PURPOSE. The immunoproteasome is known to generate peptides for antigen presentation. However, it has also been proposed to have additional functions such as stress response. The propensity of the immunoproteasome for degradation of oxidatively damaged proteins and peptides makes it interesting in the context of cataract formation and prevention. This study hypothesized that the immunoproteasome is present in human cataractous lenses and that oxidative stress will induce its expression, affect its proteolytic activity and its intracellular location in native cultured human lens epithelial cells (HLECs). METHODS. The expression of the immunoproteasome and the constitutive proteasome subunits beta i/beta 1, beta 2i/beta 2, and beta 5i/beta 5 were studied by using Western blotting. The chymotrypsin-like activity was investigated for possible oxidative stress response. Inhibitors specific for the immuno-and constitutive proteasome, ONX-0914 and MG-132, respectively, were used to study their relative contributions to total proteasome activity. The intracellular location of the proteasomal subunits beta 5i and beta 5 was studied by immunocytochemistry. RESULTs. Immunoproteasome subunits were detected both in the lens epithelium and in the lens fibers derived from cataract surgery. Oxidative stress to cultured HLECs upregulated the immunoproteasome but not the constitutive proteasome. The chymotrypsin-like activity decreased with increased oxidative stress and the two proteasome types contributed equally to total proteasome activity. Immunocytochemical labeling of subunit beta 5i showed mainly cytosolic localization, whereas subunit beta 5 was localized predominantly in the nucleus. H2O2-induced challenge increased the expression of the immunoproteasome. CONCLUSIONS. The present findings indicate a role for the immunoproteasome in oxidative stress management in the lens.