Isolation of human monoclonal antibodies that potently neutralize human cytomegalovirus infection by targeting different epitopes on the gH/gL/UL128-131A complex

Human cytomegalovirus vaccine based on the envelope gH/gL pentamer complex

Human Cytomegalovirus (HCMV) utilizes two different pathways for host cell entry. HCMV entry into fibroblasts requires glycoproteins gB and gH/gL, whereas HCMV entry into epithelial and endothelial cells (EC) requires an additional complex composed of gH, gL, UL128, UL130, and UL131A, referred to as the gH/gL-pentamer complex (gH/gL-PC). While there are no established correlates of protection against HCMV, antibodies are thought to be important in controlling infection. Neutralizing antibodies (NAb) that prevent gH/gL-PC mediated entry into EC are candidates to be assessed for in vivo protective function. However, these potent NAb are predominantly directed against conformational epitopes derived from the assembled gH/gL-PC. To address these concerns, we constructed Modified Vaccinia Ankara (MVA) viruses co-expressing all five gH/gL-PC subunits (MVA-gH/gL-PC), subsets of gH/gL-PC subunits (gH/gL or UL128/UL130/UL131A), or the gB subunit from HCMV strain TB40/E. We provide evidence for cell surface expression and assembly of complexes expressing full-length gH or gB, or their secretion when the corresponding transmembrane domains are deleted. Mice or rhesus macaques (RM) were vaccinated three times with MVA recombinants and serum NAb titers that prevented 50% infection of human EC or fibroblasts by HCMV TB40/E were determined. NAb responses induced by MVA-gH/gL-PC blocked HCMV infection of EC with potencies that were two orders of magnitude greater than those induced by MVA expressing gH/gL, UL128-UL131A, or gB. In addition, MVA-gH/gL-PC induced NAb responses that were durable and efficacious to prevent HCMV infection of Hofbauer macrophages, a fetal-derived cell localized within the placenta. NAb were also detectable in saliva of vaccinated RM and reached serum peak levels comparable to NAb titers found in HCMV hyperimmune globulins. This vaccine based on a translational poxvirus platform co-delivers all five HCMV gH/gL-PC subunits to achieve robust humoral responses that neutralize HCMV infection of EC, placental macrophages and fibroblasts, properties of potential value in a prophylactic vaccine.

Human cytomegalovirus (HCMV) fetal infection during pregnancy and infection of immunocompromised patients are both clinical problems considered extremely important by the Institute of Medicine. Limited efficacy against primary HCMV infection was found using a subunit vaccine based on glycoprotein B, an important neutralizing antibody determinant blocking HCMV entry into fibroblasts. The HCMV field has been transformed by the discovery that a five-member (pentamer) protein complex is a required factor for epithelial and endothelial cell entry and indispensable for transmission as shown in non-human primates. Targeting HCMV with antibodies specific to the pentamer may interrupt horizontal and vertical transmission. We describe an innovative vaccine strategy to induce serum neutralizing antibodies of impressive magnitude against HCMV in two animal models. Using an attenuated poxvirus vector system, we demonstrate that co-expression of all five pentamer components is significantly more potent to induce serum neutralizing antibodies than subunit subsets of the complex or glycoprotein B, reaching peak levels comparable to HCMV hyperimmune globulin. A vaccine that elicits systemic and mucosal antibody responses that prevents infection of multiple cell types crucial to natural history of HCMV infection could play a role in preventing congenital HCMV infection and control of infection in immunocompromised patients.

The immunology of human cytomegalovirus latency: could latent infection be cleared by novel immunotherapeutic strategies?

While the host immune response following primary human cytomegalovirus (HCMV) infection is generally effective at stopping virus replication and dissemination, virus is never cleared by the host and like all herpesviruses, persists for life. At least in part, this persistence is known to be facilitated by the ability of HCMV to establish latency in myeloid cells in which infection is essentially silent with, importantly, a total lack of new virus production. However, although the viral transcription programme during latency is much suppressed, a number of viral genes are expressed during latent infection at the protein level and many of these have been shown to have profound effects on the latent cell and its environment. Intriguingly, many of these latency-associated genes are also expressed during lytic infection. Therefore, why the same potent host immune responses generated during lytic infection to these viral gene products are not recognized during latency, thereby allowing clearance of latently infected cells, is far from clear. Reactivation from latency is also a major cause of HCMV-mediated disease, particularly in the immune compromised and immune naive, and is also likely to be a major source of virus in chronic subclinical HCMV infection which has been suggested to be associated with long-term diseases such as atherosclerosis and some neoplasias. Consequently, understanding latency and why latently infected cells appear to be immunoprivileged is crucial for an understanding of the pathogenesis of HCMV and may help to design strategies to eliminate latent virus reservoirs, at least in certain clinical settings.

New pharmacological strategies to fight enveloped viruses

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Many emerging or known, chronic viral diseases are caused by enveloped viruses.

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The review discusses research driven development of antivirals that became recently available or are in clinical evaluation.

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The main focus is on antiviral strategies with a broader therapeutic range, and on novel immune based therapeutics.

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Broad-spectrum antivirals will help to react faster to newly emerging viral diseases.

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Targeting immune cells against infected cells can restore immune responses in chronic infections.

Enveloped viruses pose an important health threat because most of the persistent and many emerging viruses are enveloped. In particular, newly emerging viruses create a need to develop broad-spectrum antivirals, which usually are obtained by targeting host cell factors. Persistent viruses have developed efficient strategies to escape host immune control, and treatment options are limited. Targeting host cell factors essential for virus persistence, or immune-based therapies provide alternative approaches. In this review, we therefore focus on recent developments to generate antivirals targeting host cell factors or immune-based therapeutic approaches to fight infections with enveloped viruses.

Cytomegalovirus vaccines

An effective cytomegalovirus (CMV) vaccine could prevent the majority of birth defects caused by congenital CMV infections. Candidate vaccines in clinical evaluation include live attenuated, protein subunit, DNA, and viral-vectored approaches. Subunit approaches have focused on the CMV proteins pp65 and IE1 as important inducers of cytotoxic T cells and glycoprotein B (gB) as an important inducer of neutralizing antibodies. A vaccine comprised of recombinant gB protein with MF59 adjuvant reduced the incidence of primary infection by 50%. Recent revelations regarding CMV entry pathways into different cell types suggest a possible course for improvement. A 5-subunit pentameric complex is uniquely required for endothelial and epithelial cell entry. Sera from naturally infected subjects contain high-potency neutralizing activities specific for this complex, whereas the gB/MF59 vaccine fails to induce comparable neutralizing activities. A vaccine's ability to induce salivary antibodies that neutralize epithelial cell entry may be especially important for preventing oral transmission as the first cells infected are presumably epithelial cells of the oral mucosa. In addition, recent evidence suggests that antibodies can inhibit postentry CMV spread between endothelial and epithelial cells. Such activities may serve to limit viral replication in tissues or impair dissemination to the placenta and fetus. Thus, inclusion of epitopes derived from the pentameric complex may provide enhanced efficacy by inducing potent neutralizing/spread-inhibiting antibodies that target virus replication in a broad spectrum of cell types. Next-generation vaccine candidates in preclinical development incorporate peptides, subunits, or multisubunit complexes representing parts or all of the pentameric complex. Approaches include peptides, recombinant proteins, DNA, replication-defective viral vectors, genetically disabled CMV, and inactivated CMV virions. The diversity of novel strategies under development engenders optimism that a successful candidate will emerge.

