Metabolic Rewiring Of Macrophages Promotes Anti-Tumor Activity In Pancreatic Cancer

Macrophages abound in the tumor microenvironment of pancreatic cancer and other solid malignancies. Although macrophages typically promote tumorigenesis, they also represent key targets for immunotherapy approaches, which aim to: 1) deplete macrophages, 2) inhibit their activity, or 3) redirect them toward an anti-tumor role. Redirecting macrophages is commonly described as a phenotypic shift from M2 (anti-inflammatory) to M1 (pro-inflammatory) polarization. However, macrophage phenotypes have grown increasingly diverse and only loosely describe functional roles. Here we examine the anti-tumor functions of macrophages – their ability to engulf and kill tumor cells – and the metabolic dependencies of this process, using a syngeneic and fully immunocompetent tumor model of pancreatic ductal adenocarcinoma (PDAC). In PDAC cells derived from KPC (KrasLSL.G12D/+; Trp53R172H/+; Pdx-Cre) mice, the anti-phagocytic receptor CD47 was knocked out using transient CRISPR-Cas9 expression. CD47 is overexpressed by tumor cells to suppress macrophage anti-tumor activity and escape cancer immunosurveillance. Despite the critical role of CD47, we found that complete loss of CD47 in PDAC cells did not induce macrophage engulfment in vitro, nor did it impact tumor growth in vivo. We hypothesized that macrophages required an activated state, and found that ODN1826, a CpG oligonucleotide, could stimulate macrophages to engulf tumor cells whether they expressed CD47 or not, and without inducing classical M1 or M2 markers. Moreover, CpG treatment of tumor-bearing mice induced potent anti-tumor responses that required macrophages, but not lymphocytes, natural killer cells, or dendritic cells. CpG activation was found to promote oxidative respiration dependent on fatty acid oxidation, along with rewiring of the Krebs cycle through the activity of ATP citrate lyase. Together, these changes represented a hybrid of M1 and M2 metabolisms, and were critical for macrophage engulfment of PDAC cells and anti-tumor activity. Our findings indicate that immune activation of macrophages alter their metabolic state rather than their M1/M2 phenotype to enable them to overcome inhibitory CD47 and carry out anti-tumor activity.