Background/Purpose: The dysregulation of immune checkpoint receptors on T cells and antigen presenting cells (APCs) drives autoimmunity while receptor agonism is expected to restore immune homeostasis in diseases such as rheumatoid arthritis, systemic lupus erythematosus, Sjogren’s syndrome, and giant cell arteritis. PD-1 is a checkpoint receptor expressed on T cells, including Tfh and Treg subtypes. Strong PD-1 agonism requires super-clustering, which can be achieved by therapeutic antibodies binding to Fc gamma receptors (FcγR) on APCs. S-4321 is a novel dual-cell bidirectional (DcB) PD-1:FcγRIIb antibody that agonizes the inhibitory PD-1 receptor on T cells (Fab domain) and selectively engages the inhibitory FcγRIIb on APCs (Fc domain), eliciting an inhibitory signal at the immune cell synapse to re-establish homeostasis.

Methods: Staphylococcal enterotoxin B (SEB) activated human PBMCs were cultured with S-4321 and benchmark PD-1 agonist antibodies. T cell PD-1 expression was measured by flow cytometry and IL-2 production by Meso Scale Discovery (MSD) assay. Induced Tregs (iTregs) were differentiated in vitro by culturing human CD4 naïve T cells with TGFβ and IL-2 in the presence of S-4321. A graft versus host disease (GvHD) model was generated by transferring T cells isolated from a human PD-1 knock-in mouse into a B6D2F1/J model. Immunophenotyping was performed by flow cytometry and cytokine production measured by ELISPOT and Luminex assays. S-4321 function was tested pre-clinically in cynomolgus monkeys.

Results: S-4321 achieves prolonged agonism by binding to PD-1 with low affinity (similar to PD-L1) thus allowing for the retention of PD-1 expression on the T cell surface. This ensures continued presence of target and maintains a regulatory check on effector cell activity. In contrast, higher affinity PD-1 agonists induce a marked decrease in surface PD-1, associated with increased IL-2 production in SEB activated cultures. S-4321 also selectively engages FcγRIIb, avoiding the induction of proinflammatory cytokines and undesirable depletion of PD-1 expressing T cells, such as Tregs, by antibody-dependent cellular cytotoxicity (ADCC) through engagement of activating FcγR.

S-4321 promotes iTregs in the presence of TGFβ and IL-2 (p< 0.05) in vitro. Using the B6D2F1/J model of murine GvHD, prophylactic treatment with S-4321 reduces engrafted T cell expansion and proinflammatory cytokine production (p< 0.05). Cynomolgus monkey data demonstrate dose-proportional exposure and ~70% bioavailability of S-4321 with subcutaneous dosing.

Conclusion: S-4321 is a novel DcB PD-1:FcγRIIb agonist that engages inhibitory receptors on both sides of the T cell-APC synapse. It agonizes PD-1 without causing target or cell depletion and its high bioavailability allows for convenient administration. In vivo, S-4321 decreases T cell activation that contributes to the pathogenesis of autoimmune diseases and avoids the depletion of Tregs, important for restoring peripheral tolerance.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/s-4321-a-novel-dual-cell-bidirectional-pd-1fc%ce%b3riib-selective-agonist-antibody-for-the-treatment-of-autoimmune-disease/