Human NK cells and subsets of T cells or NKT cells express the orphan C-type lectin receptor CD161 (NKR-P1A) of unknown function. In contrast to rodents that possess several NKR-P1 genes coding for either activating or inhibitory receptors, the nature of signals delivered by the single human NKR-P1A receptor is still to be clarified. In this article, we show that the lectin-like transcript 1 (LLT1) molecule is a ligand for the CD161 receptor. Engagement of CD161 on NK cells with LLT1 expressed on target cells inhibited NK cell-mediated cytotoxicity and IFN-γ secretion. Conversely, LLT1/CD161 interaction in the presence of a TCR signal enhanced IFN-γ production by T cells. These findings identify a novel ligand/receptor pair that differentially regulate NK and T cell functions.

Human cytomegalovirus (HCMV) causes lifelong, persistent infections and its survival is under intense, continuous selective pressure from the immune system. A key aspect of HCMV's capacity for survival lies in immune avoidance. In this context, cells undergoing productive infection exhibit remarkable resistance to natural killer (NK) cell-mediated cytolysis in vitro. To date, six genes encoding proteins (UL16, UL18, UL40, UL83, UL141 and UL142) and one encoding a microRNA (miR-UL112) have been identified as capable of suppressing NK cell recognition. Even though HCMV infection efficiently activates expression of ligands for the NK cell activating receptor NKG2D, at least three functions (UL16, UL142 and miR-UL112) act in concert to suppress presentation of these ligands on the cell surface. Although HCMV downregulates expression of endogenous MHC-I, it encodes an MHC-I homologue (UL18) and also upregulates the expression of cellular HLA-E through the action of UL40. The disruption of normal intercellular connections exposes ligands for NK cell activating receptors on the cell surface, notably CD155. HCMV overcomes this vulnerability by encoding a function (UL141) that acts post-translationally to suppress cell surface expression of CD155. The mechanisms by which HCMV systematically evades (or, more properly, modulates) NK cell recognition constitutes an area of growing understanding that is enhancing our appreciation of the basic mechanisms of NK cell function in humans.

Disruption of the endothelial barrier by tumour-derived secreted factors is a critical step in cancer cell extravasation and metastasis. Here, by comparative proteomic analysis of melanoma secretomes, we identify the matricellular protein SPARC as a novel tumour-derived vascular permeability factor. SPARC deficiency abrogates tumour-initiated permeability of lung capillaries and prevents extravasation, whereas SPARC overexpression enhances vascular leakiness, extravasation and lung metastasis. SPARC-induced paracellular permeability is dependent on the endothelial VCAM1 receptor and p38 MAPK signalling. Blocking VCAM1 impedes melanoma-induced endothelial permeability and extravasation. The clinical relevance of our findings is highlighted by high levels of SPARC detected in tumour from human pulmonary melanoma lesions. Our study establishes tumour-produced SPARC and VCAM1 as regulators of cancer extravasation, revealing a novel targetable interaction for prevention of metastasis.

NKG2D plays a major role in controlling immune responses through the regulation of natural killer (NK) cells, αβ and γδ T-cell function. This activating receptor recognizes eight distinct ligands (the MHC Class I polypeptide-related sequences (MIC) A andB, and UL16-binding proteins (ULBP)1–6) induced by cellular stress to promote recognition cells perturbed by malignant transformation or microbial infection. Studies into human cytomegalovirus (HCMV) have aided both the identification and characterization of NKG2D ligands (NKG2DLs). HCMV immediate early (IE) gene up regulates NKGDLs, and we now describe the differential activation of ULBP2 and MICA/B by IE1 and IE2 respectively. Despite activation by IE functions, HCMV effectively suppressed cell surface expression of NKGDLs through both the early and late phases of infection. The immune evasion functions UL16, UL142, and microRNA(miR)-UL112 are known to target NKG2DLs. While infection with a UL16 deletion mutant caused the expected increase in MICB and ULBP2 cell surface expression, deletion of UL142 did not have a similar impact on its target, MICA. We therefore performed a systematic screen of the viral genome to search of addition functions that targeted MICA. US18 and US20 were identified as novel NK cell evasion functions capable of acting independently to promote MICA degradation by lysosomal degradation. The most dramatic effect on MICA expression was achieved when US18 and US20 acted in concert. US18 and US20 are the first members of the US12 gene family to have been assigned a function. The US12 family has 10 members encoded sequentially through US12–US21; a genetic arrangement, which is suggestive of an ‘accordion’ expansion of an ancestral gene in response to a selective pressure. This expansion must have be an ancient event as the whole family is conserved across simian cytomegaloviruses from old world monkeys. The evolutionary benefit bestowed by the combinatorial effect of US18 and US20 on MICA may have contributed to sustaining the US12 gene family.

The inhibitory leukocyte Ig-like receptor 1 (LIR-1, also known as ILT2, CD85j, or LILRB1) was identified by its high affinity for the human CMV (HCMV) MHC class I homolog gpUL18. The role of this LIR-1-gpUL18 interaction in modulating NK recognition during HCMV infection has previously not been clearly defined. In this study, LIR-1+ NKL cell-mediated cytotoxicity was shown to be inhibited by transduction of targets with a replication-deficient adenovirus vector encoding UL18 (RAd-UL18). Fibroblasts infected with an HCMV UL18 mutant (ΔUL18) also exhibited enhanced susceptibility to NKL killing relative to cells infected with the parental virus. In additional cytolysis assays, UL18-mediated protection was also evident in the context of adenovirus vector transduction and HCMV infection of autologous fibroblast targets using IFN-α-activated NK bulk cultures derived from a donor with a high frequency of LIR-1+ NK cells. A single LIR-1high NK clone derived from this donor was inhibited by UL18, while 3 of 24 clones were activated. CD107 mobilization assays revealed that LIR-1+ NK cells were consistently inhibited by UL18 in all tested donors, but this effect was often masked in the global response by UL18-mediated activation of a subset of LIR-1− NK cells. Although Ab-blocking experiments support UL18 inhibition being induced by a direct interaction with LIR-1, the UL18-mediated activation is LIR-1 independent.

