The Antiapoptotic Protein Mcl-1 Controls the Type of Cell Death in Theiler's Virus-Infected BHK-21 Cells

Theiler's murine encephalomyelitis virus (TMEV) results in a persistent central nervous system infection (CNS) and immunemediated demyelination in mice. TMEV largely persists in macrophages (M phi s) in the CNS, and infected M phi s in vitro undergo apoptosis, whereas the infection of other rodent cells produces necrosis. We have found that necrosis is the dominant form of cell death in BeAn virus-infected BHK-21 cells but that similar to 20% of cells undergo apoptosis. Mcl-1 was highly expressed in BHK-21 cells, and protein levels decreased upon infection, consistent with onset of apoptosis. In infected BHK-21 cells in which Mcl-1 expression was knocked down using silencing RNAs there was a 3-fold increase in apoptotic cell death compared to parental cells. The apoptotic program switched on by BeAn virus is similar to that in mouse M phi s, with hallmarks of activation of the intrinsic apoptotic pathway in a tumor suppressor protein p53-dependent manner. Infection of stable Mc1-1-knockdown cells led to restricted virus titers and increased physical to infectious particle (PFU) ratios, with additional data suggesting that a late step in the viral life cycle after viral RNA replication, protein synthesis, and polyprotein processing is affected by apoptosis. Together, these results indicate that Mcl-1 acts as a critical prosurvival factor that protects against apoptosis and allows high yields of infectious virus in BHK-21 cells.