Priming of NK cell anti-viral effector mechanisms by direct recognition of human cytomegalovirus

Investigating the relationship between the number and activity of natural killer cells with increased cytomegalovirus and CMV disease after kidney transplantation

BACKGROUND

Cytomegalovirus (CMV) infections caused by the cytomegalovirus are one of the most common problems in patients after kidney transplant. We examined the association of the relationship between the number and activity of natural killer cells with increased cytomegalovirus and its related disease after kidney transplantation.

MATERIAL AND METHODS

In this analytical study, 58 new transplant patients in the Labbafinejad Hospital, who did not have any evidence of CMV infection, were evaluated based on the number and percentage of CD56+/16+, CD56+/16-, and CD69+ Natural Killer (NK) cells.

RESULTS

The results of this study showed that CD16+ and CD56+ cells in the group of CMV Ag-positive patients are less than negative patients (p = 0.003) and the difference between the two groups are significant (p = 0.01). However, CD69+ cells did not differ significantly between the two groups (p = 0.1). Moreover, the absolute number of CD16+ and CD56+ cells declined significantly after infection with CMV unlike the CMV Ag - group(p = 0.003).

DISCUSSION

These results indicate that kidney transplant patients suffering from CMV infection after transplantation have a significantly reduced total number of NK cells. On the other hand, a slight decrease in the number of NK subgroups was observed with an increase in the peak serum levels of cyclosporine. As a consequence of these findings, it can be assumed that more dosage and a higher level of the drug will result in more severe immunosuppression and, consequently, increased susceptibility to CMV infections. Thus, taking the right dose of the drug would prevent viral infections and immune system from over-activation.

Prospects of Cytomegalovirus-Specific T-Cell Receptors in Clinical Diagnosis and Therapy

Human cytomegalovirus (HCMV) is responsible for widespread infections worldwide. In immunocompetent individuals it is typically latent, while infection or reactivation in immunocompromised individuals can result in severe clinical symptoms or even death. Although there has been significant progress in the treatment and diagnosis of HCMV infection in recent years, numerous shortcomings and developmental limitations persist. There is an urgent need to develop innovative, safe, and effective treatments, as well as to explore early and timely diagnostic strategies for HCMV infection. Cell-mediated immune responses are the primary factor controlling HCMV infection and replication, but the protective role of humoral immune responses remains controversial. T-cells, key effector cells of the cellular immune system, are critical for clearing and preventing HCMV infection. The T-cell receptor (TCR) lies at the heart of T-cell immune responses, and its diversity enables the immune system to differentiate between self and non-self. Given the significant influence of cellular immunity on human health and the indispensable role of the TCR in T-cell immune responses, we posit that the impact of TCR on the development of novel diagnostic and prognostic methods, as well as on patient monitoring and management of clinical HCMV infection, will be far-reaching and profound. High-throughput and single-cell sequencing technologies have facilitated unprecedented quantitative detection of TCR diversity. With these current sequencing technologies, researchers have already obtained a vast number of TCR sequences. It is plausible that in the near future studies on TCR repertoires will be instrumental in assessing vaccine efficacy, immunotherapeutic strategies, and the early diagnosis of HCMV infection.

[Assesment of specific T-cell immunity to SARS-CoV-2 virus antigens in COVID-19 reconvalescents]

INTRODUCTION

The development of the COVID-19 pandemic has stimulated the scientific research aimed at studying of the mechanisms of formation the immunity against SARS-CoV-2. Currently, there is a need to develop a domestic simple and cost-effective specific method suitable for monitoring of T-cell response against SARS-CoV-2 in reconvalescents and vaccinated individuals.

AIM

Development of a screening method for evaluation specific T-cell immunity against SARS-CoV-2.

MATERIALS AND METHODS

Total 40 individuals who had mild to moderate COVID-19 and 20 healthy volunteers who did not have a history of this disease were examined. The presence and levels of IgG and IgM antibodies to SARS-CoV-2 were identified in participants sera by ELISA using the diagnostic kits from JSC Vector-Best (Novosibirsk, Russian Federation). Antigenic stimulation of mononuclear cells was carried out on commercial plates with adsorbed whole-virion inactivated SARS-CoV-2 antigen (State Research Center of Virology and Biotechnology VECTOR Novosibirsk, Russian Federation). The concentration of IFN- was measured in ELISA using the test systems from JSC Vector-Best (Novosibirsk, Russian Federation). The immunophenotyping of lymphocytes was performed on a flow cytometer Cytomics FC500 (Beckman Coulter, USA). Statistical data processing was carried out using the Microsoft Excel and STATISTICA 10 software package.

