Age dependence of viral expression. Electron microscopic and immunoperoxidase studies of Measles virus replication in mice.

The hamster neurotropic strain of measles virus inoculated intracerebrally into BALB/c mice causes a uniformly fatal encephalitis with production of infectious virus in 1- to 2-day-old suckling mice but a 31 per cent mortality with production of measles virus antigen without production of infectious virus in 4-week-weanling mice (8). These two groups of animals were studied using routine electron microscopy and the immunoperoxidase technique in order to determine the nature of the measles virus antigen produced in each situation. Suckling animals showed numerous cytoplasmic viral inclusions in neurons and glia. This nucleocapsid material stained specifically with rabbit antiserum to measles virus antigen overlayed with peroxidase-conjugated goat antirabbit gamma-globulin. Histologic and electron microscopic study of weanling mice revealed rare necrotic cells and occasional loose clusters of neurons with depolymerized ribosomes. Immunoperoxidase staining showed diffuse cytoplasmic staining of similar groups of neurons and dendritic processes without evidence of nucleocapsid material. These differences in virus replication appear to be a function of the maturation of the host cell rather than the measles virus. Hamster neurotropic virus infection of the weanling mouse is an example of a potentially fatal virus infection in which viral replication is defective at a stage prior to assembly of nucleocapsid material; thus, no direct morphologic evidence that the cause of the clinical disease is a viral infection is present.