During Infection, Theiler's Virions Are Cleaved by Caspases and Disassembled into Pentamers.

Infected macrophages in spinal cords of mice persistently infected with Theiler’s murine encephalomyelitis virus (TMEV) undergo apoptosis, resulting in restricted virus yields, as do infected macrophages in culture. Apoptosis of murine macrophages in culture occurs via the intrinsic pathway later in infection (>10 h postinfection [p.i.]) after maximal virus titers (150 to 200 PFU/ cell) have been reached, with loss of most infectious virus (<5 PFU/cell) by 20 to 24 h p.i. Here, we show that BeAn virus RNA replication, translation, polyprotein processing into final protein products, and assembly of protomers and pentamers in infected M1-D macrophages did not differ from those processes in TMEV-infected BHK-21 cells, which undergo necroptosis. However, the initial difference from BHK-21 cell infection was seen at 10 to 12 h p.i., where virions from the 160S peak in sucrose gradients had incompletely processed VP0 (compared to that in infected BHK-21 cells). Thereafter, there was a gradual loss of the 160S virion peak in sucrose gradients, with replacement by a 216S peak that was observed to contain pentamers among lipid debris in negatively stained grids by electron microscopy. After infection or incubation of purified virions with activated caspase-3 in vitro, 13- and 17-kDa capsid peptide fragments were observed and were predicted by algorithms to contain cleavage sites within proteins by cysteine-dependent aspartate-directed proteases. These findings suggest that caspase cleavage of sites in exposed capsid loops of assembled virions occurs contemporaneously with the onset and progression of apoptosis later in the infection.