By Nana O 
A groundbreaking study spearheaded by researchers at the Johns Hopkins Kimmel Cancer Center Bloomberg~Kimmel Institute for Cancer Immunotherapy and the Johns Hopkins University School of Medicine has illuminated a critical, previously unrecognized partnership within aggressive glioblastoma, the most common and deadliest form of primary brain cancer. The findings, published on January 17th in the prestigious journal Science, reveal a distinct population of immune cells that actively support the growth and recalcitrance of these treatment-resistant tumors, offering a vital new avenue for therapeutic intervention.
Unveiling the Glioblastoma Microenvironment
Glioblastoma, a grade 4 astrocytoma, is notorious for its rapid progression and devastating prognosis, with a median survival rate of only about 15 months even with aggressive treatment. Its ability to infiltrate surrounding brain tissue and resist conventional therapies like surgery, radiation, and chemotherapy stems from a complex interplay of cellular components within the tumor microenvironment. For years, scientists have sought to understand the intricate cellular dynamics that allow these tumors to thrive and evade the body’s own defenses.
This new research delved into the innermost workings of glioblastoma, specifically focusing on the cellular makeup of the most aggressive, grade 4 tumors. Employing a cutting-edge technology known as spatial genomics, which allows for the simultaneous analysis of genetic material and its location within tissue, the investigators sought to identify unique immune cell subtypes present in these formidable cancers.
The Discovery of a Symbiotic Partnership
The team’s meticulous analysis uncovered a significant co-localization between glioblastoma stem cells and a specific type of immunosuppressive immune cell called myeloid-derived suppressor cells (MDSCs). Glioblastoma stem cells, though comprising a small fraction of the tumor (estimated at 5% to 10%), are considered the "seeds" of the tumor, responsible for its continuous renewal, growth, and ultimately, its aggressive nature.
"Tumor stem cells represent only 5% to 10% of the tumor, but they’re the critical cells that are renewing and generating the rest of the tumor and are essentially responsible for the aggressiveness of the tumor," explained senior study author Drew Pardoll, M.D., Ph.D., the Martin D. Abeloff Professor of Cancer Research and director of the Bloomberg~Kimmel Institute for Cancer Immunotherapy. "We found that the myeloid-derived suppressor cells and tumor stem cells literally were in the same place – a region described by pathologists in the 1980s as the pseudopalisading region. There was a very intimate connection."
This intimate connection, the researchers discovered, is not merely spatial but functional, forming a symbiotic relationship where each cell type actively fuels the other’s proliferation and the overall tumor’s malignancy.
Methodological Advancements: Spatial Genomics and Single-Cell Sequencing
To achieve this breakthrough, the researchers employed a multi-pronged approach. Initially, they performed single-cell RNA sequencing on tissue samples from 33 different types of brain tumors, ranging from low-grade to high-grade, to meticulously characterize their cellular components. This process identified two distinct populations of MDSCs within IDH-wildtype glioblastomas, the most common subtype.
Subsequently, the team utilized spatial transcriptomics, a sophisticated technique that maps gene expression patterns across hundreds of thousands of individual cells within their tissue context. This allowed them to examine over 750,000 immune cells and more than 350,000 tumor and associated cells in the glioblastoma samples. The spatial transcriptomics data confirmed the critical finding: MDSCs were found to be intimately located alongside the glioblastoma stem cells.
"Glioblastoma is a highly aggressive brain tumor with a remarkable ability to evade the immune system, which has made immune-based therapies largely ineffective to this point," stated first and co-corresponding author Christina Jackson, M.D., an assistant professor of neurosurgery at the Perelman School of Medicine at the University of Pennsylvania, who was at Johns Hopkins during the research. "Our study revealed a distinct subset of immune cells, known as myeloid-derived suppressor cells, that promote glioblastoma growth, providing new insights into how the tumor interacts with the immune system. By identifying these cells and their role, we hope to uncover new therapeutic targets and lay the groundwork for more effective treatments."
The Mechanisms of Mutual Support
The researchers meticulously unraveled the specific molecular dialogue occurring between glioblastoma stem cells and MDSCs. They found that the tumor stem cells were actively producing chemical signals, known as chemokines, which acted as beacons to attract MDSCs to the tumor site. Furthermore, these stem cells were also secreting growth and activation factors specifically designed to bolster the MDSCs.
In a reciprocal fashion, the MDSCs, once recruited and activated, were themselves producing growth factors that directly benefited the tumor cells, creating a self-sustaining engine of tumor progression.
