Model of the accumulation of Treg cells in human tumors. Model of the accumulation of Tregcells in human tumors. One of the possible scenarios for how Treg-cell attraction to the tumor site and expansion of Treg cells occurs may be the release of CCL22 as well as H-ferritin by tumor cells and tumor-infiltrating macrophages leading to the accumulation of CCR4+ naive Treg cells in the tumor microenvironment. Interaction with PGE2-induced tolerogenic DCs then gives rise to the differentiation and peripheral expansion of naive Treg cells into memory Treg cells. Together with tolerogenic DCs, these Treg cells than inhibit the generation of effector T cells, resulting in the induction of tolerance against the tumor. Marc Beyer, and Joachim L. Schultze Blood 2006;108:804-811 ©2006 by American Society of Hematology