Overexpression of the proteasome subunits LMP2, LMP7, and MECL-1, but not PA28 alpha/beta, enhances the presentation of an immunodominant lymphocyticchoriomeningitis virus T cell epitope

The proteasome is a large protease complex that generates most of the pepti de ligands of MHC class I molecules either in their final form or in the fo rm of N-terminally extended precursors. Upon the stimulation of cells with IFN-gamma, three constitutively expressed subunits of the 20S proteasome ar e replaced by the inducible subunits LMP2 (low-molecular mass polypeptide 2 ), LMP7, and MECL-1 (multicatalytic endopeptidase complex-like-1) to form s o-called immunoproteasomes. We show in this study that overexpression of th ese three subunits in triple transfectants led to a marked enhancement in t he H-2L(d)-restricted presentation of the immunodominant nonameric epitope NP118, which is derived from the nucleoprotein (NP) of lymphocytic choriome ningitis virus. Overexpression of the alpha and beta subunits of the IFN-ga mma-inducible proteasome regulator PA28, in contrast, did not have a compar able effect. In vitro, immunoproteasomes as compared with constitutive prot easomes generated higher amounts of 11- and 12-mer fragments containing the NP118 epitope, These are likely to be cytosolic precursors of NP118, as a proline anchor residue in the second position of NP118 may interfere with T AP-mediated transport, of the nonameric epitope itself. In conclusion, we p rovide evidence that up-regulation of the three inducible subunits, LMP2, L MP7, and MECL-1, can result in a marked improvement of Ag presentation and that, depending on the epitope, PA28 and immunoproteasomes may differential ly affect Ag processing.