Round cell variant of measles virus: spontaneous conversion from productive to cell-associated state of infection.

A subacute sclerosing panencephalitis strain of measles virus was found to be composed of two viral variants distinguishable by their cytopathic effects in Vero cells. One of the variants was similar to defective cell-associated measles virus strains, whereas the other was highly productive of viral progeny but did not induce cell fusion. Cloning of the variants by an agarose plaque procedure revealed a consistent and rapid interconversion of the variants into one another. While the mechanism of this interconversion remains unknown, analysis of the expression of viral antigens by the variants using indirect immunofluorescence with monoclonal antibodies specific for measles structural antigens suggested that the interconversion involved modulation of the expression of the viral matrix or M antigen.