RESULTS

Stimulation of mononuclear cells isolated from the peripheral blood with whole-virion inactivated SARS-CoV-2 antigen fixed at the bottom of the wells of a polystyrene plate showed a significantly higher median response in terms of IFN- production in 40 people who had history of COVID-19 compared to 20 healthy blood donors (172.1 [34.3575.1] pg/ml versus 15.4 [6.925.8] pg/ml, p 0.0001). There was no difference in median IFN- levels in supernatants collected from unstimulated mononuclear cells from COVID-19 reconvalescents and healthy donors (2.7 [0.411.4] pg/ml versus 0.8 [0.023.3] pg/ml, p 0.05). The overall sensitivity and specificity of this method were 73% (95% CI 5888%) and 100% (95% CI 100100%), respectively, at a cut-off of 50 pg/ml.

CONCLUSION

The developed method for assessment of the cellular immune response to SARS-CoV-2 can be used as a screening method for monitoring the T-cell response in a population against a new coronavirus infection in recovered people.

Natural killer cells and immune-checkpoint inhibitor therapy: Current knowledge and new challenges

The discovery of immune checkpoints (ICs) and the development of specific blockers to relieve immune effector cells from this inhibiting mechanism has changed the view of anti-cancer therapy. In addition to cytotoxic T lymphocyte antigen 4 (CTLA4) and programmed death 1 (PD1), classical ICs of T lymphocytes and recently described also on a fraction of natural killer (NK) cells, several NK cell receptors, including killer immunoglobulin-like inhibitory receptors (KIRs) and NGK2A, have been recognized as checkpoint members typical of the NK cell population. This offers the opportunity of a dual-checkpoint inhibition approach, targeting classical and non-classical ICs and leading to a synergistic therapeutic effect. In this review, we will overview and discuss this new perspective, focusing on the most relevant candidates for this role among the variety of potential NK ICs. Beside listing and defining classical ICs expressed also by NK cells, or non-classical ICs either on T or on NK cells, we will address their role in NK cell survival, chronic stimulation or functional exhaustion, and the potential relevance of this phenomenon on anti-tumor immune response. Furthermore, NK ICs will be proposed as possible new targets for the development of efficient combined immunotherapy, not forgetting the relevant concerns that may be raised on NK IC blockade. Finally, the impact of epigenetic drugs in such a complex therapeutic picture will be briefly addressed.

microRNA, a Subtle Indicator of Human Cytomegalovirus against Host Immune Cells

Human cytomegalovirus (HCMV) is a double-stranded DNA virus that belongs to the β-herpesvirus family and infects 40–90% of the adult population worldwide. HCMV infection is usually asymptomatic in healthy individuals but causes serious problems in immunocompromised people. We restricted this narrative review (PubMed, January 2022) to demonstrate the interaction and molecular mechanisms between the virus and host immune cells with a focus on HCMV-encoded miRNAs. We found a series of HCMV-encoded miRNAs (e.g., miR-UL112 and miR-UL148D) are explicitly involved in the regulation of viral DNA replication, immune evasion, as well as host cell fate. MiRNA-targeted therapies have been explored for the treatment of atherosclerosis, cardiovascular disease, cancer, diabetes, and hepatitis C virus infection. It is feasible to develop an alternative vaccine to restart peripheral immunity or to inhibit HCMV activity, which may contribute to the antiviral intervention for serious HCMV-related diseases.

