Inhibition Of The Immunoproteasome Subunit LMP7 Decreases Alloresponses In Both MHC-Mismatched and miHA-Disparate Murine Models Of Graft Versus Host Disease

Background Proteasome inhibition has been studied and used as a therapeutic target in the treatment of autoimmune disorders and multiple myeloma. Immune system cells, especially antigen-presenting cells, express a higher basal level of immunoproteasomes, which are used to generate peptides that can be processed to fit in the groove of MHC class I molecules. ONX 0914 is a selective inhibitor of the immmunoproteasome that blocks LMP7-specific presentation of MHC-I restricted antigens, whereas PR-825 is a specific inhibitor of the b5 catalytic subunit generated by the constitutive proteasome. In previous work, we found that ONX 0914 administration early after transplantation significantly improved the survival of recipient mice in an MHC-matched minor histocompatibility antigen (miHA)-disparate (B10.BR -> CBA, lethally irradiated) murine model of graft-versus-host disease (GVHD). To further elucidate the mechanism of action, we compared alloreactive responses via IFN-g production in mixed lymphocyte reactions (MLR) in which stimulator cells or responder T cells were pretreated with either ONX 0914 or PR-825. MLR were conducted for the B10.BR anti-C56BL/6y (B6) MHC-mismatched, the CD8+ T cell-mediated B10.BR anti-CBA, and the CD4+ T cell-mediated B6 anti-BALB.B MHC-matched/ miHA-disparate strain combinations. Methods Stimulator splenocytes were treated for one hour with ONX 0914 (300 nM) or PR-825 (125 nM) before or immediately after exposure to irradiation (30 Gy) prior to initiation of the MLR. In addition, to try to optimize putative in vivo drug regimens, we determined the timing of release/degradation of antigenic peptides presented in the context of MHC-I. To this end, the MC57G fibrosarcoma cell line was transiently transfected with a GFP tagged SIINFEKL plasmid, cells were treated with different drug regimens and exposure times of ONX 0914 or PR-825, and monitored over 48 hours for surface expression of MHC-I presented SIINFEKL peptide using flow cytometry. Results ELISpot assays showed a statistically significant decrease in the number of IFN-g producing cells when stimulators (B6 splenocytes) were pretreated with ONX 0914 compared to pretreated responder (B10.BR) cells (60.21% vs. 1.75% respectively, p< 0.01). In the miHA disparate combinations, the percentage decrease in IFN-g+ spots was ∼30% when stimulators were treated with ONX 0914, whereas only a ∼15% decrease was observed with PR-825 pretreatment. Furthermore, IFN-g production was not dependent upon the timing of exposure to irradiation, as ELISpot counts were equally decreased when drug pretreatments were performed before irradiation or just after exposure. Antigenic peptide presentation was maximally decreased in transfected MC57G cells 48 h after treatment (i.e., SIINFEKL % in GFP+ MC57G cells treated with DMSO was equal to 83% vs. 76% when cells were exposed to ONX 0914 [300 nM, for 24 h]. This result suggests that for in vivo application, pretreatment of recipient mice with ONX 0914 to decrease miHA presentation by host cells (24-72 hours prior to bone marrow transplantation) may provide further amelioration of GVHD development. Conclusion Taken together, these data suggest that downregulation of LMP7-mediated presentation of MHC-I restricted antigens by host cells likely modulated stimulation and IFN-g production of donor T cells in vivo, rather than acting on effector cells directly, and accounted in part for the improved survival rate experienced by recipient mice treated with ONX 0914. Disclosures: Matos: Onyx Pharmaceuticals: Research Funding. Dziopa:Onyx Pharmaceuticals: Research Funding. Kirk:Onyx Pharmaceuticals: Employment. Korngold:Onyx Pharmaceuticals: Research Funding. Zilberberg:Onyx Pharmaceuticals: Research Funding.