Influence of human cytomegalovirus infection on the NK cell receptor repertoire in children

NKG2C zygosity influences CD94/NKG2C receptor function and the NK-cell compartment redistribution in response to human cytomegalovirus

Human cytomegalovirus (HCMV) infection promotes a persistent expansion of a functionally competent NK-cell subset expressing the activating CD94/NKG2C receptor. Factors underlying the wide variability of this effect observed in HCMV-seropositive healthy individuals and exacerbated in immunocompromized patients are uncertain. A deletion of the NKG2C gene has been reported, and an apparent relation of NKG2C genotype with circulating NKG2C(+) NK-cell numbers was observed in HCMV(+) children. We have assessed the influence of NKG2C gene dose on the NK-cell repertoire in a cohort of young healthy adults (N = 130, median age 19 years). Our results revealed a relation of NKG2C copy number with surface receptor levels and with NKG2C(+) NK-cell numbers in HCMV(+) subjects, independently of HLA-E dimorphism. Functional studies showed quantitative differences in signaling (i.e. iCa(2+) influx), degranulation, and IL-15-dependent proliferation, in response to NKG2C engagement, between NK cells from NKG2C(+/+) and hemizygous subjects. These observations provide a mechanistic interpretation on the way the NKG2C genotype influences steady-state NKG2C(+) NK-cell numbers, further supporting an active involvement of the receptor in the HCMV-induced reconfiguration of the NK-cell compartment. The putative implications of NKG2C zygosity over viral control and other clinical variables deserve attention.

Epstein-Barr virus coinfection in children boosts cytomegalovirus-induced differentiation of natural killer cells

During childhood, infections with cytomegalovirus (CMV) and Epstein-Barr virus (EBV) can occur in close temporal proximity. Active, as well as latent, CMV infection is associated with enlarged subsets of differentiated natural killer (NK) and cytotoxic T cells. How EBV infection may influence CMV-driven immune differentiation is not known. We found that EBV coinfection selectively influenced the NK cell compartment of CMV-seropositive (CMV(+)) children. Coinfected children had significantly higher proportions of peripheral-blood NKG2C(+) NK cells than CMV(+) EBV(-) children. Ex vivo NK cell degranulation after target cell stimulation and plasma IL-15 levels were significantly higher in CMV(+) children. EBV coinfection was related to the highest levels of plasma interleukin-15 (IL-15) and IL-12p70. Remarkably, in vitro EBV infection of peripheral blood mononuclear cells (PBMC) from EBV(-) CMV(+) children increased NKG2C(+) NK cell proportions. A similar tendency was seen in cocultures of PBMC with EBV(+) lymphoblastoid B-cell lines (LCL) and IL-15. After K562 challenge, NKG2C(+) NK cells excelled in regard to degranulation and production of gamma interferon, regardless of whether there was previous coculture with LCL. Taken together, our data suggest that dual latency with these herpesviruses during childhood could contribute to an in vivo environment supporting differentiation and maintenance of distinct NK cell populations. This viral imprint may affect subsequent immune responses through altered distributions of effector cells.

Neonatal natural killer cell function: relevance to antiviral immune defense

Neonates are particularly susceptible to various pathogens compared to adults, which is attributed in part to their immature innate and adaptive immunity. Natural killer cells provide first-line innate immune reactions against virus-infected cells without prior sensitization. This review updates phenotypic and functional deficiencies of neonatal cells compared to their adult counterparts and their clinical implications.

Cytomegalovirus infection in infants admitted to a neonatal intensive care unit

OBJECTIVES

To determine the incidence of congenital cytomegalovirus (CMV) infection and the frequency of postnatal infection in a neonatal intensive care unit (NICU).

METHODS

Urine samples of 135 infants who were admitted to the NICU during a 6 month period were evaluated to detect CMV using a nested PCR assay. A breast milk sample was obtained to determine viral excretion. Clinical characteristics of infected and non-infected infants were compared.