Impaired Antibody-Dependent Cellular Cytotoxicity in a Spanish Cohort of Patients With COVID-19 Admitted to the ICU

SARS-CoV-2 infection causes COVID-19, ranging from mild to critical disease in symptomatic subjects. It is essential to better understand the immunologic responses occurring in patients with the most severe outcomes. In this study, parameters related to the humoral immune response elicited against SARS-CoV-2 were analysed in 61 patients with different presentations of COVID-19 who were recruited in Hospitals and Primary Healthcare Centres in Madrid, Spain, during the first pandemic peak between April and June 2020. Subjects were allocated as mild patients without hospitalization, severe patients hospitalized or critical patients requiring ICU assistance. Critical patients showed significantly enhanced levels of B cells with memory and plasmablast phenotypes, as well as higher levels of antibodies against SARS-CoV-2 with neutralization ability, which were particularly increased in male gender. Despite all this, antibody-dependent cell-mediated cytotoxicity was defective in these individuals. Besides, patients with critical COVID-19 also showed increased IgG levels against herpesvirus such as CMV, EBV, HSV-1 and VZV, as well as detectable CMV and EBV viremia in plasma. Altogether, these results suggest an enhanced but ineffectual immune response in patients with critical COVID-19 that allowed latent herpesvirus reactivation. These findings should be considered during the clinical management of these patients due to the potential contribution to the most severe disease during SARS-CoV-2 infection.

Human NK cells, their receptors and function

NK cells are cytotoxic components of innate lymphoid cells (ILC) that provide a first line of defense against viral infections and contribute to control tumor growth and metastasis. Their function is finely regulated by an array of HLA-specific and non-HLA-specific inhibitory and activating receptors which allow to discriminate between healthy and altered cells. Human NK cells gained a major attention in recent years because of the important progresses in understanding their biology and of some promising data in tumor therapy. In this review, we will outline well-established issues of human NK cells and discuss some of the open questions, debates, and recent advances regarding their origin, differentiation, and tissue distribution. Newly defined NK cell specializations, including the impact of inhibitory checkpoints on their function, their crosstalk with other cell types, and the remarkable adaptive features acquired in response to certain virus infections will also be discussed.

HLA-DR-expressing NK cells: Effective killers suspected for antigen presentation

HLA-DR-expressing cells comprise an intriguing group of NK cells, which combine phenotypic characteristics of both NK cells and dendritic cells. These cells can be found in humans and mice; they are present in blood and tissues in healthy conditions and can expand in a spectrum of pathologies. HLA-DR⁺ NK cells are functionally active: they produce proinflammatory cytokines, degranulate, and easily proliferate in response to stimuli. Additionally, HLA-DR⁺ NK cells seem able to take in and then present certain antigens to CD4⁺ and CD8⁺ T cells, inducing their activation and proliferation, which puts them closer to professional antigen-presenting cells. It appears that these NK cells should be considerable players of the innate immune system, both due to their functional activity and regulation of the innate and adaptive immune responses. In this review, for the first time, we provide a detailed description and analysis of the available data characterizing phenotypic, developmental, and functional features of the HLA-DR⁺ NK cells in a healthy condition and a disease.

Human NK Cells and Herpesviruses: Mechanisms of Recognition, Response and Adaptation

NK cells contribute to early defenses against viruses through their inborn abilities that include sensing of PAMPs and inflammatory signals such as cytokines or chemokines, recognition, and killing of infected cells through activating surface receptors engagement. Moreover, they support adaptive responses via Ab-dependent mechanisms, triggered by CD16, and DC editing. Their fundamental role in anti-viral responses has been unveiled in patients with NK cell deficiencies suffering from severe Herpesvirus infections. Notably, these infections, often occurring as primary infections early in life, can be efficiently cleared by NK, T, and B cells in healthy hosts. Herpesviruses however, generate a complicated balance with the host immune system through their latency cycle moving between immune control and viral reactivation. This lifelong challenge has contributed to the development of numerous evasion mechanisms by Herpesviruses, many of which devoted to elude NK cell surveillance from viral reactivations rather than primary infections. This delicate equilibrium can be altered in proportions of healthy individuals promoting virus reactivation and, more often, in immunocompromised subjects. However, the constant stimulus provided by virus-host interplay has also favored NK-cell adaptation to Herpesviruses. During anti-HCMV responses, NK cells can reshape their receptor repertoire and function, through epigenetic remodeling, and acquire adaptive traits such as longevity and clonal expansion abilities. The major mechanisms of recognition and effector responses employed by NK cells against Herpesviruses, related to their genomic organization will be addressed, including those allowing NK cells to generate memory-like responses. In addition, the mechanisms underlying virus reactivation or control will be discussed.

