Figure 1:

Schematic of immune checkpoint inhibitor activity; anti-CTLA-4 (left) and anti-PD-1/PD-L1 (right). T cell activation requires 1) engagement of a T cell receptor (TCR) with an antigen presented in the context of a major histocompatibility complex (MHC), and 2) a second signal consisting of engagement of CD28 with B7. CTLA-4 opposes this second signal, by binding to B7 at higher affinity than CD28, thus limiting T cell activation. Blocking CTLA-4 pharmacologically, therefore “removes the brakes” on T cell activation and allows for unopposed engagement of the second signal. PD-1, a receptor on T cells, binds PD-L1, expressed on a variety of cells including tumor cells and tumor-infiltrating macrophages, which triggers a cascade of T cell inhibitor processes known as T cell exhaustion. Blocking either side of this interaction (with PD-1 or PD-L1 targeting antibodies) prevents this engagement, precludes T cell exhaustion, and permits anti-tumor activity