THE PROTEASOME-SPECIFIC INHIBITOR LACTACYSTIN BLOCKS PRESENTATION OF CYTOTOXIC T-LYMPHOCYTE EPITOPES IN HUMAN AND MURINE CELLS

We describe the effect of the proteasome specific inhibitor lactacysti n on the metabolic stability of influenza nucleoprotein (NP) and on th e generation of antigens presented by human and murine class I molecul es of the major histocompatibility complex to cytotoxic T lymphocytes (CTL). We show that cells treated with lactacystin fail to present inf luenza antigens to influenza-specific CTL, but retain the capacity to present defined epitopes expressed as peptides intracellularly by reco mbinant vaccinia viruses. This block in antigen presentation can be ov ercome by expressing the viral protein within the lumen of the endopla smic reticulum, confirming the specificity of lactacystin for cytosoli c proteases. We also show that the effect of lactacystin on antigen pr esentation correlates with the block of breakdown of a rapidly degrade d form of the influenza NP linked to ubiquitin. These results demonstr ate that proteasome dependent degradation plays an important role in t he cytosolic generation of CTL epitopes.