TGFB1 and neoplasm: Thus, CTLA-4 not only directly induces T cell function downregulation but also contributes to Treg-mediated suppression of anti-tumor immune responses.552 Treg cells have complex functions including direct suppression of DC cell function, inhibition of effector T cells, secretion of TGF-β, and competition for IL-2 and other cytokines through CD25.