Increased bindings of C/EBP-beta at the -165 kb Zeb2 enhancer  upon systemic IL-6 exposure suppress cDC1 development
Type 1 conventional dendritic cells (cDC1) are required for effective CD8 T cell responses to many viruses and tumors and for effective checkpoint blockade immunotherapy. Recently, cytokines produced in association with certain tumors were reported to impair anti-tumor immune responses by reducing the abundance of cDC1. However, the reported mechanism of this reduction remains unclear, attributed either to reduced cDC1 development or decreased peripheral cDC1 survival. Here we show that tumor-derived IL-6 blocks cDC1 development from both in murine and human systems. We show that mechanism of this blockade is the IL-6-dependent increase in C/EBPß expression in the common dendritic cell progenitor (CDP). C/EBPß and NFIL3 compete for binding to sites in the -165 kb Zeb2 enhancer, and support or repress Zeb2 expression respectively. At homeostastis, pre-cDC1 specification occurs upon Nfil3 induction and consequent Zeb2 suppression.  However, IL-6 strongly induces C/EBPß expression in CDPs and increase C/EBPß bindings at the -165 kb Zeb2 enhancer, thereby preventing normal NFIL3-dependent pre-cDC1 specification.  Importantly, the ability of IL-6 to block cDC1 development is dependent on the presence of C/EBPß binding sites in the -165 kb Zeb2 enhancer, as this effect is lost in ?1+2+3 mutnat mice in which these binding sites are mutated. These results explain how tumor-associated IL-6 suppresses cDC1 development and suggest therapeutic approaches preventing abnormal C/EBPß induction in CDPs may help reestablish cDC1 development to enhance anti-tumor immunity. Overall design: Analysis of C/EBPß bindings by CUT&RUN in Hoxb8-transformed BM progenitor cell line upon IL-6 stimulation