Pam2 lipopeptides systemically increase myeloid-derived suppressor cells through TLR2 signaling

Myeloid-derived suppressor cells (MDSCs) are immature myeloid cells that exhibit potent immunosuppressive activity. They are increased in tumor-bearing hosts and contribute to tumor development. Toll-like receptors (TLRs) on MDSCs may modulate the tumor-supporting properties of MDSCs through pattern-recognition. Pam2 lipopeptides represented by Pam2CSK4 serve as a TLR2 agonist to exert anti-tumor function by dendritic cell (DC)-priming that leads to NK cell activation and cytotoxic T cell proliferation. On the other hand, TLR2 enhances tumor cell progression/invasion by activating tumor-infiltrating macrophages. How MDSCs respond to TLR2 agonists has not yet been determined. In this study, we found intravenous administration of Pam2CSK4 systemically up-regulated the frequency of MDSCs in EG7 tumor-bearing mice. The frequency of tumor-infiltrating MDSCs was accordingly increased in response to Pam2CSK4. MDSCs were not increased by Pam2CSK4 stimuli in TLR2 knockout (KO) mice. Adoptive transfer experiments using CFSE-labeled MDSCs revealed that the TLR2-positive MDSCs survived long in tumor-bearing mice in response to Pam2CSK4 treatment. Since the increased MDSC population sustained immune-suppressive properties, our study suggests that Pam2CSK4-triggered TLR2 activation enhances the MDSC potential and suppress antitumor immune response in tumor microenvironment.