Under certain culture conditions, a nuclear antigen, apparently the Epstein-Barr (EB) virus-associated nuclear antigen (EBNA) was detected in leucocytes three days after infection with the QIMR-WIL strain of EB virus. Proliferation of these EBNA-positive cells was confirmed by five days and by two weeks the majority of the cells in the culture were EBNA-positive. These initial events were influenced by the serological status of the leucocyte donor and the presence of adult fibroblasts (inhibition of transformation) and subsequent events by the serological status of the leucocyte donor and initial cell concentration (regression of transformation).
Transformation of a non-adherent leucocyte population from donors with antibody to EB virus (seropositive donors) was reversibly inhibited on adult human fibroblasts. EBNA was detected in inhibited cultures at a maximum level two weeks post-infection after which it gradually decreased. Reversal of inhibition at 28 days either by addition of phytohaemagglutinin or by removal of the leucocytes from the adult fibroblasts, resulted in a prompt increase in the percentage of EBNA-positive cells and typical lymphoblastoid outgrowth. Inhibition was independent of virus multiplicity and of the serological status of the fibroblast donor but was strictly dependent on the serological status of the leucocyte donor. The results indicate that in the EB virus-infection of non-adherent leucocytes from seropositive donors on adult fibroblasts a block, resulting in the inhibition of transformation, occurs between the production of EBNA and the onset of autonomous proliferation of infected cells.
EB virus-infected leucocytes from nine adult seronegative donors, 12 cord blood donors and 19 seropositive donors cultured at low cell concentration showed an increase in EBNA-positive cells during a four week observation period and readily yielded cell lines on subculture at any time during this period. In contrast, leucocytes from 18 adult seropositive donors cultured at high cell concentration developed foci within the first 1-2 weeks but thereafter regressed completely and subcultures made at four weeks never gave rise to cell lines. Fractionation of leucocytes into resetting and non-rosetting fractions showed that the effect was strictly dependent on the presence in the cultures of resetting cells from the seropositive donor. The strength of regression in a leucocyte preparation could be assayed in terms of the proportion of resetting cells which must be added to EB virus-infected non-rosetting cells in order to establish regression in the culture. Regression was shown to be independent of the presence of monocytes, was not mediated by soluble factors, and did not require the presence of viral envelope material on the surface of virus-infected B cells. The results show that regression is an in vitro expression of T cell-mediated immunity to EB virus which the majority if not all seropositive individuals possess.