Natural killer cells can inhibit the transmission of human cytomegalovirus in cell culture by using mechanisms from innate and adaptive immune responses

Expanded clinical-grade NK cells exhibit stronger effects than primary NK cells against HCMV infection

Cytomegalovirus (CMV) reactivation remains a common complication and leads to high mortality in patients who undergo allogeneic hematopoietic stem cell transplantation (allo-HSCT). Early natural killer (NK) cell reconstitution may protect against the development of human CMV (HCMV) infection post-HSCT. Our previous data showed that ex vivo mbIL21/4-1BBL-expanded NK cells exhibited high cytotoxicity against leukemia cells. Nevertheless, whether expanded NK cells have stronger anti-HCMV function is unknown. Herein, we compared the anti-HCMV functions of ex vivo expanded NK cells and primary NK cells. Expanded NK cells showed higher expression of activating receptors, chemokine receptors and adhesion molecules; stronger cytotoxicity against HCMV-infected fibroblasts; and better inhibition of HCMV propagation in vitro than primary NK cells. In HCMV-infected humanized mice, expanded NK cell infusion resulted in higher NK cell persistence and more effective tissue HCMV elimination than primary NK cell infusion. A clinical cohort of 20 post-HSCT patients who underwent adoptive NK cell infusion had a significantly lower cumulative incidence of HCMV infection (HR = 0.54, 95% CI = 0.32-0.93, p = 0.042) and refractory HCMV infection (HR = 0.34, 95% CI = 0.18-0.65, p = 0.009) than controls and better NK cell reconstitution on day 30 post NK cell infusion. In conclusion, expanded NK cells exhibit stronger effects than primary NK cells against HCMV infection both in vivo and in vitro.

ADCC-activating antibodies correlate with decreased risk of congenital human cytomegalovirus transmission

Human cytomegalovirus (HCMV) is the most common vertically transmitted infection worldwide, yet there are no vaccines or therapeutics to prevent congenital HCMV (cCMV) infection. Emerging evidence indicates that antibody Fc effector functions may be a previously underappreciated component of maternal immunity against HCMV. We recently reported that antibody-dependent cellular phagocytosis (ADCP) and IgG activation of FcγRI/FcγRII were associated with protection against cCMV transmission, leading us to hypothesize that additional Fc-mediated antibody functions may be important. In this same cohort of HCMV-transmitting (n = 41) and nontransmitting (n = 40) mother-infant dyads, we report that higher maternal sera antibody-dependent cellular cytotoxicity (ADCC) activation is also associated with lower risk of cCMV transmission. We investigated the relationship between ADCC and IgG responses against 9 viral antigens and found that ADCC activation correlated most strongly with sera IgG binding to the HCMV immunoevasin protein UL16. Moreover, we determined that higher UL16-specific IgG binding and FcγRIII/CD16 engagement were associated with the greatest risk reduction in cCMV transmission. Our findings indicate that ADCC-activating antibodies against targets such as UL16 may represent an important protective maternal immune response against cCMV infection that can guide future HCMV correlates studies and vaccine or antibody-based therapeutic development.

ADCC-activating antibodies correlate with protection against congenital human cytomegalovirus infection

Human cytomegalovirus (HCMV) is the most common vertically transmitted infection worldwide, yet there are no licensed vaccines or therapeutics to prevent congenital HCMV (cCMV) infection. Emerging evidence from studies of natural infection and HCMV vaccine trials indicates that antibody Fc effector functions may defend against HCMV infection. We previously reported that antibody-dependent cellular phagocytosis (ADCP) and IgG activation of FcγRI/FcγRII were associated with reduced risk of cCMV transmission, leading us to hypothesize that other Fc-mediated antibody functions may also contribute to protection. In this same cohort of HCMV transmitting (n = 41) and non-transmitting (n = 40) mother-infant dyads, we found that higher maternal sera antibody-dependent cellular cytotoxicity (ADCC) activation was also associated with decreased risk of cCMV infection. We determined that NK cell-mediated ADCC responses correlated strongly with anti-HCMV IgG FcγRIII/CD16 activation and IgG binding to the HCMV immunoevasin protein UL16. Notably, anti-UL16 IgG binding and engagement of FcγRIII/CD16 were higher in non-transmitting versus transmitting dyads and interacted significantly with ADCC responses. These findings indicate that ADCC-activating antibodies against novel targets such as UL16 may represent an important protective maternal immune response against cCMV infection, which can guide future HCMV correlates studies and vaccine development.

