. Although the NKR-P1B(+) NK cells can produce effector molecules such as IFNs and granzymes, their proliferation is inhibited during infection. A similar phenotype in MCMV-infected Clr-b-deficient mice, which lack the ligand for NKR-P1B, suggests the involvement of ligands other than the host Clr-b. Most interestingly, genetic deficiency of the NKR-P1B, but not Clr-b, results in accelerated virus clearance and recovery from MCMV infection. This study is particularly significant because the mouse NKR-P1B:Clr-b receptor:ligand system represents the closest homolog of the human NKR-P1A:LLT1 system and may have a direct relevance to human CMV infection.

Solution structure of the lymphocyte receptor Nkrp1a reveals a distinct conformation of the long loop region as compared to in the crystal structure

Mouse Nkrp1a receptor is a C-type lectin-like receptor expressed on the surface of natural killer cells that play an important role against virally infected and tumor cells. The recently solved crystal structure of Nkrp1a raises questions about a long loop region which was uniquely extended from the central region in the crystal. To understand the functional significance of the loop, the solution structure of Nkrp1a using nuclear magnetic resonance (NMR) spectroscopy was determined. A notable difference between the crystal and NMR structure of Nkrp1a appears in the conformation of the long loop region. While the extended loop points away from the central core and mediates formation of a domain swapped dimer in the crystal, the solution structure is monomeric with the loop tightly anchored to the central region. The findings described the first solution structure in the Nkrp1 family and revealed intriguing similarities and differences to the crystal structure. Proteins 2016; 84:1304-1311. © 2016 Wiley Periodicals, Inc.

NK cell education via nonclassical MHC and non-MHC ligands

Natural killer (NK) cell education, a process for achieving functional maturation and self-tolerance, has been previously defined by the interaction between self-major histocompatibility complex class I (MHC-I) molecules and their specific inhibitory receptors. Over the past several years, growing evidence has highlighted the important roles of nonclassical MHC-I and non-MHC-I molecules in NK cell education. Herein, we review the current knowledge of NK cell education, with a particular focus on nonclassical MHC-I- and non-MHC-I-dependent education, and compare them with the classical MHC-I-dependent education theory. In addition, we update and extend this theory by presenting the 'Confining Model', discussing cis and trans characteristics, reassessing quantity and quality control, and elucidating the redundancy of NK cell education in tumor and virus infection.

Coevolution of MHC genes (LMP/TAP/class Ia, NKT-class Ib, NKp30-B7H6): lessons from cold-blooded vertebrates

Comparative immunology provides the long view of what is conserved across all vertebrate taxa versus what is specific to particular organisms or group of organisms. Regarding the major histocompatibility complex (MHC) and coevolution, three striking cases have been revealed in cold-blooded vertebrates: lineages of class Ia antigen-processing and -presenting genes, evolutionary conservation of NKT-class Ib recognition, and the ancient emergence of the natural cytotoxicity receptor NKp30 and its ligand B7H6. While coevolution of transporter associated with antigen processing (TAP) and class Ia has been documented in endothermic birds and two mammals, lineages of LMP7 are restricted to ectotherms. The unambiguous discovery of natural killer T (NKT) cells in Xenopus demonstrated that NKT cells are not restricted to mammals and are likely to have emerged at the same time in evolution as classical α/β and γ/δ T cells. NK cell receptors evolve at a rapid rate, and orthologues are nearly impossible to identify in different vertebrate classes. By contrast, we have detected NKp30 in all gnathostomes, except in species where it was lost. The recently discovered ligand of NKp30, B7H6, shows strong signs of coevolution with NKp30 throughout evolution, i.e. coincident loss or expansion of both genes in some species. NKp30 also offers an attractive IgSF candidate for the invasion of the RAG transposon, which is believed to have initiated T-cell receptor/immunoglobulin adaptive immunity. Besides reviewing these intriguing features of MHC evolution and coevolution, we offer suggestions for future studies and propose a model for the primordial or proto MHC.

