Properties of IgG subclasses to human cytomegalovirus

The IgG subclass of antiviral antibodies to human cytomegalovirus (CMV) is mainly of IgG1 type. Most CMV seropositive sera also have virus-specific IgG3, but of a lower titre as measured by enzyme-linked immunosorbent assay (ELISA). We studied the reactivity pattern of these two IgG subclasses to CMV structural polypeptides in order to define how virus-specific IgG1 and IgG3 contribute to the neutralization of CMV. Neutralization of CMV was performed with CMV IgG1 and IgG3 separated from CMV seropositive human sera on a protein A Sepharose gel. Both IgG1 and IgG3 have a neutralizing capacity. IgG3 had a 10-fold better neutralizing effect than IgG1 when related to the ELISA titre. In order to analyse the specific reactivities, CMV Towne virion polypeptides were separated by electrophoresis and transferred to nitrocellulose. Using mouse monoclonals to human IgG1 and IgG3 in combination with a biotin-streptavidin system, the reactivities of the subclasses were examined. IgG1 and IgG3 appeared to react with the same structural polypeptides. The strongest IgG1 reactivities were obtained with CMV polypeptides of apparent molecular weights of 145, 80, 64, 56, 52 and 27.5 kDa. The CMV IgG3 reactivity was restricted compared to IgG1, the strongest and most frequent reactivities occurring to polypeptides of 145 and 80 kDa.