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Application of Saccharomyces cerevisiae to improve pediatric acute lymphoblastic leukemia outcomes Gynn, Matthew

Abstract

Pediatric acute lymphoblastic leukemia (ALL) is the most commonly diagnosed childhood cancer in Canada and continues to need improvements in the care patients receive. Saccharomyces cerevisiae (S. cerevisiae) is a generally nonpathogenic organism that has the potential to affect both stages of leukemia (preleukemia and leukemia) and improve ALL outcomes by acting as a drug delivery vehicle for L-ASNase and/or influence leukemogenesis by inducing early-life, antileukemic immune activity. Mild infectious exposures during infancy and childhood has consistently been shown to influence leukemogenesis. In mice, early-life infections can deplete preleukemic cells and prevent preleukemia developing into leukemia. If preleukemia develops into ALL, children undergo chemotherapy that includes the enzyme L-asparaginase (L-ASNase). ALL cells characteristically stop expressing the enzyme asparagine synthetase (AS) and rely on extracellular sources for L-ASN. L-ASNase depletes extracellular L-ASN, selectively starving ALL cells of L-ASN, and consequently inducing their apoptosis. To evaluate the efficacy of S. cerevisiae to improve L-ASNase therapy, it was first engineered to constitutively express cell-wall associated L-ASNase-II (AEY – asparaginase expressing yeast). The cell-wall association may improve ALL therapy by shielding L-ASNase from immune detection and consequently reducing the cost, toxicity, and immunogenicity of L-ASNase therapy. In vitro, AEY co-cultured with ALL cell lines can deplete L-ASN levels leading to apoptosis and inhibited cell growth in the asparagine synthetase (AS)-negative cell line RS4;11, but not the AS-positive cell lines BV173 and 697. The AEY biomass required to yield a therapeutic L-ASNase dose exceeds the current ethical limitations for further study in vivo. To determine if S. cerevisiae can activate early-life, antileukemic immune activity, S. cerevisiae was injected intraperitoneally into day-6-old Eμ-RET mice and the effect on preleukemic burden was assessed. S. cerevisiae is unable to activate the IL-17A and Natural Killer (NK) cell-dependent immune response that has been shown to deplete preleukemic cells. Additionally, both in vitro and in vivo stimulation of NK cells with IL-17A does not directly lead to their activation. Further investigation into the mechanism leading to the activation of antileukemic NK cells may uncover new immunotherapeutic approaches for ALL.

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Attribution-NonCommercial-NoDerivatives 4.0 International

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