Voluntary Running Suppresses Tumor Growth through Epinephrine- and IL-6-Dependent NK Cell Mobilization and Redistribution

Regular exercise reduces the risk of cancer and disease recurrence. Yet the mechanisms behind this protection remain to be elucidated. In this study, tumor-bearing mice randomized to voluntary wheel running showed over 60% reduction in tumor incidence and growth across five different tumor models. Microarray analysis revealed training-induced upregulation of pathways associated with immune function. NK cell infiltration was significantly increased in tumors from running mice, whereas depletion of NK cells enhanced tumor growth and blunted the beneficial effects of exercise. Mechanistic analyses showed that NK cells were mobilized by epinephrine, and blockade of β-adrenergic signaling blunted training-dependent tumor inhibition. Moreover, epinephrine induced a selective mobilization of IL-6-sensitive NK cells, and IL-6-blocking antibodies blunted training-induced tumor suppression, intratumoral NK cell infiltration, and NK cell activation. Together, these results link exercise, epinephrine, and IL-6 to NK cell mobilization and redistribution, and ultimately to control of tumor growth.

Gene expression signature in mouse thyroid tissue after (131)I and (211)At exposure

BACKGROUND

(131)I and (211)At are used in nuclear medicine and accumulate in the thyroid gland and may impact normal thyroid function. The aim of this study was to determine transcriptional profile variations, assess the impact on cellular activity, and identify genes with biomarker properties in thyroid tissue after (131)I and (211)At administration in mice.

METHODS

To further investigate thyroid tissue transcriptional responses to (131)I and (211)At administration, we generated a new transcriptional dataset that includes re-evaluated raw intensity values from our previous (131)I and (211)At studies. Differential transcriptional profiles were identified by comparing treated and mock-treated samples using Nexus Expression 3.0 software. Further data analysis was performed using R/Bioconductor and IPA.

RESULTS

A total of 1144 genes were regulated. Hierarchical clustering subdivided the groups into two clusters containing the lowest and highest absorbed dose levels, respectively, and revealed similar transcriptional regulation patterns for many kallikrein-related genes. Twenty-seven of the 1144 genes were recurrently regulated after (131)I and (211)At exposure and divided into six clusters. Several signalling pathways were affected, including calcium, integrin-linked kinase, and thyroid cancer signalling, and the peroxisomal proliferator-activated receptor network.

CONCLUSIONS

Substantial changes in transcriptional regulation were shown in (131)I and (211)At-treated samples, and 27 genes were identified as potential biomarkers for (131)I and (211)At exposure. Clustering revealed distinct differences between transcriptional profiles of both similar and different exposures, demonstrating the necessity for better understanding of radiation-induced effects on cellular activity. Additionally, ionizing radiation-induced changes in kallikrein gene expression and identified canonical pathways should be further assessed.

Recombinant Expression, In Vitro Refolding and Characterizing Disulfide Bonds of a Mouse Inhibitory C-Type Lectin-Like Receptor Nkrp1b

As a part of the innate immunity, NK (Natural Killer) cells provide an early immune response to different stimuli, e.g. viral infections and tumor growths. However, their functions are more complex; they play an important role in reproduction, alloimmunity, autoimmunity and allergic diseases. NK cell activities require an intricate system of regulation that is ensured by many different receptors on a cell surface which integrate signals from interacting cells and soluble factors. One way to understand NK cell biology is through the structure of NK receptors, which can reveal ligand binding conditions. We present a modified protocol for recombinant expression in Escherichia coli and in vitro refolding of the ligand-binding domain of the inhibitory Nkrp1b (SJL/J) protein. Nkrp1b identity and folding was confirmed using mass spectrometry (accurate mass of the intact protein and evaluation of disulfide bonds) and one-dimensional nuclear magnetic resonance spectroscopy. The intention is to provide the basis for conducting structural studies of the inhibitory Nkrp1b protein, since only the activating Nkrp1a receptor structure is known.

