Modulation of natural killer cell cytotoxicity in human cytomegalovirus infection: the role of endogenous class I major histocompatibility complex and a viral class I homolog

Lessons from Acquired Natural Immunity and Clinical Trials to Inform Next-Generation Human Cytomegalovirus Vaccine Development

Human cytomegalovirus (HCMV) infection, the most common cause of congenital disease globally, affecting an estimated 1 million newborns annually, can result in lifelong sequelae in infants, such as sensorineural hearing loss and brain damage. HCMV infection also leads to a significant disease burden in immunocompromised individuals. Hence, an effective HCMV vaccine is urgently needed to prevent infection and HCMV-associated diseases. Unfortunately, despite more than five decades of vaccine development, no successful HCMV vaccine is available. This review summarizes what we have learned from acquired natural immunity, including innate and adaptive immunity; the successes and failures of HCMV vaccine human clinical trials; the progress in related animal models; and the analysis of protective immune responses during natural infection and vaccination settings. Finally, we propose novel vaccine strategies that will harness the knowledge of protective immunity and employ new technology and vaccine concepts to inform next-generation HCMV vaccine development.

NK cell dysfunction is linked with disease severity in SARS-CoV-2 patients

SARS-CoV-2 first raised from Wuhan City, Hubei Province in November 2019. The respiratory disorder, cough, weakness, fever are the main clinical symptoms of coronavirus disease 2019 (COVID-19) patients. Natural Killer (NK) cells as a first defense barrier of innate immune system have an essential role in early defense against pulmonary virus. They kill the infected cells by inducing apoptosis or the degranulation of perforin and granzymes. Collectively, NK cells function are coordinated by the transmitted signals from activating and inhibitory receptors. It is clear that the cytotoxic function of NK cells is disrupted in COVID-19 patients due to the dysregulation of activating and inhibitory receptors. Therefore, better understanding of the activating and inhibitory receptors mechanism could facilitate the treatment strategy in clinic. To improve the efficacy of immunotherapy in COVID-19 patients, the functional detail of NK cell and manipulation of their key checkpoints are gathered in current review.

Prevention of Congenital Cytomegalovirus Infection with Vaccines: State of the Art

Cytomegalovirus (CMV) is the most common cause of congenital infection and non-genetic sensorineural hearing loss in childhood. Up to 2% of neonates, with the highest percentages found in developing countries, are congenitally infected with CMV. At birth, most of these infants are asymptomatic. However, approximately 10% have signs and symptoms of the disease, and 40–60% of symptomatic neonates will later develop permanent neurologic sequelae. To reduce congenital CMV (cCMV) infection, a vaccine able to prevent primary infection is essential. In this narrative review, actual ongoing research about the development of a CMV vaccine is discussed. The progressive increase in knowledge on the ways in which the host’s immune system and CMV relate has made it possible to clarify that the development of a vaccine that is certainly capable of reducing the risk of cCMV infection, and preventing both primary and nonprimary infections is extremely difficult. Many of the ways in which the virus evades the immune system and causes cCMV infection are not yet fully understood, especially in cases of nonprimary infection. Moreover, the schedule that should be recommended and that subjects must be vaccinated to obtain the greatest effect have not been precisely defined. Further studies are needed before the problem of cCMV infection and its related challenges can be totally solved.

Elucidation of host-virus surfaceome interactions using spatial proteotyping

The cellular surfaceome and its residing extracellularly exposed proteins are involved in a multitude of molecular signaling processes across the viral infection cycle. Successful viral propagation, including viral entry, immune evasion, virion release and viral spread rely on dynamic molecular interactions with the surfaceome. Decoding of these viral-host surfaceome interactions using advanced technologies enabled the discovery of fundamental new functional insights into cellular and viral biology. In this review, we highlight recently developed experimental strategies, with a focus on spatial proteotyping technologies, aiding in the rational design of theranostic strategies to combat viral infections.

Understanding human γδ T cell biology toward a better management of cytomegalovirus infection

Cytomegalovirus (CMV) infection is responsible for significant morbidity and mortality in immunocompromised patients, namely solid organ and hematopoietic cell transplant recipients, and can induce congenital infection in neonates. There is currently an unmet need for new management and treatment strategies. Establishment of an anti-CMV immune response is critical in order to control CMV infection. The two main human T cells involved in HCMV-specific response are αβ and non-Vγ9Vδ2 T cells that belong to γδ T cell compartment. CMV-induced non-Vγ9Vδ2 T cells harbor a specific clonal expansion and a phenotypic signature, and display effector functions against CMV. So far, only two main molecular mechanisms underlying CMV sensing have been identified. Non-Vγ9Vδ2 T cells can be activated either by stress-induced surface expression of the γδT cell receptor (TCR) ligand annexin A2, or by a multimolecular stress signature composed of the γδTCR ligand endothelial protein C receptor and co-stimulatory signals such as the ICAM-1-LFA-1 axis. All this basic knowledge can be harnessed to improve the clinical management of CMV infection in at-risk patients. In particular, non-Vγ9Vδ2 T cell monitoring could help better stratify the risk of infection and move forward a personalized medicine. Moreover, recent advances in cell therapy protocols open the way for a non-Vγ9Vδ2 T cell therapy in immunocompromised patients.

Cytomegalovirus-vectored vaccines for HIV and other pathogens

: The use of cytomegalovirus (CMV) as a vaccine vector to express antigens against multiple infectious diseases, including simian immunodeficiency virus, Ebola virus, plasmodium, and mycobacterium tuberculosis, in rhesus macaques has generated extraordinary levels of protective immunity against subsequent pathogenic challenge. Moreover, the mechanisms of immune protection have altered paradigms about viral vector-mediated immunity against ectopically expressed vaccine antigens. Further optimization of CMV-vectored vaccines, particularly as this approach moves to human clinical trials will be augmented by a more complete understanding of how CMV engenders mechanisms of immune protection. This review summarizes the particulars of the specific CMV vaccine vector that has been used to date (rhesus CMV strain 68-1) in relation to CMV natural history.

