The research, led by Shujie Zeng, Leyan Cao, Wenxuan Dong, Leping Li, Zihao Zhang and Peng Jin at institutions including Shandong Provincial Hospital, Shandong Cancer Hospital and Institute, and Zhongshan Hospital of Fudan University, focuses on a protein called semaphorin 4D, abbreviated SEMA4D. Semaphorins were originally discovered as guidance cues that steer growing nerve fibers to their correct destinations, but over the past two decades they have been recognized as versatile signaling molecules in the immune system and in cancer. SEMA4D in particular has attracted attention because high levels of the protein in tumor cells are associated with an immunosuppressive tumor microenvironment, meaning a milieu in which immune cells are present but rendered ineffective, or excluded from the tumor altogether.

What has remained poorly understood is how the conversation between tumor cells and macrophages regulates SEMA4D and, crucially, how the soluble, freely circulating form of the protein is generated inside living tumors. The new work addresses this gap by combining laboratory co-culture experiments with genetically engineered mouse models, allowing the researchers to trace the entire regulatory axis from tumor-derived SEMA4D to macrophage behavior to T cell function, and finally to tumor growth itself.

The team’s in vitro experiments used co-culture systems in which tumor cells and macrophages were grown together, alongside models in which SEMA4D production was either increased or shut down. These experiments revealed that tumor-derived SEMA4D pushes tumor-associated macrophages, the macrophages that populate tumors, toward the M2 phenotype. Macrophage polarization is a central concept in tumor immunology. M1 macrophages are classically activated, inflammatory cells that present antigens, produce pro-inflammatory signals and support anti-tumor immunity. M2 macrophages, by contrast, are associated with tissue repair, wound healing and, in the context of cancer, immunosuppression. A tumor microenvironment dominated by M2 macrophages is one in which cytotoxic CD8-positive T cells, the immune system’s primary tumor-killing weapons, are inhibited in both their infiltration into the tumor and their effector function once they arrive.

The study found precisely this pattern: tumor-derived SEMA4D drove M2 polarization of tumor-associated macrophages and suppressed the infiltration and function of CD8-positive T cells. But the most consequential finding concerned the mechanism by which SEMA4D becomes soluble. SEMA4D normally sits on the cell surface as a membrane-bound protein. For it to diffuse through the tumor microenvironment and act at a distance, it must be cleaved from the cell surface, and the identity of the molecular scissors responsible in tumors has been a key open question.

The answer, according to the new research, is matrix metalloproteinase 14, or MMP14, an enzyme produced by the tumor-associated macrophages themselves. MMP14 is a membrane-anchored member of the matrix metalloproteinase family, enzymes best known for remodeling the extracellular matrix, the protein scaffold that gives tissues their structure. The study demonstrates that TAM-derived MMP14 promotes the generation of soluble SEMA4D, thereby converting a membrane-bound tumor protein into a diffusible immunosuppressive signal. In other words, the macrophages recruited into the tumor are not passive bystanders; they are equipped with the enzymatic machinery that activates the tumor’s immune-evasion program.

To establish this mechanism in living animals rather than in culture dishes, the researchers used a macrophage-specific conditional knockout mouse model, in which the gene encoding MMP14 was deleted only in macrophages, leaving the enzyme intact in other cell types. This genetic precision matters, because MMP14 is expressed in many tissues and a global knockout would produce broad developmental and physiological effects that would obscure the specific role of the macrophage-derived enzyme in tumor immunity. With MMP14 removed from macrophages, the levels of soluble SEMA4D in the tumor microenvironment dropped significantly.

The consequences of that reduction were dramatic. Genetic ablation of MMP14 in tumor-associated macrophages reversed the M2 polarization of macrophages and promoted robust infiltration of both M1 macrophages and CD8-positive T cells into the tumor core, the innermost and most immunologically hostile region of the tumor. Immune exclusion, the physical barrier that keeps cytotoxic T cells at the tumor periphery, is one of the major reasons why immunotherapies such as immune checkpoint inhibitors fail in many patients. By dissolving this barrier at its enzymatic source, the MMP14 knockout effectively opened the tumor to immune attack, and the mice showed tumor regression as a result.

Perhaps the most convincing element of the study is the rescue experiment. To demonstrate that soluble SEMA4D is not merely correlated with the observed immune changes but is causally responsible for them, the researchers supplemented the MMP14 knockout mice with recombinant soluble SEMA4D, the purified protein produced in the laboratory. The result was unambiguous: exogenous soluble SEMA4D fully abrogated the anti-tumor benefits of MMP14 deletion, re-establishing the immunosuppressive tumor microenvironment despite the absence of the macrophage enzyme. This kind of loss-of-function and rescue design is the gold standard for establishing causal mechanisms in vivo, and it strongly supports the conclusion that the macrophage-MMP14-SEMA4D axis is a genuine driver of immune exclusion rather than a secondary correlate of tumor biology.

The therapeutic implications are considerable. Current immunotherapies, including checkpoint inhibitors targeting PD-1 and PD-L1, depend on the presence of functional T cells within the tumor. Patients whose tumors are immunologically cold, lacking T cell infiltration, rarely respond. The new findings suggest that targeting the macrophage-MMP14-SEMA4D regulatory axis could convert cold tumors into hot ones by removing the soluble signal that keeps T cells out, potentially sensitizing tumors to existing immunotherapies. SEMA4D-targeting agents are already being explored in clinical development for other indications, and the study provides a mechanistic rationale for combining such approaches with strategies that modulate macrophage behavior. The authors note that targeting this axis represents a promising therapeutic strategy for enhancing antitumor immunity. As with all preclinical mouse studies, translation to human cancers will require further validation, but the identification of a specific, druggable enzyme acting as the gatekeeper of an immunosuppressive signal offers a concrete molecular target in the ongoing effort to make immunotherapy work for more patients.

Subject of Research: The role of macrophage-derived MMP14 in generating soluble semaphorin 4D and shaping immunosuppression in the tumor microenvironment

Article Title: Macrophage-derived MMP14 fosters immunosuppressive tumor microenvironment by regulating soluble semaphorin 4D

Article References: Zeng, S., Cao, L., Dong, W., Li, L., Zhang, Z., & Jin, P. (2026). Macrophage-derived MMP14 fosters immunosuppressive tumor microenvironment by regulating soluble semaphorin 4D. Cell Death Discovery. https://doi.org/10.1038/s41420-026-03393-4

Image Credits: AI Generated

DOI: 10.1038/s41420-026-03393-4

Keywords: macrophages, MMP14, semaphorin 4D, tumor microenvironment, immunosuppression, CD8-positive T cells, M2 polarization, immune exclusion, cancer immunotherapy, tumor-associated macrophages, Cell Death Discovery, conditional knockout