Evasion of apoptosis by DNA viruses

Apoptosis is a form of cell death distinct from necrosis which plays an important role in processes such as homoeostasis and the elimination of damaged cells. It can be triggered by a variety of stimuli including DNA damage and cytotoxic T lymphocyte activity, both of which may be induced in the course of a viral infection. Initially, induction of apoptosis may occur through pathways which have also been shown to be activated on disturbance of the cell cycle or damage to cellular DNA. At later time points during the course of infection, apoptosis can also be triggered by cytokines and immune effector cells. Apoptosis of the host cell before the completion of the viral replication cycle may limit the number of progeny and the spread of infection. The importance of apoptosis as an antiviral defence is illustrated by the presence of multiple pathways for apoptosis induction and inhibition in both the host and virus. In this review, the inhibition of apoptosis is described in adenovirus and poxvirus infection. These examples illustrate two of the divergent paths by which viruses may avoid the apoptotic response.