IMMUNOSUPPRESSION - PRE-CLINICAL AGENTS

N. Ogloff¹, C.J. Ong^2
1Department Of Surgery, University Of British Columbia, The Vancouver Prostate Centre, Vancouver/CANADA, ²Department Of Surgery, University Of British Columbia, The Vancouver Prostate Centre, Vancouver/BC/CANADA

Body: Introduction: Advances in immunosuppressive therapy over the past decade have led to dramatic improvements in graft survival. The introduction of potent immunosuppressive agents such as calcineurininhibitors (CNIs), cyclosporine and tacrolimus, has revolutionized organ transplantation leading to exceptionally low acute rejection rates and excellent graft survival. However, despite short-termsuccess, long-term graft attrition continues to be a significant problem due in part to significant adverse impact of CNIs on renal function and cardiovascular disease. Therefore, the identificationof novel agents that can effectively suppress immune responses in transplantation without associated long term toxicities for treatment of organ rejection represents a major unmet medical need. Insearch of novel immunosuppressive agents, we looked to nature for inspiration. One key natural mechanism by which the body regulates inflammation and immunosuppression is through the expression ofimmunoregulatory enzymes, arginase and indoleamine 2,3-dioxygenase (IDO). Arginase and IDO suppress immune responses by catabolizing the amino acids, arginine and tryptophan, respectively, creating alow amino acid environment. Mechanistically, amino acid depletion leads to an increase in uncharged tRNAs that which in turn leads to activation of GCN2 kinase and phosphorylation of eIF2a; resultingin inhibition of protein translation. Activation of the GCN2 pathway in T-cells has been linked to tolerance induction, autoimmunity and transplant tolerance through regulation by IDO on antigenpresenting cells. Ultimately, activation of the GCN2 pathway in T-cells leads to suppression of T-cell function. Therefore, we hypothesized that aminoacyl tRNA synthetase inhibitors may represent anovel class of immunosuppressive agents that may act as pharmacological mimetics of arginase and IDO by inducing an increase in intracellular levels of uncharged tRNA and activating the GCN2 pathwayin immune cells. Results: As proof of concept, we screened the small molecule drug database for potent inhibitors of mammalian tRNA synthetases and identified Borrelidin as a potent inhibitor ofhuman threonyl tRNA synthetase. Through in vitro and in vivo studies, we found that Borrelidin indeed activates the GCN2 pathway and importantly, preferentially inhibits activation of immune cellsover non-immune cells. Borrelidin inhibits proliferation of allogeneic lymphocyte stimulated mouse primary T-cells in vitro with an EC50 of 1.6ng/ml. Additionally borrelidin has been shown todecrease IL-2, IL-4 and INFγ cytokine production and prevent upregulation of CD25 & CD69 in anti-CD3/anti-CD28 stimulated T-cells. In addition, we have found that Borrelidin potentlyinhibits B cell proliferation as well as macrophage activation. In vivo, Borrelidin potently inhibits cutaneous anaphylaxis in mice suggesting a potential therapeutic application for treatment ofatopic diseases. Conclusions: Collectively, our results suggest that aminoacyl tRNA synthetase inhibitors such as Borrelidin can be used as activators of the GCN2 pathway to suppress both adaptiveand innate immune responses for the treatment of organ rejection and other immunological diseases.

Disclosure: All authors have declared no conflicts of interest.