Enhancing Immune Therapy by Modulating Cell Death

Abstract

Colorectal cancer is the second leading cause of cancer mortality in the United States. Treatments for colorectal cancer become limited as the cancer progresses. The most effective treatment is surgery, which is most effective while the tumor is in the early stages. Treatments used for advanced colorectal cancer includes chemoradiation and in some cases immune checkpoint blockade (ICB) immunotherapies, however, only a small percentage of colorectal cancers respond to ICB therapies. One way to potentially increase ICB efficacy may be to target myeloid-derived suppressor cells (MDSCs). MDSC accumulation is a known hallmark of cancer, with their key function being immune suppression via multiple mechanisms. In this study it is reported that MDSC accumulation is regulated by TNF-RIP1-mediated necroptosis. It was determined that inhibition of DNA methyl transferases (DNMTs) with Decitabine (DAC) abolished MDSC accumulation and increased activation of antigen-specific cytotoxic T lymphocytes (CTLs) in tumor-bearing mice. Furthermore, recombinant TNF induced MDSC cell death in a dose and RIP1-dependent manner. These data show that autocrine IL6 activates the STAT3-DNMT axis to epigenetically silence the TNF-RIP1 necroptosis pathway to sustain MDSC survival and accumulation in cancer. Targeting the TNF-RIP1 necroptosis is potentially an effective approach to suppress MDSCs and activate tumor reactive CTLs in the tumor microenvironment. Another hallmark of colorectal cancer is the loss of FAS expression, the death receptor for FASL on CTLs. However, it is unknown whether restoring FAS expression alone is sufficient to suppress colorectal cancer development. Codon usage optimized mouse and human FAS cDNAs were designed, synthesized, and cloned into a plasmid. The plasmid was then encapsulated within cationic lipids to formulate nanoparticle DOTAP-Chol-Fas. Overexpression of codon usage-optimized FAS in metastatic mouse colon-tumor cells enabled FASL induced elimination of FAS+ tumor cells in vitro, suppressed colon tumor growth, and increased the median survival rate of tumor-bearing mice in vivo. Overexpression of codon-optimized FAS-induced FAS receptor auto-oligomerization and tumor cell auto-apoptosis in metastatic human colon tumor cells. DOTAP-Chol-hFAS therapy is also sufficient to suppress metastatic human colon tumor xenograft growth in athymic mice, stopping PDX tumor growth in vivo. Thus this study determined that delivery of FAS DNA nanoparticles is sufficient to suppress human colon tumor growth in vivo.