Ray et al. generated conditional macrophage-associated CD206 knock-in reporter mice. Early (after tumor implant) CD206 ablation in reporter mice reduced tumor control and led to direct deletion of TAMS enriched for CXCL9 expression, and indirect intratumoral decline of CXCR3 (CXCL9 receptor)+ NK- and antigen-specific CD8+ T-cells, lymphocyte expression of Xcl1 and Flt3l (attractant and growth factor for cDC1s), and cDC1s. Human gene signatures of CD206hi macrophage and tumor-reactive stimulatory cDC1s correlated, and skewing towards CD206hi macrophages predicted favorable survival outcomes for patients with cancer, challenging the immunosuppressive M2 macrophage paradigm.
Contributed by Paula Hochman
ABSTRACT: Tumor-associated macrophages (TAMs) are frequently and simplistically categorized as immunosuppressive, and one molecule prominently used to highlight their so-called 'M2' state is the surface protein CD206. However, direct evidence of the impact of macrophages remains impaired by the lack of sufficiently penetrant and specific tools to manipulate them in vivo. We thus made a novel conditional CD206 knock-in mouse to specifically visualize and/or deplete these TAMs. Early depletion of CD206+ macrophages and monocytes (here, 'MonoMacs') strikingly led to an indirect loss of a key anti-tumor network of NK cells, conventional type I dendritic cells (cDC1) and CD8 T cells. Among myeloid cells, we found that the CD206+ TAMs are the primary producers of CXCL9, the well-established chemoattractant for CXCR3-expressing NK and CD8 T cells. In contrast, a population of stress-responsive TAMs ("Hypoxic" or Spp1 +) and immature monocytes, which remain following depletion, expressed vastly diminished levels of CXCL9. We confirmed that the missing NK and CD8 T cells are the primary producers of the cDC1-attracting chemokine Xcl1 and cDC1 growth factor Flt3l . Consistent with the loss of this critical network, CD206+ TAM depletion decreased tumor control in mice. Likewise, in humans, the CD206+ MonoMac signature correlated robustly with stimulatory cDC1 signature genes. Together, these findings negate the classification of CD206+ macrophages as immunosuppressive and instead illuminate the role of this majority of TAMs in organizing a critical tumor-reactive archetype of immunity.