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 neutr alizing antibodies against human cytomegalovirus (HCMV), making it an attra ctive molecule for active and passive immunoprophylaxis, The region between aa 552 and 635 of HCMV gB (termed AD-1) has been identified as the immunod ominant target for the humoral immune response following natural infection. AD-1 represents a complex domain which requires a minimal continuous seque nce of more than 70 aa for antibody binding, Neutralizing as well as non-ne utralizing antibodies can bind to AD-1 in a competitive fashion. The fine s pecificity of AD-1-binding monoclonal antibodies (MAbs) and affinity-purifi ed human polyclonal antibodies was analysed by using recombinant proteins c ontaining single amino acid substitutions spanning the entire AD-1 domain. Our results revealed that all MAbs had individual patterns of binding to th e mutant proteins indicating the presence of a considerable number of disti nct antibody-binding sites on AD-1. The neutralization capacity of antibodi es could not be predicted from their binding pattern to AD-1 mutant protein s. Polyclonal human antibodies purified from different convalescent sera sh owed identical binding patterns to the mutant proteins suggesting that the combined antibody specificities present in human sera are comparable betwee n individuals. Neutralization capacities of polyclonal human AD-1 antibodie s did not exceed 50% indicating that, during natural infection, a considera ble proportion of non-neutralizing antibodies are induced and thus might pr ovide an effective mechanism to evade complete virus neutralization.