Colorectal cancer (CRC) remains one of the leading causes of cancer death worldwide. While treatments such as surgery and chemotherapy form the backbone of care, many patients develop resistance to these approaches, driving researchers to explore new strategies like immunotherapy.
Immunotherapy works by helping the body's own immune system recognize and destroy cancer cells. Treatments called immune checkpoint inhibitors (ICIs), which target pathways such as PD-1/PD-L1, have transformed the treatment of some cancers. However, in CRC, especially tumors classified as microsatellite stable (MSS), the response rate to these drugs remains frustratingly low.
A major reason for this low response is the immunosuppressive tumor microenvironment (TME). The TME is the complex ecosystem surrounding a tumor that includes immune cells, blood vessels, and signaling molecules. In CRC, this environment is shaped to protect the tumor rather than destroy it.
Within this suppressive environment, a type of immune cell called tumor-associated macrophages (TAMs) plays a central and harmful role. TAMs are highly plastic cells that can shift between pro-inflammatory, cancer-fighting states (called M1) and anti-inflammatory, cancer-promoting states (called M2). In CRC, tumor-derived signals push TAMs toward the M2 state, where they actively suppress the immune response and promote tumor growth.
The ubiquitin-proteasome system is a fundamental cellular machinery that controls which proteins are active and which are degraded. A group of enzymes called deubiquitinating enzymes (DUBs) fine-tune this process by removing the molecular tags that mark proteins for destruction. Several DUBs have been recognized as important players in cancer biology.
Among DUBs, USP14 (Ubiquitin-Specific Protease 14) had previously been linked to promoting cancer cell growth and drug resistance in CRC. It does this partly by stabilizing proteins that help cancer cells evade the immune system, including IDO1, which suppresses anti-tumor immunity.
What was not known was whether USP14 also plays a role inside the immune cells of the TME, specifically inside TAMs. The research team hypothesized that USP14 might be a critical driver of the M2-like immunosuppressive state in colon cancer TAMs, making it a promising new therapeutic target.
To test this, the researchers first performed an unbiased screen of 68 DUB-related genes across macrophage populations isolated from mouse colon tumors compared to laboratory-derived M1 and M2 macrophages. This approach allowed them to identify which DUBs were uniquely overactive specifically inside the tumor environment, rather than in normal immune states.
The screening experiment revealed a striking finding: USP14 was specifically and dramatically overexpressed in TAMs isolated from colon tumors. Compared to normal macrophages (M0), USP14 mRNA levels were approximately 12.8-fold higher in tumor-associated macrophages. This was confirmed at the protein level through western blot analysis.
To determine whether USP14 is functionally tied to the M2 immunosuppressive state, the team treated macrophages with a small molecule inhibitor of USP14 called IU1. Treatment with IU1 significantly reduced the expression of CD206, a marker of the M2 pro-tumor state, both in isolated TAMs and in laboratory-derived M2 macrophages. Silencing USP14 with genetic tools (siRNA) produced the same effect, confirming the causal link.
The clinical relevance of these findings was supported by analysis of human cancer data from The Cancer Genome Atlas (TCGA). USP14 was found to be significantly elevated in human colon adenocarcinoma (COAD) tumor tissues compared to normal adjacent tissue. Elevated USP14 correlated with signatures of immunosuppression, including increased infiltration of M2 macrophages, regulatory T cells (Tregs), and myeloid-derived suppressor cells (MDSCs).
Single-cell RNA sequencing data from human CRC confirmed that USP14 expression is predominantly enriched in macrophages within the tumor microenvironment. Together, these findings established USP14 as a functionally important and clinically relevant regulator of immunosuppression specifically within the colon cancer TME.
Moving from cell culture to living animals, the researchers treated MC38 colon-tumor-bearing mice with the USP14 inhibitor IU1. The results were dramatic: IU1 inhibited tumor growth by 78%, reducing average tumor volume from 1100 mm3 in controls to just 235 mm3 in treated mice by day 21.
To understand how USP14 inhibition achieved this, the team performed detailed immune profiling of the tumor microenvironment. They found a profound shift in the myeloid compartment: M2-polarized TAMs decreased by approximately 2.5-fold, while M1-polarized macrophages increased significantly. The number of myeloid-derived suppressor cells, another immunosuppressive population, also declined sharply.
This myeloid reprogramming had powerful downstream effects on the adaptive immune system. The frequency of tumor-killing CD8+ cytotoxic T lymphocytes (CTLs) increased 1.5-fold. More importantly, functional subsets of these cells expanded dramatically: IFN-gamma-producing CTLs rose 2.7-fold, and activated CD8+ T cells (CD69+) increased 3.1-fold.
Cytokine profiling of the tumor tissue confirmed this immunological shift. Pro-inflammatory cytokines including IFN-gamma, TNF-alpha, IL-2, and IL-12 were significantly elevated, while the immunosuppressive cytokine IL-10 was reduced by 50%. These findings paint a clear picture: blocking USP14 transforms the tumor microenvironment from a state that protects the cancer into one that attacks it.
