When Jiabo Li examined glioblastoma tumors under the microscope, he found something striking: immune cells meant to fight cancer were almost entirely absent. Instead, the tumors were packed with macrophages—white blood cells that had been hijacked to feed cancer growth rather than suppress it. This discovery led Li and his colleagues at the University of Pittsburgh to uncover a molecular chain of events that turns helpful immune cells into cancer's accomplices, and their findings, published in the Journal of Clinical Investigation, point toward a possible way to stop it.
Glioblastoma is the most common malignant brain tumor in adults, and it is relentlessly fatal. Despite decades of research, survival rates have barely budged, and newer immunotherapies—including checkpoint inhibitors that have transformed treatment for other cancers—have failed to help. The reason, according to the study's senior author Daniel Brat, is that glioblastoma builds an environment specifically designed to silence the immune system. "They're barely there," Brat said of the cytotoxic T-cells that should be attacking the tumor. "They are a minuscule population within the tumor and are excluded. Until we find a way to increase their numbers and function, we're going to have a challenging time improving outcomes."
The team identified a receptor called CLEC5A as a central driver of this immunosuppressive environment. CLEC5A was highly expressed in macrophages clustered in oxygen-deprived, dying regions of the tumor—and patients with high levels of the receptor had worse outcomes. The researchers then traced what activates CLEC5A: glioblastoma cells produce a protein called podoplanin (PDPN), which binds to the receptor and triggers a signaling cascade known as the Syk-JAK-STAT3 pathway. This cascade reprograms macrophages into an immunosuppressive state, dampening the body's defenses against the tumor.
"It's the onset of necrosis that turns these tumors into an immunosuppressive environment," Brat explained. "By most studies, this is the most macrophage-rich signature of all cancers. Unfortunately, those macrophages are not fighting the disease, they're promoting it."
The researchers tested whether blocking this pathway could change the outcome. In mouse models of glioblastoma, removing CLEC5A or inhibiting the Syk signaling protein slowed tumor growth, reduced immune suppression, and extended survival. Crucially, several Syk inhibitors are already FDA-approved for other conditions, raising the possibility that this approach could move from lab to clinic more quickly than a entirely new drug would allow.
The team cautions that targeting this pathway alone may not be enough. Their next step is to combine CLEC5A inhibition with therapies that actively boost T-cell responses—working to reverse immunosuppression while simultaneously energizing the immune system's cancer fighters. "Glioblastoma has been incredibly challenging to tackle," Brat said. "But understanding how these tumors create an immunosuppressive environment gives us a path forward."