Human Cytomegalovirus Antigen Presentation by HLA-DR+ NKG2C+ Adaptive NK Cells Specifically Activates Polyfunctional Effector Memory CD4+ T Lymphocytes

Natural killer (NK) cells play a dual role in the defense against viral pathogens by directly lysing infected cells as well as by regulating anti-viral T cell immunity. Infection by human cytomegalovirus (HCMV) promotes a persistent expansion of NKG2C+ adaptive NK cells which have been shown to display enhanced antibody-dependent responses against infected targets and associated to viral control in transplanted patients. Based on gene expression data showing increased transcription of CIITA and several genes related to the MHC class II pathway in adaptive NK cells, we explored their putative capacity for antigen presentation to CD4+ T cells. Phenotypic analysis confirmed a preferential steady-state expression of HLA-DR by circulating NKG2C+ adaptive NK cells in healthy individuals. Expression of HLA-DR in NKG2C+ adaptive NK cells was variable and unrelated to the expression of activation (i.e., CD69 and CD25) or differentiation (i.e., FcRγ chain, CD57) markers, remaining stable over time at the individual level. Incubation of purified NK cells with HCMV complexed with serum specific antibodies induced an up-regulation of surface HLA-DR concomitant to CD16 loss whereas no changes in CD80/CD86 co-stimulatory ligands were detected. In addition, surface CX3CR1 decreased upon antigen-loading while HLA-DR+ NK cells maintained a CCR7-, CXCR3^low homing profile. Remarkably, HCMV-loaded purified NK cells activated autologous CD4+ T cells in an HLA-DR dependent manner. The fraction of T lymphocytes activated by antigen-loaded NK cells was smaller than that stimulated by monocyte-derived dendritic cells, corresponding to CD28-negative effector-memory CD4+ T cells with cytotoxic potential. Antigen presentation by NK cells activated a polyfunctional CD4+ T cell response characterized by degranulation (CD107a) and the secretion of Th1 cytokines (IFNγ and TNFα). Overall, our data discloses the capacity of NKG2C+ adaptive NK cells to process and present HCMV antigens to memory CD4+ cytotoxic T cells, directly regulating their response to the viral infection.

Pathogen recognition by NK cells amplifies the pro-inflammatory cytokine production of monocyte-derived DC via IFN-γ

Background

Besides their prominent role in the elimination of infected or malignantly transformed cells, natural killer (NK) cells serve as modulators of adaptive immune responses. Enhancing bidirectional crosstalk between NK cells and dendritic cells (DC) is considered a promising tool to potentiate cancer vaccines. We investigated to what extent direct sensing of viral and bacterial motifs by NK cells contributes to the response of inflammatory DC against the same pathogenic stimulus.

Results

We demonstrated that sensing of bacterial and viral PAMPs by NK cells contributes to DC cytokine production via NK cell-derived soluble factors. This enhancement of DC cytokine production was dependent on the pattern recognition receptor (PRR) agonist but also on the cytokine environment in which NK cells recognized the pathogen, indicating the importance of accessory cell activation for this mechanism. We showed in blocking experiments that NK cell-mediated amplification of DC cytokine secretion is dependent on NK cell-derived IFN-γ irrespective of the PRR that is sensed by the NK cell.

Conclusions

These findings illustrate the importance of bidirectional interaction between different PRR-expressing immune cells, which can have implications on the selection of adjuvants for vaccination strategies.

Electronic supplementary material

The online version of this article (10.1186/s12865-018-0247-y) contains supplementary material, which is available to authorized users.

Toll-like receptors genes polymorphisms and the occurrence of HCMV infection among pregnant women

Background

Human cytomegalovirus (HCMV) is the most common cause of intrauterine infections worldwide. The toll-like receptors (TLRs) have been reported as important factors in immune response against HCMV. Particularly, TLR2, TLR4 and TLR9 have been shown to be involved in antiviral immunity. Evaluation of the role of single nucleotide polymorphisms (SNPs), located within TLR2, TLR4 and TLR9 genes, in the development of human cytomegalovirus (HCMV) infection in pregnant women and their fetuses and neonates, was performed.

