CCL2 and glioblastoma: CSC actively recruit Treg cells to the tumour microenvironment through the secretion of chemokines, such as CCL1, CCL2, and CCL5 (Chang et al., 2016; Xu et al., 2017; Chen et al., 2018; You et al., 2018; Su et al., 2019), a process that has been validated in a mouse glioblastoma model, in which CCL2 relies on the expression of CCR4 to mediate the Treg cell trafficking (Chang et al., 2016).