Modulation of Clr Ligand Expression and NKR-P1 Receptor Function during Murine Cytomegalovirus Infection

Viruses are known to induce pathological cellular states that render infected cells susceptible or resistant to immune recognition. Here, we characterize an MHC-I-independent natural killer (NK) cell recognition mechanism that involves modulation of inhibitory NKR-P1B:Clr-b receptor-ligand interactions in response to mouse cytomegalovirus (MCMV) infection. We demonstrate that mouse Clr-b expression on healthy cells is rapidly lost at the cell surface and transcript levels in a time- and dose-dependent manner upon MCMV infection. In addition, cross-species infections using rat cytomegalovirus (RCMV) infection of mouse fibroblasts and MCMV infection of rat fibroblasts suggest that this response is conserved during host-pathogen interactions. Active viral infection appears to be necessary for Clr-b loss, as cellular stimulation using UV-inactivated whole virus or agonists of many innate pattern recognition receptors failed to elicit efficient Clr-b downregulation. Notably, Clr-b loss could be partially blocked by titrated cycloheximide treatment, suggesting that early viral or nascent host proteins are required for Clr-b downregulation. Interestingly, reporter cell assays suggest that MCMV may encode a novel Clr-b-independent immunoevasin that functionally engages the NKR-P1B receptor. Together, these data suggest that Clr-b modulation is a conserved innate host cell response to virus infection that is subverted by multiple CMV immune evasion strategies.

Attrition of TCR Vα7.2+ CD161++ MAIT cells in HIV-tuberculosis co-infection is associated with elevated levels of PD-1 expression

Mucosal-associated invariant T (MAIT) cells are evolutionarily conserved antimicrobial MR1-restricted CD8⁺ T cells co-expressing the semi-invariant TCR Vα7.2, and are numerous in the blood and mucosal tissues of humans. MAIT cells appear to undergo exhaustion in chronic viral infections. However, their role in human immunodeficiency virus type 1 (HIV-1) mono-infection and HIV/tuberculosis (TB) co-infection have seldom been elaborately investigated. We conducted a cross-sectional study to investigate the frequencies and phenotypes of CD161⁺⁺CD8⁺ T cells among anti-retroviral therapy (ART)/anti-TB therapy (ATT) treatment-naïve HIV/TB co-infected, ART/TB treated HIV/TB co-infected, ART naïve HIV-infected, ART-treated HIV-infected patients, and HIV negative healthy controls (HCs) by flow cytometry. Our data revealed that the frequency of MAIT cells was severely depleted in HIV mono- and HIV/TB co-infections. Further, PD-1 expression on MAIT cells was significantly increased in HIV mono- and HIV-TB co-infected patients. The frequency of MAIT cells did not show any significant increase despite the initiation of ART and/or ATT. Majority of the MAIT cells in HCs showed a significant increase in CCR6 expression as compared to HIV/TB co-infections. No marked difference was seen with expressions of chemokine co-receptor CCR5 and CD103 among the study groups. Decrease of CCR6 expression appears to explain why HIV-infected patients display weakened mucosal immune responses.

Dedicated immunosensing of the mouse intestinal epithelium facilitated by a pair of genetically coupled lectin-like receptors

The integrity of the intestinal epithelium is constantly surveyed by a peculiar subset of innate-like T lymphocytes embedded in the epithelial cell layer, hence called intestinal intraepithelial lymphocytes (IELs). IELs are thought to act as "first-line" sentinels sensing the state of adjacent epithelial cells via both T-cell receptors and auxiliary receptors. Auxiliary receptors modulating IEL activity include C-type lectin-like receptors encoded in the natural killer gene complex such as NKG2D. Here, we report that the CTLR Nkrp1g is expressed by a subpopulation of mouse CD103(+) IELs allowing immunosensing of the intestinal epithelium through ligation of the genetically coupled CTLR Clr-f that is almost exclusively expressed on differentiated intestinal epithelial cells (IECs). Most of these Nkrp1g-expressing IELs exhibit a γδTCR(bright)Nkg2a(-) phenotype and are intimately associated with the intestinal epithelium. As Clr-f expression strongly inhibits effector functions of Nkrp1g-expressing cells and is upregulated upon poly(I:C) challenge, Clr-f molecules may quench reactivity of these IELs towards the epithelial barrier that is constantly provoked by microbial and antigenic stimuli. Altogether, we here newly characterize a genetically linked C-type lectin-like receptor/ligand pair with a highly restricted tissue expression that apparently evolved to allow for a dedicated immunosurveillance of the mouse intestinal epithelium.