Methods

The study was performed for 131 pregnant women, including 66 patients infected with HCMV during pregnancy, and 65 age-matched control pregnant individuals. The patients were selected to the study, based on serological status of anti-HCMV IgG and IgM antibodies and on the presence of viral DNA in their body fluids. Genotypes in TLR2 2258 A > G, TLR4 896 G > A and 1196 C > T and TLR9 2848 G > A SNPs were determined by self-designed nested PCR-RFLP assays. Randomly selected PCR products, representative for distinct genotypes in TLR SNPs, were confirmed by sequencing. A relationship between the genotypes, alleles, haplotypes and multiple variants in the studied polymorphisms, and the occurrence of HCMV infection in pregnant women and their offsprings, was determined, using a logistic regression model.

Results

Genotypes in all the analyzed polymorphisms preserved the Hardy-Weinberg equilibrium in pregnant women, both infected and uninfected with HCMV (P > 0.050). GG homozygotic and GA heterozygotic status in TLR9 2848 G > A SNP decreased significantly the occurrence of HCMV infection (OR 0.44 95% CI 0.21–0.94 in the dominant model, P ≤ 0.050). The G allele in TLR9 SNP was significantly more frequent among the uninfected pregnant women than among the infected ones (χ² = 4.14, P ≤ 0.050). Considering other polymorphisms, similar frequencies of distinct genotypes, haplotypes and multiple-SNP variants were observed between the studied groups of patients.

Conclusions

TLR9 2848 G > A SNP may be associated with HCMV infection in pregnant women.

Electronic supplementary material

The online version of this article (doi:10.1186/s12985-017-0730-8) contains supplementary material, which is available to authorized users.

TLR2 2258 G>A single nucleotide polymorphism and the risk of congenital infection with human cytomegalovirus

BACKGROUND

Human cytomegalovirus (HCMV) is responsible for the most common intrauterine infections, which may be acquired congenitally from infected pregnant woman to fetus. The research was aimed to estimate the role of three single nucleotide polymorphisms (SNPs) located in TLR2 gene, and the common contribution of TLR2, and previously studied TLR4 and TLR9 SNPs, to the occurrence of congenital HCMV infection in fetuses and newborns.

METHODS

The study was performed in 20 Polish fetuses and newborns, congenitally infected with HCMV, and in 31 uninfected controls, as well as with participation of pregnant women, the mothers of 16 infected and 14 uninfected offsprings. Genotypes in TLR2 SNPs were determined, using self-designed nested PCR-RFLP assays, and confirmed by sequencing. The genotypes were tested for Hardy-Weinberg (H-W) equilibrium, and for their relationship with the development of congenital cytomegaly, using a logistic regression model. The common influence of TLR2, TLR4 and TLR9 SNPs on the occurrence of congenital disease was estimated by multiple-SNP analysis.

RESULTS

Distribution of the genotypes and alleles in TLR2 1350 T>C and 2029 C>T SNPs was similar between the studied groups of fetuses and neonates. In case of 2258 G>A polymorphism, the GA heterozygotic status was significantly more frequent in the infected cases than among the uninfected individuals (25.0% vs. 3.2%, respectively), and increased the risk of HCMV infection (OR 10.00, 95% CI 1.07-93.44; P ≤ 0.050). Similarly, the A allele within 2258 G>A polymorphism was significantly more frequent among the infected offsprings than in the uninfected ones (12.5% vs. 1.6%; P ≤ 0.050). Complex AA variants for both TLR2 2258 and TLR9 2848 G>A polymorphisms, were estimated to be at increased risk of congenital HCMV infection (OR 11.58, 95% CI 1.19-112.59; P ≤ 0.050). Additionally, significant relationships were observed between the occurrence of complex AA or GA variants for both TLR2 and TLR9 SNPs and the increased viral loads, determined in fetal amniotic fluids and in maternal blood or urine specimens (P ≤ 0.050).

CONCLUSIONS

Among various TLR2, TLR4 and TLR9 polymorphisms, TLR2 2258 G>A SNP seems to be an important factor associated with increased risk of congenital HCMV infection in Polish fetuses and neonates.

