Bridging innate and adaptive immunity: A bispecific SIRPαD1-αPCSK9 fusion protein reprograms macrophages for durable tumor clearance.

Macrophage-directed immunotherapy, exemplified by blocking the CD47-signal regulatory protein α (SIRPα) axis, can eliminate tumors by unleashing phagocytosis. However, such approaches often fail to generate durable antitumor immunity, revealing a disconnect between innate activation and effective T cell responses. Here, we identify proprotein convertase subtilisin/kexin type 9 (PCSK9) as an adaptive resistance factor induced by macrophage checkpoint blockade. Tumor-derived PCSK9 promotes lysosomal degradation of major histocompatibility complex class I (MHC-I) in macrophages, impairing antigen cross-presentation and CD8+ T cell priming-thus showing how phagocytosis-targeted therapies can paradoxically suppress adaptive immunity. To overcome this, we engineer bispecific fusion proteins co-targeting the CD47-SIRPα checkpoint and the PCSK9-MHC-I axis, among which SIRPαD1-αPCSK9 is optimal. Dual targeting synergistically enhances phagocytosis, preserves antigen cross-presentation, and reprograms tumor microenvironment toward an immunostimulatory state. Consequently, SIRPαD1-αPCSK9 activates innate and adaptive antitumor immunity, achieving potent tumor control with improved safety. These findings uncover a resistance mechanism to macrophage-centered cancer immunotherapy and establish a dual-targeting strategy bridging phagocytosis and T cell activation.