Sequestration of human cytomegalovirus by human renal and mammary epithelial cells

Urine and breast milk represent the main routes of human cytomegalovirus (HCMV) transmission but the contribution of renal and mammary epithelial cells to viral excretion remains unclear. We observed that kidney and mammary epithelial cells were permissive to HCMV infection and expressed immediate early, early and late antigens within 72 h of infection. During the first 24 h after infection, high titers of infectious virus were measured associated to the cells and in culture supernatants, independently of de novo synthesis of virus progeny. This phenomenon was not observed in HCMV-infected fibroblasts and suggested the sequestration and the release of HCMV by epithelial cells. This hypothesis was supported by confocal and electron microscopy analyses. The sequestration and progressive release of HCMV by kidney and mammary epithelial cells may play an important role in the excretion of the virus in urine and breast milk and may thereby contribute to HCMV transmission.

Human cytomegalovirus Fcγ binding proteins gp34 and gp68 antagonize Fcγ receptors I, II and III

Human cytomegalovirus (HCMV) establishes lifelong infection with recurrent episodes of virus production and shedding despite the presence of adaptive immunological memory responses including HCMV immune immunoglobulin G (IgG). Very little is known how HCMV evades from humoral and cellular IgG-dependent immune responses, the latter being executed by cells expressing surface receptors for the Fc domain of IgG (FcγRs). Remarkably, HCMV expresses the RL11-encoded gp34 and UL119-118-encoded gp68 type I transmembrane glycoproteins which bind Fcγ with nanomolar affinity. Using a newly developed FcγR activation assay, we tested if the HCMV-encoded Fcγ binding proteins (HCMV FcγRs) interfere with individual host FcγRs. In absence of gp34 or/and gp68, HCMV elicited a much stronger activation of FcγRIIIA/CD16, FcγRIIA/CD32A and FcγRI/CD64 by polyclonal HCMV-immune IgG as compared to wildtype HCMV. gp34 and gp68 co-expression culminates in the late phase of HCMV replication coinciding with the emergence of surface HCMV antigens triggering FcγRIII/CD16 responses by polyclonal HCMV-immune IgG. The gp34- and gp68-dependent inhibition of HCMV immune IgG was fully reproduced when testing the activation of primary human NK cells. Their broad antagonistic function towards FcγRIIIA, FcγRIIA and FcγRI activation was also recapitulated in a gain-of-function approach based on humanized monoclonal antibodies (trastuzumab, rituximab) and isotypes of different IgG subclasses. Surface immune-precipitation showed that both HCMV-encoded Fcγ binding proteins have the capacity to bind trastuzumab antibody-HER2 antigen complexes demonstrating simultaneous linkage of immune IgG with antigen and the HCMV inhibitors on the plasma membrane. Our studies reveal a novel strategy by which viral FcγRs can compete for immune complexes against various Fc receptors on immune cells, dampening their activation and antiviral immunity.

Herpes viruses persist lifelong continuously alternating between latency and virus production and transmission. The latter events occur despite the presence of immune IgG antibodies. IgG acts by neutralization of virions and activation of immune cells bearing one or more surface receptors, called FcγRs, recognizing the constant Fc domain of IgG. Activating FcγRs induce a wide range of immune responses, including antibody dependent cellular cytotoxicity (ADCC) of virus-infected cells by natural killer (NK) cells, cytokine secretion and the uptake of immune complexes to enhance antigen presentation to T cells. We demonstrate that the HCMV glycoproteins RL11/gp34 and UL119-118/gp68 block IgG-mediated activation of FcγRs. A novel reporter cell-based assay was used to test FcγRs individually and assess their relative susceptibility to each antagonist. This approach revealed that gp34 and gp68 block triggering of activating FcγRs, i.e. FcγRI (CD64), FcγRII (CD32A) and FcγRIII (CD16). Co-immunoprecipitation showed the formation of ternary complexes containing IgG, IgG-bound antigen and the viral antagonists on the cell surface. Assigning the redundant abilities of HCMV to hinder IgG effector responses to the viral Fc binding proteins, we discuss gp34 and gp68 as potential culprits which might contribute to the limited efficacy of therapeutic IgG against HCMV.

A new look at a poorly immunogenic neutralization epitope on cytomegalovirus glycoprotein B. Is there cause for antigen redesign?

The immune response is able to control cytomegalovirus infection in most subjects. However, in some patient groups the virus is not well contained resulting in disease and severe morbidity. The development of efficacious vaccines is therefore a high priority. Antibodies may contribute to protection against disease caused by CMV but the most efficient targets for protective humoral immunity are not precisely known. Glycoprotein B (gB) is a protein that is targeted by virus-neutralizing antibodies. One epitope on gB, AD-2, is poorly immunogenic following natural infection and vaccination. It is consequently not effectively exploited as a target for antibodies by the immune system. However, antibodies specific for this epitope, when they develop, display important functional activities that may play a role in protection against infection. In this study critical features of human antibody recognition of this epitope are re-assessed based on structural and immunochemical data. The analysis suggests that the immune system may only be able to develop an AD-2 specific antibody response through rare, very specific rearrangement events that by chance create a naïve B cell that can be recruited into an AD-2 specific immune response. These results reinvigorate the notion that if we are to be able to effectively exploit AD-2 specific humoral immunity we need to readdress the nature of the antigen incorporated into vaccines so as to more effectively recruit B cells into the response against this epitope.