Expression of NKp30, NKp46 and DNAM-1 activating receptors on resting and IL-2 activated NK cells from healthy donors according to CMV-serostatus and age

Human natural killer (NK) cells are innate lymphoid cells with capacity to kill tumor cells and virus-infected cells. According to the expression of CD56 and CD16 several NK cell subsets have been identified, a major CD56dimCD16+ subpopulation characterized by higher cytotoxic capacity, two CD56bright subsets (CD16-and CD16+) that represent different maturation stages and the fourth CD56-CD16+ subset that correspond to activated dysfunctional NK cells. Previous studies have shown quantitative changes in the frequency, phenotype and distribution of NK cell subsets depending on CMV-serostatus and age. We have analyzed the expression of NKp30, NKp46 and DNAM-1 NK activating receptors on resting and IL-2 activated NK cells from CMV-seronegative and seropositive healthy young donors and from CMV-seropositive elderly individuals. Our results showed that CMV-serostatus of healthy young donors is associated with phenotypic differences on both CD56bright and CD56dim NK cells with an increase of NKp46 and a decrease of NKp30 expression respectively. A reduced expression of DNAM-1 related to ageing and a lower NKp30 expression associated with CMV-seropositivity were observed. The expression of NKp46 and NKp30 was lower in CD57+ NK cells while the expression of DNAM-1 was increased. In vitro NK cell activation by IL-2 increased the expression of NKp46 and NKp30. In summary, both age and CMV-serostatus influence the expression of these cytotoxicity activating receptors that will have functional consequences. In elderly donors is difficult to isolate age from the effect of chronic CMV infection since in our study all elderly donors were CMV-seropositive. The possibility of modulating the expression of these activating receptors by cytokines such as IL-2 may open new opportunities for improving age-associated deterioration of NK cell function.

Antibody-mediated response of NKG2Cbright NK cells against human cytomegalovirus

Human CMV (HCMV) infection promotes a variable and persistent expansion of functionally mature NKG2C(bright) NK cells. We analyzed NKG2C(bright) NK cell responses triggered by Abs from HCMV(+) sera against HCMV-infected MRC5 fibroblasts. Specific Abs promoted the degranulation (i.e., CD107a expression) and the production of cytokines (TNF-α and IFN-γ) by a significant fraction of NK cells, exceeding the low natural cytotoxicity against HCMV-infected targets. NK cell-mediated Ab-dependent cell-mediated cytotoxicity was limited by viral Ag availability and HLA class I expression on infected cells early postinfection and increased at late stages, overcoming viral immunoevasion strategies. Moreover, the presence of specific IgG triggered the activation of NK cells against Ab-opsonized cell-free HCMV virions. As compared with NKG2A(+) NK cells, a significant proportion of NKG2C(bright) NK cells was FcεR γ-chain defective and highly responsive to Ab-driven activation, being particularly efficient in the production of antiviral cytokines, mainly TNF-α. Remarkably, the expansion of NKG2C(bright) NK cells in HCMV(+) subjects was related to the overall magnitude of TNF-α and IFN-γ cytokine secretion upon Ab-dependent and -independent activation. We show the power and sensitivity of the anti-HCMV response resulting from the cooperation between specific Abs and the NKG2C(bright) NK-cell subset. Furthermore, we disclose the proinflammatory potential of NKG2C(bright) NK cells, a variable that could influence the individual responses to other pathogens and tumors.

Human decidual macrophages and NK cells differentially express Toll-like receptors and display distinct cytokine profiles upon TLR stimulation

Maternofetal pathogen transmission is partially controlled at the level of the maternal uterine mucosa at the fetal implantation site (the decidua basalis), where maternal and fetal cells are in close contact. Toll-like receptors (TLRs) may play an important role in initiating rapid immune responses against pathogens in the decidua basalis, however the tolerant microenvironment should be preserved in order to allow fetal development. Here we investigated the expression and functionality of TLRs expressed by decidual macrophages (dMs) and NK cells (dNKs), the major decidual immune cell populations. We report for the first time that both human dMs and dNK cells express mRNAs encoding TLRs 1-9, albeit with a higher expression level in dMs. TLR2, TLR3, and TLR4 protein expression checked by flow cytometry was positive for both dMs and dNK cells. In vitro treatment of primary dMs and dNK cells with specific TLR2, TLR3, TLR4, TLR7/8, and TLR9 agonists enhanced their secretion of pro- and anti-inflammatory cytokines, as well as cytokines and chemokines involved in immune cell crosstalk. Only dNK cells released IFN-γ, whereas only dMs released IL-1β, IL-10, and IL-12. TLR9 activation of dMs resulted in a distinct pattern of cytokine expression compared to the other TLRs. The cytokine profiles expressed by dMs and dNK cells upon TLR activation are compatible with maintenance of the fetotolerant immune environment during initiation of immune responses to pathogens at the maternofetal interface.

