Immune regulatory functions and therapeutic opportunities of MIF signaling in pancreatic cancer

Background: Pancreatic ductal adenocarcinoma (PDAC) is projected to become the second leading cause of cancer-related death in the US, yet current treatments remain largely ineffective. The immunosuppressive tumor immune microenvironment (TIME), particularly tumor-associated macrophages, represents a critical barrier to therapeutic success. To identify critical targets and interactions in the PDAC TIME, we cross-referenced RNA sequencing from the KPCX autochthonous mouse model that recapitulates tumor heterogeneity with RNA sequencing from a cohort of PDAC patients. Through this analysis we identified macrophage migration inhibitory factor (MIF) as an important tumor-derived secreted factor whose function in PDAC immune evasion remains to be fully elucidated. This study investigates how MIF signaling regulates immune responses and promotes immune evasion in PDAC. Methods: To dissect MIF signaling and the relative contribution from tumor and stromal compartments, we utilized orthotopic syngeneic PDAC allografts in wild-type and Mif -/- mice. We also investigated MIF's cognate receptor, CD74, through orthotopic allografts in global CD74 -/- mice and cell-type-specific deletion models targeting macrophages/monocytes (CD74f/f;Cx3cr-Cre). We also incorporated pharmacologic inhibition of MIF in our orthotopic PDAC models. Tumor growth and immune composition were quantified by flow cytometry and immunohistochemistry. MIF signaling on macrophages was also characterized in vitro using co-cultures with Mif -/- murine PDAC cell lines 6419 and 6694. M1 (iNos, CD86) and M2 polarization (Arg1, CD206) markers were characterized using RT-qPCR. Results: Loss of MIF profoundly reduced tumor burden. Syngeneic PDAC cells implanted into Mif -/- mice demonstrated a 50% reduction in tumor growth compared to wild-type mice. Reciprocally, Mif -/- tumor cells showed a marked growth reduction in wild-type recipients, indicating MIF functions in both tumor and stromal compartments. Surprisingly, CD74 deletion, either globally or specifically in macrophages, had minimal impact on tumor growth, suggesting that MIF signals through alternative receptors or acts on other immune populations. Pharmacologic MIF inhibition reduced tumor burden by 50%, decreased pro-tumor M2-like macrophages by 50%, and increased CD8+ T cells 2-3 fold. Conclusions: Our findings demonstrate that tumor-derived MIF promotes immune evasion in PDAC through CD74-independent mechanisms and involves multiple immune cell types. MIF inhibition remodels this immunosuppressive TIME and thus represents a promising therapeutic strategy to enhance the efficacy of immunotherapy or other targeted therapies in PDAC.