The constitutive Proteasomes (c-20S) are ubiquitously expressed cellular proteases that degrade polyubiquitinated proteins and regulate cell functions. However, immunoproteasomes (IP or i-20S) are primarily expressed in immune cells. C-20S and i-20S differ only in the 3 proteolytic β subunits. We show here that in a murine heart transplant model, effector and memory T and B cells upregulate their expression of i-20S. We hypothesized that i-20S inhibition would target memory T and B cells and suppress alloimmunity without the toxicity of inhibiting c-20S.

We designed and synthesized a specific, non-covalent, small molecule inhibitor (DPLG3) of the i-20S β5i subunits. DPLG3 inhibited mouse i-20S with IC50 of 9.4 nM and 1500-fold selectivity over mouse c-20S, and inhibited human i-20S with an IC50 of 4.5 nM and 7000-fold selectivity over human c-20S. C57BL/6 recipients of BALB/c hearts treated with low dose CTLA4Ig (250 [micro]g on day 2) and 14 days injection of DPLG3 (25 mg/kg/day) exhibited indefinite heart survival compared to control (CTLA4Ig + vehicle) (MST >100 and 30 days respectively, n=6/group, p<0.05). DPLG3 significantly reduced effector CD4⁺ and CD8⁺ T cells (T_eff) coupled with a marked increase in regulatory T cells (Tregs) in spleen and draining LN. Furthermore, T_eff in DPLG3-treated mice acquired an exhaustion phenotype with an increased expression of PD1, TIM3 and LAG3, lower Th1, Th17 cytokines and lower GrB expression in CD8⁺ T_eff (P<0.05, n=8 mice/group). Furthermore, RAG^-/- mice recipients of BALB/c skin received 10 x 10⁶ CD3 Foxp3-GFP negative cells. DPLG3 treated mice (25 mg/kg/d for 7 d) showed 5-fold increase in induced Tregs in vivo. Furthermore, CD4 and CD8 T_eff exhibited more exhaustion phenotype and more apoptosis in DPLG3-treated mice (p<0.05, n= 8 mice/group). Similar results were shown in vitro. Furthermore, CD4⁺, CD8⁺ T_eff and Treg cells were sorted from DPLG3-treated and vehicle-treated mice and whole RNA seq analysis is underway.

A novel, potent, non-covalent, highly selective IP inhibitor revealed a critical role for the IP in protecting T cells against exhaustion and inhibiting induction of Tregs cells in response to alloimmune activation.

CITATION INFORMATION: Sula Karreci E, Mihali A, Uehara M, Kurdi A, Riella L, Abdi R, Fan H, Singh P, Nathan C, Lin G, Azzi J. Designing a Novel Highly Selective Immunoproteasome Inhibitor That Promotes Longterm Heart Allograft Acceptance While Inducing Immune Cell Exhaustion, Treg Induction and Memory Cell Suppression. Am J Transplant. 2016;16 (suppl 3).

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