NK Cells from RAG- or DCLRE1C-Deficient Patients Inhibit HCMV

The recombination-activating genes (RAGs) and the DNA cross-link repair 1C gene (DCLRE1C) encode the enzymes RAG1, RAG2 and Artemis. They are critical components of the V(D)J recombination machinery. V(D)J recombination is well known as a prerequisite for the development and antigen diversity of T and B cells. New findings suggested that RAG deficiency impacts the cellular fitness and function of murine NK cells. It is not known whether NK cells from severe combined immunodeficiency (SCID) patients with defective RAGs or DCLRE1C (RAGs⁻/DCLRE1C⁻-NK) are active against virus infections. Here, we evaluated the anti-HCMV activity of RAGs⁻/DCLRE1C⁻-NK cells. NK cells from six SCID patients were functional in inhibiting HCMV transmission between cells in vitro. We also investigated the expansion of HCMV-induced NK cell subset in the RAG- or DCLRE1C-deficient patients. A dynamic expansion of NKG2C⁺ NK cells in one RAG-2-deficient patient was observed post HCMV acute infection. Our study firstly reveals the antiviral activity of human RAGs⁻/ DCLRE1C⁻-NK cells.

Modulation of innate and adaptive immunity by cytomegaloviruses

The coordinated activities of innate and adaptive immunity are critical for effective protection against viruses. To counter this, some viruses have evolved sophisticated strategies to circumvent immune cell recognition. In particular, cytomegaloviruses encode large arsenals of molecules that seek to subvert T cell and natural killer cell function via a remarkable array of mechanisms. Consequently, these 'immunoevasins' play a fundamental role in shaping the nature of the immune system by driving the evolution of new immune receptors and recognition mechanisms. Here, we review the diverse strategies adopted by cytomegaloviruses to target immune pathways and outline the host's response.

A possible new pathway in natural killer cell activation also reveals the difficulty in determining human NK cell function in cancer

Immunotherapy is rapidly becoming the fourth arm of cancer treatment, and breakthrough successes have been observed in multiple malignancies. However, despite the potential for impressive anti-tumor effects, on average, only 25% of patients respond, and barriers clearly remain. Hence, uncovering innovative ways to apply immunotherapy and overcome immune resistance remains an unmet need in immuno-oncology. Natural killer (NK) cells are an attractive candidate for extending the promise of immunotherapy, although success to date has been largely limited to hematological cancers. An important study has identified novel ways in which NK cells sense and respond to tumors, and these findings may impact clinical translation of NK cells in cancer immunotherapy. Using the activating receptor NKp44, NK cells were shown to bind platelet-derived growth factor DD (PDGF-DD) which was secreted by tumors. Using transgenic mice, NKp44 binding of tumor-expressed PDGF-DD was able to limit tumor growth, and expression of natural cytotoxicity receptor-associated gene signatures (of which NKp44 is a member) was correlated to clinical outcomes. This study highlights the potential for effector-target interactions to impact immune homeostasis in previously unrecognized ways, while at the same time, underscoring the complexities inherent in pre-clinical/ translational experimental design which may confound clinical application of these interesting results.

Origins of natural killer cell memory: special creation or adaptive evolution

The few initial formative studies describing non-specific and apparently spontaneous activity of natural killer (NK) cells have since multiplied into thousands of scientific reports defining their unique capacities and means of regulation. Characterization of the array of receptors that govern NK cell education and activation revealed an unexpected relationship with the major histocompatibility molecules that NK cells originally became well known for ignoring. Proceeding true to form, NK cells continue to up-end archetypal understanding of their ever-expanding capabilities. Discovery that the NK cell repertoire is extremely diverse and can be reshaped by particular viruses into unique subsets of adaptive NK cells challenges, or at least broadens, the definition of immunological memory. This review provides an overview of studies identifying adaptive NK cells, addressing the origins of NK cell memory and introducing the heretical concept of NK cells with extensive antigenic specificity. Whether these newly apparent properties reflect adaptive utilization of known NK cell attributes and receptors or a specially creative allocation from an undefined receptor array remains to be fully determined.

Herpesviral capture of immunomodulatory host genes

Herpesviruses have acquired numerous genes from their hosts. Although these homologs are not essential for viral replication, they often have important immunomodulatory functions that ensure viral persistence in the host. Some of these viral molecules are called virokines as they mimic cellular cytokines of their host such as interleukin-10 (cIL-10). In recent years, many viral homologs of IL-10 (vIL-10s) have been discovered in the genome of members of the order Herpesvirales. For some, gene and protein structure as well as biological activity and potential use in the clinical context have been explored. Besides virokines, herpesviruses have also captured genes encoding membrane-bound host immunomodulatory proteins such as major histocompatibility complex (MHC) molecules. These viral MHC mimics also retain many of the functions of the cellular genes, in particular directly or indirectly modulating the activity of natural killer cells. The mechanisms underlying capture of cellular genes by large DNA viruses are still enigmatic. In this review, we provide an update of the advances in the field of herpesviral gene piracy and discuss possible scenarios that could explain how the gene transfer from host to viral genome was achieved.

Deciphering the killer-cell immunoglobulin-like receptor system at super-resolution for natural killer and T-cell biology

Killer-cell immunoglobulin-like receptors (KIRs) are components of two fundamental biological systems essential for human health and survival. First, they contribute to host immune responses, both innate and adaptive, through their expression by natural killer cells and T cells. Second, KIR play a key role in regulating placentation, and hence reproductive success. Analogous to the diversity of their human leucocyte antigen class I ligands, KIR are extremely polymorphic. In this review, we describe recent developments, fuelled by methodological advances, that are helping to decipher the KIR system in terms of haplotypes, polymorphisms, expression patterns and their ligand interactions. These developments are delivering deeper insight into the relevance of KIR in immune system function, evolution and disease.

