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Identification of a Continuous Neutralizing Epitope within UL128 of Human Cytomegalovirus.

ABSTRACT: As human cytomegalovirus (HCMV) is the most common infectious cause of fetal anomalies during pregnancy, development of a vaccine that prevents HCMV infection is considered a global health priority. Although HCMV immune correlates of protection are only poorly defined, neutralizing antibodies (NAb) targeting the envelope pentamer complex (PC) composed of the subunits gH, gL, UL128, UL130, and UL131A are thought to contribute to the prevention of HCMV infection. Here, we describe a continuous target sequence within UL128 that is recognized by a previously isolated potent PC-specific NAb termed 13B5. By using peptide-based scanning procedures, we identified a 13-amino-acid-long target sequence at the UL128 C terminus that binds the 13B5 antibody with an affinity similar to that of the purified PC. In addition, the 13B5 binding site is universally conserved in HCMV, contains a previously described UL128/gL interaction site, and interferes with the 13B5 neutralizing function, indicating that the 13B5 epitope sequence is located within the PC at a site of critical importance for HCMV neutralization. Vaccination of mice with peptides containing the 13B5 target sequence resulted in the robust stimulation of binding antibodies and, in a subset of immunized animals, in the induction of detectable NAb, supporting that the identified 13B5 target sequence constitutes a PC-specific neutralizing epitope. These findings provide evidence for the discovery of a continuous neutralizing epitope within the UL128 subunit of the PC that could be an important target of humoral immune responses that are involved in protection against congenital HCMV infection.IMPORTANCE Neutralizing antibodies (NAb) targeting the human cytomegalovirus (HCMV) envelope pentamer complex (PC) are thought to be important for preventing HCMV transmission from the mother to the fetus, thereby mitigating severe developmental disabilities in newborns. However, the epitope sequences within the PC that are recognized by these potentially protective antibody responses are only poorly defined. Here, we provide evidence for the existence of a highly conserved, continuous, PC-specific epitope sequence that appears to be located within the PC at a subunit interaction site of critical importance for HCMV neutralization. These discoveries provide insights into a continuous PC-specific neutralizing epitope, which could be an important target for a vaccine formulation to interfere with congenital HCMV infection.

SUBMITTER: Chiuppesi F

PROVIDER: S-EPMC5331811 | biostudies-other | 2017 Mar

REPOSITORIES: biostudies-other

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Project description:UnlabelledHuman cytomegalovirus (HCMV) is an important, ubiquitous pathogen that causes severe clinical disease in immunocompromised individuals, such as organ transplant recipients and infants infected in utero. The envelope glycoprotein B (gB) of HCMV is a major antigen for the induction of virus-neutralizing antibodies. We have begun to define target structures within gB that are recognized by virus-neutralizing antibodies. Antigenic domain 5 (AD-5) of gB has been identified as an important target for neutralizing antibodies in studies using human monoclonal antibodies (MAbs). Anti-AD-5 MAbs share a target site on gB, despite originating from different, healthy, HCMV-infected donors. Mutational analysis of AD-5 identified tyrosine 280 in combination with other surface-exposed residues (the YNND epitope) as critical for antibody binding. The YNND epitope is strictly conserved among different HCMV strains. Recombinant viruses carrying YNND mutations in AD-5 were resistant to virus-neutralizing MAbs. Competition enzyme-linked immunosorbent assays (ELISAs) with human HCMV-convalescent-phase sera from unselected donors confirmed the conserved antibody response for the YNND epitope in HCMV-infected individuals and, because a significant fraction of the gB AD-5 response was directed against the YNND epitope, further argued that this epitope is a major target of anti-AD-5 antibody responses. In addition, affinity-purified polyclonal anti-AD-5 antibodies prepared from individual sera showed reactivity to AD-5 and neutralization activity toward gB mutant viruses that were similar to those of AD-5-specific MAbs. Taken together, our data indicate that the YNND epitope represents an important target for anti-gB antibody responses as well as for anti-AD-5 virus-neutralizing antibodies.ImportanceHCMV is a major global health concern, and a vaccine to prevent HCMV disease is a widely recognized medical need. Glycoprotein B of HCMV is an important target for neutralizing antibodies and hence an interesting molecule for intervention strategies, e.g., vaccination. Mapping the target structures of neutralizing antibodies induced by naturally occurring HCMV infection can aid in defining the properties required for a protective capacity of vaccine antigens. The data presented here extend our knowledge of neutralizing epitopes within gB to include AD-5. Collectively, our data will contribute to optimal vaccine design and development of antibody-based therapies.

Project description:Human cytomegalovirus (HCMV) is a ubiquitous pathogen that causes severe clinical disease in immunosuppressed patients and congenitally infected newborn infants. Viral envelope glycoproteins represent attractive targets for vaccination or passive immunotherapy. To extend the knowledge of mechanisms of virus neutralization, monoclonal antibodies (MAbs) were generated following immunization of mice with HCMV virions. Hybridoma supernatants were screened for in vitro neutralization activity, yielding three potent MAbs, 6E3, 3C11, and 2B10. MAbs 6E3 and 3C11 blocked infection of all viral strains that were tested, while MAb 2B10 neutralized only 50% of the HCMV strains analyzed. Characterization of the MAbs using indirect immunofluorescence analyses demonstrated their reactivity with recombinantly derived gH. While MAbs 6E3 and 3C11 reacted with gH when expressed alone, 2B10 detected gH only when it was coexpressed with gB and gL. Recognition of gH by 3C11 was dependent on the expression of the entire ectodomain of gH, whereas 6E3 required residues 1 to 629 of gH. The strain-specific determinant for neutralization by Mab 2B10 was identified as a single Met→Ile amino acid polymorphism within gH, located within the central part of the protein. The polymorphism is evenly distributed among described HCMV strains. The 2B10 epitope thus represents a novel strain-specific antibody target site on gH of HCMV. The dependence of the reactivity of 2B10 on the simultaneous presence of gB/gH/gL will be of value in the structural definition of this tripartite complex. The 2B10 epitope may also represent a valuable tool for diagnostics to monitor infections/reinfections with different HCMV strains during pregnancy or after transplantation. IMPORTANCE HCMV infections are life threatening to people with compromised or immature immune systems. Understanding the antiviral antibody repertoire induced during HCMV infection is a necessary prerequisite to define protective antibody responses. Here, we report three novel anti-gH MAbs that potently neutralized HCMV infectivity. One of these MAbs (2B10) targets a novel strain-specific conformational epitope on gH that only becomes accessible upon coexpression of the minimal fusion machinery gB/gH/gL. Strain specificity is dependent on a single amino acid polymorphism within gH. Our data highlight the importance of strain-specific neutralizing antibody responses against HCMV. The 2B10 epitope may also represent a valuable tool for diagnostics to monitor infections/reinfections with different HCMV strains during pregnancy or after transplantation. In addition, the dependence of the reactivity of 2B10 on the simultaneous presence of gB/gH/gL will be of value in the structural definition of this tripartite complex.

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