A neutralizing anti-gH/gL monoclonal antibody is protective in the guinea pig model of congenital CMV infection

Human cytomegalovirus (HCMV) is the most common cause of congenital virus infection. Congenital HCMV infection occurs in 0.2–1% of all births, and causes birth defects and developmental abnormalities, including sensorineural hearing loss and developmental delay. Several key studies have established the guinea pig as a tractable model for the study of congenital HCMV infection and have shown that polyclonal antibodies can be protective [1]–[3]. In this study, we demonstrate that an anti-guinea pig CMV (GPCMV) glycoprotein H/glycoprotein L neutralizing monoclonal antibody protects against fetal infection and loss in the guinea pig. Furthermore, we have delineated the kinetics of GPCMV congenital infection, from maternal infection (salivary glands, seroconversion, placenta) to fetal infection (fetus and amniotic fluid). Our studies support the hypothesis that a neutralizing monoclonal antibody targeting an envelope GPCMV glycoprotein can protect the fetus from infection and may shed light on the therapeutic intervention of HCMV congenital infection in humans.

Human cytomegalovirus (HCMV) is the most common cause of congenital virus infection and causes developmental abnormalities, including hearing loss and developmental delay. Although there is no therapy for congenital HCMV disease, there is evidence from both human and animal studies that antibodies can have efficacy in this setting. Such studies have focused exclusively on polyclonal antibodies, in which the targets of protective antibodies are unknown. Guinea pigs have been used as a model of human maternal fetal transmission of infection because of similarities in placental anatomy between human and guinea pig. Furthermore, guinea pig CMV (GPCMV) has been demonstrated to cross the placenta and cause fetal infection and loss, similar to the effects of infection with HCMV. However, the kinetics of maternal and fetal infection in this model has not been carefully investigated. In this work, we have delineated the kinetics of maternal to fetal infection and found that congenital infection is rapid following maternal infection. Importantly, we demonstrate that a monoclonal antibody against a protein critical for viral entry protects pregnant guinea pigs against fetal infection. Thus, our studies may be informative for development of a therapeutic intervention to treat congenital HCMV infection in humans.

A randomized trial of hyperimmune globulin to prevent congenital cytomegalovirus

BACKGROUND

Congenital infection with human cytomegalovirus (CMV) is a major cause of morbidity and mortality. In an uncontrolled study published in 2005, administration of CMV-specific hyperimmune globulin to pregnant women with primary CMV infection significantly reduced the rate of intrauterine transmission, from 40% to 16%.

METHODS

We evaluated the efficacy of hyperimmune globulin in a phase 2, randomized, placebo-controlled, double-blind study. A total of 124 pregnant women with primary CMV infection at 5 to 26 weeks of gestation were randomly assigned within 6 weeks after the presumed onset of infection to receive hyperimmune globulin or placebo every 4 weeks until 36 weeks of gestation or until detection of CMV in amniotic fluid. The primary end point was congenital infection diagnosed at birth or by means of amniocentesis.

RESULTS

A total of 123 women could be evaluated in the efficacy analysis (1 woman in the placebo group withdrew). The rate of congenital infection was 30% (18 fetuses or infants of 61 women) in the hyperimmune globulin group and 44% (27 fetuses or infants of 62 women) in the placebo group (a difference of 14 percentage points; 95% confidence interval, -3 to 31; P=0.13). There was no significant difference between the two groups or, within each group, between the women who transmitted the virus and those who did not, with respect to levels of virus-specific antibodies, T-cell-mediated immune response, or viral DNA in the blood. The clinical outcome of congenital infection at birth was similar in the two groups. The number of obstetrical adverse events was higher in the hyperimmune globulin group than in the placebo group (13% vs. 2%).

CONCLUSIONS

In this study involving 123 women who could be evaluated, treatment with hyperimmune globulin did not significantly modify the course of primary CMV infection during pregnancy. (Funded by Agenzia Italiana del Farmaco; CHIP ClinicalTrials.gov number, NCT00881517; EudraCT no. 2008-006560-11.).

Using the nonhuman primate model of HCMV to guide vaccine development

The natural history of human cytomegalovirus (HCMV) is inextricably associated with mucosal surfaces. The vast preponderance of primary infections occur following mucosal exposure to infectious virions, and the high seroprevalence of HCMV throughout the world is due to long-term excretion of HCMV in bodily fluids from multiple mucosal sites. Accumulating evidence presents a model where the earliest virus-host interactions following infection dictate the long-term pattern of infection, alter innate immune responses that skew adaptive responses to enable persistence within an immune host, and are essential for reinfection of a host with prior immunity. HCMV has evolved a complex repertoire of viral functions fine-tuned to manipulate the immune environment both locally at the sites of infection and systemically within an infected host. Collectively, viral immune modulation represents a significant impediment for an HCMV vaccine. As HCMV can disseminate beyond mucosal surfaces to reinfect immune hosts, it may not matter whether prior immunity results from prior infection or immunization. A better understanding of the earliest virus-hosts interactions at mucosal surfaces may identify elements of the viral proteome that are especially susceptible to vaccine-mediated disruption and prevent challenge virus from disseminating to distal sites, particularly the maternal-fetal interface.

HCMV infection of human trophoblast progenitor cells of the placenta is neutralized by a human monoclonal antibody to glycoprotein B and not by antibodies to the pentamer complex

Human cytomegalovirus (HCMV) is the major viral cause of congenital infection and birth defects. Primary maternal infection often results in virus transmission, and symptomatic babies can have permanent neurological deficiencies and deafness. Congenital infection can also lead to intrauterine growth restriction, a defect in placental transport. HCMV replicates in primary cytotrophoblasts (CTBs), the specialized cells of the placenta, and inhibits differentiation/invasion. Human trophoblast progenitor cells (TBPCs) give rise to the mature cell types of the chorionic villi, CTBs and multi-nucleated syncytiotrophoblasts (STBs). Here we report that TBPCs are fully permissive for pathogenic and attenuated HCMV strains. Studies with a mutant virus lacking a functional pentamer complex (gH/gL/pUL128-131A) showed that virion entry into TBPCs is independent of the pentamer. In addition, infection is blocked by a potent human neutralizing monoclonal antibody (mAb), TRL345, reactive with glycoprotein B (gB), but not mAbs to the pentamer proteins pUL130/pUL131A. Functional studies revealed that neutralization of infection preserved the capacity of TBPCs to differentiate and assemble into trophospheres composed of CTBs and STBs in vitro. Our results indicate that mAbs to gB protect trophoblast progenitors of the placenta and could be included in antibody treatments developed to suppress congenital infection and prevent disease.

