Abstract
Colorectal cancer (CRC) remains largely refractory to immune-checkpoint blockade, with adenomatous polyposis coli (APC) mutations present in 80%-90% of cases. Loss of APC was previously thought to promote tumor progression mainly through deregulated Wnt/β-catenin signaling. Here, we report that APC loss leads to inhibition of CD8+ T cell infiltration and CRC immune evasion through the dephosphorylation of signal transducers and activators of transcription 1 (STAT1) by protein tyrosine phosphatase non-receptor type 13 (PTPN13), independently of β-catenin. Peptides containing the last 11 C-terminal amino acid (aa) residues of APC (APC11) bind directly to PTPN13 to block PTPN13-STAT1 interactions and facilitate STAT1 phosphorylation, interferon regulatory factor-1 (IRF1) expression, major histocompatibility complex (MHC) class I antigen presentation, and T cell intratumoral infiltration, all of which eventually inhibit tumor progression and enhance the effects of programmed cell death 1 (PD1) blockade. Thus, we have identified a previously unknown APC/PTPN13/STAT1-dependent tumor immune-suppressive mechanism. The potent tumor-suppressing effect of combining anti-PD1 antibodies with APC11 peptides provides a compelling target and rationale for future development of anti-tumor drugs for patients with CRC.