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 specific subset of immune cells that actively promote the growth and aggression of glioblastoma, the most formidable and treatment-resistant form of brain cancer. This pivotal discovery, published on January 17th in the prestigious journal Science, sheds new light on the intricate tumor microenvironment and offers potential new avenues for therapeutic intervention in a disease that has historically defied effective treatment.
Unveiling a Symbiotic Relationship in the Tumor Microenvironment
Glioblastoma, a grade 4 astrocytoma, is characterized by its rapid proliferation, invasive nature, and profound resistance to conventional therapies such as surgery, radiation, and chemotherapy. For decades, the relentless progression of this malignancy has presented a significant challenge to oncologists and researchers alike. While much focus has been placed on the tumor cells themselves, this new research delves into the critical role of the surrounding immune cells, particularly within the most aggressive subtypes of glioblastoma.
Utilizing a cutting-edge technology known as spatial genomics, which allows for the simultaneous analysis of gene expression and cellular location within tissue samples, the Johns Hopkins team embarked on a mission to characterize the immune landscape of high-grade brain tumors. Their investigation zeroed in on the unique cellular composition of glioblastomas, aiming to identify any distinct immune cell populations that might be instrumental in fueling the tumor’s malignancy.
The researchers discovered a remarkable co-localization between glioblastoma stem cells, the presumed engines of tumor renewal and recurrence, and a specific type of immunosuppressive immune cell known as myeloid-derived suppressor cells (MDSCs). This intimate proximity, often found within regions historically described by pathologists as pseudopalisading – a hallmark of glioblastoma – suggests a deeply intertwined and symbiotic relationship.
"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 symbiotic interaction appears to be a two-way street, with both cell types actively supporting and enhancing each other’s functions, ultimately driving tumor growth and perpetuating the aggressive nature of glioblastoma.
The Power of Spatial Genomics and Single-Cell Analysis
The investigation began with a comprehensive single-cell RNA sequencing analysis of tissue samples from 33 different types of brain tumors, ranging from low to high grades. This initial step allowed the researchers to identify distinct populations of MDSCs present in IDH-wild-type glioblastomas, the most common and aggressive form of the disease.
Building upon this foundation, the team employed spatial transcriptomics, a sophisticated technique that maps gene expression patterns across thousands of individual cells within their spatial context. This allowed them to examine the intricate relationships between over 750,000 immune cells and more than 350,000 tumor and associated cells within the glioblastoma samples. The findings confirmed the significant co-location 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 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."
A Detailed Look at the Molecular Dialogue
The research meticulously detailed the molecular mechanisms underlying this tumor-immune cell crosstalk. The study revealed that glioblastoma stem cells actively recruit and activate MDSCs through the secretion of specific chemical signals, known as chemokines. Furthermore, the stem cells provide growth and activation factors that prime the MDSCs for their tumor-promoting functions.
Specifically, the researchers identified two key chemokines produced by tumor stem cells: interleukin-6 (IL-6) and interleukin-8 (IL-8). These molecules are well-known players in inflammatory responses and possess receptors on MDSCs, making them potent attractants and activators.
"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.
In return for this "nourishment," the MDSCs reciprocate by secreting growth factors that further fuel the glioblastoma stem cells. A particularly significant finding was the identification of fibroblast growth factor 11 (FGF11) as a key growth factor secreted by MDSCs. This molecule had not previously been implicated in the development or progression of brain cancers, marking a novel discovery in the field.
The IDH Mutation Link and Prognostic Significance
The study also observed a striking difference in the presence of MDSCs in tumors with specific genetic mutations. Tumors harboring a mutation in the IDH1 gene, which are generally less aggressive and associated with a better prognosis, exhibited a significant paucity of MDSCs and a far lower number of cancer stem cells. This observation prompted the researchers to investigate the broader correlation between MDSC infiltration and patient survival across all brain cancers.
Leveraging the extensive National Cancer Institute’s Cancer Genome Atlas (TCGA) database, which contains a vast collection of genomic and clinical data from cancer patients, the team found a robust correlation. Tumors with fewer cancer stem cells and lower levels of MDSC infiltration were consistently associated with improved patient outcomes. This finding reinforces the critical role of this specific immune cell population in dictating the aggressive behavior and prognosis of glioblastoma.
Implications for Future Therapies
The discovery of this intricate symbiotic relationship between glioblastoma stem cells and MDSCs opens up exciting new possibilities for therapeutic intervention. By targeting either the production of key chemokines by tumor stem cells or the function of MDSCs themselves, researchers and clinicians may be able to disrupt this tumor-promoting axis.
"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 stated.
One promising avenue already being explored involves investigational bispecific antibodies. Jamie Spangler, Ph.D., an associate professor of biomedical engineering at Johns Hopkins, has developed such an antibody designed to bind to the receptors for IL-6 and IL-8. By blocking the signaling pathways mediated by these chemokines, this antibody has the potential to inhibit the recruitment and activation of MDSCs, thereby hindering tumor growth.
The identification of FGF11 as a novel growth factor secreted by MDSCs also presents another potential therapeutic target. Inhibiting the production or action of FGF11 could potentially starve the tumor stem cells and impede their regenerative capabilities.
A Collaborative Effort with Far-Reaching Support
This significant research undertaking was the result of a multidisciplinary collaboration involving numerous scientists from Johns Hopkins University and Stanford University School of Medicine. Key contributors from Johns Hopkins included 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.
The research was generously supported by grants from the National Institutes of Health (NIH), including grant numbers #F32NS108580, #R01HG010889, R01HG009518, RA37CA230400, and U07CA230691. Additional funding was provided by 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 article also disclosed potential conflicts of interest for several researchers, outlining their consulting roles, advisory positions, and financial interests in various biotechnology and pharmaceutical companies. These relationships were managed by The Johns Hopkins University in accordance with its conflict-of-interest policies, ensuring the integrity of the research.
Looking Ahead: A New Frontier in Glioblastoma Treatment
The discovery of the critical role played by MDSCs in supporting glioblastoma stem cell growth represents a significant leap forward in understanding this devastating disease. By elucidating the molecular dialogue between these cell types, the Johns Hopkins researchers have not only expanded our fundamental knowledge of brain cancer biology but have also paved the way for the development of novel, immune-based therapeutic strategies. As research continues to unravel the complexities of the tumor microenvironment, the prospect of more effective treatments for glioblastoma patients moves closer to reality. The ongoing efforts to translate these findings into clinical applications hold immense promise for improving the lives of those affected by this challenging malignancy.