The NK-92 cell line-30 years later: its impact on natural killer cell research and treatment of cancer

The NK-92 cell line, established in 1992, mirrors all the characteristics of highly active blood natural killer (NK) cells but with much broader and greater cytotoxicity. The cell line was established from the blood cells of a patient with lymphoma and has been made widely available for research since it was deposited into the American Type Culture Collection in 1998. The worldwide distribution of NK-92 cells has led to a plethora of scientific discoveries that have greatly increased the understanding of NK-cell biology. NK-92 cells also have been developed for clinical use, overcoming the challenges of obtaining and expanding NK cells from donor or patient blood. More than 100 patients with cancer have now been treated all over the world with unmodified or genetically engineered NK-92 cells. Modified cells include high-affinity Fc-receptor expressing NK-92 cells (haNK^R) and various chimeric antigen receptor targeted haNK cells (t-haNK^TM). Infusions of either unmodified or modified NK-92 cells have been reported to be safe and efficacious, leading in some cases to disease remission even in patients who had failed multiple previous lines of therapy. It is the purpose of this review to distill the plethora of scientific data on NK-92 cells and its genetic variants, highlighting relevant experimental findings that have contributed to a better understanding of NK cell biology and summarize the therapeutic potential of these cells for treatment of cancer and infections.

Adaptive NK cells undergo a dynamic modulation in response to human cytomegalovirus and recruit T cells in in vitro migration assays

Human cytomegalovirus (HCMV) reactivation remains a relevant complication after hematopoietic stem cell transplantation (HSCT) despite the great progress made in prophylaxis and treatment. Adaptive Natural Killer (NK) cells undergo a persistent reconfiguration in response to HCMV reactivation however, the exact role of adaptive NK cells in HCMV surveillance is currently unknown. We studied the relationship between HCMV reactivation and adaptive NK cells in 70 patients monitored weekly until day +100 after HSCT. Absolute cell counts of adaptive NK cells increased significantly after resolution of HCMV-reactivation compared to patients without reactivation. Patients with HCMV-reactivation had an early reconstitution of adaptive NK cells (“Responders”) and had mainly a single reactivation (75% Responders vs 48% Non-Responders). Adaptive NK cells eliminated HCMV-infected human foreskin fibroblasts (HFF) in vitro and recruited T cells in an in vitro transwell migration assay. An extensive cytokine/chemokine panel demonstrated strongly increased secretion of CXCL10/IP-10, IFN-α, IL-1α, IL-1β, IL-5, IL-7 and CCL4. Thus, adaptive NK cells may control viral spread and T cell expansion and survival during HCMV-reactivation. Taken together, we have demonstrated the potential of adaptive NK cells in the control of HCMV reactivation both by direct cytotoxicity and by recruitment of other immune cells.

Human cytomegalovirus (HCMV) is a β-herpes virus that causes life-long infection as a latent virus in its host. Its prevalence depends on socioeconomic geographical factors and can affect 50–90% of the population depending on these factors. HCMV infection is usually asymptomatic in healthy individuals. However, in patients lacking proper immune responses, such as following hematopoietic stem cell transplant (HSCT), HCMV can reactivate and increase the mortality and morbidity rates in these patients. We set to investigate the role of a population of innate cells, the adaptive Natural Killer (NK) cells, in the response to HCMV reactivation after HSCT. Our findings revealed that adaptive NK cells are modulated in response to HCMV reactivation after HSCT. Furthermore, in addition to their ability to eliminate HCMV-infected target cells after in vitro expansion, we have also shown that adaptive NK cells recruit T cells in response to co-culture with HCMV-infected target cells and identified secreted factors possibly involved in this recruitment.