To formally prove that the beneficial effects of USP14 inhibition were truly mediated through macrophages, the researchers used a macrophage depletion strategy. Mice were treated with clodronate liposomes, a well-established method that selectively eliminates macrophages, reducing them by more than 85% in the blood and 70% in tumors.
When macrophages were depleted, the ability of IU1 to activate CD8+ T cells was completely abolished. The robust expansion of IFN-gamma-producing CTLs seen with IU1 treatment vanished entirely in macrophage-depleted mice. This was direct experimental proof that USP14 inhibition works primarily by reprogramming macrophages, not through direct effects on other cell types.
The mechanistic basis of macrophage reprogramming was uncovered through RNA sequencing of TAMs isolated from treated versus untreated tumors. Gene set enrichment analysis confirmed a clear shift: M1-associated genes (Tnf, Il12b, Nos2) were upregulated, while M2-associated genes (Arg1, Mrc1, Cd163) were downregulated.
Pathway analysis identified the MAPK signaling pathway (specifically JNK and p38 kinases) as the most significantly activated by USP14 inhibition. Experiments using specific MAPK inhibitors showed that blocking JNK or p38 reversed the reprogramming effect of IU1, establishing these kinases as key molecular mediators of USP14's control over macrophage identity.
A critical and unexpected finding emerged when the researchers examined the effect of USP14 inhibition on PD-L1 expression. While IU1 treatment reprogrammed macrophages, it also specifically increased PD-L1 expression on tumor cells themselves, without changing it on macrophages or dendritic cells.
This appeared to be an adaptive resistance mechanism: as IU1 activated anti-tumor T cells (which produce IFN-gamma), the tumor cells responded by upregulating PD-L1 to suppress those T cells. Rather than being a setback, this observation revealed a clear rational opportunity: the elevated PD-L1 made tumor cells more vulnerable to anti-PD-1 immunotherapy.
The team tested this combination in mice. Animals received one of four treatments: vehicle control, IU1 alone, anti-PD-1 alone, or the combination. The combination therapy dramatically outperformed all single agents, reducing average tumor volume by approximately 90% by day 21. The synergy was striking and highly significant.
The survival benefit was equally impressive. In Kaplan-Meier survival analysis, all control mice succumbed to cancer by day 30, while 60% of mice in the combination group survived beyond day 50. Immunofluorescence of tumor sections confirmed that combination therapy produced near-complete elimination of M2 macrophages alongside the most extensive infiltration of CD8+ cytotoxic T cells of any group tested.
The study revealed an elegant sequential logic underlying the combination strategy. USP14 inhibition first reprograms suppressive TAMs into an anti-tumor M1 state and activates CD8+ T cells. These activated T cells produce IFN-gamma, which in turn induces PD-L1 upregulation on tumor cells as a defense mechanism.
Rather than undermining the therapy, this adaptive PD-L1 upregulation creates a specific molecular vulnerability on the tumor surface that can be precisely targeted by anti-PD-1 antibodies. The two drugs thus work together in a logical sequence: the first dismantles immune suppression in the myeloid compartment, and the second blocks the tumor's secondary defense response in the T cell compartment.
The researchers noted that this strategy is particularly relevant for MSS colorectal cancer, which makes up the majority of CRC cases but typically does not respond to anti-PD-1 therapy alone. By using USP14 inhibition to reshape the TME first, it may be possible to convert these cold, non-responding tumors into hot, immunotherapy-responsive ones.
Previous studies had shown USP14 plays roles in tumor cell proliferation and drug resistance. This work adds an entirely new dimension by revealing USP14's function inside the immune compartment specifically within the TME. This positions USP14 as what the authors describe as a novel myeloid-specific target that could be exploited to expand the benefit of immunotherapy to more patients.
The authors are transparent about important limitations of their study. All in vivo experiments were performed using a single syngeneic mouse model (MC38 cells implanted subcutaneously in C57BL/6 mice). While valuable for mechanistic studies, this model does not fully capture the complexity of human CRC, particularly the biology of MSS tumors or metastatic disease.
Mechanistically, while the study clearly shows that USP14 inhibition activates MAPK signaling (specifically JNK and p38) to reprogram macrophages, the direct molecular substrate of USP14 responsible for initiating this signaling remains unknown. Identifying which protein USP14 normally keeps stabilized will be an important goal for future research.
The study also focused exclusively on pharmacological inhibition using IU1. Future experiments using conditional knockout mice with macrophage-specific deletion of USP14 would provide more definitive genetic proof of the cell-specific role of this enzyme in the TME.
Despite these limitations, the study provides a compelling and well-supported preclinical rationale for USP14 as a new therapeutic target in CRC immunotherapy. The consistent results across in vitro experiments, in vivo mouse models, and human TCGA data analysis make USP14 inhibition a strong candidate for future clinical investigation, especially in combination with anti-PD-1 checkpoint therapy.