The CC′ loop of IgV domain containing immune checkpoint receptors: From a bystander to an active determinant of receptor:ligand binding

Antibodies targeting negative regulators of immune checkpoints have shown unprecedented and durable response against variety of malignancies. While the concept of blocking the negative regulators of immune checkpoints using mAbs appears to be an outstanding approach, their limited effect and several drawbacks such as resistance, poor solid tumor penetration and so on, calls for the rational design of next generation of therapeutics. Soluble isoforms of negative regulators of immune checkpoints are expressed naturally and are shown to regulate the immune response, suggesting the soluble version of these molecules and affinity-modified versions of these self-molecules could be effective lead molecules for immunotherapy. To get a better insight on hotspot regions for modification, we have analysed structures of available immune receptor:ligand complexes containing IgV domains. Interestingly, this analysis reveals that the CC′ loop of IgV domain, a loop which is distinct from CDRs which are generally utilized by antibodies to recognize antigens, plays a pivotal role in affinity modulation. Here, we present several examples of cognate partner specific conformational variation observed in CC′ loop of several checkpoint receptor:ligand complexes. In addition,in silicoswapping of CC′ loop targeting TIGIT:Nectin-2/PVR pathway corroborated well with biophysically determined affinity values for these complexes. Thus, CC′ loop appears to be a hotspot for affinity modification without affecting the specificity to their cognate receptors, an important requirement to avoid unintended interaction of these modified molecules with undesired targets.