Activation of measles virus from silently infected human lymphocytes.

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Abstract
Lymphocytes were incubated with measles virus for 4 days in the absence of a lymphocyte stimulating agent. Such nonstimulated lymphocytes, infected with measles virus, did not express the virus antigens that are detectable by cytotoxic antibodies. Approximately 1 of 5000, or even fewer, of such lymphocytes produced virus as demonstrated by the infectious center assay; in the supernate only 10-100 infectious viruses/ml were detected. No virus structures could be observed by EM. Such lymphocytes showed no reaction to phytohemagglutinin (PHA) in terms of DNA synthesis in a subsequent culture in the presence of antibodies against measles virus to prevent spreading of the infection to other cells. Although stimulation by PHA did not result in a significant increase in [3H]-thymidine incorporation, measles virus was activated; 32 h after the addition of PHA nearly 80% of the cells were killed by measles virus antibodies and complement. The number of virus-producing cells increased to approximately 1 in 300 or more, and at 72 h the virus titer in the supernate rose to 106 infectious particles/ml. This reactivation of mealses virus was still obtained when PHA was added as late as 8 or more days after the initial infection.