Role of human cytomegalovirus (HCMV)-specific antibody in HCMV-infected pregnant women

Maternal preconception immunity confers substantial protection against HCMV infection and disease to the unborn child. However, the protective role played by single components of virus-specific humoral and cellular immunity is poorly defined. Recently, it was discovered that UL128-131 gene products are essential for the virus to exert endothelial/epithelial cell tropism during natural infection. This, together with the finding that the gH-gL-UL128-131 complex can elicit early, highly potent, and long-lasting neutralizing antibody response as well as other antibodies involved in cell-to-cell spreading and virus transfer from endothelial cells to leukocytes, indicate that antibodies may indeed potentially control virus dissemination in vivo and play a role in mother-to-fetus transmission as well. Additionally, passive immunization of pregnant women with primary HCMV infection has been reported to be highly beneficial for both prevention and therapy of congenital infection in nonrandomized studies. Recently, a phase IIB, randomized, double blind, hyperimmunoglobulin vs placebo trial (CHIP study) showed a lower, although not significant, rate of transmission in the hyperimmunoglobulin arm. Ongoing phase III controlled trials as well as laboratory investigations will hopefully help in better defining the protective role of maternal antibodies.

Identification by mass spectrometry and immune response analysis of guinea pig cytomegalovirus (GPCMV) pentameric complex proteins GP129, 131 and 133

Development of a vaccine against congenital infection with human cytomegalovirus (HCMV) is a major public health priority. A potential vaccine target receiving considerable recent attention is the pentameric complex (PC) of HCMV proteins consisting of gL, gH, UL128, UL130, and UL131, since some antibodies against these target proteins are capable of potently neutralizing virus at epithelial and endothelial cell surfaces. Recently, homologous proteins have been described for guinea pig cytomegalovirus (GPCMV), consisting of gH, gL, and the GPCMV proteins GP129, GP131, and GP133. To investigate these proteins as potential vaccine targets, expression of GP129-GP133 transcripts was confirmed by reverse-transcriptase PCR. Mass spectrometry combined with western blot assays demonstrated the presence of GP129, GP131, and GP133 proteins in virus particles. Recombinant proteins corresponding to these PC proteins were generated in baculovirus, and as GST fusion proteins. Recombinant proteins were noted to be immunoreactive with convalescent sera from infected animals, suggesting that these proteins are recognized in the humoral immune response to GPCMV infection. These analyses support the study of PC-based recombinant vaccines in the GPCMV congenital infection model.

Multiplexed screening of natural humoral immunity identifies antibodies at fine specificity for complex and dynamic viral targets

Viral entry targets with therapeutic neutralizing potential are subject to multiple escape mechanisms, including antigenic drift, immune dominance of functionally irrelevant epitopes, and subtle variations in host cell mechanisms. A surprising finding of recent years is that potent neutralizing antibodies to viral epitopes independent of strain exist, but are poorly represented across the diverse human population. Identifying these antibodies and understanding the biology mediating the specific immune response is thus difficult. An effective strategy for meeting this challenge is to incorporate multiplexed antigen screening into a high throughput survey of the memory B cell repertoire from immune individuals. We used this approach to discover suites of cross-clade antibodies directed to conformational epitopes in the stalk region of the influenza A hemagglutinin (HA) protein and to select high-affinity anti-peptide antibodies to the glycoprotein B (gB) of human cytomegalovirus. In each case, our screens revealed a restricted VH and VL germline usage, including published and previously unidentified gene families. The in vivo evolution of paratope specificity with optimal neutralizing activity was understandable after correlating biological activities with kinetic binding and epitope recognition. Iterative feedback between antigen probe design based on structure and function information with high throughput multiplexed screening demonstrated a generally applicable strategy for efficient identification of safe, native, finely tuned antibodies with the potential for high genetic barriers to viral escape.

Enveloped virus-like particle expression of human cytomegalovirus glycoprotein B antigen induces antibodies with potent and broad neutralizing activity

A prophylactic vaccine to prevent the congenital transmission of human cytomegalovirus (HCMV) in newborns and to reduce life-threatening disease in immunosuppressed recipients of HCMV-infected solid organ transplants is highly desirable. Neutralizing antibodies against HCMV confer significant protection against infection, and glycoprotein B (gB) is a major target of such neutralizing antibodies. However, one shortcoming of past HCMV vaccines may have been their failure to induce high-titer persistent neutralizing antibody responses that prevent the infection of epithelial cells. We used enveloped virus-like particles (eVLPs), in which particles were produced in cells after the expression of murine leukemia virus (MLV) viral matrix protein Gag, to express either full-length CMV gB (gB eVLPs) or the full extracellular domain of CMV gB fused with the transmembrane and cytoplasmic domains from vesicular stomatitis virus (VSV)-G protein (gB-G eVLPs). gB-G-expressing eVLPs induced potent neutralizing antibodies in mice with a much greater propensity toward epithelial cell-neutralizing activity than that induced with soluble recombinant gB protein. An analysis of gB antibody binding titers and T-helper cell responses demonstrated that high neutralizing antibody titers were not simply due to enhanced immunogenicity of the gB-G eVLPs. The cells transiently transfected with gB-G but not gB plasmid formed syncytia, consistent with a prefusion gB conformation like those of infected cells and viral particles. Two of the five gB-G eVLP-induced monoclonal antibodies we examined in detail had neutralizing activities, one of which possessed particularly potent epithelial cell-neutralizing activity. These data differentiate gB-G eVLPs from gB antigens used in the past and support their use in a CMV vaccine candidate with improved neutralizing activity against epithelial cell infection.

Pentameric complex of viral glycoprotein H is the primary target for potent neutralization by a human cytomegalovirus vaccine

Human cytomegalovirus (HCMV) can cause serious morbidity/mortality in transplant patients, and congenital HCMV infection can lead to birth defects. Developing an effective HCMV vaccine is a high medical priority. One of the challenges to the efforts has been our limited understanding of the viral antigens important for protective antibodies. Receptor-mediated viral entry to endothelial/epithelial cells requires a glycoprotein H (gH) complex comprising five viral proteins (gH, gL, UL128, UL130, and UL131). This gH complex is notably missing from HCMV laboratory strains as well as HCMV vaccines previously evaluated in the clinic. To support a unique vaccine concept based on the pentameric gH complex, we established a panel of 45 monoclonal antibodies (mAbs) from a rabbit immunized with an experimental vaccine virus in which the expression of the pentameric gH complex was restored. Over one-half (25 of 45) of the mAbs have neutralizing activity. Interestingly, affinity for an antibody to bind virions was not correlated with its ability to neutralize the virus. Genetic analysis of the 45 mAbs based on their heavy- and light-chain sequences identified at least 26 B-cell linage groups characterized by distinct binding or neutralizing properties. Moreover, neutralizing antibodies possessed longer complementarity-determining region 3 for both heavy and light chains than those with no neutralizing activity. Importantly, potent neutralizing mAbs reacted to the pentameric gH complex but not to gB. Thus, the pentameric gH complex is the primary target for antiviral antibodies by vaccination.

