Checkpoints Under Traffic Control: From and to Organelles

Immune checkpoints are variegated stimulatory and inhibitory signals that are fundamental in immune homeostasis. The regulative molecules for immune checkpoints include programmed cell death protein 1 (PD1), programmed death-ligand 1 or 2 (PD-L1 or PD-L2), cytotoxic T-lymphocyte-associated protein 4 (CTLA-4), and so on. While the immune checkpoint molecules have gained soaring attention in recent years, the trafficking of them has been rarely studied. Since all of the discovered immune checkpoint molecules are transmembrane domain (TMD) proteins, they share similar pathophysiological characteristics which make studies about their trafficking and associated disorders resembled. PD-L1 is one of the most classic immune checkpoint molecules, and anti-PD1 monoantibodies have shown promising immunotherapeutic effects. PD-L1 trafficking has been particularly studied, the key regulators of which include metformin, chemokine-like factor-like MARVEL transmembrane domain-containing family member (CMTM), Huntingtin-interacting protein 1-related (HIP1R), exosomes, ALIX, polyI:C, and various post-translational modifications. Here, we focus on the checkpoints under traffic control, counting PD-L1, CTLA-4, lymphocyte-activation gene 3 (LAG-3), killer immunoglobulin-like receptors (KIRs), CD70, CD94, and attempt to shed light on the potentials of drug targets based on these findings and look forward to further studies in combinatorial therapeutic regimens in the meantime.