Constitutive proteasomes (c-20S) are ubiquitously-expressed cellular proteases that degrade polyubiquitinated proteins and regulate cell functions. Its inhibitor, bortezomib, is a plasma cell-targeted therapy used in AMR. However, to limit its off-target effects, it is dosed suboptimally. Here, we show that an inducible subset of the proteasomes, immunoproteasomes (IP or i-20S), is enriched in immunocytes such as T and B cells and replace c-20S upon activation. C-20S and i-20S differ only in 3 proteolytic subunits. We hypothesized that i-20S inhibition would target B cell activation, memory B cells, and antibody-secreting cells (ASC) and limit systemic toxicity.

We developed specific, non-covalent, small-molecule b5i inhibitors with improved pharmacokinetic properties, asparagine ethylenediamines (AsnEDAs), which are highly potent and kinetically noncompetitive. We also present a high-resolution cryo-EM structure of the human immunoproteasome with an AsnEDA PKS21004 with a resolution of 3.8Å.

We differentiated human B cells into CD19⁺CD38⁺CD27⁺ ASCs in the presence of IL-2 and TLR-7/8 agonist R848 for five days. These were then exposed to IP inhibition (IPI). The percentage of viable ASCs decreased with IPI treatment in a dose and time-dependent manner, while the PBMCs overall were not affected[mdash]explained by enhanced apoptosis of the ASCs. Apoptosis of CD19⁺ non-ASCs was slightly enhanced, but not that of CD19⁻ cells. Similarly, IPI did not affect the viability of CD19⁻ or CD19⁺ non-ASCs (5 separate experiments, *P<0.05).

C57BL/6 recipients of BALB/c hearts treated with sCTLA4-Ig (250 [micro]g on day 2) and 2-week IPI injections (25 mg/kg/day) showed indefinite heart survival compared to the control (sCTLA4-Ig + vehicle) (MST>100 vs. 30 days, n=6/group, *P<0.05). Activated andmemory B cells, and plasma cells were significantly reduced in the spleen and bone marrow of IPI-treated mice compared to the vehicle (*P<0.05, n=6-8/group). Subsequently, IPI-treatment significantly decreased DSAs measured in the serum of heart allograft recipients (*P<0.05, n=6-8/group).

In all, we designed a novel plasma-cell targeted inhibitor that will have a significant impact on the treatment of AMR.

CITATION INFORMATION: Assaker J., Murakami N., Eskandari S., Uehara M., Nathan C., Li H., Lin G., Azzi J. Designing Novel Selective Immunoproteasome Inhibitors Reveals a Critical Role of the Immunoproteasome in Protecting Antibody Secreting B Cells from Apoptosis Am J Transplant. 2017;17 (suppl 3).

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