Leukocyte Immunoglobulin-Like Receptor 1-Expressing Human Natural Killer Cell Subsets Differentially Recognize Isolates of Human Cytomegalovirus through the Viral Major Histocompatibility Complex Class I Homolog UL18

Immune responses of natural killer (NK) cell are controlled by the balance between activating and inhibitory receptors, but the expression of these receptors varies between cells within an individual. Although NK cells are a component of the innate immune system, particular NK cell subsets expressing Ly49H are positively selected and increase in frequency in response to cytomegalovirus infection in mice. Recent evidence suggests that in humans certain NK subsets also have an increased frequency in the blood of human cytomegalovirus (HCMV)-infected individuals. However, whether these subsets differ in their capacity of direct control of HCMV-infected cells remains unclear. In this study, we developed a novel in vitro assay to assess whether human NK cell subsets have differential abilities to inhibit HCMV growth and dissemination. NK cells expressing or lacking NKG2C did not display any differences in controlling viral dissemination. However, when in vitro-expanded NK cells were used, cells expressing or lacking the inhibitory receptor leukocyte immunoglobulin-like receptor 1 (LIR1) were differentially able to control dissemination. Surprisingly, the ability of LIR1⁺ NK cells to control virus spread differed between HCMV viral strains, and this phenomenon was dependent on amino acid sequences within the viral ligand UL18. Together, the results here outline an in vitro technique to compare the long-term immune responses of different human NK cell subsets and suggest, for the first time, that phenotypically defined human NK cell subsets may differentially recognize HCMV infections.

IMPORTANCE HCMV infection is ubiquitous in most populations; it is not cleared by the host after primary infection but persists for life. The innate and adaptive immune systems control the spread of virus, for which natural killer (NK) cells play a pivotal role. NK cells can respond to HCMV infection by rapid, short-term, nonspecific innate responses, but evidence from murine studies suggested that NK cells may display long-term, memory-like responses to murine cytomegalovirus infection. In this study, we developed a new assay that examines human NK cell subsets that have been suggested to play a long-term memory-like response to HCMV infection. We show that changes in an HCMV viral protein that interacts with an NK cell receptor can change the ability of NK cell subsets to control HCMV while the acquisition of another receptor has no effect on virus control.

Biology and pathogenesis of cytomegalovirus in periodontal disease

Human periodontitis is associated with a wide range of bacteria and viruses and with complex innate and adaptive immune responses. Porphyromonas gingivalis, Tannerella forsythia, Aggregatibacter actinomycetemcomitans, Treponema denticola, cytomegalovirus and other herpesviruses are major suspected pathogens of periodontitis, and a combined herpesvirus–bacterial periodontal infection can potentially explain major clinical features of the disease. Cytomegalovirus infects periodontal macrophages and T‐cells and elicits a release of interleukin‐1β and tumor necrosis factor‐α. These proinflammatory cytokines play an important role in the host defense against the virus, but they also have the potential to induce alveolar bone resorption and loss of periodontal ligament. Gingival fibroblasts infected with cytomegalovirus also exhibit diminished collagen production and release of an increased level of matrix metalloproteinases. This article reviews innate and adaptive immunity to cytomegalovirus and suggests that immune responses towards cytomegalovirus can play roles in controlling, as well as in exacerbating, destructive periodontal disease.

Classical and non-classical MHC I molecule manipulation by human cytomegalovirus: so many targets—but how many arrows in the quiver?

Major mechanisms for the recognition of pathogens by immune cells have evolved to employ classical and non-classical major histocompatibility complex class I (MHC I) molecules. Classical MHC I molecules present antigenic peptide ligands on infected cells to CD8⁺ T cells, whereas a key function for non-classical MHC I molecules is to mediate inhibitory or activating stimuli in natural killer (NK) cells. The structural diversity of MHC I puts immense pressure on persisting viruses, including cytomegaloviruses. The very large coding capacity of the human cytomegalovirus allows it to express a whole arsenal of immunoevasive factors assigned to individual MHC class I targets. This review summarizes achievements from more than two decades of intense research on how human cytomegalovirus manipulates MHC I molecules and escapes elimination by the immune system.

Two novel human cytomegalovirus NK cell evasion functions target MICA for lysosomal degradation

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.

S100A9 protein is a novel ligand for the CD85j receptor and its interaction is implicated in the control of HIV-1 replication by NK cells

Background

The reportedly broad expression of CD85j across different immune cell types suggests an importance for this molecule in the human immune system. Previous reports have shown that this receptor interacts with several HLA class-I molecules, as well as with some viral proteins. We have demonstrated that the subset of CD85j + Natural Killer (NK) cells efficiently controls human immunodeficiency virus type 1 (HIV-1) replication in monocyte-derived dendritic cells (MDDC) in vitro and this led us to hypothesize that the CD85j + NK cell-mediated anti-HIV activity in MDDC is specifically dependent on the interaction between the CD85j receptor and unknown non-HLA class-I ligand(s).

Results

In this study, we focused our efforts on the identification of these non-described ligands for CD85j. We found that the CD85j receptor interacts with a calcium-binding proteins of the S100 family; namely, S100A9. We further demonstrated that HIV-1 infection of MDDC induces a modulation of S100A9 expression on surface of the MDDC, which potentially influences the anti-HIV-1 activity of human NK cells through a mechanism involving CD85j ligation. Additionally, we showed that stimulation of NK cells with exogenous S100A9 enhances the control of HIV-1 infection in CD4+ T cells.

Conclusions

Our data show that S100A9 protein, through ligation with CD85j, can stimulate the anti-HIV-1 activity of NK cells.

Presentation of an immunodominant immediate-early CD8+ T cell epitope resists human cytomegalovirus immunoevasion

Control of human cytomegalovirus (HCMV) depends on CD8+ T cell responses that are shaped by an individual's repertoire of MHC molecules. MHC class I presentation is modulated by a set of HCMV-encoded proteins. Here we show that HCMV immunoevasins differentially impair T cell recognition of epitopes from the same viral antigen, immediate-early 1 (IE-1), that are presented by different MHC class I allotypes. In the presence of immunoevasins, HLA-A- and HLA-B-restricted T cell clones were ineffective, but HLA-C*0702-restricted T cell clones recognized and killed infected cells. Resistance of HLA-C*0702 to viral immunoevasins US2 and US11 was mediated by the alpha3 domain and C-terminal region of the HLA heavy chain. In healthy donors, HLA-C*0702-restricted T cells dominated the T cell response to IE-1. The same HLA-C allotype specifically protected infected cells from attack by NK cells that expressed a corresponding HLA-C-specific KIR. Thus, allotype-specific viral immunoevasion allows HCMV to escape control by NK cells and HLA-A- and HLA-B-restricted T cells, while the virus becomes selectively vulnerable to an immunodominant population of HLA-C-restricted T cells. Our work identifies a T cell population that may be of particular efficiency in HCMV-specific immunotherapy.

