Ontogeny of glycoprotein gB-specific antibody and neutralizing activity during natural cytomegalovirus infection

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.

Comparison of multiple DNA vaccines for protection against cytomegalovirus infection in BALB/c mice

Background

Human cytomegalovirus (HCMV) causes serious HCMV-related diseases in immunocompromised people. Vaccination is the most effective measure to control infection with the pathogen, yet no vaccine has been licensed till now. We performed a head-to-head comparison of the protective abilities of multiple DNA vaccines in murine model of murine cytomegalovirus (MCMV) infection.

Methods

Five DNA vaccines were constructed. Four encoding MCMV proteins gp34 (m04), p65 (M84), DNA helicase (M105), and immediate-early 1 protein pp89 (IE-1) , respectively, which were reported to induce CD8+ T cell responses, were compared with the one expressing gB (M55), the neutralizing antibody target antigen, for immune protection in BALB/c mice. Mice were immunized with these DNA vaccines 1 to 4 times via intramuscular injection followed by electroporation, and were subsequently infected with a lethal dose (3 × LD50) of highly virulent SG-MCMV. Specific antibodies and IFN-γ secreting splenocytes were detected by immunoblotting and ELISPOT, respectively. Protective abilities in mice provided by the vaccines were evaluated by residual virus titers in organs, survival rate and weight loss.

Results

These DNA vaccines, especially m04, M84 and IE-1, could effectively reduce the virus loads in salivary glands and spleens of mice, but they couldn’t completely clear the residual virus. Survival rates of 100% in mice after a lethal dose of MCMV infection could be reached by more than one dose of M84 vaccine or two doses of m04 or IE-1 vaccine. Immunization with M55 or M105 DNA at four doses offered mice only 62.5% survival rate after the lethal challenge.

Conclusions

The study demonstrated that DNA vaccines could effectively afford mice protection against infection with a highly virulent MCMV and that the protection offered by induced CD8+ T cell immunity might be superior to that by gB-specific antibodies. These results are valuable references for development and application of HCMV vaccines.

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.

Update on the current status of cytomegalovirus vaccines

Human cytomegalovirus (HCMV) is ubiquitous in all populations, and is the most commonly recognized cause of congenital viral infection in developed countries. On the basis of the economic costs saved and the improvement in quality of life that could potentially be conferred by a successful vaccine for prevention of congenital HCMV infection, the Institute of Medicine has identified HCMV vaccine development as a major public health priority. An effective vaccine could potentially also be beneficial in preventing or ameliorating HCMV disease in immunocompromised individuals. Although there are no licensed HCMV vaccines currently available, enormous progress has been made in the last decade, as evidenced by the recently reported results of a Phase II trial of a glycoprotein B vaccine for the prevention of HCMV infection in seronegative women of childbearing age. HCMV vaccines currently in clinical trials include: glycoprotein B subunit vaccines; alphavirus replicon particle vaccines; DNA vaccines; and live-attenuated vaccines. A variety of vaccine strategies are also being examined in preclinical systems and animal models of infection. These include: recombinant vesicular stomatitis virus vaccines; recombinant modified vaccinia virus Ankara; replication-deficient adenovirus-vectored vaccines; and recombinant live-attenuated virus vaccines generated by mutagenesis of cloned rodent CMV genomes maintained as bacterial artificial chromosomes in Escherichia coli. In this article, we provide an overview of the current state of clinical trials and preclinical development of vaccines against HCMV, with an emphasis on studies that have been conducted in the past 5 years. We also summarize a number of recent advances in the study of the biology of HCMV, particularly with respect to epithelial and endothelial cell entry of the virus, which have implications for future vaccine design.

