Analysis of antigenic properties of human herpesvirus 7 and the host immune responses

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To elucidate antigenic properties of human herpesviruses 6 and 7 (HHV-6 and -7), three monoclonal antibodies (Mabs) against HHV-6 and 13Mabs against HHV-7 were established and characterized by radio-immunoprecipitation. The 40-kDa phosphoprotein by recognized by a Mab (TK17) was the product of HHV-7 U27 gene. Among these Mabs, a Mab(IK3) which recognizes the 135-kDa late polypeptide of HHV-6 and several Mabs (TK3 and others) which recognize the 125-kDa late polypeptide of HHV-7 were selected to monitor virus growth by a dot blot antigen-detection method. Using the dot blot method, we established a neutralizing antibody assay for these viruses. The dot-blot neutralization assay is easy to perform, is highly reproducible and objective when compared with the conventional methods based on cytopathology or immunofluorescence for determining neutralization endpoints. Using this method, 55 sera from healthy adults were examined. In most individuals, neutralizing antibody titers against HHV-7 were much higher than those against HHV-6. Elevated neutralizing titers against HHV-7 might be attributed to continuous booster effects by persistent HHV-7 production in saliva. Furthermore, we monitored neutralizing antibody titers against these viruses in 15 children with documented history of exanthem subitum. Transferred antibody titers against these viruses from mothers were readily demonstrated by the neutralization test. Transferred antibody titers against HHV-7 were higher and remained longer after birth than those of HHV-6, and these findings were in accord with clinical observation that HHV-6 infection usually occurs earlier than HHV-7 infection. It is notable that no cross-reactive elevation of heterologous neutralizing antibody titer was observed.