CCL17 and neoplasm: T-regs accumulate into the tumor milieu thanks to the surface expression of CCR4, the receptor for “thymus and activation regulated chemokine” (TARC/CCL17), a factor greatly secreted by HRS cells.25 T cells stimulated with TARC acquire a regulatory function, able to silence the cytotoxic activity of CTL.26 Conversely, CTL are not influenced by TARC since they lack the surface expression of CCR4.27