RESULTS

Congenital CMV infection was confirmed in two (1.48%) infants. Post-natal infection was documented in four of 36 (11.1%) infants that were evaluated. CMV excretion was detected in 43 of 116 mothers. Gestational age of infants born to mothers who excreted CMV was shorter than that of infants of mothers with negative results (33.1 versus 34.2 weeks; p = 0.07).

CONCLUSIONS

CMV excretion in breast milk is frequent and is associated to congenital and postnatal infection. Further studies are necessary to assess the impact of CMV infection during pregnancy and neonatal outcomes.

Natural killer cell functional defects in pediatric patients with severe and recurrent herpesvirus infections

Natural killer (NK) cells play a critical role in the host defense against herpesviruses. Although herpesviruses are ubiquitous in human populations, only a minority of people experience severe recurrent infections. We hypothesize that uncharacterized NK cell functional deficits predispose individuals to more significant or frequent herpesvirus infections and reactivations. To investigate this hypothesis, we broadly analyzed NK cell phenotype and functional responses in a cohort of predominantly pediatric patients with recurrent and/or severe herpesvirus infections and compared them to a healthy control population. Our results identified no global differences in cytolysis, degranulation, interferon-γ production, or surface receptor upregulation following cytokine stimulation. However, abnormal NK cell functional responses were observed in nearly one-third of patients (including 3 with hyporesponsiveness to activating signals and 1 with markedly decreased CD11b expression associated with reduced cytotoxicity and degranulation), which might contribute to those individuals' susceptibility to herpesvirus infections.

Influence of congenital human cytomegalovirus infection and the NKG2C genotype on NK-cell subset distribution in children

Human cytomegalovirus (HCMV) has been reported to reshape the NK-cell receptor (NKR) distribution, promoting an expansion of CD94/NKG2C(+) NK and T cells. The role of NK cells in congenital HCMV infection is ill-defined. Here we studied the expression of NKR (i.e., NKG2C, NKG2A, LILRB1, CD161) and the frequency of the NKG2C gene deletion in children with past congenital infection, both symptomatic (n = 15) and asymptomatic (n = 11), including as controls children with postnatal infection (n = 11) and noninfected (n = 20). The expansion of NKG2C(+) NK cells in HCMV-infected individuals appeared particularly marked and was associated with an increased number of LILRB1(+) NK cells in cases with symptomatic congenital infection. Increased numbers of NKG2C(+), NKG2A(+), and CD161(+) T cells were also associated to HCMV infection. The NKG2C deletion frequency was comparable in children with congenital HCMV infection and controls. Remarkably, the homozygous NKG2C(+/+) genotype appeared associated with increased absolute numbers of NKG2C(+) NK cells. Moreover, HCMV-infected NKG2C(+/+) children displayed higher absolute numbers of NKG2A(+) and total NK cells than NKG2C(+/-) individuals. Our study provides novel insights on the impact of HCMV infection on the homeostasis of the NK-cell compartment in children, revealing a modulatory influence of NKG2C copy number.

Human leukocyte antigen E in human cytomegalovirus infection: friend or foe?

Human cytomegalovirus (HCMV) is a well-studied β-herpesvirus virus, which adopts a variety of strategies to evade immune surveillance. It has been reported that in HCMV-infected cells, classical major histocompatibility (MHC) class I molecules are down-regulated, but the MHC class Ib molecule human leukocyte antigen (HLA)-E is normally expressed or even overexpressed on the cell surface. HLA-E has been first described to interact with CD94/NKG2 receptors expressed mainly on the surface of natural killer (NK) cells, thus confining its role to the regulation of NK-cell function. The engagement of CD94/NKG2A with HLA-E, with a signal peptide of the HCMV glycoprotein UL40, usually induces inhibitory signals. However, HLA-E also serves as a ligand for the TCR expressed by αβCD8(+) T cells. Recognition of peptides presented by HLA-E may result in CD8(+) effector T-cell activation. These findings will help to understand more on both pathogenic and protective roles of HLA-E in HCMV infection. In this review, we discussed recent studies about the roles of HLA-E in HCMV infection.