Progress toward an elusive goal: current status of cytomegalovirus vaccines

Although infection with human cytomegalovirus (CMV) is ubiquitous and generally asymptomatic in most individuals, certain patient populations are at high risk for CMV-associated disease. These include HIV-infected individuals with AIDS, transplant patients, and newborn infants with congenital CMV infection. Immunity to CMV infection, both in the transplant setting and among women of childbearing age, plays a vital role in the control of CMV-induced injury and disease. Although immunity induced by CMV infection is not completely protective against reinfection, there is nevertheless a sound basis on which to believe that vaccination could help control CMV disease in high-risk patient populations. Evidence from several animal models of CMV infection indicates that a variety of vaccine strategies are capable of inducing immune responses sufficient to protect against CMV-associated illness following viral challenge. Vaccination has also proven effective in improving pregnancy outcomes following CMV challenge of pregnant guinea pigs, providing a 'proof-of-principle' relevant to human clinical trials of CMV vaccines. Although there are no licensed vaccines currently available for human CMV, progress toward this goal has been made, as evidenced by ongoing clinical trial testing of a number of immunization strategies. CMV vaccines currently in various stages of preclinical and clinical testing include: protein subunit vaccines; DNA vaccines; vectored vaccines using viral vectors, such as attenuated pox- and alphaviruses; peptide vaccines; and live attenuated vaccines. This review summarizes some of the obstacles that must be overcome in development of a CMV vaccine, and provides an overview of the current state of preclinical and clinical trial evaluation of vaccines for this important public health problem.

Progress made towards the development of a CMV peptide vaccine

Cytomegalovirus disease still remains a major cause of morbidity and mortality in hematopoietic stem cell transplantation recipients. The cell-mediated immune response is essential in the maintenance of latency and the resolution of primary infections. The identification of immunodominant cytomegalovirus antigens has enabled researchers to determine the best candidate antigens to be included in a cytomegalovirus vaccine. Such a vaccine would have to stimulate both a cell-mediated and humoral immune response. Recent advances have enabled the rapid identification of minimal cytotoxic epitopes required to trigger such responses. Epitope mapping to date has mainly focused on the pp65 antigen but other antigens such as IE1 are starting to be mapped. A human leukocyte antigen allele hierarchy is starting to emerge that is dependent on the alleles present in an individual; this is relevant when considering what peptides should be included in a vaccine. This review looks at the current methods available for epitope mapping and the progress that has been made to date.

Antibody responses to rhesus cytomegalovirus glycoprotein B in naturally infected rhesus macaques

Rhesus cytomegalovirus (RhCMV) exhibits strong parallels with human CMV (HCMV) in terms of nucleic and amino acid identities, natural history, and mechanisms of persistence and pathogenesis in its natural host, rhesus macaques (Macaca mulatta). To determine whether this non-human primate model would be useful to assess vaccine strategies for HCMV, host immune responses to RhCMV glycoprotein B (gB) were evaluated in RhCMV-infected monkeys. Total protein extracts were prepared from cells transiently transfected with an expression plasmid for either the full-length gB or a derivative (gBDelta, 1-680 aa) lacking both the transmembrane domain and cytoplasmic tail. Western blot analysis showed identical reactivity of macaque sera with full-length gB and its derivative gBDelta, indicating that the immunodominant epitopes of gB are contained in the extracellular portion of the protein. Using gBDelta extract as a solid phase, a sensitive and specific ELISA was established to characterize gB antibody responses in monkeys acutely and chronically infected with RhCMV. During primary infection (seroconversion), gB-specific antibodies developed concurrently and in parallel with total RhCMV-specific antibodies. However, during chronic infection gB-specific antibody responses were variable. A strong correlation was observed between neutralizing and gB-specific antibody levels in RhCMV-seropositive monkeys. Taken together, the results of this study indicate that, similar to host humoral responses to HCMV gB, anti-gB antibodies are an integral part of humoral immunity to RhCMV infection and probably play an important protective role in limiting the extent of RhCMV infection. Thus, the rhesus macaque model of HCMV infection is relevant for testing gB-based immune therapies.

Complex formation by human cytomegalovirus glycoproteins M (gpUL100) and N (gpUL73)

