Functions as a ligand of CD3E, inhibiting TCR-CD3 complex signaling to regulate T cell activation. Induces stable CD3E-NCK1 binding, thereby preventing the CD3E-ZAP70 interaction and subsequently inhibiting the activation of the downstream ERK-NFkB signaling cascade and calcium influx. . This is the function of ITPRIPL1 (ITPRIP like 1, ENSG00000198885).