1. A targeted immunoproteasome inhibitor, DPLG3, was found to have high selectivity for the immune cell proteasome and was not toxic to other cell types.
2. When administered after a heart transplant in mice, DPLG3 allowed for minimal graft rejection through a decrease in T cell activation.
Evidence Rating Level: 1 (Excellent)
Study Rundown: Organ transplants typically require the recipient to continue taking immunosuppressive medications to prevent rejection of the graft. However, the toxicity of these medications necessitates a safer option for transplant recipients. Researchers in this study synthesized DPLG3, a molecule that was modeled after a currently prescribed proteasome inhibitor and had selectivity to a subunit of the immunoproteasome.
When tested in vitro, DPLG3 selectively inhibited the β5i immunoproteasome subunit in a lymphoma cell line and did not affect other immunoproteasomes or cell types. The compound was found to decrease T cell proliferation in response to alloantigens. In a mouse model of skin transplantation, treatment with DPLG3 decreased the production of inflammatory cytokines and increased the expression of coinhibitory molecules by T cells. Next, DPLG3 was administered to mice that received a heart transplant from a mouse of another strain. These mice survived longer than the recipients that did not receive the compound, and there were few signs of rejection histologically. In addition, there were fewer effector T cells in the spleen, indicating the inhibition of an immune response. Overall, there was less lymphocyte infiltration and a decrease in T cell activation and proliferation compared to mice not treated with DPLG3. This study demonstrated that the synthesis of a selective proteasome inhibitor to prevent immune response to allografts could potentially allow for a safer and more efficacious medication for transplant recipients.
In-Depth [animal study]: An N,C-capped dipeptide, DPLG3, was synthesized to selectively bind to β5i 99,000-fold higher than β5c, a selectivity significantly greater than the currently available proteasome inhibitor. In vitro, DPLG3 inhibited β5i activity in Karpas lymphoma cells, while it had no effect on β5c activity in HepG2 human hepatoma cells. DPLG3 was not cytotoxic to human peripheral blood mononuclear cells. When T cells were isolated from the spleen of mice and treated with increasing concentrations of DPLG3, there was a concentration-dependent decrease in proliferation assessed using 3H-thymidine incorporation (p<0.05). A similar result was seen when mice that received a skin allograft were treated with 25 mg/kg of the compound. Their immune cells showed reduced proliferation in response to alloantigen, increased gene expression of coinhibitory molecules such as CTLA4 and BTLA, as well as a decrease in the pro-inflammatory cytokines IL-2 and IL-17 (p<0.05).
The efficacy of DPLG3 was then tested on C57BL/6 mice that were allografted with hearts from BALB/c mice. The recipient mice were then given 25 mg/kg DPLG3 each day for 14 days after receiving the transplant. These mice were found to have a significant decrease in the percentage of T cells in the spleen (p<0.05), demonstrating a lack of immune activation. In addition, the splenocytes demonstrated reduced responsiveness to alloimmune stimuli and there was a significant decrease in histologic signs of graft rejection. The treated mice had overall longer graft survival, with no death seen with 2 weeks of treatment with DPLG3 and a single dose of 25 µg CTLA4 Ig.
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