RNA polymerase II inhibitors dissociate antigenic peptide generation from normal viral protein synthesis: a role for nuclear translation in defective ribosomal product synthesis?

Following viral infection, cells rapidly present peptides from newly synthesized viral proteins on MHC class I molecules, likely from rapidly degraded forms of nascent proteins. The nature of these defective ribosomal products (DRiPs) remains largely undefined. Using inhibitors of RNA polymerase II that block influenza A virus neuraminidase (NA) mRNA export from the nucleus and inhibit cytoplasmic NA translation, we demonstrate a surprising disconnect between levels of NA translation and generation of SIINFEKL peptide genetically inserted into the NA stalk. A 33-fold reduction in NA expression is accompanied by only a 5-fold reduction in K(b)-SIINFEKL complex cell-surface expression, resulting in a net 6-fold increase in the overall efficiency of Ag presentation. Although the proteasome inhibitor MG132 completely blocked K(b)-SIINFEKL complex generation, we were unable to biochemically detect a MG132-dependent cohort of NA DRiPs relevant for Ag processing, suggesting that a minute population of DRiPs is a highly efficient source of antigenic peptides. These data support the idea that Ag processing uses compartmentalized translation, perhaps even in the nucleus itself, to increase the efficiency of the generation of class I peptide ligands.