CD56 negative NK cells: origin, function, and role in chronic viral disease. Trends Immunol. 2010;31:401-406. [PMID: 20829113 DOI: 10.1016/j.it.2010.08.003]

Role of NK cells in influenza infection

Within days after infection, natural killer (NK) cells are recruited to the lungs and play an essential role in the immune response against influenza infection. Through interactions with the virus itself, as well as viral-infected cells, NK cells secrete a variety of cytokines and can contain viral replication by killing infected cells early after influenza infection. However, the virus has means of evading NK cell responses, including escaping NK cell recognition through mutation of the viral hemagglutinin (HA) protein, regulating HA levels, and by directly infecting and destroying NK cells. Although much of our understanding of NK cell role in influenza infection has come from animal models, there is increasing information from human infection. Studies conducted during the 2009 H1N1 pandemic provided much needed information on the importance of NK cells during human infection and suggest that NK lymphopenia may correlate with increased disease severity. However, more information on how different influenza virus subtypes influence NK cell levels and activities, the role of the different NK cell receptors in infection, and the impact of NK cells on human infection, particularly in high risk populations is needed.

Impact of HCMV Infection on NK Cell Development and Function after HSCT

Natural Killer (NK) cell function is regulated by an array of inhibitory and activating surface receptors that during NK cell differentiation, at variance with T and B cells, do not require genetic rearrangement. Importantly, NK cells are the first lymphocyte population recovering after hematopoietic stem cell transplantation (HSCT). Thus, their role in early immunity after HSCT is considered crucial, as they can importantly contribute to protect the host from tumor recurrence and viral infections before T-cell immunity is fully recovered. In order to acquire effector functions and regulatory receptors, NK cell precursors undergo a maturation process that can be analyzed during immune reconstitution after HSCT. In this context, the occurrence of human cytomegalovirus (HCMV) infection/reactivation was shown to accelerate NK cell maturation by promoting the differentiation of high frequencies of NK cells characterized by a KIR⁺NKG2A⁻ and NKG2C⁺ mature phenotype. Thus, it appears that the development of NK cells and the distribution of NK cell receptors can be deeply influenced by HCMV infection. Moreover, in HCMV-infected subjects the emergence of so called “memory-like” or “long-lived” NK cells has been documented. These cells could play an important role in protecting from infections and maybe from relapse in patients transplanted for leukemia. All the aspects regarding the influence of HCMV infection on NK cell development will be discussed.

NKG2C gene deletion in the Mexican population and lack of association to respiratory viral infections

Expansion of a natural killer (NK) cell population that expresses NKG2C has been associated with cytomegalovirus and other viral infections. It has been suggested that this cell population may play a role in infection control. Deletion of the NKG2C gene (homozygous or heterozygous) has been reported with high prevalence in European and Asian populations. However, the effect of NKG2C genotype on NK cell responses to infection remains poorly defined. We determined the prevalence of the NKG2C deletion in a Mexican population (n = 300) and in a group of patients (n = 131) to assess whether NKG2C genotype affects the incidence of symptomatic viral infections caused by influenza or respiratory syncytial virus. The frequency of the NKG2C deletion haplotype in Mexican mestizos was significantly lower (10.3%) than that reported in other populations (17.5-21.9%). No difference in the prevalence of NKG2C deletion was observed in subjects with viral infections compared with the reference population. In addition, no differences in clinical characteristics and infection outcome were observed between patients with and without the NKG2C gene deletion. Our results indicate that copy number variation in the NKG2C gene has no impact on the severity of respiratory viral infections.

Expression of the activating receptor, NKp46 (CD335), in human natural killer and T-cell neoplasia

OBJECTIVES

To evaluate the expression of CD335 (NKp46), an activation receptor that is selectively expressed on natural killer (NK) cells.

METHODS

We assessed CD335's potential utility as a diagnostic marker in 657 cases by flow cytometry and 410 cases by immunohistochemistry.

RESULTS

We observed that CD335 was highly specific for NK cells in nonneoplastic tissues. Moreover, 61 (90%) of 68 of NK cell neoplasms demonstrated CD335 expression, whereas B-cell, myelomonocytic, and plasma cell neoplasms lacked expression. Notably, 16 (20%) of 82 mature T-cell neoplasms, particularly T-cell large granular lymphocytic leukemia, mycosis fungoides, and ALK+ anaplastic large cell lymphoma, aberrantly expressed CD335.

CONCLUSIONS

Collectively, these data support the diagnostic utility of CD335 in evaluating hematopoietic malignancies and suggest that CD335 could be a useful target for selective immunotherapy in patients with mature NK and T-cell neoplasms.

Reduced frequency of CD56 dim CD16 pos natural killer cells in pediatric systemic inflammatory response syndrome/sepsis patients

BACKGROUND

Sepsis continues to be a leading cause of death in infants and children. Natural killer (NK) cells serve as a bridge between innate and adaptive immunity, yet their role in pediatric sepsis has not been well characterized.

