Elucidation of the cross-reactive immunoglobulin M response to human herpesviruses 6 and 7 on the basis of neutralizing antibodies.

Human herpesvirus 6 (HHV-6) is closely related to HHV-7 in terms of genome organization and sequence. The cross-reactive responses between HHV-6 and HHV-7 have been reported by using immunofluorescent techniques. Recently we have shown that neutralizing (NT) antibody responses are specific to each virus and do not cross-react. We took advantage of this and used the NT antibody response to estimate the time of seroconversion to each virus and examined the pattern of humoral immune response, especially the immunoglobulin M (IgM) response, against each virus antigen in the natural course of infection with HHV-6 and HHV-7. In children who experienced HHV-6 infection first, followed by HHV-7 infection, the IgM response at the first HHV-6 infection was directed only against HHV-6, while no IgM response was directed against HHV-7 at the second HHV-7 infection. In contrast, in children who experienced HHV-7 infection first, followed by HHV-6 infection, the IgM response at the first HHV-7 infection was directed not only against HHV-7 but also against HHV-6. These data suggest that cross-reactive responses to heterologous viruses should be taken into consideration when making a diagnosis based on IgM antibody.