Immunoproteasome Upregulation Is Not Required to Control Protein Homeostasis during Viral Infection

The prime function of proteasomes is the control of protein homeostasis in cells (i.e., the removal of proteins that are not properly folded, damaged by stress conditions like reactive oxygen species formation, or degraded on the basis of regular protein turnover). During viral infection, the standard proteasome is replaced by the so-called immunoproteasome (IP) in an IFN-γ-dependent manner. It has been proposed that the IP is required to protect cell viability under conditions of IFN-induced oxidative stress. In this study, we investigated the requirement for IP to cope with the enhanced need for protein degradation during lymphocytic choriomeningitis virus (LCMV) infection in mice lacking the IP subunit LMP7. We found that IP are upregulated in the liver but not in the spleen during LCMV infection, although the total proteasome content was not altered. The expression of standard proteasome subunits is not induced in LMP7-deficient mice, indicating that enhanced proteasomal activity is not required during viral infection. Furthermore, ubiquitin accumulation, apoptosis induction, and viral titers were similar in LCMV-infected mice lacking LMP7 compared with wild-type mice. Taken together, these data indicate that the IP is not required to regulate protein homeostasis during LCMV infection.