Epstein-Barr virus infection and mitogen stimulation of normal B cells induces wild-type p53 without subsequent growth arrest or apoptosis

Infection of human B lymphocytes with Epstein-Barr virus (EBV) in vitro ind uces a G(0) to G(1) transition followed by DNA synthesis and cell division. The virus activation of the cell cycle closely mimics the antigen-dependen t normal B cell activation pathway. Infected B cells undergo blast transfor mation followed by the emergence of immortalized lymphoblastoid cell lines. Numerous cellular proteins are switched on in the infected cells, includin g p53, In view of the frequent association of wild-type p53 (wtp53) express ion with growth arrest and apoptosis, p53 expression, cell viability (absen ce of apoptosis) and cell cycle progression at the single cell level during the first week after EBV infection were assessed. The rate of EBV infectio n was scored by EBNA-5 staining between 20 and 72 h after infection and var ied between 20 and 25% of the cell population, All EBNA-5-positive blasts w ere p53-positive as well. Double staining for p53 and for DNA ends (TUNEL) revealed that p53-positivity and apoptosis were mutually exclusive. Quantif ication of the DNA content by Hoechst staining and computer-assisted image analysis showed that a fraction of the p53-positive blasts had a DNA conten t higher than 2N, indicating entry into the S/G(2) phases. Double p53 and B rdU staining of the cells, pulse-labelled with BrdU, revealed that 65% of t he p53-positive blasts were in S phase 3 days after infection. Similarly, B cell activation by CD40L and IL-4 induced p53 expression without any adver se effect on cell cycle progression. Therefore, the phenomenon is not EBV-s pecific but correlates with immunoblast activation.