Immune Landscape of CMV Infection in Cancer Patients: From "Canonical" Diseases Toward Virus-Elicited Oncomodulation

Human Cytomegalovirus (HCMV) is an immensely pervasive herpesvirus, persistently infecting high percentages of the world population. Despite the apparent robust host immune responses, HCMV is capable of replicating, evading host defenses, and establishing latency throughout life by developing multiple immune-modulatory strategies. HCMV has coexisted with humans mounting various mechanisms to evade immune cells and effectively win the HCMV-immune system battle mainly through maintaining its viral genome, impairing HLA Class I and II molecule expression, evading from natural killer (NK) cell-mediated cytotoxicity, interfering with cellular signaling, inhibiting apoptosis, escaping complement attack, and stimulating immunosuppressive cytokines (immune tolerance). HCMV expresses several gene products that modulate the host immune response and promote modifications in non-coding RNA and regulatory proteins. These changes are linked to several complications, such as immunosenescence and malignant phenotypes leading to immunosuppressive tumor microenvironment (TME) and oncomodulation. Hence, tumor survival is promoted by affecting cellular proliferation and survival, invasion, immune evasion, immunosuppression, and giving rise to angiogenic factors. Viewing HCMV-induced evasion mechanisms will play a principal role in developing novel adapted therapeutic approaches against HCMV, especially since immunotherapy has revolutionized cancer therapeutic strategies. Since tumors acquire immune evasion strategies, anti-tumor immunity could be prominently triggered by multimodal strategies to induce, on one side, immunogenic tumor apoptosis and to actively oppose the immune suppressive microenvironment, on the other side.

Human Cytomegalovirus (HCMV)-Specific Antibody Response and Development of Antibody-Dependent Cellular Cytotoxicity Against HCMV After Lung Transplantation

BACKGROUND

Human cytomegalovirus (HCMV) may cause severe infections in lung transplant recipients (LTRs). The impact of the host antibody (AB)-dependent cytotoxicity (ADCC) on HCMV is still unclear. Therefore, we analyzed the AB-response against HCMV glycoprotein B (gB) and the pentameric complex (PC) and the ADCC response in HCMV-seropositive (R+) LTRs and in seronegative recipients of positive organs (D+/R-).

METHODS

Plasma samples were collected from 35 R+ and 28 D+/R- LTRs for 1 (R+) or 2 (D+/R-) years posttransplantation and from 114 healthy control persons. The PC- and gB-specific ABs were assessed by enzyme-linked immunosorbent assay. The ADCC was analyzed by focal expansion assay and CD107 cytotoxicity assay.

RESULTS

In R+ LTRs, significantly higher gB-specific AB levels developed within 1 year posttransplantation than in controls (immunoglobulin [Ig]G1, P < .001; IgG3, P < .001). In addition, higher levels of ADCC were observed by FEA and CD107 assay in R+ patients compared with controls (P < .001). In 23 D+R- patients, HCMV-specific ABs developed. Antibody-dependent cytotoxicity became detectable 3 months posttransplantation in these, with higher ADCC observed in viremic patients. Depletion of gB- and PC-specific ABs revealed that, in particular, gB-specific Abs were associated with the ADCC response.

CONCLUSIONS

We show that a strong ADCC is elicited after transplantation and is especially based on gB-specific ABs.