METHODS

We tested the hypothesis that decreased NK cell cytotoxicity is a common feature of pediatric systemic inflammatory response syndrome (SIRS)/sepsis patients by measuring, using flow cytometry, NK cell cytotoxicity and cell surface phenotype in the peripheral blood of 38 pediatric intensive care patients who demonstrated signs and symptoms of SIRS and/or sepsis.

RESULTS

NK cell cytotoxicity was significantly reduced in peripheral blood mononuclear cells (PBMCs) of pediatric SIRS/sepsis patients as compared with healthy controls, and the percentage of CD56(dim) CD16(+) cytotoxic NK cells in PBMCs was lower in patients with SIRS/sepsis than in normal donors. However, on a per cell basis, CD56(dim) CD16(+) NK cells in patients mediated cytotoxicity as well as those in normal donors.

CONCLUSION

The NK cell dysfunction in pediatric SIRS/sepsis patients reflects a quantitative rather than a qualitative difference from healthy controls.

Obstacles to ideal anti-HIV antibody-dependent cellular cytotoxicity responses

A safe and effective vaccine against HIV is a global health priority. Large-scale phase III clinical vaccine trials based on neutralizing antibodies and cytotoxic T-lymphocytes have failed to provide protection, highlighting the lack of understanding of basic immune correlates of protection against HIV. The partial success of the RV144 vaccine trial, however, sparked an intense research effort to identify and describe the protective potential of non-neutralizing antibodies. Correlates of protection analyses have identified antibodies that induced antibody-dependent cellular cytotoxicity (ADCC) as potentially important. Despite the attractiveness of utilizing ADCC antibodies for HIV vaccine design, it is important to note that effective ADCC responses are contingent on many factors. As discussed in this review, these factors are important considerations for determining the feasibility of designing an optimal ADCC antibody-inducing vaccine construct. Important determinants of ADCC responses include characteristics of the antibody, such as isotype and subclass, antigen-specificity, titer, durability and glycosylation of the constant region. Second, ADCC immune responses are highly contingent on the natural killer (NK) cell effectors. This review will describe the current state of knowledge regarding the ontogeny of NK cells, highlighting the continuous "education" they undergo that determines their functional potential upon stimulation. Other important NK cell factors, such as constant region receptor polymorphisms, cellular exhaustion, and the effects of the cytokine milieu on cellular function, will also be covered. Finally, an exciting, but yet untested, role for NK cell-mediated ADCC lies in its potential ability to eliminate latently infected cells, which harbor the viral reservoir. The review will address the potential of a two-pronged attack, where latently infected cells are induced to express HIV antigens and then eliminated by NK cells via an ADCC mechanism, with the goal of inducing a cure.

Healthy Neonates Possess a CD56-Negative NK Cell Population with Reduced Anti-Viral Activity

Background

Neonatal Natural Killer (NK) cells show functional impairment and expansion of a CD56 negative population of uncertain significance.

Methods

NK cells were isolated from cord blood and from adult donors. NK subpopulations were identified as positive or negative for the expression of CD56 and characterized for expression of granzyme B and surface markers by multi-parameter flow cytometry. Cell function was assessed by viral suppression and cytokine production using autologous lymphocytes infected with HIV. Activating (NKp30, NKp46) and inhibitory (Siglec-7) markers in healthy infants and adults were compared with viremic HIV-infected adults.

Results

Cord blood contained increased frequencies of CD56 negative (CD56neg) NK cells with reduced expression of granzyme B and reduced production of IFNγ and the CC-class chemokines RANTES, MIP1α and MIP1β upon stimulation. Both CD56pos and CD56neg NK subpopulations showed impaired viral suppression in cord blood, with impairment most marked in the CD56neg subset. CD56neg NK cells from cord blood and HIV-infected adults shared decreased inhibitory and activating receptor expression when compared with CD56pos cells.

Conclusions

CD56neg NK cells are increased in number in normal infants and these effectors show reduced anti-viral activity. Like the expanded CD56neg population described in HIV-infected adults, these NK cells demonstrate functional impairments which may reflect inadequate development or activation.

HIV-1 Vpu interference with innate cell-mediated immune mechanisms

The HIV-1 accessory protein Vpu is emerging as a viral factor with a range of activities devoted to counteracting host innate immunity. Here, we review recent findings concerning the role of Vpu in hampering activation of cellular immune responses mediated by CD1d-restricted invariant natural killer T (iNKT) cells and natural killer (NK) cells. The two key findings are that Vpu interferes with CD1d expression and antigen presentation, and also with expression of the NK cell activation ligand NK-T and B cell antigen (NTB-A). Both these activities are mechanistically distinct from CD4 and Tetherin (BST-2) down-modulation. We summarize the mechanistic insights gained into Vpu interference with CD1d and NTB-A, as well as important challenges going forward, and discuss these mechanisms in the context of the role that iNKT and NK cells play in HIV-1 immunity and immunopathogenesis.