Coding potential of UL/b' from the initial source of rhesus cytomegalovirus Strain 68-1

Rhesus cytomegalovirus (RhCMV) 68-1 is the prototypic strain of RhCMV that has been used for pathogenesis and vaccine development. We determined the complete sequence of the RhCMV 68-1 UL/b' region directly from the original urine from which RhCMV 68-1 was isolated in 1968, and compared it to other RhCMVs. The laboratory passaged RhCMV 68-1 has inversions, deletions, and stop codons in UL/b' that are absent in the original isolate and other low passage RhCMV isolates. Fourteen of the 17 open reading frames (ORFs) in 68-1 UL/b' in the original isolate share >95% amino acid identity with low passage RhCMV. The original isolate retains 6 ORFs that encode α-chemokine-like proteins, including RhUL146 and RhUL146b that share only 92% and 81% amino acid identity, respectively, with a contemporary low passage RhCMV isolate. Identification of the original RhCMV 68-1 UL/b' sequence is important for using RhCMV 68-1 in pathogenesis and vaccine studies.

Antibody inhibition of human cytomegalovirus spread in epithelial cell cultures

Anti-cytomegalovirus (CMV) antibodies reduce the incidence of CMV transmission and ameliorate the severity of CMV-associated disease. Neutralizing activity, measured as the ability of antibodies to prevent entry of cell-free virus, is an important component of natural immunity. However, in vivo CMV amplification may occur mainly via spread between adjacent cells within tissues. Thus, inhibition of cell-to-cell spread may be important when evaluating therapeutic antibodies or humoral responses to infection or immunization. In vitro CMV cell-to-cell spread is largely resistant to antibodies in fibroblast cultures but sensitive in endothelial cell cultures. In the present study antibodies in CMV hyperimmuneglobulin or seropositive human sera inhibited CMV cell-to-cell spread in epithelial cell cultures. Spread inhibition activity was quantitated with a GFP reporter assay employing GFP-tagged epithelialtropic variants of CMV strains Towne or AD169. Measurement of spread inhibition provides an additional parameter for the evaluation of candidate vaccines or immunotherapeutics and to further characterize the role of antibodies in controlling CMV transmission and disease.

Impact of sequence variation in the UL128 locus on production of human cytomegalovirus in fibroblast and epithelial cells

The human cytomegalovirus (HCMV) virion envelope contains a complex consisting of glycoproteins gH and gL plus proteins encoded by the UL128 locus (UL128L): pUL128, pUL130, and pUL131A. UL128L is necessary for efficient infection of myeloid, epithelial, and endothelial cells but limits replication in fibroblasts. Consequently, disrupting mutations in UL128L are rapidly selected when clinical isolates are cultured in fibroblasts. In contrast, bacterial artificial chromosome (BAC)-cloned strains TB40-BAC4, FIX, and TR do not contain overt disruptions in UL128L, yet no virus reconstituted from them has been reported to acquire mutations in UL128L in vitro. We performed BAC mutagenesis and reconstitution experiments to test the hypothesis that these strains contain subtle mutations in UL128L that were acquired during passage prior to BAC cloning. Compared to strain Merlin containing wild-type UL128L, all three strains produced higher yields of cell-free virus. Moreover, TB40-BAC4 and FIX spread cell to cell more rapidly than wild-type Merlin in fibroblasts but more slowly in epithelial cells. The differential growth properties of TB40-BAC4 and FIX (but not TR) were mapped to single-nucleotide substitutions in UL128L. The substitution in TB40-BAC4 reduced the splicing efficiency of UL128, and that in FIX resulted in an amino acid substitution in UL130. Introduction of these substitutions into Merlin dramatically increased yields of cell-free virus and increased cell-to-cell spread in fibroblasts but reduced the abundance of pUL128 in the virion and the efficiency of epithelial cell infection. These substitutions appear to represent mutations in UL128L that permit virus to be propagated in fibroblasts while retaining epithelial cell tropism.

Neutralizing antibodies are unable to inhibit direct viral cell-to-cell spread of human cytomegalovirus

Infection with human cytomegalovirus (CMV) during pregnancy is the most common cause of congenital disorders, and can lead to severe life-long disabilities with associated high cost of care. Since there is no vaccine or effective treatment, current efforts are focused on identifying potent neutralizing antibodies. A panel of CMV monoclonal antibodies identified from patent applications, was synthesized and expressed in order to reproduce data from the literature showing that anti-glycoprotein B antibodies neutralized virus entry into all cell types and that anti-pentameric complex antibodies are highly potent in preventing virus entry into epithelial cells. It had not been established whether antibodies could prevent subsequent rounds of infection that are mediated primarily by direct cell-to-cell transmission. A thorough validation of a plaque reduction assay to monitor cell-to-cell spread led to the conclusion that neutralizing antibodies do not significantly inhibit plaque formation or reduce plaque size when they are added post-infection.

Sensitivity of immune response quality to influenza helix 190 antigen structure displayed on a modular virus-like particle

Biomolecular engineering enables synthesis of improved proteins through synergistic fusion of modules from unrelated biomolecules. Modularization of peptide antigen from an unrelated pathogen for presentation on a modular virus-like particle (VLP) represents a new and promising approach to synthesize safe and efficacious vaccines. Addressing a key knowledge gap in modular VLP engineering, this study investigates the underlying fundamentals affecting the ability of induced antibodies to recognize the native pathogen. Specifically, this quality of immune response is correlated to the peptide antigen module structure. We modularized a helical peptide antigen element, helix 190 (H190) from the influenza hemagglutinin (HA) receptor binding region, for presentation on murine polyomavirus VLP, using two strategies aimed to promote H190 helicity on the VLP. In the first strategy, H190 was flanked by GCN4 structure-promoting elements within the antigen module; in the second, dual H190 copies were arrayed as tandem repeats in the module. Molecular dynamics simulation predicted that tandem repeat arraying would minimize secondary structural deviation of modularized H190 from its native conformation. In vivo testing supported this finding, showing that although both modularization strategies conferred high H190-specific immunogenicity, tandem repeat arraying of H190 led to a strikingly higher immune response quality, as measured by ability to generate antibodies recognizing a recombinant HA domain and split influenza virion. These findings provide new insights into the rational engineering of VLP vaccines, and could ultimately enable safe and efficacious vaccine design as an alternative to conventional approaches necessitating pathogen cultivation.

