The proteasome, a multicatalytic protease, is responsible for the generation of most MHC class I ligands. Bortezomib, a proteasome inhibitor, is clinically approved for treatment of multiple myeloma and mantle cell myeloma. In the present study, we investigated the effect of bortezomib on viral infection. Infection of bortezomib-treated mice with the lymphocytic choriomeningitis virus (LCMV) led to a decreased cytotoxic T cell response to several LCMV-derived CD8+ T cell epitopes. Bortezomib treatment caused a reduced expansion of CD8+ T lymphocytes and increased viral titers in LCMV-infected mice. Administration of bortezomib during expansion of CD8+ T cells had no influence on the cytotoxic T cell response, suggesting that bortezomib interferes with priming of naive T cells. Indeed, determination of Ag load in spleen 4 days post infection, revealed a reduced presentation of LCMV-derived cytotoxic T cell epitopes on MHC class I molecules. In summary, we show that proteasome inhibition with bortezomib led to an increased susceptibility to viral infection, and demonstrate for the first time, that proteasome inhibitors can alter Ag processing in vivo.