Metastatic triple-negative breast cancer secretes heme metabolites to alter macrophage immune suppression and function

To test if bilirubin impacts immune cells, mouse bone-marrow derived macrophages (BMDM) were treated with exogenous bilirubin that decreased macrophage efferocytic capacity, or dead cell engulfment, in a dose dependent manner. Conversely, bilirubin treatment significantly increased the ability of BMDM to suppress activation of CD8+ splenocytes. Macrophages isolated from 66Cl-4 mammary tumors treated with SnMP to deplete intratumoral bilirubin were unable to suppress production of cytotoxic molecules Granzyme-B and Perforin in stimulated CD8+ splenocytes. 66Cl-4 mammary tumors harboring HO-1 knockdown had a significant decrease in growth rate and number of pro-metastatic CD206+ macrophages after treatment with αPD-1.

Together these results suggest that tumor HO-1 supports macrophage dysfunction and immune suppression during TNBC progression via bilirubin. Further, depletion of intratumoral bilirubin levels has the potential to dynamically impact macrophage populations, particularly in combination with immunotherapy, demonstrating an underappreciated role for this metabolic pathway and metabolite as a pro-metastatic, immunomodulators in cancer.