Natural killer cell dependent within-host competition arises during multiple MCMV infection: consequences for viral transmission and evolution

It is becoming increasingly clear that many diseases are the result of infection from multiple genetically distinct strains of a pathogen. Such multi-strain infections have the capacity to alter both disease and pathogen dynamics. Infection with multiple strains of human cytomegalovirus (HCMV) is common and has been linked to enhanced disease. Suggestions that disease enhancement in multi-strain infected patients is due to complementation have been supported by trans-complementation studies in mice during co-infection of wild type and gene knockout strains of murine CMV (MCMV). Complementation between naturally circulating strains of CMV has, however, not been assessed. In addition, many models of multi-strain infection predict that co-infecting strains will compete with each other and that this competition may contribute to selective transmission of more virulent pathogen strains. To assess the outcome of multi-strain infection, C57BL/6 mice were infected with up to four naturally circulating strains of MCMV. In this study, profound within-host competition was observed between co-infecting strains of MCMV. This competition was MCMV strain specific and resulted in the complete exclusion of certain strains of MCMV from the salivary glands of multi-strain infected mice. Competition was dependent on Ly49H⁺ natural killer (NK) cells as well as the expression of the ligand for Ly49H, the MCMV encoded product, m157. Strains of MCMV which expressed an m157 gene product capable of ligating Ly49H were outcompeted by strains of MCMV expressing variant m157 genes. Importantly, within-host competition prevented the shedding of the less virulent strains of MCMV, those recognized by Ly49H, into the saliva of multi-strain infected mice. These data demonstrate that NK cells have the strain specific recognition capacity required to meditate within-host competition between strains of MCMV. Furthermore, this within-host competition has the capacity to shape the dynamics of viral shedding and potentially select for the transmission of more virulent virus strains.

Infection of the host with multiple strains of a pathogen is common and occurs with the herpesvirus, human cytomegalovirus (HCMV). However the effects of multi-strain infection on the host and the pathogen remain poorly studied. Here we show, in a mouse model, that infection of C57BL/6 mice with multiple strains of murine CMV (MCMV) results in profound within-host competition. Competition between the strains of MCMV is dependent on Ly49H⁺ natural killer (NK) cells. The NK cell activation receptor Ly49H receptor targets certain genotypes of the viral protein, m157. During multi-strain infection, strains of MCMV encoding an m157 capable of binding Ly49H are excluded from the salivary gland and the saliva of C57BL/6 mice, allowing for the shedding of only non-Ly49H binding strains of MCMV in the saliva. This within-host competition could therefore have significant impacts on the circulation of MCMV strains, as only the most virulent MCMV strains were present in the saliva.

Human herpesviridae methods of natural killer cell evasion

Human herpesviruses cause diseases of considerable morbidity and mortality, ranging from encephalitis to hematologic malignancies. As evidence emerges about the role of innate immunity and natural killer (NK) cells in the control of herpesvirus infection, evidence of viral methods of innate immune evasion grows as well. These methods include interference with the ligands on infected cell surfaces that bind NK cell activating or inhibitory receptors. This paper summarizes the most extensively studied NK cell receptor/ligand pairs and then describes the methods of NK cell evasion used by all eight herpesviruses through these receptors and ligands. Although great strides have been made in elucidating their mechanisms, there is still a disparity between viruses in the amount of knowledge regarding innate immune evasion. Further research of herpesvirus innate immune evasion can provide insight for circumventing viral mechanisms in future therapies.

Expansion of a unique CD57⁺NKG2Chi natural killer cell subset during acute human cytomegalovirus infection

During human CMV infection, there is a preferential expansion of natural killer (NK) cells expressing the activating CD94-NKG2C receptor complex, implicating this receptor in the recognition of CMV-infected cells. We hypothesized that NK cells expanded in response to pathogens will be marked by expression of CD57, a carbohydrate antigen expressed on highly mature cells within the CD56(dim)CD16(+) NK cell compartment. Here we demonstrate the preferential expansion of a unique subset of NK cells coexpressing the activating CD94-NKG2C receptor and CD57 in CMV(+) donors. These CD57(+)NKG2C(hi) NK cells degranulated in response to stimulation through their NKG2C receptor. Furthermore, CD57(+)NKG2C(hi) NK cells preferentially lack expression of the inhibitory NKG2A receptor and the inhibitory KIR3DL1 receptor in individuals expressing its HLA-Bw4 ligand. Moreover, in solid-organ transplant recipients with active CMV infection, the percentage of CD57(+)NKG2C(hi) NK cells in the total NK cell population preferentially increased. During acute CMV infection, the NKG2C(+) NK cells proliferated, became NKG2C(hi), and finally acquired CD57. Thus, we propose that CD57 might provide a marker of "memory" NK cells that have been expanded in response to infection.

NK cells and immune "memory"

Immunological memory is a hallmark of the adaptive immune system. However, the ability to remember and respond more robustly against a second encounter with the same pathogen has been described in organisms lacking T and B cells. Recently, NK cells have been shown to mediate Ag-specific recall responses in several different model systems. Although NK cells do not rearrange the genes encoding their activating receptors, NK cells experience a selective education process during development, undergo a clonal-like expansion during virus infection, generate long-lived progeny (i.e., memory cells), and mediate more efficacious secondary responses against previously encountered pathogens--all characteristics previously ascribed only to T and B cells in mammals. This review describes past findings leading up to these new discoveries, summarizes the evidence for and characteristics of NK cell memory, and discusses the attempts and future challenges to identify these long-lived memory NK cell populations in humans.

Exploitation of herpesvirus immune evasion strategies to modify the immunogenicity of human mesenchymal stem cell transplants

BACKGROUND

Mesenchymal stem cells (MSCs) are multipotent cells residing in the connective tissue of many organs and holding great potential for tissue repair. In culture, human MSCs (hMSCs) are capable of extensive proliferation without showing chromosomal aberrations. Large numbers of hMSCs can thus be acquired from small samples of easily obtainable tissues like fat and bone marrow. MSCs can contribute to regeneration indirectly by secretion of cytokines or directly by differentiation into specialized cell types. The latter mechanism requires their long-term acceptance by the recipient. Although MSCs do not elicit immune responses in vitro, animal studies have revealed that allogeneic and xenogeneic MSCs are rejected.