An in vitro model of innate lymphoid cell function and differentiation

Innate lymphoid cells (ILC) are RAG-independent lymphocytes with important roles in innate immunity, and include group-1 (natural killer (NK) cell, ILC1), group-2 (ILC2), and group-3 (lymphoid tissue inducer (LTi), NCR(+) ILC3) subsets. Group-3 ILC express Rorγt, produce interleukin (IL)-22, and are critically important in the normal function of mucosal tissues. Here, we describe a novel model cell line for the study of ILC function and differentiation. The parental MNK cell line, derived from NKR-P1B(+) fetal thymocytes, shows a capacity to differentiate in γc cytokines. One IL-7-responsive subline, designated MNK-3, expresses Rorγt and produces high levels of IL-22 in response to IL-23 and IL-1β stimulation. MNK-3 cells display surface markers and transcript expression characteristic of group-3 ILC, including IL-7Rα (CD127), c-kit (CD117), CCR6, Thy1 (CD90), RANK, RANKL, and lymphotoxin (LTα1β2). Using an in vitro assay of LTi cell activity, MNK-3 cells induce ICAM-1 and VCAM-1 expression on stromal cells in a manner dependent upon LTα1β2 expression. A second IL-2-responsive subline, MNK-1, expresses several NK cell receptors, perforin and granzymes, and shows some cytotoxic activity. Thus, MNK-1 cells serve as a model of ILC1/NK development and differentiation, whereas MNK-3 cells provide an attractive in vitro system to study the function of ILC3/LTi cells.

Nkrp1 family, from lectins to protein interacting molecules

The C-type lectin-like receptors include the Nkrp1 protein family that regulates the activity of natural killer (NK) cells. Rat Nkrp1a was reported to bind monosaccharide moieties in a Ca²⁺-dependent manner in preference order of GalNac > GlcNAc >> Fuc >> Gal > Man. These findings established for rat Nkrp1a have been extrapolated to all additional Nkrp1 receptors and have been supported by numerous studies over the past two decades. However, since 1996 there has been controversy and another article showed lack of interactions with saccharides in 1999. Nevertheless, several high affinity saccharide ligands were synthesized in order to utilize their potential in antitumor therapy. Subsequently, protein ligands were introduced as specific binders for Nkrp1 proteins and three dimensional models of receptor/protein ligand interaction were derived from crystallographic data. Finally, for at least some members of the NK cell C-type lectin-like proteins, the “sweet story” was impaired by two reports in recent years. It has been shown that the rat Nkrp1a and CD69 do not bind saccharide ligands such as GlcNAc, GalNAc, chitotetraose and saccharide derivatives (GlcNAc-PAMAM) do not directly and specifically influence cytotoxic activity of NK cells as it was previously described.

Genetic investigation of MHC-independent missing-self recognition by mouse NK cells using an in vivo bone marrow transplantation model

MHC-I-specific receptors play a vital role in NK cell-mediated "missing-self" recognition, which contributes to NK cell activation. In contrast, MHC-independent NK recognition mechanisms are less well characterized. In this study, we investigated the role of NKR-P1B:Clr-b (Klrb1:Clec2d) interactions in determining the outcome of murine hematopoietic cell transplantation in vivo. Using a competitive transplant assay, we show that Clr-b(-/-) bone marrow (BM) cells were selectively rejected by wild-type B6 recipients, to a similar extent as H-2D(b-/-) MHC-I-deficient BM cells. Selective rejection of Clr-b(-/-) BM cells was mitigated by NK depletion of recipient mice. Competitive rejection of Clr-b(-/-) BM cells also occurred in allogeneic transplant recipients, where it was reversed by selective depletion of NKR-P1B(hi) NK cells, leaving the remaining NKR-P1B(lo) NK subset and MHC-I-dependent missing-self recognition intact. Moreover, competitive rejection of Clr-b(-/-) hematopoietic cells was abrogated in Nkrp1b-deficient recipients, which lack the receptor for Clr-b. Of interest, similar to MHC-I-deficient NK cells, Clr-b(-/-) NK cells were hyporesponsive to both NK1.1 (NKR-P1C)-stimulated and IL-12/18 cytokine-primed IFN-γ production. These findings support a unique and nonredundant role for NKR-P1B:Clr-b interactions in missing-self recognition of normal hematopoietic cells and suggest that optimal BM transplant success relies on MHC-independent tolerance mechanisms. These findings provide a model for human NKR-P1A:LLT1 (KLRB1:CLEC2D) interactions in human hematopoietic cell transplants.

