In vitro rubella virus infections of lymphocytes from normal adult humans impaired their responsiveness to phytohemagglutinin (PHA) stimulations; a situation which seemed analogous to the PHA unresponsiveness of peripheral lymphocytes from babies with the congenital rubella syndrome. Such in vitro viral infection of normal cells also decreased the synthesis of normal nucleic acids and structural proteins, and abrogated the enhanced DNA synthesis induced by pokeweed and specific antigen stimulations. Furthermore, it was shown that live rubella virus, but not ultraviolet-irradiated virus, was necessary for the impaired mitogenic responses of normal leukocytes.
These observations are interpreted to favor the view that the virus achieves its inhibitory effect on the action of mitogens by interference either directly or indirectly at an intracellular site. Such an action could reduce the functional potential of lymphocytes and impair their effectiveness as immunologically competent cells or as effectors in immunologic reactions.