The invention disclosed herein is directed to methods of identifying a polypeptide suitable for epitope liberation including, for example, the steps of identifying an epitope of interest providing a substrate polypeptide sequence including the epitope, wherein the substrate polypeptide permits processing by a proteasome contacting the substrate polypeptide with a composition including the proteasome, under conditions that support processing of the substrate polypeptide by the proteasome and assaying for liberation of the epitope. The invention further relates to vectors including a housekeeping epitope expression cassette. The housekeeping epitope(s) can be derived from a target-associated antigen, and the housekeeping epitope can be liberatable, that is capable of liberation, from a translation product of the cassette by immunoproteasome processing. The invention also relates to a method of activating a T cell comprising contacting a substrate polypeptide with an APC and contacting the APC with a T cell.
