Primary CD8 + T cell (T CD8+ ) responses to viruses are directed toward multiple Ags and shaped by both the level of Ag presentation and the underlying Ag-specific T CD8+ repertoire. The relative importance of these factors in deciding the hierarchy of T CD8+ responses and how they are influenced by the immunoproteasome are not well understood. Using an influenza infection model in mice deficient in various immunoproteasome subunits, we observe that Ag presentation and T CD8+ repertoire are altered in an epitope-specific and immunoproteasome subunit-dependent manner. More importantly, we find that the level of Ag presentation and the extent of the underlying repertoire can work either alone or in concert to determine definitively the magnitude of the individual T CD8+ responses and hence the overall T CD8+ hierarchy. Together, these results provide a clearer understanding of how immunodominance hierarchies are established.