The expansion of Tregs in tumor microenvironments is partially caused by an increased recruitment of (n)Tregs from the peripheral blood, and we found that the serum level of chemokines, including CCL19, IL-8, CXCL1 and CXCL12 (binding to CXCR2), and RANTES (binding to CCR5), was significantly increased in BC patients (P < 0.001, Fig. S2A). Here, CXCR2 is linked to breast cancer.