The envelope glycoproteins of human cytomegalovirus (HCMV) virions are incompletely characterized. We have analyzed complex formation between glycoprotein M (gM or gpUL100) and a second glycoprotein. gM-homologous proteins are conserved throughout the herpesvirus family and represent type III membrane proteins containing multiple hydrophobic sequences. In extracellular HCMV particles, gM was found to be complexed through disulfide bonds to a second protein with an apparent molecular mass of 50 to 60 kDa. The 50- to 60-kDa protein was found to be derived from reading frame UL73 of HCMV strain AD169. UL73-homologous genes are also conserved within herpesviruses. When transiently expressed by itself, the UL73 gene product consisted of a protein of 18 kDa. However, in the presence of gM, the UL73 gene product was posttranslationally modified to the 50- to 60-kDa species. Thus, gM and the UL73 gene product, which represents the gN homolog of herpesviruses, form a disulfide-linked complex in HCMV virions. Transient expression of gM and gN followed by fluorescence imaging with monoclonal antibodies against either protein demonstrated that complex formation was required for transport of the proteins from the endoplasmic reticulum to the Golgi and trans-Golgi compartments. Finally, we tested the gM-gN complex for reactivity with sera from HCMV-seropositive donors. Whereas most sera failed to react with either gM or gN when expressed alone, 62% of sera were positive for the gM-gN complex. Because a murine monoclonal antibody reactive with gN in the gM-gN complex efficiently neutralizes infectious virus, the gM-gN complex may represent a major antigenic target of antiviral antibody responses.

Early neutralizing and glycoprotein B (gB)-specific antibody responses to human cytomegalovirus (HCMV) in immunocompetent individuals with distinct clinical presentations of primary HCMV infection

BACKGROUND

Antibodies with functional anti-Human Cytomegalovirus (HCMV) activity are likely to be involved in preventing virus dissemination and thus may contribute to minimize the clinical manifestations of infection.

OBJECTIVES

To investigate the role of humoral immunity in modulating the clinical expression of primary Human Cytomegalovirus (HCMV) infection in immunocompetent persons.

STUDY DESIGN

Neutralizing (NA) and glycoprotein B (gB)-specific antibodies were quantitated in acute-phase and late-convalescence phase sera from 19 individuals who developed either HCMV mononucleosis (12) or oligosymptomatic hepatitis (seven).

RESULTS

The levels of NA in sera drawn early after infection were significantly lower in the former patients than in the latter (P=0. 032). This difference was not related to either the total serum IgG levels and anti-HCMV IgGs avidity or to the presence of higher viral loads in blood, as assessed by detecting serum HCMV DNA by PCR, in patients experiencing mononucleosis. Increased NA titers were seen in all available late-convalescence sera. In these sera, median NA levels were not significantly different among the study groups. Antibodies to HCMV gB of both IgG and IgM classes were detected in all acute-phase sera analyzed. Median anti-gB IgG and IgM titers did not differ significantly between study groups. Likewise, the IgG subclass reactivity pattern against gB was found to be similar for both groups.

CONCLUSIONS

The data revealed that an intense and early antibody response to gB developed in patients undergoing primary HCMV infection irrespective of the clinical manifestation of the disease. In contrast, a deficient NA response was observed in patients with HCMV mononucleosis versus that observed in patients displaying a milder form of disease-suggesting that the strength of NA response to HCMV generated early after infection might determine the severity of primary HCMV infection.

Antibody response to human cytomegalovirus (HCMV) glycoprotein B (gB) in AIDS patients with HCMV end-organ disease

Human cytomegalovirus (HCMV)-specific antibody responses in HIV-1 infected individuals either with or without HCMV end-organ disease were examined to determine the whether development of HCMV disease was associated with a particular deficit in the antibody response. Antiwhole HCMV, anti-glycoprotein B (gB), and neutralizing antibody levels were higher in HIV-1 infected individuals than in healthy immunocompetent subjects, particularly in patients with AIDS either with or without HCMV-associated disease. Irrespective of location and spread of HCMV disease, patients who had received anti-HCMV therapy prior to sampling exhibited significantly higher anti-gB and neutralizing antibody titers than those who remained untreated. Likewise, patients with HCMV disease who were antigenemic or viremic had significantly lower anti-gB and neutralizing antibody titers than those who tested negative in either assay. Patients with untreated HCMV disease had significantly lower antibody titers than AIDS patients without disease. Analysis of the IgG subclass antibody responses to gB revealed no significant differences among HIV-1 infected individuals. These results suggest that levels of detectable anti-gB and HCMV neutralizing antibodies are inversely related to systemic viral load. Thus, antibodies with such specificities may be relevant in preventing the establishment of HCMV-associated disease or in modulating its progression.