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

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

Assessing Anti-HCMV Cell Mediated Immune Responses in Transplant Recipients and Healthy Controls Using a Novel Functional Assay

HCMV infection, reinfection or reactivation occurs in 60% of untreated solid organ transplant (SOT) recipients. Current clinical approaches to HCMV management include pre-emptive and prophylactic antiviral treatment strategies. The introduction of immune monitoring to better stratify patients at risk of viraemia and HCMV mediated disease could improve clinical management. Current approaches quantify T cell IFNγ responses specific for predominantly IE and pp65 proteins ex vivo, as a proxy for functional control of HCMV in vivo. However, these approaches have only a limited predictive ability. We measured the IFNγ T cell responses to an expanded panel of overlapping peptide pools specific for immunodominant HCMV proteins IE1/2, pp65, pp71, gB, UL144, and US3 in a cohort of D+R- kidney transplant recipients in a longitudinal analysis. Even with this increased antigen diversity, the results show that while all patients had detectable T cell responses, this did not correlate with control of HCMV replication in some. We wished to develop an assay that could directly measure anti-HCMV cell-mediated immunity. We evaluated three approaches, stimulation of PBMC with (i) whole HCMV lysate or (ii) a defined panel of immunodominant HCMV peptides, or (iii) fully autologous infected cells co-cultured with PBMC or isolated CD8+ T cells or NK cells. Stimulation with HCMV lysate often generated non-specific antiviral responses while stimulation with immunodominant HCMV peptide pools produced responses which were not necessarily antiviral despite strong IFNγ production. We demonstrated that IFNγ was only a minor component of secreted antiviral activity. Finally, we used an antiviral assay system to measure the effect of whole PBMC, and isolated CD8+ T cells and NK cells to control HCMV in infected autologous dermal fibroblasts. The results show that both PBMC and especially CD8+ T cells from HCMV seropositive donors have highly specific antiviral activity against HCMV. In addition, we were able to show that NK cells were also antiviral, but the level of this control was highly variable between donors and not dependant on HCMV seropositivity. Using this approach, we show that non-viraemic D+R+ SOT recipients had significant and specific antiviral activity against HCMV.

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

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

Key features and homing properties of NK cells in the liver are shaped by activated iNKT cells

The contribution of natural killer (NK) cells to the clearance of hepatic viral infections is well recognized. The recently discovered heterogeneity of NK cell populations renders them interesting targets for immune interventions. Invariant natural killer T (iNKT) cells represent a key interaction partner for hepatic NK cells. The present study addressed whether characteristics of NK cells in the liver can be shaped by targeting iNKT cells. For this, the CD1d-binding pegylated glycolipid αGalCerMPEG was assessed for its ability to modulate the features of NK cells permanently or transiently residing in the liver. In vivo administration resulted in enhanced functionality of educated and highly differentiated CD27⁺ Mac-1⁺ NK cells accompanied by an increased proliferation. Improved liver homing was supported by serum-derived and cellular factors. Reduced viral loads in a mCMV infection model confirmed the beneficial effect of NK cells located in the liver upon stimulation with αGalCerMPEG. Thus, targeting iNKT cell-mediated NK cell activation in the liver represents a promising approach for the establishment of liver-directed immune interventions.

Battle between Host Immune Cellular Responses and HCMV Immune Evasion

Human cytomegalovirus (HCMV) is ubiquitously prevalent. HCMV infection is typically asymptomatic and controlled by the immune system in healthy individuals, yet HCMV can be severely pathogenic for the fetus during pregnancy and in immunocompromised persons, such as transplant recipients or HIV infected patients. HCMV has co-evolved with the hosts, developed strategies to hide from immune effector cells and to successfully survive in the human organism. One strategy for evading or delaying the immune response is maintenance of the viral genome to establish the phase of latency. Furthermore, HCMV immune evasion involves the downregulation of human leukocyte antigens (HLA)-Ia molecules to hide infected cells from T-cell recognition. HCMV expresses several proteins that are described for downregulation of the HLA class I pathway via various mechanisms. Here, we review the wide range of immune evasion mechanisms of HCMV. Understanding the mechanisms of HCMV immune evasion will contribute to the development of new customized therapeutic strategies against the virus.

