Immunoproteasome Inhibition Selectively Kills Human CD14⁺ Monocytes and as a Result Dampens IL-23 Secretion

MECL-1 (β2i), LMP2 (β1i), and LMP7 (β5i) are the proteolytically active subunits of the immunoproteasome (IP), a special type of proteasome mainly expressed in hematopoietic cells. Targeting the IP in autoimmune diseases proved to be therapeutically effective in preclinical mouse models. In endotoxin-stimulated human PBMCs, IP inhibition reduces the secretion of several proinflammatory cytokines, with the suppression of IL-23 being the most prominent. In this study, we investigated why the production of IL-23, a key mediator of inflammation in autoimmunity, is blocked when the IP is inhibited in LPS-stimulated human PBMCs. CD14⁺ monocytes could be identified as the main producers of IL-23 in LPS-stimulated PBMCs. We found that IP inhibition with the irreversible LMP7/LMP2 inhibitor ONX 0914 induced apoptosis in CD14⁺ monocytes, whereas CD4⁺, CD3⁺, CD19⁺, and CD56⁺ cells remained unaffected. A high expression of IPs renders monocytes susceptible to IP inhibition, leading to an accumulation of polyubiquitylated proteins and the induction of the unfolded protein response. Similar to IP inhibition, inducers of the unfolded protein response selectively kill CD14⁺ monocytes in human PBMCs. The blockage of the translation in CD14⁺ monocytes protects these cells from ONX 0914-induced cell death, indicating that the IP is required to maintain protein turnover in monocytes. Taken together, our data reveal why IP inhibition is particularly effective in the suppression of IL-23-driven autoimmunity.