Recent advances in designing an effective vaccine to prevent cytomegalovirus-associated clinical diseases

It is now well over a decade since the US Institute of Medicine of the National Academy of Sciences assigned the highest priority for a vaccine to prevent congenital human CMV infection, which was subsequently endorsed by the US National Vaccine Program Office. In spite of extensive efforts over many years, successful licensure of a CMV vaccine formulation remains elusive. While the understanding of immune regulation of CMV infection in healthy virus carriers and diseased patients has dramatically improved, traditional vaccine development programs have failed to exploit this knowledge. Until recently, most efforts have concentrated on designing vaccine formulations that block CMV infection through neutralizing antibodies. However, studies carried out in various disease settings, especially in transplant patients, have clearly emphasized the importance of cellular immunity and it is indeed encouraging to see that recent CMV vaccine development programs have started to incorporate this arm of the immune system. A number of new vaccine candidates have been found to be effective in preclinical studies, and are able to induce CMV-specific immune responses in clinical studies, although firm evidence for long-term efficacy is not yet available. For successful implementation of these vaccines in clinical settings, it will be important to demonstrate that the vaccine can induce effective levels of immunity for prevention of transmission of viral infection from mother to unborn baby and thus reduce CMV-related pathogenesis. For transplant recipients, vaccine strategies should be aimed at the induction of immunity that restricts viral reactivation and limits development of disease.

Desirability and feasibility of a vaccine against cytomegalovirus

Publication of a report from the Institute of Medicine in 2000 showing that a vaccine against cytomegalovirus (CMV) would likely be cost saving was very influential and encouraged the clinical evaluation of candidate vaccines. The major objective of a CMV vaccination program would be to reduce disease caused by congenital CMV infection, which is the leading viral cause of sensorineural hearing loss and neurodevelopmental delay. CMV has challenges as a vaccine target because it is a herpesvirus, it persists lifelong despite host immunity, infected individuals can be reinfected with new strains, overt disease occurs in those with immature or impaired immune systems and persons with this infection do not usually report symptoms. Nevertheless, natural immunity against CMV provides some protection against infection and disease, natural history studies have defined the serological and molecular biological techniques needed for endpoints in future clinical trials of vaccines and CMV is not highly communicable, suggesting that it may not be necessary to achieve very high levels of population immunity through vaccination in order to affect transmission. Three phase 2 CMV vaccine studies have been completed in the last 3 years and all report encouraging outcomes. A key international meeting was organized by the Food and Drug Administration in January 2012 at which interested parties from regulatory bodies, industry and academia discussed and prioritised designs for phase 2 and phase 3 clinical trials. Vaccines able to prevent primary infection with CMV and to boost the immune response of those already infected are desirable. The major target populations for a CMV vaccine include women of childbearing age and adolescents. Toddlers represent another potential population, since an effect of vaccine in this age group could potentially decrease transmission to adults. In addition, prospective recipients of transplants and patients with AIDS would be expected to benefit.

Characterization of the guinea pig CMV gH/gL/GP129/GP131/GP133 complex in infection and spread

In human cytomegalovirus (HCMV), the UL128-131A locus plays an essential role in cellular tropism and spread. Here, we report the complete annotation of the GP129-133 locus from guinea pig cytomegalovirus (GPCMV) and the discovery of the UL131A homolog, named GP133. We have found that similar to HCMV the GP129-133 proteins form a pentamer complex with the GPCMV glycoproteins gH and gL. In addition, we find that the GP129-133 proteins play a critical role in entry as the GP129-133 deletion mutant shows a defect in both endothelial and fibroblast cell entry. Although the GP129-133 deletion strain can propagate in vitro, we find that the deletion fails to spread in vivo. Interestingly, the wildtype strain can spontaneously give rise to the GP129-133 deletion strain during in vivo spread, suggesting genetic instability at this locus.

Fetal human cytomegalovirus transmission correlates with delayed maternal antibodies to gH/gL/pUL128-130-131 complex during primary infection

Primary human cytomegalovirus (HCMV) infections during pregnancy are associated with a high risk of virus transmission to the fetus. To identify correlates of intrauterine HCMV transmission, serial serum samples from HCMV transmitter and non-transmitter pregnant women with primary HCMV infection were analyzed for the presence of neutralizing antibodies against different glycoproteins and glycoprotein complexes, which are known to mediate entry into distinct types of host cells. Neutralizing activity was detected in the sera early after primary infection; absorption with a soluble pentameric complex formed by gH/gL/pUL128-131, but not with gH/gL dimer or with gB, abolished the capacity of sera to neutralize infection of epithelial cells. Importantly, an early, high antibody response to pentamer antigenic sites was associated with a significantly reduced risk of HCMV transmission to the fetus. This association is consistent with the high in vitro inhibition of HCMV infection of epithelial/endothelial cells as well as cell-to-cell spreading and virus transfer to leukocytes by anti-pentamer antibodies. Taken together, these findings indicate that the HCMV pentamer complex is a major target of the antibody-mediated maternal immunity.

Immune regulation of human herpesviruses and its implications for human transplantation

Human herpesviruses including cytomegalovirus, Epstein-Barr virus, HHV6, HHV7, HHV8, Herpes simplex virus (HSV)-1 and HSV-2 and varicella zoster virus (VZV) have developed an intricate relationship with the human immune system. This is characterized by the interplay between viral immune evasion mechanisms that promote the establishment of a lifelong persistent infection and the induction of a broad humoral and cellular immune response, which prevents the establishment of viral disease. Understanding the immune parameters that control herpesvirus infection, and the strategies the viruses use to evade immune recognition, has been critical in understanding why immunological dysfunction in transplant patients can lead to disease, and in the development of immunological strategies to prevent and control herpesvirus associated diseases.

Broadly neutralizing antiviral antibodies

A fascinating aspect of viral evolution relates to the ability of viruses to escape the adaptive immune response. The widely held view has been that the great variability of viral glycoproteins would be an absolute obstacle to the development of antibody-based therapies or vaccines that could confer broad and long-lasting protection. In the past five years, new approaches have been developed to interrogate human memory B cells and plasma cells with high efficiency and to isolate several broadly neutralizing antiviral antibodies against highly variable pathogens such as HIV-1 and influenza virus. These antibodies not only provide new tools for prophylaxis and therapy for viral diseases but also identify conserved epitopes that may be used to design new vaccines capable of conferring broader protection.

