Control of cell cycle entry and apoptosis in B lymphocytes infected by Epstein-Barr virus.

Infection of human B cells with Epstein-Barr virus (EBV) results in activation of the cell cycle and cell growth. To interpret the mechanisms by which EBV activates the cell, we have assayed many proteins involved in control of the G0 and G1 phases of the cell cycle and regulation of apoptosis. In EBV infection most of the changes, including the early induction of cyclin D2, are dependent on expression of EBV genes, but an alteration in the E2F-4 profile was partly independent of viral gene expression, presumably occurring in response to signal transduction activated when the virus binds to its receptor, CD21. By comparing the expression of genes controlling apoptosis, including those encoding several members of the BCL-2 family of proteins, the known relative resistance of EBV-immortalized B-cell lines to apoptosis induced by low serum was found to correlate with expression of both BCL-2 and A20. A20 can be regulated by the NF-kappaB transcription factor, which is known to be activated by the EBV LMP-1 protein. Quantitative assays demonstrated a direct temporal relationship between LMP-1 protein levels and active NF-kappaB during the time course of infection.

Cell cycle arrest following exposure of EBV-immortalised B-cells to gamma irradiation correlates with inhibition of cdk2 activity.

The exposure of Epstein-Barr virus immortalised B cells (LCLs) to the genotoxic effects of gamma irradiation causes a decreased proliferation of the cells. The early events in this process have been investigated here. The induction of p53 expression correlates with a cell cycle arrest in the G1 and G2/M phases of the cell cycle within 24 h of exposure. The molecular mechanism governing the decreased proliferation appears to involve the induction of the cyclin dependent kinase (cdk) inhibitor p21CIP1 and its functional association with cdk2.

Epstein-Barr virus exploits the normal cell pathway to regulate Rb activity during the immortalisation of primary B-cells.

Infection of human primary B-lymphocytes with Epstein-Barr virus (EBV) drives quiescent cells into continual proliferation and results in the outgrowth of immortal cell lines. This requires re-programming of the mechanisms that, in the absence of appropriate antigenic stimulation, normally prevent the proliferation of B-lymphocytes. Since the Retinoblastoma protein (pRb) and its relatives, p107 and p130, play critical roles in controlling the mammalian cell division cycle, we have investigated the expression and phosphorylation status of these proteins following EBV immortalisation of primary B-lymphocytes. In this report, we show that EBV drives the hyperphosphorylation of pRb. This is achieved by a strategy involving the altered expression of several components of the signal transduction pathway that normally regulates the phosphorylation status of pRb, including the up regulation of a number of cyclins and cyclin-dependent kinases and the down regulation of a subset of cyclin-dependent kinase inhibitors. The net result is the formation of active cyclin-dependent kinase complexes that are capable of phosphorylating and inactivating pRb. The results presented here identify the activation of a normal signal transduction pathway as an important component of the strategy used by EBV to drive cell proliferation.