We found that the cancerous pancreas harbors a markedly more abundant microbiome compared with normal pancreas in both mice and humans, and select bacteria are differentially increased in the tumorous pancreas compared with gut. Ablation of the microbiome protects against preinvasive and invasive pancreatic ductal adenocarcinoma (PDA), whereas transfer of bacteria from PDA-bearing hosts, but not controls, reverses tumor protection. Bacterial ablation was associated with immunogenic reprogramming of the PDA tumor microenvironment, including a reduction in myeloid-derived suppressor cells and an increase in M1 macrophage differentiation, promoting TH1 differentiation of CD4 T cells and CD8 T-cell activation. Bacterial ablation also enabled efficacy for checkpoint-targeted immunotherapy by upregulating PD-1 expression. Mechanistically, the PDA microbiome generated a tolerogenic immune program by differentially activating select Toll-like receptors in monocytic cells. These data suggest that endogenous microbiota promote the crippling immune-suppression characteristic of PDA and that the microbiome has potential as a therapeutic target in the modulation of disease progression. We found that a distinct and abundant microbiome drives suppressive monocytic cellular differentiation in pancreatic cancer via selective Toll-like receptor ligation leading to T-cell anergy. Targeting the microbiome protects against oncogenesis, reverses intratumoral immune tolerance, and enables efficacy for checkpoint-based immunotherapy. These data have implications for understanding immune suppression in pancreatic cancer and its reversal in the clinic. .

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Bacterial dysbiosis has emerged as an accomplice to carcinogenesis in malignancies such as colon and liver cancer, and we have recently implicated the microbiome in the pathogenesis of pancreatic ductal adenocarcinoma (PDA) 1. However, the mycobiome has not been clearly implicated in tumorigenesis. We found that fungi migrate from the gut lumen to the pancreas. PDA tumors harbored a ~3000-fold increase in fungi compared to normal pancreas in both mice and humans. The composition of the PDA mycobiome was distinct from that of gut or normal pancreas based on alpha and beta diversity indices. Specifically, the fungal community infiltrating PDA tumors was markedly enriched for Malassezia in both mice and humans. Fungal ablation was tumor-protective in slowly progressive and invasive models of PDA whereas repopulation with Malassezia-but not Candida, Saccharomyces, or Aspergillus-accelerated oncogenesis. In parallel, we discovered that ligation of mannose-binding lectin (MBL), which binds fungal wall glycans to activate the complement cascade, was required for oncogenic progression whereas MBL or C3 deletion in the extra-tumoral compartment or C3aR knockdown in tumor cells were protective. Further, reprogramming of the mycobiome did not alter PDA progression in Mbl or C3 deficient mice. Collectively, our work shows that pathogenic fungi promote PDA by driving the complement cascade via MBL activation.

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