Cell-to-Cell Spread Blocking Activity Is Extremely Limited in the Sera of Herpes Simplex Virus 1 (HSV-1)- and HSV-2-Infected Subjects

Herpes simplex virus 1 (HSV-1) and HSV-2 can evade serum antibody-mediated neutralization through cell-to-cell transmission mechanisms, which represent one of the central steps in disease reactivation. To address the role of humoral immunity in controlling HSV-1 and HSV-2 replication, we analyzed serum samples from 44 HSV-1 and HSV-2 seropositive subjects by evaluating (i) their efficiency in binding both the purified viral particles and recombinant gD and gB viral glycoproteins, (ii) their neutralizing activity, and (iii) their capacity to inhibit the cell-to-cell virus passage in vitro All of the sera were capable of binding gD, gB, and whole virions, and all sera significantly neutralized cell-free virus. However, neither whole sera nor purified serum IgG fraction was able to inhibit significantly cell-to-cell virus spreading in in vitro post-virus-entry infectious assays. Conversely, when spiked with an already described anti-gD human monoclonal neutralizing antibody capable of inhibiting HSV-1 and -2 cell-to-cell transmission, each serum boosted both its neutralizing and post-virus-entry inhibitory activity, with no interference exerted by serum antibody subpopulations.IMPORTANCE Despite its importance in the physiopathology of HSV-1 and -2 infections, the cell-to-cell spreading mechanism is still poorly understood. The data shown here suggest that infection-elicited neutralizing antibodies capable of inhibiting cell-to-cell virus spread can be underrepresented in most infected subjects. These observations can be of great help in better understanding the role of humoral immunity in controlling virus reactivation and in the perspective of developing novel therapeutic strategies, studying novel correlates of protection, and designing effective vaccines.

Human monocyte-derived macrophages inhibit HCMV spread independent of classical antiviral cytokines

Infection of healthy individuals with human cytomegalovirus (HCMV) is usually unnoticed and results in life-long latency, whereas HCMV reactivation as well as infection of newborns or immunocompromised patients can cause life-threatening disease. To better understand HCMV pathogenesis we studied mechanisms that restrict HCMV spread. We discovered that HCMV-infected cells can directly trigger plasmacytoid dendritic cells (pDC) to mount antiviral type I interferon (IFN-I) responses, even in the absence of cell-free virus. In contrast, monocyte-derived cells only expressed IFN-I when stimulated by cell-free HCMV, or upon encounter of HCMV-infected cells that already produced cell-free virus. Nevertheless, also in the absence of cell-free virus, i.e., upon co-culture of infected epithelial/endothelial cells and monocyte-derived macrophages (moMΦ) or dendritic cells (moDC), antiviral responses were induced that limited HCMV spread. The induction of this antiviral effect was dependent on cell-cell contact, whereas cell-free supernatants from co-culture experiments also inhibited virus spread, implying that soluble factors were critically needed. Interestingly, the antiviral effect was independent of IFN-γ, TNF-α, and IFN-I as indicated by cytokine inhibition experiments using neutralizing antibodies or the vaccinia virus-derived soluble IFN-I binding protein B18R, which traps human IFN-α and IFN-β. In conclusion, our results indicate that human macrophages and dendritic cells can limit HCMV spread by IFN-I dependent as well as independent mechanisms, whereas the latter ones might be particularly relevant for the restriction of HCMV transmission via cell-to-cell spread.