The mouse NKR-P1B:Clr-b recognition system is a negative regulator of innate immune responses

NKR-P1B is a homodimeric type II transmembrane C-type lectinlike receptor that inhibits natural killer (NK) cell function upon interaction with its cognate C-type lectin-related ligand, Clr-b. The NKR-P1B:Clr-b interaction represents a major histocompatibility complex class I (MHC-I)-independent missing-self recognition system that monitors cellular Clr-b levels. We have generated NKR-P1B(B6)-deficient (Nkrp1b(-/-)) mice to study the role of NKR-P1B in NK cell development and function in vivo. NK cell inhibition by Clr-b is abolished in Nkrp1b(-/-) mice, confirming the inhibitory nature of NKR-P1B(B6). Inhibitory receptors also promote NK cell tolerance and responsiveness to stimulation; hence, NK cells expressing NKR-P1B(B6) and Ly49C/I display augmented responsiveness to activating signals vs NK cells expressing either or none of the receptors. In addition, Nkrp1b(-/-) mice are defective in rejecting cells lacking Clr-b, supporting a role for NKR-P1B(B6) in MHC-I-independent missing-self recognition of Clr-b in vivo. In contrast, MHC-I-dependent missing-self recognition is preserved in Nkrp1b(-/-) mice. Interestingly, spontaneous myc-induced B lymphoma cells may selectively use NKR-P1B:Clr-b interactions to escape immune surveillance by wild-type, but not Nkrp1b(-/-), NK cells. These data provide direct genetic evidence of a role for NKR-P1B in NK cell tolerance and MHC-I-independent missing-self recognition.

A negative feedback modulator of antigen processing evolved from a frameshift in the cowpox virus genome

Coevolution of viruses and their hosts represents a dynamic molecular battle between the immune system and viral factors that mediate immune evasion. After the abandonment of smallpox vaccination, cowpox virus infections are an emerging zoonotic health threat, especially for immunocompromised patients. Here we delineate the mechanistic basis of how cowpox viral CPXV012 interferes with MHC class I antigen processing. This type II membrane protein inhibits the coreTAP complex at the step after peptide binding and peptide-induced conformational change, in blocking ATP binding and hydrolysis. Distinct from other immune evasion mechanisms, TAP inhibition is mediated by a short ER-lumenal fragment of CPXV012, which results from a frameshift in the cowpox virus genome. Tethered to the ER membrane, this fragment mimics a high ER-lumenal peptide concentration, thus provoking a trans-inhibition of antigen translocation as supply for MHC I loading. These findings illuminate the evolution of viral immune modulators and the basis of a fine-balanced regulation of antigen processing.

Virus-infected or malignant transformed cells are eliminated by cytotoxic T lymphocytes, which recognize antigenic peptide epitopes in complex with major histocompatibility complex class I (MHC I) molecules at the cell surface. The majority of such peptides are derived from proteasomal degradation in the cytosol and are then translocated into the ER lumen in an energy-consuming reaction via the transporter associated with antigen processing (TAP), which delivers the peptides onto MHC I molecules as final acceptors. Viruses have evolved sophisticated strategies to escape this immune surveillance. Here we show that the cowpox viral protein CPXV012 inhibits the ER peptide translocation machinery by allosterically blocking ATP binding and hydrolysis by TAP. The short ER resident active domain of the viral protein evolved from a reading frame shift in the cowpox virus genome and exploits the ER-lumenal negative feedback peptide sensor of TAP. This CPXV012-induced conformational arrest of TAP is signaled by a unique communication across the ER membrane to the cytosolic motor domains of the peptide pump. Furthermore, this study provides the rare opportunity to decipher on a molecular level how nature plays hide and seek with a pathogen and its host.

