Enhancement of Anti-Leukemia Immunity by Leukemia–Derived Exosomes Via Downregulation of TGF-β1 Expression

Background/Aims: Minimal residual leukemia cells (MRLs) are difficult to eradicate through traditional treatment and therefore remain to be a major threat to the long-term survival of leukemia patients. Tumor-derived exosomes (TEXs), which carry tumor associated antigens (TAA), may be a potential cell-free tumor vaccine for the specific eradication of MRLs. However, TEXs are intended to be less immunogenic due to exosomal TGF-β1. To further optimize the efficacy of TEX-based vaccines, we investigated whether exosomes from TGF-β1 silenced leukemia cells (LEXTGF-β1si) had an increased potential to induce a specific antitumor effect compared with non-modified exosomes. Methods: Exosomal TGF-β1 was downregulated via lentiviral shRNA silencing of TGF-β1 in leukemia cells. The characteristics of LEXTGF-β1si were determined via electron microscopy, western blot analysis, and flow cytometry. The antitumor effect of LEXTGF-β1si was evaluated by detecting the properties of LEXTGF-β1si-pulsed dendritic cells (DCs), CD4+ T-cell proliferation, Th1 cytokine secretion, specific CTL activity, and NK cell function. Moreover, to verify the superiority of LEXTGF-β1si immunization, LEXTGF-β1si was subcutaneously injected into DBA/2 mice: either followed by tumor challenge or tumor bearing. Results: The lentiviral shRNA silencing of TGF-β1 in parental leukemia cells successfully downregulated the TGF-β1 level in leukemia cell derived exosomes (LEX). LEXTGF-β1si was uptaken by DCs and was more potent in promoting DC function by upregulating the surface expression of costimulatory factors and MHC class II molecules, while inducing the secretion of IL-12p70 and TNF-α. Furthermore, immunization with LEXTGF-β1si facilitated CD4+ T-cell proliferation and Th1 cytokine secretion, and stimulated stronger specific cytotoxic lymphocyte (CTL) response and nature killer (NK) cell cytotoxicity more efficiently compared to non-modified LEX. In mice models, immunization with LEXTGF-β1si resulted in a more potent capability to inhibit tumor growth and to prolong survival, suggesting that LEXTGF-β1si was more effective in both protective and therapeutic antitumor tests than non-modified LEX. Conclusions: These data suggested that down-regulation of exosomal TGF-β1 effectively induced potent anti-tumor immunity. Our strategy of optimizing exosome vaccine may have promising potential for leukemia immunotherapy.