Fig. 3

Immune checkpoint and its characterizations. a Immune checkpoint classification. Immune checkpoints are classified as one-way immune checkpoint and two-way immune checkpoint based on signal 2 direction and are further divided into stimulatory and inhibitory immune checkpoints. b. One-way immune checkpoint. The one-way immune checkpoint only involves forward signaling in signal 2. The ligation of co-stimulatory pair molecules triggers forward stimulatory immune checkpoint inducing TC proliferation, whereas the inhibitory immune checkpoint induces TC suppression or death. c Two-way immune checkpoint. The two-way immune checkpoint involves both forward and reverse stimulatory signaling. The reverse stimulatory immune checkpoint induces either TC proliferation or MC (APC) differentiation/inflammation. The inhibitory immune checkpoint leads to TC suppression/death or APC death. Words in red emphasize our newly proposed signal. Abbreviations: APC antigen present cell, MC monocyte, RF risk factor, sCD40L soluble CD40 ligand, TC T cell