METHODOLOGY/PRINCIPAL FINDINGS

We aim to overcome MSC immune rejection through permanent down-regulation of major histocompatibility complex (MHC) class I proteins on the surface of these MHC class II-negative cells through the use of viral immune evasion proteins. Transduction of hMSCs with a retroviral vector encoding the human cytomegalovirus US11 protein resulted in strong inhibition of MHC class I surface expression. When transplanted into immunocompetent mice, persistence of the US11-expressing and HLA-ABC-negative hMSCs at levels resembling those found in immunodeficient (i.e., NOD/SCID) mice could be attained provided that recipients' natural killer (NK) cells were depleted prior to cell transplantation.

CONCLUSIONS/SIGNIFICANCE

Our findings demonstrate the potential utility of herpesviral immunoevasins to prevent rejection of xenogeneic MSCs. The observation that down-regulation of MHC class I surface expression renders hMSCs vulnerable to NK cell recognition and cytolysis implies that multiple viral immune evasion proteins are likely required to make hMSCs non-immunogenic and thereby universally transplantable.

Activating and inhibitory receptors of natural killer cells

Natural killer (NK) cells are potent immune effector cells that can respond to infection and cancer, as well as allowing maternal adaptation to pregnancy. In response to malignant transformation or pathogenic invasion, NK cells can secrete cytokine and may be directly cytolytic, as well as exerting effects indirectly through other cells of the immune system. To recognize and respond to inflamed or infected tissues, NK cells express a variety of activating and inhibitory receptors including NKG2D, Ly49 or KIR, CD94-NKG2 heterodimers and natural cytotoxicity receptors, as well as co-stimulatory receptors. These receptors recognize cellular stress ligands as well as major histocompatibility complex class I and related molecules, which can lead to NK cell responses. Importantly, NK cells must remain tolerant of healthy tissue, and some of these receptors can also prevent activation of NK cells. In this review, we describe the expression of prominent NK cell receptors, as well as expression of their ligands and their role in immune responses. In addition, we describe the main signaling pathways used by NK cell receptors. Although we now appreciate that NK cell biology is more complicated than first thought, there are still facets of their biology that remain unclear. These will be highlighted and discussed in this review.

Enhancement of natural killer cell activity of aged mice by modified arabinoxylan rice bran (MGN-3/Biobran)

The present study is aimed to examine the possibility of enhancement of natural killer (NK) cell activity in aged C57BL/6 and C3H mice using MGN-3, a modified arabinoxylan from rice bran. Intraperitoneal injection of MGN-3 (10 mg kg−1 per day) caused a remarkable increase in the peritoneal NK activity as early as 2 days (35.2 lytic units), and the level remained elevated through day 14. The control aged mice had a level of 5.8 lytic units. Enhancement in NK activity was associated with an increase in both the binding capacity of NK cells to tumour targets and in the granular content as measured by BLT-esterase activity. Treatment did not alter the percentage of peritoneal NK cells. Data showed that peritoneal macrophages inhibit NK activity. In conclusion, MGN-3 enhances murine NK activity of aged mice and may be useful for enhancing NK function in aged humans.

Natural killer cells: integrating diversity with function

The key role of natural killer cells in many aspects of the immune response is now being recognized. The last decade has seen an exponential increase in our understanding of the workings of these cells. Receptor diversity is crucial in allowing natural killer cells to respond effectively to a variety of different pathogens. This article reviews aspects of natural killer cell diversity that combine to generate populations of functional natural killer cells that exist within both the individual and throughout the population at large.

A live guinea pig cytomegalovirus vaccine deleted of three putative immune evasion genes is highly attenuated but remains immunogenic in a vaccine/challenge model of congenital cytomegalovirus infection

Live attenuated vaccines for prevention of congenital cytomegalovirus infections encode numerous immune evasion genes. Their removal could potentially improve vaccine safety and efficacy. To test this hypothesis, three genes encoding MHC class I homologs (presumed NK evasins) were deleted from the guinea pig cytomegalovirus genome and the resulting virus, 3DX, was evaluated as a live attenuated vaccine in the guinea pig congenital infection model. 3DX was attenuated in vivo but not in vitro. Vaccination with 3DX produced elevated cytokine levels and higher antibody titers than wild type (WT) virus while avidity and neutralizing titers were similar. Protection, assessed by maternal viral loads and pup mortality following pathogenic viral challenge during pregnancy, was comparable between 3DX and WT and significant compared to naïve animals. These results suggest that the safety and perhaps efficacy of live attenuated human cytomegalovirus vaccines could be enhanced by deletion of viral immunomodulatory genes.

Immunobiology of human cytomegalovirus: from bench to bedside

SUMMARY

Following primary infection, human cytomegalovirus (HCMV) establishes lifelong latency and periodically reactivates without causing symptoms in healthy individuals. In the absence of an adequate host-derived immune response, this fine balance of permitting viral reactivation without causing pathogenesis is disrupted, and HCMV can subsequently cause invasive disease and an array of damaging indirect immunological effects. Over the last decade, our knowledge of the immune response to HCMV infection in healthy virus carriers and diseased individuals has allowed us to translate these findings to develop better diagnostic tools and therapeutic strategies. The application of these emerging technologies in the clinical setting is likely to provide opportunities for better management of patients with HCMV-associated diseases.

Feeling manipulated: cytomegalovirus immune manipulation

No one likes to feel like they have been manipulated, but in the case of cytomegalovirus (CMV) immune manipulation, we do not really have much choice. Whether you call it CMV immune modulation, manipulation, or evasion, the bottom line is that CMV alters the immune response in such a way to allow the establishment of latency with lifelong shedding. With millions of years of coevolution within their hosts, CMVs, like other herpesviruses, encode numerous proteins that can broadly influence the magnitude and quality of both innate and adaptive immune responses. These viral proteins include both homologues of host proteins, such as MHC class I or chemokine homologues, and proteins with little similarity to any other known proteins, such as the chemokine binding protein. Although a strong immune response is launched against CMV, these virally encoded proteins can interfere with the host's ability to efficiently recognize and clear virus, while others induce or alter specific immune responses to benefit viral replication or spread within the host. Modulation of host immunity allows survival of both the virus and the host. One way of describing it would be a kind of "mutually assured survival" (as opposed to MAD, Mutually Assured Destruction). Evaluation of this relationship provides important insights into the life cycle of CMV as well as a greater understanding of the complexity of the immune response to pathogens in general.