Assessment of copy-number variation in the NKG2C receptor gene in a single-tube and characterization of a reference cell panel, using standard polymerase chain reaction

Natural killer (NK) and T-lymphocytes monitor human leukocyte antigen (HLA)-E expression through CD94:NKG2 heterodimers. Structural polymorphism is not a hallmark for NK-complex genes on chromosome 12, except for complete NKG2C deletion in some humans. We present a method for fast, simple and accurate assessment of NKG2C copy-number variation - presence or absence in the genome of an NKG2C gene, in homo- or heterozygosis, is detected by a single conventional polymerase chain reaction that yields amplicons of different lengths in each genotype. We have also determined the NKG2C genotypes of a reference cell panel comprising 13 NK- and tumour-cell lines and 39 Epstein-Barr virus transformed cells from the International Histocompatibility Workshop. Our results should facilitate research on the importance of NKG2C and its deletion for immunity.

Human papillomavirus immunization is associated with increased expression of different innate immune regulatory receptors

Human papillomavirus (HPV) is able to inhibit the secretion of gamma interferon (IFN-γ) and the expression of some immune innate cell receptors. Immunoglobulin-like transcript 2 (ILT2) is a regulatory receptor that seems to participate in the pathogenesis of viral infections. We have studied the expression and function of ILT2 and the expression of other NK cell receptors in 23 healthy women before and after immunization with the quadrivalent HPV (type 6/11/16/18) vaccine (Gardasil). Receptor expression was analyzed by flow cytometry in freshly isolated peripheral blood mononuclear cells as well as after in vitro stimulation with the quadrivalent HPV (type 6/11/16/18) vaccine. In addition, the regulatory function of ILT2 on cell proliferation and IFN-γ production was analyzed. We found a significant increase in the expression of ILT2 by NK and CD3(+) CD56(+) lymphocytes and monocytes after quadrivalent HPV (type 6/11/16/18) vaccine immunization. In addition, the in vitro stimulation with the quadrivalent HPV (type 6/11/16/18) vaccine also increased the proportion of CD3(-) CD56(+) ILT2(+) NK cells. Although the inhibitory function of ILT2 on cell proliferation was enhanced after HPV immunization, the in vitro engagement of this receptor did not affect the synthesis of IFN-γ induced by HPV. Finally, a significant increase in the expression of NKG2D, NKp30, and NKp46 by NK and CD3(+) CD56(+) lymphocytes was detected after quadrivalent HPV (type 6/11/16/18) vaccine immunization. Our data indicate that HPV immunization is associated with significant changes in the expression and function of different innate immune receptors, including ILT2, which may participate in the protective effect of HPV vaccines.

Cytomegalovirus reactivation after allogeneic transplantation promotes a lasting increase in educated NKG2C+ natural killer cells with potent function

During mouse cytomegalovirus (CMV) infection, a population of Ly49H(+) natural killer (NK) cells expands and is responsible for disease clearance through the induction of a "memory NK-cell response." Whether similar events occur in human CMV infection is unknown. In the present study, we characterized the kinetics of the NK-cell response to CMV reactivation in human recipients after hematopoietic cell transplantation. During acute infection, NKG2C(+) NK cells expanded and were potent producers of IFNγ. NKG2C(+) NK cells predominately expressed killer cell immunoglobulin-like receptor, and self-killer cell immunoglobulin-like receptors were required for robust IFNγ production. During the first year after transplantation, CMV reactivation induced a more mature phenotype characterized by an increase in CD56(dim) NK cells. Strikingly, increased frequencies of NKG2C(+) NK cells persisted and continued to increase in recipients who reactivated CMV, whereas these cells remained at low frequency in recipients without CMV reactivation. Persisting NKG2C(+) NK cells lacked NKG2A, expressed CD158b, preferentially acquired CD57, and were potent producers of IFNγ during the first year after transplantation. Recipients who reactivated CMV also expressed higher amounts of IFNγ, T-bet, and IL-15Rα mRNA transcripts. Our findings support the emerging concept that CMV-induced innate memory-cell populations may contribute to malignant disease relapse protection and infectious disease control long after transplantation.

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.