New extracellular factors in glioblastoma multiforme development: neurotensin, growth differentiation factor-15, sphingosine-1-phosphate and cytomegalovirus infection

Recent years have seen considerable progress in understanding the biochemistry of cancer. For example, more significance is now assigned to the tumor microenvironment, especially with regard to intercellular signaling in the tumor niche which depends on many factors secreted by tumor cells. In addition, great progress has been made in understanding the influence of factors such as neurotensin, growth differentiation factor-15 (GDF-15), sphingosine-1-phosphate (S1P), and infection with cytomegalovirus (CMV) on the 'hallmarks of cancer' in glioblastoma multiforme. Therefore, in the present work we describe the influence of these factors on the proliferation and apoptosis of neoplastic cells, cancer stem cells, angiogenesis, migration and invasion, and cancer immune evasion in a glioblastoma multiforme tumor. In particular, we discuss the effect of neurotensin, GDF-15, S1P (including the drug FTY720), and infection with CMV on tumor-associated macrophages (TAM), microglial cells, neutrophil and regulatory T cells (T_reg), on the tumor microenvironment. In order to better understand the role of the aforementioned factors in tumoral processes, we outline the latest models of intratumoral heterogeneity in glioblastoma multiforme. Based on the most recent reports, we discuss the problems of multi-drug therapy in treating glioblastoma multiforme.

Association between γ marker, human leucocyte antigens and killer immunoglobulin-like receptors and the natural course of human cytomegalovirus infection: a pilot study performed in a Sicilian population

Natural killer (NK) cells provide a major defence against human cytomegalovirus (HCMV) infection through the interaction of their surface receptors, including the activating and inhibitory killer immunoglobulin-like receptors (KIRs), and human leucocyte antigen (HLA) class I molecules. Also γ marker (GM) allotypes, able to influence the NK antibody-dependent cell-mediated cytotoxicity, appear to be involved in the immunological control of virus infections, including HCMV. In some cases, their contribution requires epistatic interaction with other genes of the immune system, such as HLA. In the present report, with the aim of gaining insight into the immune mechanisms controlling HCMV, we have studied the possible associations among humoral and NK responses, and HCMV infections. In a previous study we assessed whether the KIR and HLA repertoire might influence the risk of developing symptomatic (n = 60) or asymptomatic (n = 60) disease after primary HCMV infection in the immunocompetent host. In the present study, the immunocompetent patients with primary symptomatic HCMV infection were genotyped for GM3/17 and GM23 allotypes, along with the 60 participants with a previous asymptomatic infection as controls. Notwithstanding the presence of missing data record, advanced missing data recovery techniques were able to show that individuals carrying the GM23 allotypes, both homozygous and heterozygous, GM17/17, HLA-C2 and Bw4^T KIR-ligand groups are associated with the risk of developing symptomatic infection. Our findings on the role of both cellular and humoral immunity in the control of HCMV infection should be of value in guiding efforts to reduce HCMV-associated health complications in the elderly, including immunosenescence, and in transplantation.

Peptide vaccination in the presence of adjuvants in patients after hematopoietic stem cell transplantation with CD4+ T cell reconstitution elicits consistent CD8+ T cell responses

Rationale: Patients receiving an allogeneic stem cell graft from cytomegalovirus (CMV) seronegative donors are particularly prone to CMV reactivation with a high risk of disease and mortality. Therefore we developed and manufactured a novel vaccine and initiated a clinical phase I trial with a CMV phosphoprotein 65 (CMVpp65)-derived peptide. Methods: Ten patients after allogeneic stem cell transplantation received four vaccinations at a biweekly interval. All patients were monitored for CMVpp65 antigenemia. Flow cytometry for CMV-specific CD8+ and γδ T cells as well as neutralizing anti-CMV antibodies were correlated to clinical parameters. Results: The vaccination was well tolerated. Seven of nine patients cleared CMVpp65 antigenemia after four vaccinations and are still free from antigenemia to this day. Two patients with CMV reactivation showed persisting CMV antigenemia. One patient received prophylactic vaccination and did not develop antigenemia. An increase of up to six-fold in frequency of both CMV-specific CD8+ T cells and/or Vδ2negative γδ T cells was detected. Titers of neutralizing antibodies increased up to the tenfold. Humoral and cellular immune responses correlated with clearance of CMV. Conclusion: In summary, CMVpp65 peptide vaccination for patients after allogeneic stem cell transplantation at high risk for CMV reactivation was safe, well tolerated and clinically encouraging. A study in solid-organ transplant patients is ongoing.

