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Although therapeutic immunoglobulin G (IgG) antibodies that regulate the activity of immune checkpoints bring innovation to the field of immuno-oncology, they are still limited in their efficiency to infiltrate the tumor microenvironment due to their large molecular size (150 kDa) and the necessity of additional engineering works to ablate effector functions for antibodies targeting immune cells. To address these issues, the human PD-1 (hPD-1) ectodomain, a small protein moiety of 14−17 kDa, has been considered as a therapeutic agent. Here, we used bacterial display-based high-throughput directed evolution to successfully isolate glycan-controlled (aglycosylated or only single-N-linked glycosylated) human PD-1 variants exhibiting over 1000-fold increased hPD-L1 binding affinity compared to that of wild-type hPD-1. The resulting hPD-1 variants, aglycosylated JYQ12 and JYQ12-2 with a single-N-linked glycan chain, showed exceptionally high binding affinity to hPD-L1 and very high affinity to both hPD-L2 and mPD-L1. Moreover, the JYQ12-2 efficiently potentiated the proliferation of human T cells. hPD-1 variants with significantly improved binding affinities for hPD-1 ligands could be used as effective therapeutics or diagnostics that can be differentiated from large-sized IgG antibody-based molecules.

The immunoglobulin G (IgG) molecule has a long circulating serum half‐life (~3 weeks) through pH‐ dependent FcRn binding‐mediated recycling. To hijack the intracellular trafficking and recycling mechanism of IgG as a way to extend serum persistence of non‐antibody therapeutic proteins, we have evolved the ectodomain of a low‐affinity human FcγRIIa for enhanced binding to the lower hinge and upper CH2 region of IgG, which is very far from the FcRn binding site (CH2–CH3 interface). High‐throughput library screening enabled isolation of an FcγRIIa variant (2A45.1) with 32‐fold increased binding affinity to human IgG1 Fc (equilibrium dissociation constant: 9.04 × 10−7 M for wild type FcγRIIa and 2.82 × 10−8 M for 2A45.1) and significantly improved affinity to mouse serum IgG compared to wild type human FcγRIIa. The in vivo pharmacokinetic profile of PD‐L1 fused with engineered FcγRIIa (PD‐L1–2A45.1) was compared with that of PD‐L1 fused with wild type FcγRIIa (PD‐L1–wild type FcγRIIa) and human PD‐L1 in mice. PD‐L1–2A45.1 showed 11.7‐ and 9.7‐fold prolonged circulating half‐life (t1/2) compared to PD‐L1 when administered intravenously and intraperitoneally, respectively. In addition, the AUCinf of PD‐L1–2A45.1 was two‐fold higher compared to that of PD‐L1–wild type FcγRIIa. These results demonstrate that engineered FcγRIIa fusion offers a novel and successful strategy for prolonging serum half‐life of therapeutic proteins.

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