Interleukin-27 (IL-27), a heterodimeric cytokine belonging to the IL-12 family. IL-27 is produced by antigen-presenting cells and exerts complex immunoregulatory effects within the tumor microenvironment. While it can have initial activating effects on T cells, sustained IL-27 signaling often promotes an immunosuppressive state. It can induce immune checkpoint molecules (e.g., PD-L1), drive the differentiation of regulatory T cells (Tregs), and suppress effector T cell function.