MMP9 and prostate cancer: The secreted factors from these infiltrating immune cells in the prostate cancer TME such as cytokine CCL2 and CCL5 from TAMs, and metalloprotease MMP9 from TANs and mast cells, and inactivation of cytotoxic CD8+ T cells which can be mediated by Treg all work in unison to maintain the tumor’s ability to grow, migrate and invade.