Tim-3 expression defines regulatory T cells in human tumors

Tim-3, a member of the novel Tim (T cell immunoglobulin and mucin domain) family, has been reported to negatively regulate the immune responses against viral infection and had implications for autoimmune disease. However, the nature and role of Tim-3⁺ CD4 T cells in human tumors remain largely unknown. In the present study, we characterized Tim-3⁺ CD4 T cells in 100 specimens from human hepatocellular, cervical, colorectal and ovarian carcinoma patients. Compared with peripheral blood and nontumor-infiltrating lymphocytes, the lymphocytes isolated from the corresponding tumor tissues of hepatocellular, cervical, colorectal and ovarian carcinoma patients contained significantly greater proportion of Tim-3⁺ CD4 T cells. The majority of tumor-derived Tim-3⁺ CD4 T cells exhibited an impaired capacity to produce IFN-γ and IL-2, but expressed higher levels of CD25, Foxp3, CTLA-4 and GITR than their Tim-3⁻ CD4 T cell counterparts. In contrast, most Tim-3⁺ CD4 T cells isolated from the paired nontumor tissues and peripheral blood did not express these molecules. Moreover, tumor-derived Tim-3⁺ CD4 T cells, but not tumor-derived Tim-3⁻ CD4 T cells, significantly suppressed the proliferation of autologous CD8⁺ T cells in vitro. Notably, multi-color immunofluorescence and confocal microscopy demonstrated that Tim-3⁺Foxp3⁺CD4⁺ cells were preferentially distributed in the tumor nest rather than the peritumoral stroma of hepatocellular carcinoma. Together, our data indicate that Tim-3-expressing CD4 T cells in human tumors could represent the functional regulatory T cells which contribute to the formation of the immune-suppressive tumor micromilieu.

The search for an HIV cure: tackling latent infection

Strategies to eliminate infectious HIV that persists despite present treatments and with the potential to cure HIV infection are of great interest. One patient seems to have been cured of HIV infection after receiving a bone marrow transplant with cells resistant to the virus, although this strategy is not viable for large numbers of infected people. Several clinical trials are underway in which drugs are being used to activate cells that harbour latent HIV. In a recent study, investigators showed that activation of latent HIV infection in patients on antiretroviral therapy could be achieved with a single dose of vorinostat, a licensed anticancer drug that inhibits histone deacetylase. Although far from a cure, such studies provide some guidance towards the logical next steps for research. Clinical studies that use a longer duration of drug dosing, alternative agents, combination approaches, gene therapy, and immune-modulation approaches are all underway.

Peripheral blood NK cells from breast cancer patients are tumor-induced composite subsets

Human NK lymphocytes are involved in antitumor immunity. The therapeutic potential of this population against cancers has stimulated their study and led to the discovery of several NK cell subsets, each of which is endowed with different immunoregulatory functions. We have previously reported that NK cell functions are profoundly altered in advanced breast cancer patients. In this study, we show that these tumor-mediated alterations also variably affect NK cell subsets. We found that in addition to the known human CD56(dim)CD16(+), CD56(bright)CD16(-), and CD56(-)CD16(+) NK cell subsets, two additional subsets, namely the CD56(bright)CD16(+) and CD56(dim)CD16(-) subsets, were increased in the peripheral blood of patients with advanced invasive breast cancers. These subsets corresponded to the main two subsets found at the tumor site. The extensive phenotype of these subsets revealed an "à la carte" pattern of expression for the various NK receptors, functional molecules, adhesion molecules, and chemokine receptors, depending on the subset. We next compared these subsets to known NK cell populations endowed with specific phenotypic characteristics, but also with functional properties. Our data show that advanced breast cancer patients have an increased proportion of more immature and noncytotoxic NK cell subsets in their peripheral blood, which might account for at least part of the low cytotoxic functions observed in these patients. They reveal a major heterogeneity and plasticity of the NK cell compartment, which are both tightly linked to the microenvironment. The identification of NK cell subsets endowed with particular functional capabilities might help monitor residual antitumor NK cell-mediated responses in breast cancer patients.

Short communication: NKG2C+ NK cells contribute to increases in CD16+CD56- cells in HIV type 1+ individuals with high plasma viral load

Chronic HIV-1 infection results in the expansion of both NKG2C+ and CD16+CD56- human natural killer cells. NKG2C+ cells proliferate in response to human cytomegalovirus (HCMV) and expansion of the dysfunctional CD56-CD16+ natural killer (NK) cells is associated with HIV-1 viremia. Here we report an association between increased proportions of CD56-CD16+ NK cells in viremic HIV-1+ individuals and an increased contribution of NKG2C+ cells to this subset. These data, in addition to anti-HCMV IgG serology, indicate a potential contribution of both HCMV and HIV-1 to NK cell dysfunction in HIV-1-infected individuals.