Activation of Innate and Adaptive Immunity by a Recombinant Human Cytomegalovirus Strain Expressing an NKG2D Ligand

Development of an effective vaccine against human cytomegalovirus (HCMV) is a need of utmost medical importance. Generally, it is believed that a live attenuated vaccine would best provide protective immunity against this tenacious pathogen. Here, we propose a strategy for an HCMV vaccine that aims at the simultaneous activation of innate and adaptive immune responses. An HCMV strain expressing the host ligand ULBP2 for the NKG2D receptor was found to be susceptible to control by natural killer (NK) cells, and preserved the ability to stimulate HCMV-specific T cells. Infection with the ULBP2-expressing HCMV strain caused diminished cell surface levels of MHC class I molecules. While expression of the NKG2D ligand increased the cytolytic activity of NK cells, NKG2D engagement in CD8+ T cells provided co-stimulation and compensated for lower MHC class I expression. Altogether, our data indicate that triggering of both arms of the immune system is a promising approach applicable to the generation of a live attenuated HCMV vaccine.

Human cytomegalovirus (CMV) is a major cause of morbidity and mortality in congenitally infected newborns and immunocompromised individuals, indicating an utmost need for a vaccine to protect these vulnerable groups. Recent experimental studies in animal models, including non-human primates, have shown that attenuated CMVs trigger a potent immune response and are attractive vaccine candidates. However, an effective CMV vaccine is still not available. Here, we demonstrate that rational engineering of a live attenuated human CMV vaccine candidate is feasible. We equipped a CMV strain with an immunostimulatory molecule that is a ligand for an activating receptor present on both Natural Killer cells and CD8+ T cells. Moreover, we deleted several immunoevasins involved in downregulation of MHC class I molecules and of a ligand for Natural Killer cells in order to elicit stronger immune responses. In vitro assays using human immune cells and a first assessment in a humanized mouse model in vivo suggest that the generated CMV strain is attenuated and has the ability to induce a virus-specific immune response. Our study proposes this novel approach for the development of a rationally engineered CMV vaccine.

Association of Human Immunoglobulin G1 Heavy Chain Variants With Neutralization Capacity and Antibody-Dependent Cellular Cytotoxicity Against Human Cytomegalovirus

Human cytomegalovirus (HCMV) infection is limited by HCMV-specific antibody functions. Here the association between the genetic marker (GM) 3/17 variants in the immunoglobulin G1 (IgG1) heavy chain constant region, virus neutralization, and natural killer (NK)-cell activation was investigated. In 100 HCMV-seropositive individuals, the GM3/17 polymorphism, serum 50% HCMV antibody neutralization titer (NT50), and in vitro HCMV-specific antibody NK-cell activation were assessed. The HCMV NT50 was higher in heterozygous GM3/17 persons than in GM3/3 persons (P = .0276). Furthermore, individuals expressing GM3/17 exhibited significantly higher NK-cell activation than persons carrying GM3/3 (P < .0001) or GM17/17 (P = .0095). Thus, persons expressing GM3/17 have potentially a selective advantage in HCMV defense.

Phenotypic characterization of human cytomegalovirus strains in cell cultures based on their transmission kinetics

