Blockade of PF4-induced Th1-Treg polarization enhances anti-tumor immunity

The tumor microenvironment (TME) contains various immune-suppressive cells such as regulatory T cells (Tregs) and M2-like tumor associated macrophages (TAMs) that express the enzyme arginase I (Arg1). T helper 1-polarized Treg (Th1-Treg) is a Treg subset that markedly accumulate in tumor tissues, suppressing anti-tumor immunity. However, little is known about the mechanism behind the abundant presence of Th1-Tregs in TME. Here we show that Arg1-expressing TAMs (Arg1+ TAMs) play critical roles for the high Th1-Treg ratio in TME. Selective depletion of Arg1+ TAMs using the VeDTR system inhibited tumor growth and concurrently reduced the Th1-Treg ratio in TME. Notably, Arg1+ TAMs secreted platelet factor 4 (PF4) that polarized Tregs to Th1-Tregs in a CXCR3-dependent manner. Both genetic PF4 inactivation and PF4 neutralization hindered Th1-Treg accumulation in TME, consequently suppressing tumor growth. Collectively, our study highlights the importance of M2-like TAM-produced PF4 for high Th1-Treg levels in TME to suppress anti-tumor immunity, and demonstrates PF4 neutralization as a potential cancer immunotherapeutic strategy by intervening the M2-like TAM/Th1-Treg axis.