Efficiency of transformation of lymphocytes by Epstein-Barr virus.

We explored host cell and viral factors which influence the susceptibility of lymphocytes to immortalization by the Epstein-Barr virus (EBV). We found that immortalization by the B95-8 strain of EBV follows “one-hit” kinetics. With a limiting dilution technique, approximately 1 in 20 virus particles is transforming when assayed on human umbilical cord leukocytes (HUCL). Mixed mononuclear leukocytes from adult human and marmoset blood are 100- and 1000-fold, respectively, less sensitive to immortalization than HUCL. Pretreatment of cord blood leukocytes with phytohemagglutinin results in a 50% increase in their transformability. Lipopolysaccharide (LPS) from Escherichia coli increases the number of transformation events observed by 300–500%. LPS appears to enhance transformation by mechanisms other than stimulation of cellular DNA synthesis. A transformed center assay employing either autochthonous lymphocytes or human placental cells as feeders was used to determine the number of umbilical cord cells which were immortalized. At multiplicities of infection of 30–100 particles per cell, multiplicities which saturate nearly all the susceptible cells, approximately 1 in 200 to 1 in 500 mixed mononuclear cells transformed. When the mixed population of cells was depleted of T lymphocytes, there was a fourfold increase in the number of transformed cells. When correction is made for the plating efficiency of transformed cord blood leukocytes, it is calculated that at least 10% of virus-exposed cells establish permanent lines. This number is probably still an underestimate since we have not yet used a population which is composed entirely of susceptible cells.