Conophyllidine is a bisindole alkaloid and functions as a selective inhibitor of M2 polarization. It inhibits histone acetylation by targeting the histone acetyltransferase domain of the P300/CBP proteins. The IC50 of Conophyllidine for IL-4-induced arginase inhibition is 0.31 μM. This compound effectively induces tumor-associated macrophages (TAMs) to shift from an anti-inflammatory to an inflammatory state, thereby enhancing the recruitment and function of cytotoxic CD8+ T cells in the tumor microenvironment. Conophyllidine is useful for studying tumor-associated macrophages.