We established a new 'transmission kinetic assay (TKA)' to quantify the human cytomegalovirus (HCMV) transmission between cells in vitro and to phenotypically characterize HCMV strains based on their mode of transmission by flow cytometric analysis. On one hand we used the genetically modified HCMV strain TB40/E-delUL16-GFP, and on the other hand, clinical isolates. When twofold diluted infecting cells were seeded to a constant number of uninfected cells, the transmission of virus on each day (day 0-5) followed a strictly linear pattern, which was characterized by a linear equation. The slope of this linear equation represents 'the number of newly infected cells per infecting cell'. To standardize the TKA, the slopes of the different days were plotted against the corresponding days. This resulted in a new linear equation with a new slope value, which characterizes the transmission kinetics. To differentiate cell-associated and cell-free modes of transmission, we introduced HCMV neutralizing antibodies into the system. The slope was 0.9 (±0.5) when the virus exhibited only cell-associated transmission and was 4.1 (±0.7) when the virus exhibited both modes of transmission. TKA was then applied to different clinical isolates and they were phenotypically characterized based on their modes of transmission. Apart from the quantitative analysis of HCMV transmission and the phenotypical characterization of clinical isolates, the TKA was applied to quantify the inhibition of clinical isolates transmission by immune cells and to study the effect of cytokine (IL-2) on immune cells inhibiting HCMV transmission.

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

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

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

Interleukin-2 from Adaptive T Cells Enhances Natural Killer Cell Activity against Human Cytomegalovirus-Infected Macrophages

Control of human cytomegalovirus (HCMV) requires a continuous immune surveillance, thus HCMV is the most important viral pathogen in severely immunocompromised individuals. Both innate and adaptive immunity contribute to the control of HCMV. Here, we report that peripheral blood natural killer cells (PBNKs) from HCMV-seropositive donors showed an enhanced activity toward HCMV-infected autologous macrophages. However, this enhanced response was abolished when purified NK cells were applied as effectors. We demonstrate that this enhanced PBNK activity was dependent on the interleukin-2 (IL-2) secretion of CD4(+) T cells when reexposed to the virus. Purified T cells enhanced the activity of purified NK cells in response to HCMV-infected macrophages. This effect could be suppressed by IL-2 blocking. Our findings not only extend the knowledge on the immune surveillance in HCMV-namely, that NK cell-mediated innate immunity can be enhanced by a preexisting T cell antiviral immunity-but also indicate a potential clinical implication for patients at risk for severe HCMV manifestations due to immunosuppressive drugs, which mainly suppress IL-2 production and T cell responsiveness.

IMPORTANCE

Human cytomegalovirus (HCMV) is never cleared by the host after primary infection but instead establishes a lifelong latent infection with possible reactivations when the host's immunity becomes suppressed. Both innate immunity and adaptive immunity are important for the control of viral infections. Natural killer (NK) cells are main innate effectors providing a rapid response to virus-infected cells. Virus-specific T cells are the main adaptive effectors that are critical for the control of the latent infection and limitation of reinfection. In this study, we found that IL-2 secreted by adaptive CD4(+) T cells after reexposure to HCMV enhances the activity of NK cells in response to HCMV-infected target cells. This is the first direct evidence that the adaptive T cells can help NK cells to act against HCMV infection.

Increase in Peripheral CD3-CD56brightCD16- Natural Killer Cells in Hashimoto's Thyroiditis Associated with HHV-6 Infection

Hashimoto's thyroiditis (HT) is a very common autoimmune disease of the thyroid. In addition to genetic background, several viruses, including herpesviruses, have been suggested to play a role as possible environmental triggers of disease, but conclusive data are still lacking. Previous results showed that HT patients have an increased cellular immune response directed against the HHV-6 U94 protein and increased NK activity directed against HHV-6 infected thyrocytes.In this study, we characterized the antiviral antibody response and the NK cells activity and subtype in HHV-6 infected HT patients. The results showed that HT subjects have increased prevalence and titer of anti-U94 antibodies and a higher amount of CD3-CD56(bright)CD16(-)NK cell percentages compared to controls. Furthermore, the cell activation of CD3(-)CD56(bright) NK cells in HT patients significantly correlates with TPO and Tg Ab levels.The results suggest that HHV-6 might contribute to HT development, increasing NK cell secretion of inflammatory cytokines that could sustain the persistence of an inflammatory status in HT patients.