Human cytomegalovirus UL18 utilizes US6 for evading the NK and T-cell responses

Human cytomegalovirus (HCMV) US6 glycoprotein inhibits TAP function, resulting in down-regulation of MHC class I molecules at the cell surface. Cells lacking MHC class I molecules are susceptible to NK cell lysis. HCMV expresses UL18, a MHC class I homolog that functions as a surrogate to prevent host cell lysis. Despite a high level of sequence and structural homology between UL18 and MHC class I molecules, surface expression of MHC class I, but not UL18, is down regulated by US6. Here, we describe a mechanism of action by which HCMV UL18 avoids attack by the self-derived TAP inhibitor US6. UL18 abrogates US6 inhibition of ATP binding by TAP and, thereby, restores TAP-mediated peptide translocation. In addition, UL18 together with US6 interferes with the physical association between MHC class I molecules and TAP that is required for optimal peptide loading. Thus, regardless of the recovery of TAP function, surface expression of MHC class I molecules remains decreased. UL18 represents a unique immune evasion protein that has evolved to evade both the NK and the T cell immune responses.

HCMV establishes a lifelong latent infection and causes serious disease in immunocompromised individuals. Cytotoxic T lymphocytes (CTL) and natural killer (NK) cells are the primary effectors for the immune defense against HCMV. However, HCMV has evolved to evade both the innate and adaptive cellular immunity to viral infection. HCMV US6 glycoprotein inhibits TAP function, resulting in down-regulation of MHC class I, while HCMV UL18 is an MHC class I homolog that functions as a surrogate to prevent host cell lysis. Despite significant sequence and structural homology between UL18 and MHC class I molecules, US6 down regulates surface expression of MHC class I, but not UL18. Here, we describe a mechanism by which UL18 circumvents the self-derived TAP inhibitor, US6. UL18 abrogates US6 inhibition of TAP-ATP binding and restores TAP-mediated peptide translocation, thereby making peptides available for the assembly and subsequent surface expression of UL18. Together UL18 and US6 inhibit binding of MHC class I to TAP, thus down regulating surface expression of MHC class I molecules. UL18 represents a unique immune evasion protein resistant to both the NK and T cell immune responses. Our data provide a molecular basis for persistent HCMV infection and will aid in the development of a therapeutic vaccine.

Structure of UL18, a peptide-binding viral MHC mimic, bound to a host inhibitory receptor

UL18 is a human cytomegalovirus class I MHC (MHCI) homolog that binds the host inhibitory receptor LIR-1 and the only known viral MHC homolog that presents peptides. The 2.2-A structure of a LIR-1/UL18/peptide complex reveals increased contacts and optimal surface complementarity in the LIR-1/UL18 interface compared with LIR/MHCI interfaces, resulting in a >1,000-fold higher affinity. Despite sharing only approximately 25% sequence identity, UL18's structure and peptide binding are surprisingly similar to host MHCI. The crystal structure suggests that most of the UL18 surface, except where LIR-1 and the host-derived light chain bind, is covered by carbohydrates attached to 13 potential N-glycosylation sites, thereby preventing access to bound peptide and association with most MHCI-binding proteins. The LIR-1/UL18 structure demonstrates how a viral protein evolves from its host ancestor to impede unwanted interactions while preserving and improving its receptor-binding site.

Differential initiation of innate immune responses induced by human cytomegalovirus entry into fibroblast cells

Infection of permissive fibroblasts with human CMV (HCMV, AD169) is accompanied by a robust activation of innate immune defense. In this study, we show that inflammatory cytokine (IC) secretion and activation of the type I IFN pathway (alphabeta IFN) are initiated through distinct mechanisms. HCMV is recognized by TLR2 leading to the NF-kappaB activation and IC secretion. However, the IFN response to HCMV is not a TLR2-dependent process, as a dominant negative TLR2 does not affect the antiviral response to infection. Additionally, bafilomycin, an endosomal acidification inhibitor, has no effect on HCMV-induced IFN responses suggesting that IFN signaling is independent of endosomal resident TLRs. By contrast, disruption of lipid rafts by depletion of cellular cholesterol inhibits both HCMV entry as well as IFN responses. Cholesterol depletion had no effect on the induction of ICs by HCMV, illustrating a biological distinction at the cellular level with the initiation of innate immune pathways. Furthermore, HCMV entry inhibitors block IFN responses but not IC signaling. In particular, blocking the interaction of HCMV with beta(1) integrin diminished IFN signaling, suggesting that this virus-cell interaction or subsequent downstream steps in the entry pathway are critical for downstream signal transduction events. These data show that HCMV entry and IFN signaling are coordinated processes that require cholesterol-rich microdomains, whereas IC signaling is activated through outright sensing via TLR2. These findings further highlight the complexity and sophistication of innate immune responses at the earliest points in HCMV infection.

Modulation of natural killer cells by human cytomegalovirus

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.

Immune evasion of natural killer cells by viruses

Natural killer (NK) cells are important in the host resistance to viral infections. They are among the first cells to sense the release of proinflammatory cytokines, as well as the downregulation of surface MHC class I molecules and molecules induced by viral invasion of cells. Various viral functions have evolved to counter NK cell responses illustrating the evolutionary struggles between viruses and NK cells. Ligands for NK cell receptors are primary targets for viral immunoevasion. In order to counteract NK cell activation via the 'missing self'-axis, viruses encode proteins which serve as ligands for inhibitory NK cell receptors. Viruses also downmodulate the ligands for the activating NK cell receptors and encode soluble ligands which block these receptors. In addition to viral immunoregulatory proteins, regulatory RNAs can also inhibit the expression of ligands for NK cell receptors. Improving our understanding of viral regulation of NK cell function could be essential for designing more efficient measures in the prophylaxis and treatment of virus-induced pathology.