The impact of HIV-1 infection and exposure on natural killer (NK) cell phenotype in Kenyan infants during the first year of life

Natural killer (NK) cells play an important role in the containment of HIV replication during primary infection, though their functions are impaired during chronic HIV infection. Infants experience more rapid HIV disease progression than adults, but contributions of infant NK cells to containing HIV infection are unknown. The aim of this study was to determine the impact of HIV infection on infant NK cell phenotype by evaluating samples and data from a cohort study of women and their infants, conducted in Nairobi, Kenya between 1999 and 2003. The percentage and phenotype of NK cells was evaluated longitudinally by multi-parameter flow cytometry over the first year of life in HIV-infected (HIV+, = 16), HIV-exposed uninfected (HIV-EU, n = 6), and healthy unexposed controls (HIV–, n = 4). At birth, NK subset distributions based on expression of CD56 and CD16 did not differ between HIV+, HIV-EU, or HIV– infants. However, HIV infection was associated with a subsequent decline in NK cells as a percentage of total lymphocytes (p < 0.001), and an expanding proportion of CD56-CD16+ NK cells (p < 0.001). Activated CD38^brightCD69+ NK cells were more frequent in the HIV+ infants, followed by HIV-EU and HIV- infants, in both CD56^dim (p = 0.005) and CD56^bright compartments (p = 0.03). HIV infection and exposure was also associated with a significant decline in the percentage of perforin-expressing NK cells in the CD56^dim compartment over the first year of life, with HIV+ infants losing approximately 2.5% (p < 0.001) and HIV-EU infants losing 3.0% (p = 0.01) of perforin+ cells per month. Thus, infant HIV infection is associated with alterations in NK cell subsets, activation, and cytolytic potential that could contribute to their poor control over HIV infection. Furthermore, exposure to HIV infection in infants who escaped infection is also associated with alterations in NK cells that may contribute to the reduced ability to fight infections that is observed in HIV-EU infants.

Human natural killer cell maturation defect supports in vivo CD56(bright) to CD56(dim) lineage development

Two populations of human natural killer (NK) cells can be identified in peripheral blood. The majority are CD3⁻CD56^dim cells while the minority exhibits a CD3⁻CD56^bright phenotype. In vitro evidence indicates that CD56^bright cells are precursors of CD56^dim cells, but in vivo evidence is lacking. Here, we studied NK cells from a patient that suffered from a melanoma and opportunistic fungal infection during childhood. The patient exhibited a stable phenotype characterized by a reduction in the frequency of peripheral blood CD3⁻CD56^dim NK cells, accompanied by an overt increase in the frequency and absolute number of CD3⁻CD56^bright cells. These NK cells exhibited similar expression of perforin, CD57 and CD158, the major activating receptors CD16, NKp46, NKG2D, DNAM-1, and 2B4, as well as the inhibitory receptor CD94/NKG2A, on both CD56^bright and CD56^dim NK cells as healthy controls. Also, both NK cell subpopulations produced IFN-γ upon stimulation with cytokines, and CD3⁻CD56^dim NK cells degranulated in response to cytokines or K562 cells. However, upon stimulation with cytokines, a substantial fraction of CD56^dim cells failed to up-regulate CD57 and CD158, showed a reduction in the percentage of CD16⁺ cells, and CD56^bright cells did not down-regulate CD62L, suggesting that CD56^dim cells could not acquire a terminally differentiated phenotype and that CD56^bright cells exhibit a maturation defect that might result in a potential altered migration pattern. These observations, support the notion that NK cells of this patient display a maturation/activation defect that precludes the generation of mature NK cells at a normal rate accompanied by CD56^dim NK cells that cannot completely acquire a terminally differentiated phenotype. Thus, our results provide evidence that support the concept that in vivo CD56^bright NK cells differentiate into CD56^dim NK cells, and contribute to further understand human NK cell ontogeny.

NK cells in healthy aging and age-associated diseases

NK cells exhibit the highest cytotoxic capacity within the immune system. Alteration of their number or functionality may have a deep impact on overall immunity. This is of particular relevance in aging where the elderly population becomes more susceptible to infection, cancer, autoimmune diseases, and neurodegenerative diseases amongst others. As the fraction of elderly increases worldwide, it becomes urgent to better understand the aging of the immune system to prevent and cure the elderly population. For this, a better understanding of the function and phenotype of the different immune cells and their subsets is necessary. We review here NK cell functions and phenotype in healthy aging as well as in various age-associated diseases.

