MAO-A deficiency in immune cells resulted in suppressed tumor growth (Fig. 2b, c), altered TAM polarization (i.e., downregulation of immunosuppressive markers such as CD206, Fig. 2d; and upregulation of immunostimulatory markers such as CD69, CD86 and MHC class II I-Ab; Fig. 2e, f and Supplementary Fig. 2a), and enhanced tumor-infiltrating CD8+ T-cell activation (i.e., increased production of cytotoxic molecules such as Granzyme B; Supplementary Fig. 2b), indicating that MAO-A directly regulates immune cell antitumor activity, in particular TAM polarization and T-cell antitumor reactivity. Here, CD8A is linked to neoplasm.