Some of the mechanisms by which CD25⁺CD4⁺ Tregs suppress the activation and proliferation of other T cells. The inhibition between these 2 T-cell subtypes occurs as either contact-dependent (suppressor synapse) or -independent manners. APCs play an important role in this interaction. (A) Tregs can suppress the functions of Teffs via direct contact. (B) Tregs can suppress the functions of Teffs via releasing inhibitory cytokines. (C) Tregs can complete with Teffs for the costimulatory signals of APCs. (D) Tregs can suppress APCs and thus prevent the stimulation of Teffs by APCs.