Virologically suppressed HIV patients show activation of NK cells and persistent innate immune activation

FcRγ is an ITAM-containing adaptor required for CD16 signaling and function in NK cells. We have previously shown that NK cells from HIV patients receiving combination antiretroviral therapy (cART) have decreased FcRγ expression, but the factors causing this are unknown. We conducted a cross-sectional study of cART-naive viremic patients (ART(-)), virologically suppressed patients receiving cART (ART(+)), and HIV-uninfected controls. CD8(+) T cells were activated, as assessed by CD38(+)HLA-DR(+) expression, in ART(-) patients (p < 0.0001), which was significantly reduced in ART(+) patients (p = 0.0005). In contrast, CD38(+)HLA-DR(+) NK cells were elevated in ART(-) patients (p = 0.0001) but did not decrease in ART(+) patients (p = 0.88). NK cells from both ART(-) and ART(+) patients showed high levels of spontaneous degranulation in ex vivo whole blood assays as well as decreased CD16 expression (p = 0.0001 and p = 0.0025, respectively), FcRγ mRNA (p < 0.0001 for both groups), FcRγ protein expression (p = 0.0016 and p < 0.0001, respectively), and CD16-dependent Syk phosphorylation (p = 0.0001 and p = 0.003, respectively). HIV-infected subjects showed alterations in NK activation, degranulation, CD16 expression and signaling, and elevated plasma markers of inflammation and macrophage activation, that is, neopterin and sCD14, which remained elevated in ART(+) patients. Alterations in NK cell measures did not correlate with viral load or CD4 counts. These data show that in HIV patients who achieve viral suppression following cART, NK cell activation persists. This suggests that NK cells respond to factors different from those driving T cell activation, but which are associated with inflammation in HIV patients.

Innate immunosenescence: effect of aging on cells and receptors of the innate immune system in humans

Components of the innate immune response, including neutrophils and macrophages, are the first line of defense against infections. Their role is to initiate an inflammatory response, phagocyte and kill pathogens, recruit natural killer cells (NK), and facilitate the maturation and migration of dendritic cells that will initiate the adaptive immune response. Extraordinary advances have been made in the last decade on the knowledge of the receptors and mechanisms used by cells of the innate immunity not only to sense and eliminate the pathogen but also to communicate each other and collaborate with cells of adaptive immunity to mount an effective immune response. The analysis of innate immunity in elderly humans has evidenced that aging has a profound impact on the phenotype and functions of these cells. Thus altered expression and/or function of innate immunity receptors and signal transduction leading to defective activation and decreased chemotaxis, phagocytosis and intracellular killing of pathogens have been described. The phenotype and function of NK cells from elderly individuals show significant changes that are compatible with remodeling of the different NK subsets, with a decrease in the CD56bright subpopulation and accumulation of the CD56dim cells, in particular those differentiated NK cells that co-express CD57, as well as a decreased expression of activating natural cytotoxicity receptors. These alterations can be responsible of the decreased production of cytokines and the lower per-cell cytotoxicity observed in the elderly. Considering the relevance of these cells in the initiation of the immune response, the possibility to reactivate the function of innate immune cells should be considered in order to improve the response to pathogens and to vaccination in the elderly.

HIV infection and aging of the innate immune system

The increased life expectancy of HIV-infected individuals due to improved treatment has revealed an unexpected increase in non-AIDS comorbidities that are typically associated with older age including cardiovascular disease, dementia and frailty. The majority of these diseases arise as the result of dysregulated systemic inflammation, and both the aged and HIV-infected individuals exhibit elevated basal levels of inflammation. In the elderly, increased inflammation and age-related diseases are associated with a state of impaired immunity called immunosenescence, which is thought to result from a lifetime of immune stimulation. It is now apparent that HIV induces premature immunosenescence within T-cells; however, the impact of HIV on aging of cells of the innate arm of the immune system is unknown. Innate immune cells play a central role in inflammation and are thus critical for the pathogenesis of inflammatory diseases. Limited evidence suggests HIV infection mimics age-related changes to innate immune cells; however, the extent of this effect and the mechanism underlying these changes remain to be defined. This review focuses on the impact of HIV infection on the function and aging of innate immune cells and discusses potential drivers of premature immunosenescence including chronic endotoxaemia, residual viraemia, telomere attrition and altered cellular signalling.

Human Herpesvirus 8 (HHV8) sequentially shapes the NK cell repertoire during the course of asymptomatic infection and Kaposi sarcoma

The contribution of innate immunity to immunosurveillance of the oncogenic Human Herpes Virus 8 (HHV8) has not been studied in depth. We investigated NK cell phenotype and function in 70 HHV8-infected subjects, either asymptomatic carriers or having developed Kaposi's sarcoma (KS). Our results revealed substantial alterations of the NK cell receptor repertoire in healthy HHV8 carriers, with reduced expression of NKp30, NKp46 and CD161 receptors. In addition, down-modulation of the activating NKG2D receptor, associated with impaired NK-cell lytic capacity, was observed in patients with active KS. Resolution of KS after treatment was accompanied with restoration of NKG2D levels and NK cell activity. HHV8-latently infected endothelial cells overexpressed ligands of several NK cell receptors, including NKG2D ligands. The strong expression of NKG2D ligands by tumor cells was confirmed in situ by immunohistochemical staining of KS biopsies. However, no tumor-infiltrating NK cells were detected, suggesting a defect in NK cell homing or survival in the KS microenvironment. Among the known KS-derived immunoregulatory factors, we identified prostaglandin E2 (PGE2) as a critical element responsible for the down-modulation of NKG2D expression on resting NK cells. Moreover, PGE2 prevented up-regulation of the NKG2D and NKp30 receptors on IL-15-activated NK cells, and inhibited the IL-15-induced proliferation and survival of NK cells. Altogether, our observations are consistent with distinct immunoevasion mechanisms that allow HHV8 to escape NK cell responses stepwise, first at early stages of infection to facilitate the maintenance of viral latency, and later to promote tumor cell growth through suppression of NKG2D-mediated functions. Importantly, our results provide additional support to the use of PGE2 inhibitors as an attractive approach to treat aggressive KS, as they could restore activation and survival of tumoricidal NK cells.

