Engineered CTLA-4 targeting chimeras for targeted protein degradation and immune synapse reprogramming

Despite the success of immune checkpoint blockade in cancer therapy, durable responses are still limited by the immunosuppressive tumor microenvironment, creating a need for strategies that both deplete suppressive signals and restore T cell activity.

To address this, the authors introduce CTLA-4-targeting chimeras (cTACs), a class of synthetic molecules that harness CTLA-4-mediated trans-endocytosis (TE) to achieve targeted protein degradation across cellular boundaries. The lead candidate, cTAC4.0, consists of the extracellular domain of CD80 fused to an anti-programmed death-ligand 1 (PD-L1) fragment via a human immunoglobulin G4 (IgG4) heavy chain. Unlike existing protein degraders that act within cancer cells through house-keeping pathways, cTAC4.0 functions in trans between immune and tumor cells.

Mechanistically, cTAC4.0 redirects the natural CD80-CTLA-4 internalization pathway to capture and degrade PD-L1 from tumor cells into T cells. It can simultaneously convert PD-L1-mediated suppression into CD28-driven co-stimulation, depending on the relative availability of CTLA-4 or CD28. The authors show that cTAC4.0 promotes CTLA-4-dependent internalization and lysosomal degradation of both membrane-bound and soluble PD-L1, reduces surface PD-L1 levels on target cells, and activates T cells in a CD28-dependent manner.

Functionally, cTAC4.0 enhances the cytotoxicity of both bispecific T cell engagers and T cell receptor (TCR)-engineered T cells in vitro. When combined with these engineered T cells, it suppresses tumor growth in vivo in a glioblastoma xenograft model.

By coupling immune checkpoint degradation with T cell activation in an immune synapse-enriched manner, the cTAC platform establishes a paradigm for intercellular protein degradation systems that simultaneously deplete suppressive ligands and enhance T cell function, offering a potential route to overcome resistance in cancer immunotherapy.