Mucosal-associated invariant T-cells: new players in anti-bacterial immunity

Mucosal-associated invariant T (MAIT) cells are an innate-like T-cell population involved in anti-bacterial immunity. In human beings, MAIT cells are abundant, comprising ~10% of the CD8⁺ T-cell compartment in blood. They are enriched at mucosal sites and are particularly prevalent within the liver. MAIT cells are defined by the expression of a semi-invariant T-cell receptor (Vα7.2-Jα33/12/20) and are restricted by the non-polymorphic, highly evolutionarily conserved MHC class Ib molecule, MHC-related protein (MR)1. MR1 has recently been shown to present an unstable pyrimidine intermediate derived from a biosynthetic precursor of riboflavin; riboflavin biosynthesis occurs in many bacteria but not in human beings. Consistent with this, MAIT cells are responsive to riboflavin-metabolizing bacteria, including Salmonella. In mouse models, MAIT cells have been shown to play a non-redundant role in anti-bacterial immunity, including against Escherichia coli, Klebsiella pneumoniae, and Mycobacterium bovis BCG. In human beings, MAIT cells are decreased in frequency in the blood of patients with tuberculosis or pneumonia, and their frequency has been inversely correlated with the risk of subsequent systemic bacterial infection in patients in intensive care. Intriguingly, MAIT cells are also depleted from the blood early in HIV infection and fail to recover with anti-retroviral therapy, which may contribute to the susceptibility of patients infected with HIV to certain bacterial infections, including non-typhoidal Salmonella. In this review, we will discuss what is currently known about MAIT cells, the role that Salmonella has played in elucidating MAIT cell restriction and function, and the role MAIT cells might play in the control of Salmonella infection.

Complexity and Diversity of the NKR-P1:Clr (Klrb1:Clec2) Recognition Systems

The NKR-P1 receptors were identified as prototypical natural killer (NK) cell surface antigens and later shown to be conserved from rodents to humans on NK cells and subsets of T cells. C-type lectin-like in nature, they were originally shown to be capable of activating NK cell function and to recognize ligands on tumor cells. However, certain family members have subsequently been shown to be capable of inhibiting NK cell activity, and to recognize proteins encoded by a family of genetically linked C-type lectin-related ligands. Some of these ligands are expressed by normal, healthy cells, and modulated during transformation, infection, and cellular stress, while other ligands are upregulated during the immune response and during pathological circumstances. Here, we discuss historical and recent developments in NKR-P1 biology that demonstrate this NK receptor–ligand system to be far more complex and diverse than originally anticipated.

The leukocyte activation receptor CD69 controls T cell differentiation through its interaction with galectin-1

CD69 is involved in immune cell homeostasis, regulating the T cell-mediated immune response through the control of Th17 cell differentiation. However, natural ligands for CD69 have not yet been described. Using recombinant fusion proteins containing the extracellular domain of CD69, we have detected the presence of a ligand(s) for CD69 on human dendritic cells (DCs). Pulldown followed by mass spectrometry analyses of CD69-binding moieties on DCs identified galectin-1 as a CD69 counterreceptor. Surface plasmon resonance and anti-CD69 blocking analyses demonstrated a direct and specific interaction between CD69 and galectin-1 that was carbohydrate dependent. Functional assays with both human and mouse T cells demonstrated the role of CD69 in the negative effect of galectin-1 on Th17 differentiation. Our findings identify CD69 and galectin-1 to be a novel regulatory receptor-ligand pair that modulates Th17 effector cell differentiation and function.

