Aggressive breast cancers secrete heme metabolites to alter macrophage immune suppression and function

The heme catabolism pathway is often elevated in aggressive cancers; however, the impact of this pathway and some of its byproducts on the tumor microenvironment remain largely unknown. In human breast cancers, tumor expression of the heme catabolizing enzyme heme oxygenase-1 (HO-1/HMOX1) is positively associated with macrophage abundance. In mouse mammary tumors, knockdown of Hmox1 significantly decreased tumor growth and lung metastasis. Analysis of mammary tumor interstitial fluid compared to matching plasma revealed that the heme metabolite bilirubin was elevated intratumorally, which could be partially reversed via Hmox1 knockdown. Further investigation revealed that bilirubin nearly ablates macrophage engulfment of dead tumor cells and significantly increases macrophage T cell suppression. Mammary tumors harboring Hmox1 knockdown had a significant decrease in tumor growth rate and number of pro-metastatic CD206⁺ macrophages upon treatment with αPD-1. Depletion of intratumoral bilirubin levels impacts pro-tumor macrophage populations, particularly in combination with immunotherapy, demonstrating that heme catabolism and bilirubin act as immunomodulators in cancer.