Natural Killer (NK) cells are part of the innate immune response against virus infections and tumors. Their activation is the net result of signals emanating from a panel of inhibitory and activating receptors recognizing specific ligands on target cells. Human Herpes Virus 8 (HHV8) is an oncogenic virus responsible of Kaposi Sarcoma (KS), a multifocal angiogenic tumor. How NK cells contribute to the control of infection by HHV8 infection and development of KS, is unclear. In this paper, we show different strategies used by HHV8 to escape NK cell response. Patients with asymptomatic infection or KS have down-modulated expression of NKp30, NKp46 and CD161 receptors. In addition, patients with active KS show additional down-modulation of the NKG2D activating receptor, associated with impaired NK-cell cytotoxicity against target cells. Resolution of KS correlates with regained NKG2D expression and cytotoxic function. We present evidence that down-modulation of NKG2D is mediated by inflammatory prostaglandin E2 (PGE2), known to be released by KS cells, and show that PGE2 acts by preventing IL-15-mediated activation of NK cells. These results strongly support the use of PGE2 inhibitors as an attractive approach to treat active KS.

Effects on innate immunity of a therapeutic dendritic cell-based vaccine for HIV-1 infection

Changes in natural killer (NK) cells according to their phenotype and expression of certain regulatory receptors were analyzed in a double-blind, controlled study of antiretroviral therapy (ART)-untreated HIV-seropositive patients, who had been vaccinated with monocyte-derived dendritic cells pulsed with inactivated HIV-1 autologous virus. This work extends other recently published studies of the same group of HIV-1(+) vaccinated patients, which demonstrated that the viral load significantly decreases and correlates inversely with an increase in HIV-specific T-cell responses in vaccinated patients, but not in controls who received placebo. Our results indicate that this vaccine raises the level of the NK CD56(neg) cell subpopulation, while levels of the NK CD56(dim) and NK CD56(bright) cells expressing the inhibitory receptor CD85j/ILT-2 fell in vaccinated patients. Taken together, these results suggest that this vaccine might enhance innate immunity by amplifying the inflammatory and cytolytic capacity.

Phenotypic and functional heterogeneity of human NK cells developing after umbilical cord blood transplantation: a role for human cytomegalovirus?

Natural killer (NK) cells play a crucial role in early immunity after hematopoietic stem cell transplantation because they are the first lymphocyte subset recovering after the allograft. In this study, we analyzed the development of NK cells after intrabone umbilical cord blood (CB) transplantation in 18 adult patients with hematologic malignancies. Our data indicate that, also in this transplantation setting, NK cells are the first lymphoid population detectable in peripheral blood. However, different patterns of NK-cell development could be identified. Indeed, in a group of patients, a relevant fraction of NK cells expressed a mature phenotype characterized by the KIR(+)NKG2A(-) signature 3-6 months after transplantation. In other patients, most NK cells maintained an immature phenotype even after 12 months. A possible role for cytomegalovirus in the promotion of NK-cell development was suggested by the observation that a more rapid NK-cell maturation together with expansion of NKG2C(+) NK cells was confined to patients experiencing cytomegalovirus reactivation. In a fraction of these patients, an aberrant and hyporesponsive CD56(-)CD16(+)p75/AIRM1(-) NK-cell subset (mostly KIR(+)NKG2A(-)) reminiscent of that described in patients with viremic HIV was detected. Our data support the concept that cytomegalovirus infection may drive NK-cell development after umbilical CB transplantation.

NK cells prevalence, subsets and function in viral hepatitis C

Innate immunity appears to play an important role in the pathogenesis of viral hepatitis C. Among various cell subsets of this immunity natural killer (NK) cells raised particular interest. These cells are abundant in liver, possess significant cytotoxic potential and show links with adaptive immunity. They play important role, particularly in the acute phase of viral infections, including hepatitis C. They exhibit various types of receptors, either inhibitory or activating, that are able to react with distinct ligands on infected cells. Homozygosity of some receptors, namely KIR2DL3 reacting with recipient HLA-C1 antigens is a herald of good prognosis in hepatitis C virus (HCV) infection. In the early stage of the latter, both the prevalence and the cytotoxicity of NK cells are increased. Their inhibitory receptors are down regulated whereas activating ones are up regulated. Interferon-γ secreted by NK56(+bright) NK cells has a direct cytotoxic effect on infected hepatocytes. In contrast, in the chronic phase of HCV liver disease both, the prevalence and function of NK cells are impaired. Nevertheless, their cytotoxicity contributes to liver injury. Cells show change in the polarization profile from NK1 to NK2, manifested by secretion of immunosuppressive cytokines. Some HCV peptides are inhibitory for NK cells leading to the reduction of their antiviral activity. The unwanted effects of HCV peptides can be at least partly reversed by the antiviral therapy.

