IL-10 is a cytokine with multiple, pleiotropic, effects in immunoregulation and inflammation. It downregulates the expression of Th1 cytokines, MHC class II antigens, and co-stimulatory molecules on macrophages. It also enhances B cell survival, proliferation, and antibody production. IL-10 can block NF-κB activity, and is involved in the regulation of the JAK-STAT signaling pathway. Over time a more nuanced picture of IL-10's function has emerged as treatment of tumor bearing mice has been shown to inhibit tumor metastasis. Additional investigation by multiple laboratories has generated data that further supports IL-10's immunostimulatory capacity in an immunoncology context. Expression of IL-10 from transfected tumor cell lines in IL-10 transgenic mice[30] or dosing with IL-10 leads to control of primary tumor growth and decreased metastatic burden. More recently, PEGylated recombinant murine IL-10 (PEG-rMuIL-10) has been shown to induce IFNγ and CD8+ T cell dependent anti-tumor immunity. More specifically, PEGylated recombinant human IL-10 (PEG-rHuIL-10) has been shown to enhance CD8+ T cell secretion of the cytotoxic molecules Granzyme B and Perforin and potentiate T cell receptor dependent IFNγ secretion. A study in mice has shown that IL-10 is also produced by mast cells, counteracting the inflammatory effect that these cells have at the site of an allergic reaction.