CCL17 and neoplasm: Mechanistically, expression of lymphoid homing receptors including CCR4, CCR6, and also CD62L on Treg cells during cancer development leads to the gradually increased number of tumor-associated Treg cells (38) through secretion of CCL17 and CCL22 (ligands for CCR4) by tumor cells as chemotactic factors in early stages of gastric cancer (39).