Changes in cytokine levels and NK cell activation associated with influenza

Several studies have highlighted the important role played by murine natural killer (NK) cells in the control of influenza infection. However, human NK cell responses in acute influenza infection, including infection with the 2009 pandemic H1N1 influenza virus, are poorly documented. Here, we examined changes in NK cell phenotype and function and plasma cytokine levels associated with influenza infection and vaccination. We show that absolute numbers of peripheral blood NK cells, and particularly those of CD56^bright NK cells, decreased upon acute influenza infection while this NK cell subset expanded following intramuscular influenza vaccination. NK cells exposed to influenza antigens were activated, with higher proportions of NK cells expressing CD69 in study subjects infected with seasonal influenza strains. Vaccination led to increased levels of CD25+ NK cells, and notably CD56^bright CD25+ NK cells, whereas decreased amounts of this subset were present in the peripheral blood of influenza infected individuals, and predominantly in study subjects infected with the 2009 pandemic H1N1 influenza virus. Finally, acute influenza infection was associated with low plasma concentrations of inflammatory cytokines, including IFN-γ, MIP-1β, IL-2 and IL-15, and high levels of the anti-inflammatory cytokines IL-10 and IL-1ra. Altogether, these data suggest a role for the CD56^bright NK cell subset in the response to influenza, potentially involving their recruitment to infected tissues and a local production and/or uptake of inflammatory cytokines.

NK cells and their receptors during viral infections

Increasing evidence indicates the importance of human natural killer (NK) cells in the immune response against certain viral infections. In the present article, we summarize information on NK cell responses against several viruses and on the nature of NK cell receptor–ligand interactions involved in these responses. Recent studies indicate that NK cells display functional features that are normally attributed exclusively to cells of the adaptive immune system. In this context, experiments both in mice and humans suggest the existence of long-lived NK cells that expand during viral infections and retain a ‘memory’ of previous exposure to a specific antigen. However, further studies are necessary to better define the characteristics of these long-lived NK cell populations and their role in viral infections.

Biomarkers of immune dysfunction following combination antiretroviral therapy for HIV infection

Combination antiretroviral therapy (cART) has significantly reduced morbidity and mortality of HIV-infected patients, yet their life expectancy remains reduced compared with the general population. Most HIV-infected patients receiving cART have some persistent immune dysfunction characterized by chronic immune activation and premature aging of the immune system. Here we review biomarkers of T-cell activation (CD69, -25 and -38, HLA-DR, and soluble CD26 and -30); generalized immune activation (C-reactive protein, IL-6 and D-dimer); microbial translocation (lipopolysaccharide, 16S rDNA, lipopolysaccharide-binding protein and soluble CD14); and immune dysfunction of specific cellular subsets (T cells, natural killer cells and monocytes) in HIV-infected patients on cART and their relationship to adverse clinical outcomes including impaired CD4 T-cell recovery, as well as non-AIDS clinical events, such as cardiovascular disease.

Human NK cells in acute myeloid leukaemia patients: analysis of NK cell-activating receptors and their ligands

Natural killer (NK) cell activation is strictly regulated to ensure that healthy cells are preserved, but tumour-transformed or virus-infected cells are recognized and eliminated. To carry out this selective killing, NK cells have an ample repertoire of receptors on their surface. Signalling by inhibitory and activating receptors by interaction with their ligands will determine whether the NK cell becomes activated and kills the target cell. Here, we show reduced expression of NKp46, NKp30, DNAM-1, CD244 and CD94/NKG2C activating receptors on NK cells from acute myeloid leukaemia patients. This reduction may be induced by chronic exposure to their ligands on leukaemic blasts. The analysis of ligands for NK cell-activating receptors showed that leukaemic blasts from the majority of patients express ligands for NK cell-activating receptors. DNAM-1 ligands are frequently expressed on blasts, whereas the expression of the NKG2D ligand MICA/B is found in half of the patients and CD48, a ligand for CD244, in only one-fourth of the patients. The decreased expression of NK cell-activating receptors and/or the heterogeneous expression of ligands for major receptors on leukaemic blasts can lead to an inadequate tumour immunosurveillance by NK cells. A better knowledge of the activating receptor repertoire on NK cells and their putative ligands on blasts together with the possibility to modulate their expression will open new possibilities for the use of NK cells in immunotherapy against leukaemia.