Association of Atherosclerosis With Expression of the LILRB1 Receptor By Human NK and T-Cells Supports the Infectious Burden Hypothesis

Objective—

The contribution of human cytomegalovirus (HCMV) to vascular disease may depend on features of the immune response not reflected by the detection of specific antibodies. Persistent HCMV infection in healthy blood donors has been associated with changes in the distribution of NK cell receptors (NKR). The putative relationship among HCMV infection, NKR distribution, subclinical atherosclerosis, and coronary heart disease was assessed.

Methods and Results—

NKR expression was compared in acute myocardial infarction (AMI) patients ( n =70) and a population-based control sample ( n =209). The relationship between NKR expression and carotid intima-media thickness (CIMT) in controls ( n =149) was also studied. HCMV infection was associated with higher proportions of NKG2C+ and LILRB1+ NK and T-cells. In contrast, only LILRB1+ NK and CD56+ T-cells were found to be increased in AMI patients, independent of age, sex, conventional vascular risk factors, and HCMV seropositivity. Remarkably, LILRB1 expression in NK and T-cells significantly correlated with CIMT in controls.

Conclusion—

The association of overt and subclinical atherosclerotic disease with LILRB1+ NK and T-cells likely reflects a relationship between the immune challenge by infections and cardiovascular disease risk, without attributing a dominant role for HCMV. Our findings may lead to the identification of novel biomarkers of vascular disease.

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.

Natural killer cell responses to infections in early life

Natural killer (NK) cells are an important component of innate immune responses to infectious diseases. They mediate protection by being able to rapidly lyse infected cells and produce cytokines (primarily interferon-γ) that shape innate and adaptive immune responses. This review summarizes current knowledge on the phenotype and functional abilities of NK cells from healthy newborns/infants and on NK cell responses against viral, bacterial and protozoan infections in early life. Interestingly, NK cell blood counts are higher in newborns than in adults but they do not display striking differences in phenotype, except for an increased frequency of expression of the inhibitory CD94/NKG2A receptor. They display some inherent functional defects, mainly a lower cytolytic capacity that may contribute to the immaturity of the neonatal immune system. Changes in circulating levels of NK cells observed during pediatric infections and the ability of NK cells from newborns and children to produce interferon-γ at the encounter with pathogens indicate that NK cells participate in the immune response to infectious diseases in early life. Unfortunately, information is currently insufficient to assess whether these NK cell responses really contribute to control infections, either vertically transmitted or acquired in infancy.

Genetic labeling reveals altered turnover and stability of innate lymphocytes in latent mouse cytomegalovirus infection

Mouse CMV (MCMV) infection rapidly induces the proliferation of NK cells, which correlates with immunological protection. Whether NK cells primed during acute response against MCMV are maintained for the long term is not known. In this study, we used TcrdH2BeGFP mice in which maturing NK cells are genetically labeled with a pulse of very stable histone-2B-eGFP. In this system, we found that the reporter protein was diluted out upon NK cell division during acute MCMV infection. At the same time, mature NK cells in uninfected mice showed only very limited turnover in vivo. Three months after primary infection when MCMV latency was established, the majority of peripheral NK cells still displayed a higher record of proliferation than NK cells in mock-infected controls. This observation included both Ly49H(+) and Ly49H(-) NK cells. Conversely, naive NK cells did not show more proliferation after transfer into latently MCMV-infected mice than that after transfer into mock-infected control mice. This indicated that the observed alterations of the NK cell compartment in MCMV latency were "legacy" (i.e., resulting from prior events during the initial immune response). Together, these results suggest that antiviral immune responses induce sustained alterations of innate lymphocyte populations that extend far beyond the first days of acute infection.

Modulation of natural killer cell activity by viruses

Since their discovery, our understanding of NK cells has evolved from branding them marginal innate immunity cells to key players in anti-viral and anti-tumor immunity. Importance of NK cells in control of various viral infections is perhaps best illustrated by the existence of plethora of viral mechanisms aimed to modulate their function. These mechanisms include not only virally encoded immunoevasion proteins but also viral miRNA. Moreover, the evidence has been accumulated supporting the role of viral immunoevasion of NK cells in viral pathogenesis in vivo.