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 identified a distinct population of immune cells that actively support the growth and aggressiveness of a particularly challenging form of brain cancer. This discovery sheds new light on glioblastoma, an aggressive, treatment-resistant brain tumor, and offers potential new avenues for therapeutic intervention. The findings were published on January 17th in the prestigious scientific journal Science.
Unveiling the Tumor’s Immune Allies
Glioblastoma, classified as a grade 4 brain tumor, represents the most severe and life-threatening form of primary brain cancer. Its notoriously aggressive nature and profound resistance to conventional treatments have long perplexed oncologists and researchers. A key challenge in treating glioblastoma stems from its ability to effectively evade the body’s immune system, a characteristic that has historically rendered many immunotherapies ineffective.
The Johns Hopkins-led team embarked on a mission to dissect the intricate cellular landscape of these formidable tumors, specifically focusing on the less-understood immune cell subtypes present in the most advanced glioblastomas. Leveraging a cutting-edge technology known as spatial genomics, which allows for the detailed mapping of gene expression within the physical context of tissue, the researchers were able to pinpoint a critical symbiotic relationship.
Their investigations revealed that glioblastoma stem cells, the self-renewing cells responsible for generating and sustaining the tumor’s growth, were not only present but also intimately co-localized with a specific type of immunosuppressive immune cell: myeloid-derived suppressor cells (MDSCs). This spatial proximity suggests a direct and active interaction, a finding that challenges previous assumptions about the tumor’s immune microenvironment.
The Symbiotic Dance: A Mutualistic Fueling Mechanism
The research elucidated a sophisticated mutualistic relationship between these two cell types. Glioblastoma stem cells, which constitute a small but critically important fraction of the tumor (estimated at 5% to 10%), play a pivotal role in driving tumor progression and aggressiveness. These stem cells were found to actively recruit MDSCs to their vicinity by secreting specific chemical signals, known as chemokines. Furthermore, the tumor stem cells were observed to provide growth and activation factors essential for the MDSCs’ function.
In a reciprocal fashion, the MDSCs, in turn, were discovered to secrete growth factors that directly nourish and promote the proliferation of the glioblastoma stem cells, thereby perpetuating the tumor’s relentless expansion. This intricate feedback loop creates a powerful, self-sustaining engine for tumor growth and resistance.
"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 Dr. Drew Pardoll, the senior study author, the Martin D. Abeloff Professor of Cancer Research, co-director of the Mark Foundation Center for Advanced Genomics and Imaging, 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."
The pseudopalisading region, a histological hallmark of glioblastoma, refers to the characteristic arrangement of tumor cells that surround areas of necrosis (dead tissue). This discovery of MDSCs within these critical zones underscores their direct involvement in the tumor’s core machinery.
Technological Advancements Enabling Discovery
The breakthrough was made possible by the application of advanced technologies. The research team initially performed single-cell RNA sequencing on tissue samples from 33 different types of brain tumors, ranging from low-grade to high-grade malignancies. This analysis helped to identify distinct populations of immune cells, including two specific subtypes of MDSCs found in IDH-wild-type (IDH-WT) glioblastomas, the most common and aggressive form.
Subsequently, the researchers employed spatial transcriptomics, a sophisticated technique that analyzes gene expression patterns across hundreds of thousands of individual cells within their original tissue architecture. This allowed them to map the precise locations and interactions of over 750,000 immune cells and more than 350,000 tumor and associated cells within the glioblastoma samples. The spatial transcriptomics data unequivocally confirmed the co-localization of MDSCs with tumor stem cells.
"Glioblastoma is a highly aggressive brain tumor with remarkable ability to evade the immune system, which has made immune-based therapies largely ineffective to this point," stated Dr. Christina Jackson, the first and co-corresponding author of the study. Dr. Jackson, an assistant professor of neurosurgery at the Perelman School of Medicine at the University of Pennsylvania, was affiliated with Johns Hopkins during the research period. "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."
Pinpointing the Molecular Messengers
The researchers delved deeper to identify the specific molecular signals exchanged between tumor stem cells and MDSCs. They discovered that tumor stem cells produce chemokines, such as interleukin-6 (IL-6) and interleukin-8 (IL-8), which act as potent attractants for MDSCs. These interleukins are well-known players in inflammatory responses, and MDSCs possess specific receptors designed to bind to 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, highlighting the crucial roles of these cytokines.
On the other side of this cellular dialogue, MDSCs were found to secrete a growth factor called fibroblast growth factor 11 (FGF11). This molecule had not previously been implicated in the progression of brain cancers or other malignancies, representing a novel finding in cancer biology. The secretion of FGF11 by MDSCs further reinforces their supportive role in fueling tumor stem cell activity.
Implications for Treatment and Patient Outcomes
The study also provided a crucial comparative analysis. Tumors harboring a mutation in the IDH1 gene, which are generally less aggressive than IDH-WT glioblastomas, showed a marked deficiency in MDSCs and a significantly lower presence of cancer stem cells. This observation prompted the researchers to investigate the broader correlation between MDSC infiltration and patient survival across all types of brain cancers.
Utilizing the extensive National Cancer Institute’s Cancer Genome Atlas (TCGA) database, which contains a vast collection of cancer samples and associated genomic data, the team confirmed a strong correlation: tumors with fewer cancer stem cells and lower levels of MDSCs were associated with better patient outcomes. This finding solidifies the role of these cellular components as key determinants of glioblastoma’s aggressive phenotype and its impact on patient survival.
A Glimmer of Hope for Future Therapies
While further research is essential to fully comprehend the complex interplay between these cells, this discovery opens up promising avenues for developing novel therapeutic strategies. The identification of specific molecular targets, such as IL-6, IL-8, and FGF11, offers the potential to disrupt the symbiotic relationship and thereby inhibit tumor growth.
"The work is exciting in that it suggests additional potential targets to block in treatment of these aggressive brain tumors," Dr. Pardoll remarked.
One such promising development is an investigational bispecific antibody engineered by Dr. Jamie Spangler, an associate professor of biomedical engineering at Johns Hopkins. This antibody is designed to bind to the receptors for both IL-6 and IL-8, effectively blocking their signaling pathways and potentially interrupting the recruitment and activation of MDSCs by tumor stem cells. Such targeted therapies could represent a significant advancement in the fight against glioblastoma.
The collaborative nature of this research is further highlighted by the extensive list of contributing authors from Johns Hopkins, as well as collaborators from the Stanford University School of Medicine. The study was generously supported by a multitude of grants and foundations, including the National Institutes of Health, the Neurosurgery Research Education Foundation, the Bloomberg~Kimmel Institute for Cancer Immunotherapy, the Mark Foundation for Cancer Research, and others, underscoring the significant scientific and societal investment in tackling this devastating disease.
The disclosure of potential conflicts of interest by several researchers involved in the study, including consulting roles and financial interests in companies related to cancer diagnostics and therapeutics, demonstrates the robust ethical framework guiding such research and ensures transparency in the scientific process. These relationships are managed by Johns Hopkins University in accordance with its established conflict-of-interest policies, ensuring that scientific integrity remains paramount.
This discovery represents a significant step forward in understanding the intricate biology of glioblastoma and offers a beacon of hope for patients and their families facing this formidable diagnosis. By unraveling the symbiotic relationship between tumor stem cells and immune cells, researchers are paving the way for more effective and targeted treatments that could ultimately improve outcomes for those battling aggressive brain cancers.