Applications for a dual fluorescent human cytomegalovirus in the analysis of viral entry

The existence of cell type-specific entry pathways of human cytomegalovirus is an unresolved question as the course of viral entry in different cell types is still not fully understood. To gain more insight into these processes, we generated a dual fluorescent HCMV, where the capsid-associated tegument protein pp150 is labelled with EGFP and the envelope glycoprotein gM with mCherry. This dual labelled virus allows for the separate tracking of the viral envelope fusing with a cellular membrane and the viral capsid during its movement from the cellular membrane to the nucleus. We describe two applications for this virus in the analysis of viral entry: (a) Dynamic live-cell imaging allows for the visualization of viral de-envelopment and transport processes within the living cell. (b) Imaging of cell cultures fixed at different time points after infection enables a more comprehensive statistical analysis of the kinetics of viral entry events such as adsorption, fusion, and nuclear translocation. The techniques are described on the example of fibroblasts and endothelial cells, but can be adapted to other cell types as well. Furthermore, these protocols could provide suggestions for the establishment of live cell applications to other viruses.

Vectored co-delivery of human cytomegalovirus gH and gL proteins elicits potent complement-independent neutralizing antibodies

Human cytomegalovirus (hCMV) is prevalent worldwide with infection generally being asymptomatic. Nevertheless, hCMV infection can lead to significant morbidity and mortality. Primary infection of seronegative women or reactivation/re-infection of seropositive women during pregnancy can result in transmission to the fetus, leading to severe neurological defects. In addition, hCMV is the most common viral infection in immunosuppressed organ transplant recipients and can produce serious complications. Hence, a safe and effective vaccine to prevent hCMV infection is an unmet medical need. Neutralizing antibodies to several hCMV glycoproteins, and complexes thereof, have been identified in individuals following hCMV infection. Interestingly, a portion of the CMV-specific neutralizing antibody responses are directed to epitopes found on glycoprotein complexes but not the individual proteins. Using an alphavirus replicon particle (VRP) vaccine platform, we showed that bicistronic VRPs encoding hCMV gH and gL glycoproteins produce gH/gL complexes in vitro. Furthermore, mice vaccinated with these gH/gL-expressing VRPs produced broadly cross-reactive complement-independent neutralizing antibodies to hCMV. These neutralizing antibody responses were of higher titer than those elicited in mice vaccinated with monocistronic VRPs encoding gH or gL antigens, and they were substantially more potent than those raised by VRPs encoding gB. These findings underscore the utility of co-delivery of glycoprotein components such as gH and gL for eliciting potent, broadly neutralizing immune responses against hCMV, and indicate that the gH/gL complex represents a potential target for future hCMV vaccine development.

Tools to therapeutically harness the human antibody response

The natural human antibody response is a rich source of highly specific, neutralizing and self-tolerant therapeutic reagents. Recent advances have been made in isolating and characterizing monoclonal antibodies that are generated in response to natural infection or vaccination. Studies of the human antibody response have led to the discovery of crucial epitopes that could serve as new targets in vaccine design and in the creation of potentially powerful immunotherapies. With a focus on influenza virus and HIV, herein we summarize the technological tools used to identify and characterize human monoclonal antibodies and describe how these tools might be used to fight infectious diseases.

A vaccine based on the rhesus cytomegalovirus UL128 complex induces broadly neutralizing antibodies in rhesus macaques

Neutralizing antibodies (NAb) are important for interfering with horizontal transmission of human cytomegalovirus (HCMV) leading to primary and congenital HCMV infection. Recent findings have shown that a pentameric virion complex formed by the glycoproteins gH/gL, UL128, UL130, and UL131A (UL128C) is required for HCMV entry into epithelial/endothelial cells (Epi/EC) and is the target of potent NAb in HCMV-seropositive individuals. Using bacterial artificial chromosome technology, we have generated a modified vaccinia Ankara virus (MVA) that stably coexpresses all 5 rhesus CMV (RhCMV) proteins homologous to HCMV UL128C, termed MVA-RhUL128C. Coimmunoprecipitation confirmed the interaction of RhgH with the other 4 RhCMV subunits of the pentameric complex. All 8 RhCMV-naïve rhesus macaques (RM) vaccinated with MVA-RhUL128C developed NAb that blocked infection of monkey kidney epithelial cells (MKE) and rhesus fibroblasts. NAb titers induced by MVA-RhUL128C measured on both cell types at 2 to 6 weeks postvaccination were comparable to levels observed in naturally infected RM. In contrast, MVA expressing a subset of RhUL128C proteins or RhgB glycoprotein only minimally stimulated NAb that inhibited infection of MKE. In addition, following subcutaneous RhCMV challenge at 8 weeks postvaccination, animals vaccinated with MVA-RhUL128C showed reduced plasma viral loads. These results indicate that MVA expressing the RhUL128C induces NAb inhibiting RhCMV entry into both Epi/EC and fibroblasts and limits RhCMV replication in RM. This novel approach is the first step in developing a prophylactic HCMV vaccine designed to interfere with virus entry into major cell types permissive for viral replication, a required property of an effective vaccine.

Vaccination with synthetic constructs expressing cytomegalovirus immunogens is highly T cell immunogenic in mice

There is no licensed vaccine or cure for human cytomegalovirus (CMV), a ubiquitous β-herpesvirus infecting 60-95% of adults worldwide. Infection can cause congenital abnormalities, result in severe disease in immunocompromised patients, and is a major impediment during successful organ transplantation. In addition, it has been associated with numerous inflammatory diseases and cancers, as well as being implicated in the development of essential hypertension, a major risk factor for heart disease. To date, limited data regarding the identification of immunogenic viral targets has frustrated CMV vaccine development. Based upon promising clinical data suggesting an important role for T cells in protecting against disease in the transplantation setting, we designed a novel panel of highly-optimized synthetic vaccines encoding major CMV proteins and evaluated their immune potential in murine studies. Vaccination induced robust CD8+ and CD4+ T cells of great epitopic breadth as extensively analyzed using a novel modified T cell assay described herein. Together with improved levels of CMV-specific T cells as driven by a vaccine, further immune evaluation of each target is warranted. The present model provides an important tool for guiding future immunization strategies against CMV.

Restoration of viral epithelial tropism improves immunogenicity in rabbits and rhesus macaques for a whole virion vaccine of human cytomegalovirus

Maternal immunity to human cytomegalovirus (HCMV) prior to conception is ~70% protective against congenital transmission and in utero infection of HCMV. Both functional antibodies capable of neutralizing virus and effective T-cells are believed to be important for the protection. Previous HCMV vaccines have rarely been shown able to induce neutralizing antibody titers comparable to those seen in naturally infected HCMV seropositive subjects. Recent studies link a glycoprotein H (gH) complex to receptor-mediated viral entry of endothelial/epithelial cells and leukocytes. This pentameric gH complex, composed of five proteins (gH, gL, UL128, UL130 and UL131 proteins), is notably missing in all HCMV vaccine previously evaluated in clinic. Here we showed that a HCMV virus, with restored expression of the pentameric gH complex, can induce 10-fold higher neutralizing antibody titers than an attenuated AD169 virus or a recombinant glycoprotein B vaccine in multiple animal species in which viral replication is not expected. Encouragingly, the peak neutralizing titers post vaccination in rabbits and monkeys were within 2-4-fold of the levels determined in HCMV seropositive subjects. Functional antibodies by vaccination could further be improved when formulated with a novel adjuvant, and the titers of the antiviral antibodies were sustained in rabbits for over a year after vaccination. These results indicate that the pentameric gH complex is associated with greatly improved functional antibodies following vaccination, and support a vaccine concept based on a nonreplicating whole HCMV with the pentameric gH-associated epithelial tropism restored.

