Figure 2: Extrinsic model for antigen-induced T cell receptor downmodulation. The scheme depicts the internalisation of the TCR during antigen presentation after its interaction with the p-MHC complex as depicted. After T cell activation, PD-1 is expressed on the surface of the T cell undergoing antigen presentation, where it binds to PD-L1 expressed by the APC as shown in the figure. Engaged PD-1 recruits phosphatases (SHP) that will terminate TCR signal transduction. Additionally, the E3 ubiquitin Cbl-b is up-regulated and triggers TCR internalisation.