TLR/NCR/KIR: Which One to Use and When?

By means of a complex receptor array, Natural killer (NK) cells can recognize variable patterns of ligands and regulate or amplify accordingly their effector functions. Such NK receptors include old, rather conserved, molecules, such as toll-like receptors (TLRs), which enable NK cells to respond both to viral and bacterial products, and newer and evolving molecules, such as killer Ig-like receptors and natural cytotoxicity receptors, which control NK cytotoxicity and are responsible for the elimination of virus-infected or tumor cells without damaging self-unaltered cells. In addition, to rapidly gain new functions NK cells also can acquire new receptors by trogocytosis. Thus, NK cells may have adapted their receptors to different functional needs making them able to play a key role in the modulation of critical events occurring in several compartments of human body (primarily in SLCs but also in decidua during pregnancy). In this review, we will discuss on how the various types of receptors can be used to address specific functions in different immunological contexts.

Human NK cell response to pathogens

NK cells represent important effectors of the innate immunity in the protection of an individual from microbes. During an NK-mediated anti-microbial response, the final fate (survival or death) of a potential infected target cell depends primarily on the type and the number of receptor/ligand interactions occurring at the effector/target immune synapse. The identification of an array of receptors involved in NK cell triggering has been crucial for a better understanding of the NK cell biology. In this context, NCR play a predominant role in NK cell activation during the process of natural cytotoxicity. Regarding the NK-mediated pathogen recognition and NK cell activation, an emerging concept is represented by the involvement of TLRs and activating KIRs. NK cells express certain TLRs in common with other innate cell types. This would mean that specific TLR ligands are able to promote the simultaneous and synergistic stimulation of these innate cells, providing a coordinated mechanism for regulating the initiation and amplification of immune responses. Evidences have been accumulated indicating that viral infections may have a significant impact on NK cell maturation, promoting the expansion of phenotypically and functionally aberrant NK cell subpopulations. For example, during chronic HIV-infection, an abnormal expansion of a dysfunctional CD56neg NK cell subset has been detected that may explain, at least in part, the defective NK cell-mediated antiviral activity. An analogous imbalance of NK cell subsets has been detected in patients receiving HSCT to cure high risk leukemias and experiencing HCMV infection/reactivation. Remarkably, NK cells developing after CMV reactivation may contain "memory-like" or "long-lived" NK cells that could exert a potent anti-leukemia effect.

KIR and HLA interactions are associated with control of primary CMV infection in solid organ transplant recipients

Cytomegalovirus (CMV) infection remains a major source of morbidity and mortality in solid organ transplant recipients. Killer immunoglobulin-like receptors(KIR) are genetically polymorphic natural killer(NK) cell receptors important in antiviral responses. A retrospective, single-center cohort study was performed to study the interaction of KIR genotype and primary control of CMV infection after transplantation.Time to first CMV viremia was determined for a cohort of 531 CMV serology donor positive/recipient negative solid organ transplant recipients. Of the KIR genes,KIR2DL3 and KIR2DS2 were most strongly associated with time to CMV viremia in random survival forest analysis. As KIR2DL3 and KIR2DS2 both interact with HLA-C1, these interactions were evaluated. Seventy six recipients were found to be positive for both KIR2DL3 and KIR2DS2 and expressed only HLA-C1 antigens in both recipient and donor. These patients had a substantially reduced hazard of CMV viremia in the first year after solid organ transplantation (hazard ratio 0.44, 95% CI 0.27–0.72, p=0.0012). In KIR2DL3+/KIR2DS2+/HLA-C1/1 recipients who received an organ from a non-C1/1 donor, this protective effect was not observed. These results improve our understanding of human NK cell function in primary CMV infection after transplant.

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.