Acts as a negative regulator of T-cell receptor (TCR) signaling by direct dephosphorylation of the Src family kinases LCK and FYN, ITAMs of the TCRz/CD3 complex, as well as ZAP70, VAV, VCP and other key signaling molecules.Associates with and probably dephosphorylates CBL. Dephosphorylates LCK at its activating 'Tyr-394' residue.