 However, when a T cell is activated in a lymphoid organ through cytokine and TCR signaling, CD69 is expressed and forms a complex with S1P1 (not S1P3 or S1P5).This association is dependent on the interaction between the CD69 transmembrane domain and helix-4 of S1P1.Following formation of this complex, S1P1 is internalized and is destroyed within the cell, inhibiting its ability to bind S1P and initiate downstream signaling.