PBMC activation via the ERK and STAT signaling pathways enhances the anti-tumor activity of Staphylococcal enterotoxin A

Staphylococcal enterotoxin A (SEA) is well known as a superantigen and is highly potent in activating T lymphocytes. And it has been used clinically as an immunomodifier in the treatment of a number of tumors for years. However, the mechanism of its action remains largely unclear. In this study, SEA was found to significantly inhibit the proliferation and induce the death of human lung carcinoma A549 cells when co-cultured with human peripheral blood mononuclear cells (PBMCs). SEA could also induce the proliferation of human PBMCs and stimulate human PBMCs to release a wide range of cytokines that have broad anti-tumor activities such as IFN-γ, TNF-α, IL-2. Furthermore, SEA was found in PBMCs to induce a rapid and long-lasting phosphorylation of extracellular signal-regulated kinases (ERKs) which was significantly inhibited by MEK/ERK pathway inhibitors U0126 and PD0325901, and a late onset of phosphorylation of signal transducers and activators of transcription (STATs) which was significantly inhibited by a pan-JAK inhibitor Pyridone 6 (P6). Unexpectedly constitutive ERK or STATs phosphorylation was also significantly inhibited by P6 or U0126 in a dose-dependent manner, respectively. Summing up, our data reveal SEA may function as a novel protein drug used for cancer immunotherapy via inducing activation of PBMCs, immune cell crosstalk-dependent activation of ERK and STATs, and production of tumor-suppressive cytokines.