The Adenoviridae family includes clinically important viruses associated with acute episodes of pharyngitis, conjunctivitis, diarrhea, and cystitis in immunocompetent individuals. In immunosuppressed patients, particularly pediatric transplant recipients, adenoviruses (Ads) are capable of systemic viremic spread that produces significant levels of morbidity and mortality. More than 50 human Ads have been described, of which the common species C serotypes 2 and 5 (HAdV-2 and HAdV-5) have been studied most intensively (52), and demonstrated to encode functions that act in vitro and in vivo to promote evasion of both innate and adaptive host immune responses (11). Immunomodulatory functions are concentrated in the early transcription unit E3 (33, 51). An E3 protein, E3/19K, became an immunological paradigm when it was demonstrated to impede cell surface antigen presentation by retaining the major histocompatibility class I complex (MHC-I) in the endoplasmic reticulum (ER) (1,8,10,17). E3/ 10.4K and E3/14.5K (the RID complex) together remove Fas, tumor necrosis factor-related apoptosis-inducing ligand receptors 1 and 2, and the epidermal growth factor receptor from the cell surface and promote their subsequent degradation in lysosomes (5,12,18,24,44,45,51). Moreover, E3/14.7K suppresses inflammation and cytolysis by inhibiting tumor necrosis factor alphamediated signaling independently of the RID complex (11,26).We are interested in the role of NK cells in controlling Ad infections. NK cells are a heterogeneous population of cells expressing a wide range of activating and inhibitory receptors. NKG2D is a homodimeric activating receptor that is ubiquitously expressed on NK cells, ␥␦ T cells, and NKT cells and also on certain ␣␤ T-cell subsets (3). To date, seven human cellular NKG2D ligands (NKG2DLs) have been identified: the MHC-I chain-related A (MICA) and B (MICB) proteins, the UL16 binding proteins ULBP1 to -3, and the retinoic acid early inducible proteins E (ULBP4) and G (2, 3, 16). While MICA and MICB are closely related, sharing 84% amino acid (aa) identity and similar tertiary structures (25, 32), they are also highly polymorphic, with 61 MICA and 30 MICB alleles having been described (IMGT;

Human cytomegalovirus (HCMV) UL141 induces protection against natural killer cell-mediated cytolysis by downregulating cell surface expression of CD155 (nectin-like molecule 5; poliovirus receptor), a ligand for the activating receptor DNAM-1 (CD226). However, DNAM-1 is also recognized to bind a second ligand, CD112 (nectin-2). We now show that HCMV targets CD112 for proteasome-mediated degradation by 48 h post-infection, thus removing both activating ligands for DNAM-1 from the cell surface during productive infection. Significantly, cell surface expression of both CD112 and CD155 was restored when UL141 was deleted from the HCMV genome. While gpUL141 alone is sufficient to mediate retention of CD155 in the endoplasmic reticulum, UL141 requires assistance from additional HCMV-encoded functions to suppress expression of CD112.

Aberrant extracellular matrix (ECM) deposition and stiffening is a physical hallmark of several solid cancers and is associated with therapy failure. BRAF-mutant melanomas treated with BRAF and MEK inhibitors almost invariably develop resistance that is frequently associated with transcriptional reprogramming and a de-differentiated cell state. Melanoma cells secrete their own ECM proteins, an event that is promoted by oncogenic BRAF inhibition. Yet, the contribution of cancer cell-derived ECM and tumor mechanics to drug adaptation and therapy resistance remains poorly understood. Here, we show that melanoma cells can adapt to targeted therapies through a mechanosignaling loop involving the autocrine remodeling of a drug-protective ECM.Analyses revealed that therapy resistant cells associated with a mesenchymal dedifferentiated state displayed elevated responsiveness to collagen stiffening and forcemediated ECM remodeling through activation of actin-dependent mechanosensors Yesassociated protein (YAP) and Myocardin-related transcription factor (MRTF). Shortterm inhibition of MAPK pathway also induced mechanosignaling associated with deposition and remodeling of an aligned fibrillar matrix. This provided a favored ECM reorganization that promoted tolerance to BRAF inhibition in a YAP and MRTFdependent manner. Matrix remodeling and tumor stiffening were also observed in vivo upon exposure of BRAF-mutant melanoma cell lines or patient-derived xenograft models to MAPK pathway inhibition. Importantly, pharmacological targeting of YAP reversed treatment-induced excessive collagen deposition, leading to enhancement of BRAF inhibitor efficacy. We conclude that MAPK pathway targeting therapies mechanically reprogram melanoma cells to confer a drug-protective matrix environment. Preventing melanoma cell mechanical reprogramming might be a promising therapeutic strategy for patients on targeted therapies. SIGNIFICANCEThese findings reveal a biomechanical adaptation of melanoma cells to oncogenic BRAF pathway inhibition, which fuels a YAP/MRTF-dependent feed-forward loop associated with tumor stiffening, mechanosensing and therapy resistance.