Targeted immunosuppression with a tissue-selective PD-1 agonist prevents rejection of transplanted hearts

Introduction/Rationale: Systemic immunosuppression to prevent allograft rejection or treat autoimmune disease can risk infection and malignancy. To uncouple therapeutic immunosuppression from systemic toxicity, we engineered Location-Specific T-cell Inhibitory Molecules (LoSTIMs), Fc-silent bispecific antibodies that activate PD-1 and inhibit T-cells only when they engage a predefined tissue surface antigen. We hypothesized that tissue-selective PD-1 agonism could protect allografts from rejection while sparing systemic immunity.

Methods: We profiled PD-1 agonism, epitope specificity, and binding kinetics of anti-PD-1 antibodies, then engineered Fc-silent bispecific LoSTIMs. We evaluated antigen-dependent PD-1 agonism in Jurkat reporter assays and inhibition of T-cell activation in primary T cells. We tested a donor MHC class I (H-2Kb)-specific LoSTIM in fully mismatched heterotopic cardiac transplant models, assessing graft survival and immune cell phenotype by flow cytometry and spatial transcriptomics.

Results: LoSTIMs acted as conditional PD-1 agonists, inhibiting T cells only when engaging target antigen, and were FcγR independent, enabling Fc-silent designs that do not deplete PD-1+ T-cells and preserve physiologic immunity outside the target tissue. Systemic treatment with H-2Kb-specific LoSTIM durably prevented rejection of C57BL/6 allografts in BALB/c recipients, whereas monospecific anti-PD-1 or anti-H-2Kb did not. Flow cytometry and spatial transcriptomics showed reduced donor-specific priming and graft infiltration, loss of effector function in graft-infiltrating CD8+ T cells, and a shift toward less inflammatory, regulatory/progenitor-like T-cell populations. Third-party C3H cardiac allografts (H-2Kk) lacking the H-2Kb surface antigen were not protected, consistent with tissue-selective immunosuppression.

Conclusion: LoSTIMs provide systemically administered, tissue-restricted PD-1 agonism that prevents allograft rejection without global immune suppression and may be generalizable to other immune diseases.