Evolutionary struggles between NK cells and viruses

Natural killer (NK) cells are well recognized for their ability to provide a first line of defence against viral pathogens and they are increasingly being implicated in immune responses against certain bacterial and parasitic infections. Reciprocally, viruses have devised numerous strategies to evade the activation of NK cells and have influenced the evolution of NK-cell receptors and their ligands. NK cells contribute to host defence by their ability to rapidly secrete cytokines and chemokines, as well as to directly kill infected host cells. In addition to their participation in the immediate innate immune response against infection, interactions between NK cells and dendritic cells shape the nature of the subsequent adaptive immune response to pathogens.

Immune modulation by the human cytomegalovirus-encoded molecule UL18, a mystery yet to be solved

Human cytomegalovirus infects human populations at a high frequency worldwide. During the long coevolution of virus and host, a fine balance has developed between viral immune evasion strategies and defense mechanisms of the immune system. Human cytomegalovirus encodes multiple proteins involved in the evasion of immune recognition, among them UL18, a MHC class I homologue. Despite almost 20 years of research and the discovery of a broadly expressed inhibitory receptor for this protein, its function in immune modulation is not clear yet. Recent data suggest that besides inhibitory effects on various immune cells, UL18 may also act as an activating component during CMV infection. In this review, we provide an overview of the biology of UL18 and discuss several attempts to shed light on its function.

Human cytomegalovirus regulates surface expression of the viral protein UL18 by means of two motifs present in the cytoplasmic tail

UL18 is a trans-membrane viral protein expressed on human cytomegalovirus (HCMV)-infected cells, and its surface expression determines the interaction of infected cells with lymphocytes expressing the CD85j (LIR-1/ILT2) receptor. We previously showed that the UL18-CD85j interaction elicits activation of T lymphocytes. However, in in vitro cell models UL18 displays mostly undetectable surface expression. Thus, we asked how surface expression of UL18 is regulated. Domain-swapping experiments and construction of specific mutants demonstrated that two motifs on its cytoplasmic tail, homologous to YXXPhi and KKXX consensus sequences, respectively, are responsible for impairing UL18 surface expression. However, the presence of the whole HCMV genome, granted by HCMV infection of human fibroblasts, restored surface expression of either UL18 or chimeric proteins carrying the UL18 cytoplasmic tail, starting from the third day after infection. It is of note that the two motifs responsible for cytoplasmic retention are identical in all 17 HCMV strains examined. We disclosed a control mechanism used by the HCMV to regulate the availability of UL18 on the infected-cell surface to allow interaction with its ligand on T and NK cells.

Dissecting the structural determinants of the interaction between the human cytomegalovirus UL18 protein and the CD85j immune receptor

UL18 is a glycoprotein encoded by the human cytomegalovirus genome and is thought to play a pivotal role during human cytomegalovirus infection, although its exact function is still a matter of debate. UL18 shares structural similarity with MHC class I and binds the receptor CD85j on immune cells. Besides UL18, CD85j binds MHC class I molecules. The binding properties of CD85j to MHC class I molecules have been thoroughly studied. Conversely, very little information is available on the CD85j/UL18 complex, namely that UL18 binds CD85j through its alpha3 domain with an affinity that is approximately 1000-fold higher than the MHC class I affinity for CD85j. Deeper knowledge of features of the UL18/CD85j complex would help to disclose the function of UL18 when it binds to CD85j. In this study we first demonstrated that the UL18alpha3 domain is not sufficient per se for binding and that beta2-microglobulin is necessary for UL18-CD85j interaction. We then dissected structural determinants of binding UL18 to CD85j. To this end, we constructed a three-dimensional model of the complex. The model was used to design mutants in selected regions of the putative interaction interface, the effects of which were measured on binding. Six regions in both the alpha2 and alpha3 domains and specific amino acids within them were identified that are potentially involved in the UL18-CD85j interaction. The higher affinity of UL18 to CD85j, compared with MHC class I, seems to be due not to additional interaction regions but to an overall better fit of the two molecules.

Macrophage migration inhibitory factor has a MHC class I-like motif and function

Macrophage migration inhibitory factor (MIF) is found in immune-privileged sites and inhibits cytotoxicity mediated by CD3-ve lymphokine-activated killer cells (LAK). The mechanism by which MIF attenuates LAK cytotoxicity is unknown. We provide evidence that MIF has a major histocompatibility complex (MHC) class I-like motif. A monoclonal antibody (OX18) that binds a conserved region of rat MHC class I proteins binds native MIF. Anti-MIF polyclonal antibodies bind MHC class I. Epitope mapping suggests OX18 binds a loop of MHC class I bound by several receptors for MHC class I. A sequence (PRPEG) within the proposed OX18-binding site on MHC class I exists with a short insertion in MIF. OX18 does not bind MIF that is denatured by SDS-PAGE. This suggests the OX18 epitope is dependent on higher order structure in MIF. Interestingly, MIF inhibits binding of tetramers of MHC class I (H2D(b)) to LAK cells, suggesting it may bind to receptors for MHC class I. MIF may be an example where small regions of MHC class I are used by endogenous and viral proteins to control cytotoxicity mediated by immune cells.

Murine cytomegalovirus regulation of NKG2D ligands

Human cytomegalovirus (HCMV) is a ubiquitous pathogen that causes morbidity risk in immunologically suppressed and immunodeficient patients including congenital infections. Approaches to curb the consequences of HCMV infections are restricted by a lack of complete understanding of viral pathogenesis. The infection of mice with murine cytomegalovirus (MCMV) as a model of HCMV infection has been particularly useful in elucidating the role of innate and adaptive immune response mechanisms. A large number of cytomegalovirus genes modulate the innate and the adaptive host immune response. The products of several MCMV genes are involved in subverting the natural killer (NK) cell response by down-modulating cellular ligands for the NKG2D receptor expressed on NK cells and CD8(+) T cells. Mutant viruses lacking these immunoevasion genes are attenuated with respect to virus growth in vivo. Given the importance of the NKG2D receptor in controlling both NK- and T cell-mediated immunity, it is of tremendous importance to understand the molecular mechanisms and consequences of viral regulation of the NKG2D ligands.

