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Three lymphoblastoid cell lines, of human, squirrel monkey, and marmoset origin, all transformed by the same strain of Epstein-Barr virus (EBV), differed markedly in their content of infectious virus. Single cell clones were obtained from each line to learn whether these differences were dependent upon factors shared by all cells in each line or upon factors present only in a proportion of the total cell population. A total of 17 primary clones were examined: 6 human, 6 squirrel monkey, and 5 marmoset. Cloning efficiency on human placental cell feeder layers varied from 16 to 24%. EBV antiserum, present in the cloning suspension, was shown to neutralize all extracellular virus. 15 of the 17 clones released EBV as measured by the transformation assay. Titers of infectious virus released by daughter clones paralleled titers of virus in the parent line. The median virus titers from human, squirrel monkey, and marmoset clones were respectively 10(1.5), 10(3.0), and 10(4.3) 50% transforming doses per 0.2 ml. The median yield of virus from clones of the three species was, respectively, 4, 96, and 786 transforming units per 1,000 cells containing viral capsid antigen. Two nonproducer clones (one human and one squirrel monkey) did not release infectious virus after treatment with 5'-bromodeoxyuridine, or with X ray followed by co-cultivation with marmoset leukocytes. The nonproducer clones could not be superinfected by biologically active EBV. These results show that differences in production of infectious EBV among the lines tested are reflected in the majority of cells of these lines. The data imply that the mechanism for regulation of the expression of the EBV genome is cellular rather than viral in origin. There are presumably genetic differences among primate species in this regulatory process.

COMPARISON OF THE YIELD OF INFECTIOUS VIRUS FROM CLONES OF HUMAN AND SIMIAN LYMPHOBLASTOID LINES TRANSFORMED BY EPSTEIN-BARR VIRUS