Antibody-mediated neutralization of cytomegalovirus: modulation of efficacy induced through the IgG constant region.

Antibodies can neutralize the infectious properties of human cytomegalovirus (CMV). In vivo, the major neutralization determinants are located on glycoprotein B (gB). Recombinant human antibodies, that carry different constant regions (IgG1, IgG3 and the synthetic variant IgG3mA) against two of these epitopes were investigated for their ability to recruit the complement cascade for destruction of the virus. It was shown that all variants of an antibody against the antigenic domain (AD)-2 epitope displayed a similar neutralization activity despite the fact that improved C1q binding was observed for IgG3 and IgG3mA over the IgG1 variant. In contrast, an antibody against the AD-1 epitope carrying the normal IgG3 constant region, was less efficient than its IgG1 counterpart in neutralizing the virus in the absence of complement. However, it restored its activity in the presence of complement to the level of the naturally occurring IgG1 version. The same antibody was substantially more potent in neutralizing the virus in the presence of complement if it carried the IgG3mA constant region. This demonstrates the importance of the constant domain for the biological activity of AD-1 specific antibodies, a factor that should be taken into account when using antibody-based therapeutics or when inducing antibodies by vaccination.

The antigenic domain 1 of human cytomegalovirus glycoprotein B contains an intramolecular disulphide bond.

Glycoprotein B (gB, gpUL55) is the major antigen recognized by the neutralizing humoral immune response against human cytomegalovirus (HCMV). The immunodominant region on gB is the antigenic domain 1 (AD-1), a complex structure that requires a minimal continuous sequence of more than 75 amino acids (aa 552-635) for antibody binding. In this study, the structural requirements for antibody binding to AD-1 have been determined. The domain was expressed in prokaryotic and eukaryotic systems and analysed in immunoblots under reducing and non-reducing conditions. In addition, AD-1 was purified in an immunologically active form and the concentration of sulphydryl groups was determined. The data clearly show that the only form that is recognized by antibodies is a disulphide-linked monomer of AD-1. The disulphide bond is formed between cysteines at amino acid positions 573 and 610 of gB.

Antigenic domain 1 of human cytomegalovirus glycoprotein B induces a multitude of different antibodies which, when combined, results in incomplete virus neutralization.

Glycoprotein B (gB, gpUL55) is the major antigen for the induction of neutralizing antibodies against human cytomegalovirus (HCMV), making it an attractive molecule for active and passive immunoprophylaxis. The region between aa 552 and 635 of HCMV gB (termed AD-1) has been identified as the immunodominant target for the humoral immune response following natural infection. AD-1 represents a complex domain which requires a minimal continuous sequence of more than 70 aa for antibody binding. Neutralizing as well as non-neutralizing antibodies can bind to AD-1 in a competitive fashion. The fine specificity of AD-1-binding monoclonal antibodies (MAbs) and affinity-purified human polyclonal antibodies was analysed by using recombinant proteins containing single amino acid substitutions spanning the entire AD-1 domain. Our results revealed that all MAbs had individual patterns of binding to the mutant proteins indicating the presence of a considerable number of distinct antibody-binding sites on AD-1. The neutralization capacity of antibodies could not be predicted from their binding pattern to AD-1 mutant proteins. Polyclonal human antibodies purified from different convalescent sera showed identical binding patterns to the mutant proteins suggesting that the combined antibody specificities present in human sera are comparable between individuals. Neutralization capacities of polyclonal human AD-1 antibodies did not exceed 50% indicating that, during natural infection, a considerable proportion of non-neutralizing antibodies are induced and thus might provide an effective mechanism to evade complete virus neutralization.