Two key molecular signals identified as crucial players in this communication were Interleukin-6 (IL-6) and Interleukin-8 (IL-8). These cytokines are well-known for their roles in inflammatory responses, and importantly, MDSCs possess specific receptors for them.
"IL-8 is one of the major attractants to bring the MDSCs to the tumor, and IL-6 is one of the major activators of the MDSCs," Dr. Pardoll elaborated.
A Novel Player in Tumor Nourishment
Adding another layer to this complex interaction, the study identified a previously uncharacterized mechanism of tumor sustenance. The MDSCs were found to secrete a growth factor called Fibroblast Growth Factor 11 (FGF11). Significantly, FGF11 had never before been implicated in the growth or progression of brain cancers, or indeed other cancers, highlighting the novelty of this discovery. This finding suggests that MDSCs not only suppress anti-tumor immunity but also actively contribute to the direct nourishment and proliferation of glioblastoma stem cells.
Correlation with Tumor Aggressiveness and Patient Survival
The research team also explored the implications of their findings in the context of different glioblastoma subtypes and patient outcomes. They observed that tumors with a mutation in the IDH1 gene, which are generally less aggressive than their IDH-wildtype counterparts, exhibited a near absence of MDSCs and significantly fewer cancer stem cells.
This observation prompted a broader investigation into the correlation between MDSC infiltration and patient survival across various brain cancers. Leveraging the extensive National Cancer Institute’s Cancer Genome Atlas (TCGA) database, which houses a vast repository of cancer genomic and clinical data, the researchers found a strong and consistent correlation: patients with fewer cancer stem cells and lower levels of MDSCs in their tumors generally experienced better survival outcomes. This correlation underscores the detrimental role of this cellular alliance in dictating the clinical trajectory of brain tumors.
Implications for Future Therapies
The identification of this symbiotic relationship between glioblastoma stem cells and MDSCs opens up promising new avenues for therapeutic development. By targeting either the signals that attract and activate MDSCs, or the factors that MDSCs produce to support tumor growth, it may be possible to disrupt this detrimental alliance and halt tumor progression.
"While additional studies are needed to further understand these cellular interactions, the work is exciting in that it suggests additional potential targets to block in treatment of these aggressive brain tumors," Dr. Pardoll noted.
One such promising approach is already under development. Jamie Spangler, Ph.D., an associate professor of biomedical engineering at Johns Hopkins, has engineered an investigational bispecific antibody. This antibody is designed to simultaneously bind to the receptors for both IL-6 and IL-8, effectively blocking the signaling pathways that attract and activate MDSCs. This represents a tangible step towards translating these fundamental research findings into clinical applications.
A Collaborative Effort and Future Directions
This pivotal research represents a significant collaborative effort, involving a multidisciplinary team of scientists. The study co-authors include Christopher Cherry, Sadhana Bom, Arbor Dykema, Rulin Wang, Elizabeth Thompson, Ming Zhang, Runzhe Li, Zhicheng Ji, Wenpin Hou, Wentao Zhan, Hao Zhang, John Choi, Ajay Vaghasia, Landon Hansen, Kate Jones, Fausto Rodriguez, Jon Weingart, Calixto-Hope Lucas, Jonathan Powell, Jennifer Elisseeff, Srinivasan Yegnasubramanian, Chetan Bettegowda, and Hongkai Ji, all from Johns Hopkins. Researchers from Stanford University School of Medicine also contributed to the work.
The research was generously supported by grants from the National Institutes of Health, the Neurosurgery Research Education Foundation, the Bloomberg~Kimmel Institute for Cancer Immunotherapy, the Mark Foundation for Cancer Research, a Burroughs Wellcome Career Award for Medical Scientists, the Commonwealth Foundation, the Maryland Cigarette Restitution Fund, and the NIH Pioneer Award.
The study also acknowledged potential conflicts of interest, with several researchers having consulting roles, advisory positions, or financial interests in biotechnology and pharmaceutical companies. These relationships are managed by The Johns Hopkins University in accordance with its conflict-of-interest policies, ensuring transparency and ethical conduct.
The discovery of this symbiotic relationship between glioblastoma stem cells and MDSCs marks a significant leap forward in our understanding of brain cancer biology. It offers a beacon of hope for developing novel and more effective therapeutic strategies against this formidable disease, potentially transforming the outlook for patients facing glioblastoma. Further research will undoubtedly build upon these findings, aiming to translate this intricate cellular dance into life-saving clinical interventions.
