MHC class I antigen processing of an Adenovirus CTL epitope is linked to the levels of immunoproteasomes in infected cells

Proteasomes are the major source for the generation of peptides bound by MH C class I molecules, To study the functional relevance of the IFN-gamma-ind ucible proteasome subunits low molecular mass protein 2 (LMP2), LMP7, and m ouse embryonal cell (MEC) ligand 1 in Ag processing and concomitantly that of immunoproteasomes, we established the tetracycline-regulated mouse cell line MEC217, allowing the titrable formation of immunoproteasomes. Infectio n of MEC217 cells with Adenovirus type 5 (Ad5) and analysis of Ag presentat ion with Ad5-specific CTL showed that cells containing immunoproteasomes pr ocessed the viral early 1B protein (E1B)-derived epitope E1B(192-200) with increased efficiency, thus allowing a faster detection of viral entry in in duced cells. Importantly, optimal CTL activation was already achieved at su bmaximal immunosubunit expression. In contrast, digestion of E1B-polypeptid e with purified proteasomes in vitro yielded E1B(192-200) at quantities tha t were proportional to the relative contents of immunosubunits. Our data pr ovide evidence that the IFN-gamma-inducible proteasome subunits, when prese nt at relatively low levels as at initial stages of infection, already incr ease the efficiency of antigenic peptide generation and thereby enhance MHC class I Ag processing in infected cells.