Cathepsin D-Mediated MHC-I Degradation Contributes to Immune Evasion in Microsatellite Stable Colorectal Cancer

Microsatellite stable (MSS) colorectal cancer (CRC) is often characterized as a "cold" tumor, exhibiting minimal responsiveness to monotherapy with PD-1 antibodies. The underlying mechanisms of this intrinsic unresponsiveness to immunotherapy have been elusive. Here, we report that cathepsin D (CTSD) is highly expressed in MSS CRC, significantly contributing to its resistance to immunotherapy. Specifically, CTSD facilitates immune evasion by shielding cancer cells from cytotoxic T-cell-mediated killing. Mechanistically, as a protease, CTSD interacts with the a2 domain of the major histocompatibility complex class I (MHC-I) molecule via its catalytic domain's light chain, promoting the degradation of MHC-I through lysosomal pathways and disrupting the recycling of MHC-I to the cell surface. Notably, deletion or pharmacological inhibition of CTSD with pepstatin A prevents this immune evasion and enhances the efficacy of anti-PD-1 antibodies. Collectively, these findings highlight the role of CTSD in immune evasion and provide a compelling rationale for the development of a novel combination therapy involving CTSD inhibition and anti-PD-1 immunotherapy in CRC. Overall design: To investigate the functional role of CTSD in CRC, NC and CTSD knockdown CT26 tumors. To elucidate the impact of CTSD depletion on the immune microenvironment of MSS CRC, we conducted single-cell RNA sequencing (scRNA-seq) to profile transcriptomic alterations in shCTSD versus shNC CT26 syngeneic tumors. After stringent quality control, we obtained 68,159 high-quality single-cell transcriptomes from both tumor groups