A study examined whether macrophage PABPC4 is linked to colorectal cancer progression and immune suppression through SPP1 and CD8+ T-cell activity. In mouse CRC models, a myeloid-specific Pabpc4 conditional knockout, called C4cKO, was associated with lower subcutaneous tumor growth and less intraperitoneal dissemination. Hepatic metastasis showed no obvious change.
The signal appeared in the data
Compared with normal tissues, PABPC4 was among the most increased PABP proteins in CRC-related data. Higher PABPC4 expression was consistently associated with poorer CRC survival, and PABPC4 was upregulated in the AOM/DSS mouse model. Single-cell RNA sequencing of 20 clinical CRC specimens found that PABPC4 expression increased with tumor stage and was predominantly detected in macrophages.
How the link was tested
The mouse model used myeloid-specific Pabpc4 conditional knockouts, and animals were randomized to treatment groups after genotyping. Macrophage RNA sequencing compared four conditions: control, C4cKO, and the corresponding conditioned-medium groups, with three biological replicates per group. Experiments were independently repeated on three occasions. The analysis used two-tailed t tests, one- or two-way analysis of variance, Wilcoxon rank-sum tests for omics data, and Gehan-Breslow-Wilcoxon or log-rank tests for survival.
The immune mix shifted
In C4cKO tumors, total macrophages, M2-like macrophages and MDSCs were fewer, while tumor-infiltrating T cells and CD8+ T cells expressing IFNγ, TNFα and GzmB were more numerous. In macrophage assays, PABPC4 deficiency was associated with higher TNFα and lower Arg1 and IL10 expression. It was not associated with altered macrophage phagocytosis.
Depletion tests changed the picture
In intraperitoneal tumor models, the lower tumor burden associated with C4cKO was lost after CD8+ T-cell depletion. Macrophage depletion also removed the reported C4cKO antitumor effect, whereas CD4+ T-cell depletion did not. Adoptive transfer of C4cKO macrophages was associated with reduced intraperitoneal dissemination.
SPP1 offered a possible mechanism
With SPP1 restored, the C4cKO-associated changes in macrophage polarization and CD8+ T-cell immunity were no longer seen, and the lower-tumor phenotype in peritoneal metastasis was largely abrogated. In molecular tests, Spp1 mRNA had a shorter half-life in Pabpc4-knockout macrophages, meaning the message persisted for less time. RNA-binding and reporter assays supported a direct interaction between PABPC4 and the Spp1 3′UTR, a region of the RNA, with the wild-type reporter favored over a mutant reporter.
The authors interpret these findings as a model in which PABPC4 helps stabilize Spp1 mRNA, preserves an immunosuppressive M2-like macrophage state and dampens CD8+ T-cell effector activity through CD44. In macrophage and CD8+ T-cell co-cultures, CD44 blockade attenuated the C4cKO-associated increase in CD8+ T-cell effector molecules and decrease in PD-1. This is evidence from the tested system, not a patient treatment result.
Human data were suggestive, not decisive
In human CRC tissue and computational cohorts, higher macrophage PABPC4-SPP1 axis activity was associated with tumor-tissue marker upregulation, T-cell dysfunction, immune-checkpoint activity, poorer predicted checkpoint-blockade response and unfavorable overall survival. The human tissue analysis included 17 samples per group, and the TCGA-COAD survival comparison included 223 patients per group. These were observational associations, and the checkpoint-blockade result was predicted rather than a direct measure of treatment benefit.
Important boundaries
Those human results do not show that the axis causes immunotherapy resistance or poor survival in patients. The mouse result also had a clear boundary: C4cKO was not associated with an obvious change in hepatic metastasis. The authors caution that the study's thioglycollate-elicited peritoneal macrophages and bone-marrow-derived macrophages differentiated in vitro do not fully reproduce tumor-associated macrophages. They suggest incomplete Cre recombination in Kupffer cells and the hepatic immune environment as possible reasons for the liver result. Most key findings were reported as qualitative directions, without numerical effect sizes, confidence intervals or exact p values.
Paper data and sources
Original title: Macrophage PABPC4-SPP1 Axis Orchestrates Immunosuppression in Colorectal Cancer.
Authors: Meng Wang, Fang Yang, Yuan Gao et al.
Journal/Repository: Advanced science (Weinheim, Baden-Wurttemberg, Germany)
Status: Peer-reviewed
First online: 2026-08-21
DOI: 10.1002/advs.77361
Original paper