Structural basis for recognition of cellular and viral ligands by NK cell receptors

Natural killer (NK) cells are key components of innate immune responses to tumors and viral infections. NK cell function is regulated by NK cell receptors that recognize both cellular and viral ligands, including major histocompatibility complex (MHC), MHC-like, and non-MHC molecules. These receptors include Ly49s, killer immunoglobulin-like receptors, leukocyte immunoglobulin-like receptors, and NKG2A/CD94, which bind MHC class I (MHC-I) molecules, and NKG2D, which binds MHC-I paralogs such as the stress-induced proteins MICA and ULBP. In addition, certain viruses have evolved MHC-like immunoevasins, such as UL18 and m157 from cytomegalovirus, that act as decoy ligands for NK receptors. A growing number of NK receptor–ligand interaction pairs involving non-MHC molecules have also been identified, including NKp30–B7-H6, killer cell lectin-like receptor G1–cadherin, and NKp80–AICL. Here, we describe crystal structures determined to date of NK cell receptors bound to MHC, MHC-related, and non-MHC ligands. Collectively, these structures reveal the diverse solutions that NK receptors have developed to recognize these molecules, thereby enabling the regulation of NK cytolytic activity by both host and viral ligands.

Advances in NKG2D ligand recognition and responses by NK cells

The natural killer (NK) group 2 member D (NKG2D) is an activating immune receptor expressed on NK cells, cytotoxic T cells and a subset of other T cells. It has an important role in the recognition and lysis of a variety of infected and tumor cells. Despite significant gains in our understanding of NKG2D, the relevance of NKG2D and its ligands in human diseases has only recently started to emerge. Here, we present an overview of the recent advances in NKG2D biology, discuss the expression of NKG2D ligands in cancer patients and evaluate the diagnostic and prognostic potential of NKG2D ligands.

The CD200-CD200R1 inhibitory signaling pathway: immune regulation and host-pathogen interactions

The CD200:CD200R1 inhibitory signaling pathway has been implicated in playing a prominent role in limiting inflammation in a wide range of inflammatory diseases. CD200R1 signaling inhibits the expression of proinflammatory molecules including tumor necrosis factor, interferons, and inducible nitric oxide synthase in response to selected stimuli. Unsurprisingly, due to the regulatory role that CD200R1 plays in multiple inflammatory pathways, an increasing number of parasitic, bacterial, and viral pathogens exploit this pathway to suppress host defenses. A complete understanding of the pathways regulated by CD200R1 signaling and the diverse mechanisms that pathogens have evolved to manipulate the CD200:CD200R1 pathway can help identify clinical situations where targeting this interaction can be of therapeutic benefit. In this review, we compare CD200R1 to other pathogen-targeted inhibitory receptors and highlight how this signaling pathway is utilized by a diverse number of pathogens and, therefore, may represent a novel targeting strategy for the treatment of infectious diseases.

Modulation of NK cell function by genetically coupled C-type lectin-like receptor/ligand pairs encoded in the human natural killer gene complex

Functional responses of natural killer (NK) cells including eradication of “harmful” cells and modulation of immune responses are regulated by a broad variety of activating and inhibitory NK receptors. Whereas the leukocyte receptor complex (LRC) encodes for NK receptors of the immunoglobulin superfamily, genes of C-type lectin-like NK receptors are clustered in the mammalian natural killer gene complex (NKC). Besides the thoroughly studied C-type lectin-like receptors NKG2D, CD94/NKG2x, and members of the murine Ly49 subfamily, the NKC also encodes for NK receptors of the less characterized NKRP1 subfamily. The prototypic mouse NKRP1 receptor is Nkrp1c (also known as NK1.1), while human members of the NKRP1 subfamily are NKRP1A, NKp80, and NKp65. The latter are not straight homologs of mouse NKRP1 receptors, but share distinct subfamily-specific traits classifying them as members of the NKRP1 subfamily. Ligands of the human NKPR1 receptors are likewise C-type lectin-like glycoproteins belonging to the CLEC2 subfamily (i.e., LLT1, AICL, and KACL), and are encoded in the NKC in tight genetic linkage to their respective receptors. Similarly, certain members of the mouse NKRP1 subfamily interact with genetically coupled CLEC2 glycoproteins, while the reasons for this intriguing tight genetic linkage remain unknown. Recent studies provided new and unique insights into the expression, interaction, and signaling of NKRP1 receptors and their ligands, thereby substantially advancing our understanding of their function and biology. Here, we review our current knowledge on NKRP1 receptors and their genetically linked CLEC2 ligands with an emphasis on the human receptor/ligand pairs NKRP1A-LLT1, NKp80-AICL, and NKp65-KACL.