Natural killer cells in spontaneous control of HIV infection

PURPOSE OF REVIEW

Over the last decade our understanding of the role of natural killer cells in HIV infection has changed dramatically due to strong epidemiological, phenotypic, and functional data providing evidence for their involvement in antiviral control. Here we review the current literature on natural killer cells in the control of HIV infection, with a specific focus on their role in HIV controllers, individuals that spontaneously control HIV replication in the absence of antiretroviral therapy.

RECENT FINDINGS

Differences between progressors and controllers are highlighted in the context of genetic influences, natural killer cell phenotypes, function and dysregulation. Also, recent findings on the role of natural killer cell-mediated antibody-dependent cellular cytotoxicity in HIV control are summarized.

SUMMARY

This evolving understanding of the complex biology of natural killer cells and their multifaceted role in HIV infection offer exciting new approaches for future vaccine strategies. Furthermore, the specific natural killer cell phenotype and function observed in controllers may guide new vaccine modalities that specifically harness the antiviral power of natural killer cells as adjuvants, or as direct effectors.

Alteration of inhibitory and activating NK cell receptor expression on NK cells in HIV-infected Chinese

Natural killer (NK) cell function, based on the expression of activating and inhibitory natural killer receptors (NKRs), may become abnormal during human immunodeficiency virus (HIV) infection. In this study, we investigated changes in receptor expression with individual and combinational analysis on NK cell subsets in HIV-infected Chinese. The results showed that natural killer group 2 member D (NKG2D) expression on total NK cells decreased significantly in HIV infection, while the expressions of natural killer group 2 member A (NKG2A) and killer cell immunoglobulin-like receptor, three domains, long cytoplasmic tail 1 (KIR3DL1) on total NK cells were not significantly different between any of the groups including HIV-positive treatment-naïve group, AIDS treatment-naïve group, HAART-treatment AIDS group and HIV-negative control group. Individual analysis of NKG2A(+) and KIR3DL1(+) cells revealed no significant differences in expression in any NK cell subsets between any of the groups, but the combinational analysis of NKG2D(-)NKG2A(+), and NKG2D(-)KIR3DL1(+) on the NK CD56(dim) cell subset in the AIDS group were increased compared to the HIV-negative control group. On the contrary, NKG2D(-)NKG2A(+) expression on the CD56(bright) subset decreased in the AIDS group compared to the control group. Highly active antiretroviral therapy (HAART) treatment almost completely restored the levels of these receptor expressions. The results indicate that the distinct alteration of activating and inhibitory NKR expression on NK cells and its subsets occurred during HIV progression. Moreover, the imbalanced change of activating and inhibitory NKRs on NK cells and its subsets may explain the impaired NK cell immunity in HIV infected individuals.

NK cells and CD1d-restricted NKT cells respond in different ways with divergent kinetics to IL-2 treatment in primary HIV-1 infection

Cytokine immunotherapy is being evaluated as adjunct treatment in infectious diseases. The effects on innate and adaptive immunity in vivo are insufficiently known. Here, we investigate whether combination treatment with antiretroviral therapy (ART) and Interleukin-2 (IL-2) of patients with primary HIV-1 infection induces sustained increases in circulating NKT cell and NK cell numbers and effector functions and investigate how changes are coordinated in the two compartments. Patients with primary HIV-1 infection starting ART were analyzed for numbers, phenotype and function of NKT cells, NK cells and dendritic cells (DC) in peripheral blood before, during and after IL-2 treatment. NKT cells expanded during IL-2 treatment as expected from previous studies. However, their response to α-galactosyl ceramide antigen were retained but not boosted. Myeloid DC did not change their numbers or CD1d-expression during treatment. In contrast, the NK cell compartment responded with rapid expansion of the CD56(dim) effector subset and enhanced IFNγ production. Expansions of NKT cells and NK cells retracted back towards baseline values at 12 months after IL-2 treatment ended. In summary, NKT cells and NK cells respond to IL-2 treatment with different kinetics. Effects on cellular function are distinct between the cell types and the effects appear not to be sustained after IL-2 treatment ends. These results improve our understanding of the effects of cytokine immunotherapy on innate cellular immunity in early HIV-1 infection.

Rapid expansion and long-term persistence of elevated NK cell numbers in humans infected with hantavirus

Acute hantavirus infection in humans triggers a rapid expansion and long-term persistence of NK cells.

Natural killer (NK) cells are known to mount a rapid response to several virus infections. In experimental models of acute viral infection, this response has been characterized by prompt NK cell activation and expansion followed by rapid contraction. In contrast to experimental model systems, much less is known about NK cell responses to acute viral infections in humans. We demonstrate that NK cells can rapidly expand and persist at highly elevated levels for >60 d after human hantavirus infection. A large part of the expanding NK cells expressed the activating receptor NKG2C and were functional in terms of expressing a licensing inhibitory killer cell immunoglobulin-like receptor (KIR) and ability to respond to target cell stimulation. These results demonstrate that NK cells can expand and remain elevated in numbers for a prolonged period of time in humans after a virus infection. In time, this response extends far beyond what is considered normal for an innate immune response.