Cytomegalovirus impairs cytotrophoblast-induced lymphangiogenesis and vascular remodeling in an in vivo human placentation model

We investigated human cytomegalovirus pathogenesis by comparing infection with the low-passage, endotheliotropic strain VR1814 and the attenuated laboratory strain AD169 in human placental villi as explants in vitro and xenografts transplanted into kidney capsules of SCID mice (ie, mice with severe combined immunodeficiency). In this in vivo human placentation model, human cytotrophoblasts invade the renal parenchyma, remodel resident arteries, and induce a robust lymphangiogenic response. VR1814 replicated in villous and cell column cytotrophoblasts and reduced formation of anchoring villi in vitro. In xenografts, infected cytotrophoblasts had a severely diminished capacity to invade and remodel resident arteries. Infiltrating lymphatic endothelial cells proliferated, aggregated, and failed to form lymphatic vessels. In contrast, AD169 grew poorly in cytotrophoblasts in explants, and anchoring villi formed normally in vitro. Likewise, viral replication was impaired in xenografts, and cytotrophoblasts retained invasive capacity, but some partially remodeled blood vessels incorporated lymphatic endothelial cells and were permeable to blood. The expression of both vascular endothelial growth factor (VEGF)-C and basic fibroblast growth factor increased in VR1814-infected explants, whereas VEGF-A and soluble VEGF receptor-3 increased in those infected with AD169. Our results suggest that viral replication and paracrine factors could undermine vascular remodeling and cytotrophoblast-induced lymphangiogenesis, contributing to bleeding, hypoxia, and edema in pregnancies complicated by congenital human cytomegalovirus infection.

Interplay between human cytomegalovirus and intrinsic/innate host responses: a complex bidirectional relationship

The interaction between human cytomegalovirus (HCMV) and its host is a complex process that begins with viral attachment and entry into host cells, culminating in the development of a specific adaptive response that clears the acute infection but fails to eradicate HCMV. We review the viral and cellular partners that mediate early host responses to HCMV with regard to the interaction between structural components of virions (viral glycoproteins) and cellular receptors (attachment/entry receptors, toll-like receptors, and other nucleic acid sensors) or intrinsic factors (PML, hDaxx, Sp100, viperin, interferon inducible protein 16), the reactions of innate immune cells (antigen presenting cells and natural killer cells), the numerous mechanisms of viral immunoevasion, and the potential exploitation of events that are associated with early phases of virus-host interplay as a therapeutic strategy.

Host immune responses to a viral immune modulating protein: immunogenicity of viral interleukin-10 in rhesus cytomegalovirus-infected rhesus macaques

Background

Considerable evidence has accumulated that multiple viruses, bacteria, and protozoa manipulate interleukin-10 (IL-10)-mediated signaling through the IL-10 receptor (IL-10R) in ways that could enable establishment of a persistent microbial infection. This suggests that inhibition of pathogen targeting of IL-10/IL-10R signaling could prevent microbial persistence. Human cytomegalovirus (HCMV) and rhesus cytomegalovirus (RhCMV) express a viral interleukin-10 (cmvIL-10 and rhcmvIL-10, respectively) with comparable immune modulating properties in vitro to that of their host's cellular IL-10 (cIL-10). A prior study noted that rhcmvIL-10 alters innate and adaptive immunity to RhCMV in vivo, consistent with a central role for rhcmvIL-10 during acute virus-host interactions. Since cmvIL-10 and rhcmvIL-10 are extremely divergent from the cIL-10 of their respective hosts, vaccine-mediated neutralization of their function could inhibit establishment of viral persistence without inhibition of cIL-10.

Methods and Findings

As a prelude to evaluating cmvIL-10-based vaccines in humans, the rhesus macaque model of HCMV was used to interrogate peripheral and mucosal immune responses to rhcmvIL-10 in RhCMV-infected animals. ELISA were used to detect rhcmvIL-10-binding antibodies in plasma and saliva, and an IL-12-based bioassay was used to quantify plasma antibodies that neutralized rhcmvIL-10 function. rhcmvIL-10 is highly immunogenic during RhCMV infection, stimulating high avidity rhcmvIL-10-binding antibodies in the plasma of all infected animals. Most infected animals also exhibited plasma antibodies that partially neutralized rhcmvIL-10 function but did not cross-neutralize the function of rhesus cIL-10. Notably, minimally detectable rhcmvIL-10-binding antibodies were detected in saliva.

Conclusion

This study demonstrates that rhcmvIL-10, as a surrogate for cmvIL-10, is a viable vaccine candidate because (1) it is highly immunogenic during natural RhCMV infection, and (2) neutralizing antibodies to rhcmvIL-10 do not cross-react with rhesus cIL-10. Exceedingly low rhcmvIL-10 antibodies in saliva further suggest that the oral mucosa, which is critical in RhCMV natural history, is associated with suboptimal anti-rhcmvIL-10 antibody responses.

Herpes virus fusion and entry: a story with many characters

Herpesviridae comprise a large family of enveloped DNA viruses all of whom employ orthologs of the same three glycoproteins, gB, gH and gL. Additionally, herpesviruses often employ accessory proteins to bind receptors and/or bind the heterodimer gH/gL or even to determine cell tropism. Sorting out how these proteins function has been resolved to a large extent by structural biology coupled with supporting biochemical and biologic evidence. Together with the G protein of vesicular stomatitis virus, gB is a charter member of the Class III fusion proteins. Unlike VSV G, gB only functions when partnered with gH/gL. However, gH/gL does not resemble any known viral fusion protein and there is evidence that its function is to upregulate the fusogenic activity of gB. In the case of herpes simplex virus, gH/gL itself is upregulated into an active state by the conformational change that occurs when gD, the receptor binding protein, binds one of its receptors. In this review we focus primarily on prototypes of the three subfamilies of herpesviruses. We will present our model for how herpes simplex virus (HSV) regulates fusion in series of highly regulated steps. Our model highlights what is known and also provides a framework to address mechanistic questions about fusion by HSV and herpesviruses in general.