Tumor-derived retinoic acid regulates intratumoral monocyte differentiation to promote immune suppression and resistance to immune checkpoint blockade

The immunosuppressive tumor microenvironment (TME) is a major barrier to immunotherapy. Within solid tumors, why monocytes preferentially differentiate into immunosuppressive tumor associated macrophages (TAMs) but not immunostimulatory dendritic cells (DCs) remains unclear. Using multiple murine sarcoma models, we found that the TME induced retinoic acid (RA) production by tumor cells, which polarized intratumoral monocyte differentiation towards TAMs and away from DCs via suppression of DC-promoting transcription factor Irf4. Genetic inhibition of RA production by tumor cells or pharmacologic inhibition of RA signaling within TME increased stimulatory monocyte-derived cells, enhanced T cell-dependent anti-tumor immunity and demonstrated striking synergy with immune checkpoint blockade. Further, RA responsive gene signature in human monocytes correlated with an immunosuppressive TME in multiple human tumors. RA has been long considered as an anti-cancer agent, but our work demonstrates its tumorigenic capability via myeloid-mediated immune suppression and provides proof of concept for targeting this pathway for tumor immunotherapy. Overall design: Cas9 Control or Raldh1/3 DKO tumors (n=4 per group) were harvested on Day 11 to generate single cell suspensions (as described in Materials and Methods) and CD45+ live cells were FACS sorted. Subsequently, 10x Genomics Controller and the v3 Library and Gel Bead kit (10x Genomics) were used to obtain single-cell emulsions. 10x 3' v3 kit protocol was followed as described to generate RNA sequencing libraries. The generated libraries were sequenced using an Illumina NovaSeq SP.