Human cytomegalovirus-derived protein UL18 alters the phenotype and function of monocyte-derived dendritic cells

Human cytomegalovirus (HCMV) encodes the MHC class I-like molecule UL18, which binds with high affinity to the leukocyte Ig-like receptor-1 (LIR-1), an inhibitory receptor commonly expressed on myeloid cells and subsets of NK and T cells. The exact role of UL18 is not known, in particular in relation to its proposed role in HCMV immune escape. Given the ubiquitous expression of LIR-1 on dendritic cells (DCs), we hypothesized that UL18 may affect DC function. To study the effects of UL18 on DC, we made use of UL18 fusion proteins. We demonstrate that UL18 fusion proteins inhibit the chemotaxis of DCs. Furthermore, UL18 interfered with CD40 ligand-induced maturation of DCs, resulting in reduced allogeneic T cell proliferation. Finally, we demonstrate that UL18 proteins up-regulate the expression of the maturation marker CD83 on immature monocyte-derived DCs and induce cytokine production. The capacity of UL18 to affect the function and the phenotype of DCs suggests a novel role for this HCMV-derived protein.

Cytomegalovirus immune evasion

Human cytomegalovirus (HCMV) has become a paradigm for viral immune evasion due to its unique multitude of immune-modulatory strategies. HCMV modulates the innate as well as adaptive immune response at every step of its life cycle. It dampens the induction of antiviral interferon-induced genes by several mechanisms. Further striking is the multitude of genes and strategies devoted to modulating and escaping the cellular immune response. Several genes are independently capable of inhibiting antigen presentation to cytolytic T cells by downregulating MHC class I. Recent data revealed an astounding variety of methods in triggering or inhibiting activatory and inhibitory receptors found on NK cells, NKT cells, T cells as well as auxiliary cells of the immune system. The multitude and complexity of these mechanisms is fascinating and continues to reveal novel insights into the host-pathogen interaction and novel cell biological and immunological concepts.

Structural elements underlying the high binding affinity of human cytomegalovirus UL18 to leukocyte immunoglobulin-like receptor-1

Human cytomegalovirus (HCMV) encodes UL18, a major histocompatibility complex (MHC) class I homologue that binds to the leukocyte immunoglobulin-like receptor (LIR)-1 (also called ILT2/CD85j/LILRB1), an inhibitory receptor expressed on myeloid and lymphoid immune cells. The molecular basis underlying the high affinity binding of UL18 to LIR-1, compared to MHC class I molecules (MHC-I), is unclear. Based on a comparative structural analysis of a molecular model of UL18 with the crystal structure of the HLA-A2/LIR-1 complex, we identified three regions in UL18 influencing interaction with LIR-1. Comparison of the relative binding affinities of mutated UL18 proteins to LIR-1 demonstrated the importance of specific residues in each region. Substitution of residues K42/A43 and Q202, localized in the alpha1 and alpha3 domains, respectively, reduced binding affinity to LIR-1 nearly by half. The model also suggested the formation of an additional disulfide bridge in the alpha3 domain of UL18 between residues C240 and C255, not present in MHC-I. Substitution of either cysteine residue prevented association of UL18 to beta2m, abolishing binding to LIR-1. All observed differences in binding affinities translated directly into functional consequences in terms of inhibition of IFN-gamma production by T cells, mediated through the UL18-LIR-1 interaction. The larger amount of interacting regions, combined with an increased stability of the alpha3 and beta2m domains allow a higher recognition affinity of UL18 by LIR-1.

The human cytomegalovirus MHC class I homolog UL18 inhibits LIR-1+ but activates LIR-1- NK cells

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 (DeltaUL18) 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-alpha-activated NK bulk cultures derived from a donor with a high frequency of LIR-1(+) NK cells. A single LIR-1(high) 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.

Reduced natural killer (NK) function associated with high-risk myelodysplastic syndrome (MDS) and reduced expression of activating NK receptors

Myelodysplastic syndromes (MDS) are characterized by ineffective hematopoiesis with potential for progression to acute myeloid leukemia (AML). We compared natural killer (NK) cytolytic function in 48 MDS patients with 37 healthy donors and found reduced activity in the patient population (K562 cytolysis, 19% +/- 21% SD versus 40% +/- 17%) (P < .001). NK cytotoxicity in MDS patients was reduced against 3 disparate tumor targets with differential activating receptor requirement, suggesting global defects in NK function. Reduced NK function in MDS was significantly associated with higher International Prognostic Score (P = .01), abnormal karyotype (P = .05), the presence of excess blasts (P = .01), and age-adjusted bone marrow hypercellularity (P = .04). MDS patients had a display of the activating receptor NKp30, and NKG2D down-regulation closely correlated with impaired NK function (P = .001). NKG2D ligands (MICA and MICB) were expressed on CD34(+) cells from bone marrow of 30% of MDS patients and a leukemic cell line derived from an MDS patient (MDS1). Collectively, these findings suggest that impairment of NK cytolytic function derives in part from reduced activating NK receptors such as NKG2D in association with disease progression. Evasion of NK immunosurveillance may have importance for MDS disease progression.

Increased expression of leukocyte Ig-like receptor-1 and activating role of UL18 in the response to cytomegalovirus infection

NK and T cells are important for combating CMV infection. Some NK and T cells express leukocyte Ig-like receptor-1 (LIR-1), an inhibitory receptor recognizing MHC class I and the CMV-encoded homolog UL18. We previously demonstrated an early increase in LIR-1-expressing blood lymphocytes in lung-transplanted patients later developing CMV disease. We now show that NK and T cells account for the observed LIR-1 augmentation. Coincubation of PBMC from CMV-seropositive donors with virus-infected lung fibroblasts led to a T cell-dependent secretion of IFN-gamma, produced mainly by LIR-1(+) T cells and by NK cells. Cytokine production during coculture with fibroblasts infected with virus containing the UL18 gene was augmented compared with the UL18 deletion virus, suggesting a stimulatory role for UL18. However, purified UL18Fc proteins inhibited IFN-gamma production of LIR-1(+) T cells. We propose that cytokine production in the transplant induces NK and T cells to express LIR-1, which may predispose to CMV disease by MHC/LIR-1-mediated suppression. Although the UL18/LIR-1 interaction could inhibit T cell responses, this unlikely plays a role in response to infected cells. Instead, our data point to an activating role for viral UL18 during infection, where indirect intracellular effects cannot be excluded.