The proteasome controls diverse cellular functions, among them signal transduction for inflammatory cytokine release, antigen presentation, and the ability of plasma cells to secrete antibodies without dying from accumulation of misfolded immunoglobulins. Thus the proteasome could be an attractive target for treating autoimmune disease. However, it would be necessary to restrict proteasome inhibition to cells of the immune system, because inhibition of proteasomes in all cells is toxic. I have pioneered in the development of inhibitors that target certain proteasomes but not others. In my past work, I introduced the first inhibitors of bacterial proteasomes that spared those of the host. In the present application, I have developed inhibitors that are highly specific for host immunoproteasomes over host constitutive proteasomes. These inhibitors could open a new path to the treatment of autoimmune disorders.

The proteasome that is constitutively expressed in the cytoplasm and nucleus of eukaryotic cells is responsible for the degradation of the majority of cellular proteins. Inside the proteasome reside two copies each of three proteases with distinct specificities, called ß1 (caspase-like), ß2 (tryptic-like) and ß5 (chymotryptic like). However, lymphocytes in general and cells that respond to interferon-? express the immunoproteasome, in which the proteases are the products of different genes: ß1i, ß2i and ß5i. Mice with combined deficiency ß1i, ß2i and ß5i are viable, fertile and healthy but express a different antigenic peptide repertoire than wild type mice. These findings suggest that selective inhibition of the immunoproteasome would impact the immune system but would otherwise be largely nontoxic.

The proteasome inhibitor first approved by the FDA for treatment of malignancy, Bortezomib, inhibits both the constitutive proteasome and the immunoproteasome. In mice with autoimmune disease, Bortezomib reduced the severity of glomerulonephritis, plasma cell proliferation, ds-DNA autoantibody formation and IFN-a production by TLR-activated plasmacytoid dendritic cells. However, such a broad-spectrum inhibitor is too toxic for treatment of non-malignant disease. Inhibitors that are selective for the immunoproteasome are expected to be equally if not more efficacious in treating autoimmune disease, and less toxic. An inhibitor called ONX 0914 that is relatively selective for the immunoproteasome has undergone preclinical studies, but it acts irreversibly and was still toxic. Here I introduce the first inhibitors that act both with high selectivity and with full reversibility on the human immunoproteasome. This application proposes to characterize these compounds further and test them in mice with lupus-like disease.