By Nana O 
A groundbreaking study led by investigators 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 and previously unappreciated population of immune cells that actively support the growth and aggressiveness of glioblastomas, the most formidable and treatment-resistant form of brain cancer. This discovery, published on January 17th in the prestigious journal Science, offers critical new insights into the complex tumor microenvironment and opens promising avenues for novel therapeutic strategies.
Unraveling the Tumor Microenvironment: A Focus on Glioblastoma Stem Cells
Glioblastomas, classified as grade 4 brain tumors, are notoriously challenging to treat due to their rapid proliferation, invasive nature, and profound ability to evade the immune system. While significant progress has been made in understanding the genetic mutations driving these cancers, the intricate cellular ecosystem within the tumor itself has remained a significant area of investigation. This latest research specifically focused on identifying unique immune cell subtypes present in the most aggressive glioblastomas, employing cutting-edge spatial genomics technology.
The researchers’ meticulous investigation revealed a crucial co-localization between glioblastoma stem cells (GSCs) and a specific type of immunosuppressive immune cell known as myeloid-derived suppressor cells (MDSCs). GSCs, though comprising a relatively small percentage of the overall tumor mass (estimated at 5% to 10%), are widely recognized as the "engine" of tumor growth, responsible for its continuous renewal and generation. Their aggressive nature dictates the overall malignancy of the tumor.
"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," stated 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."
The Power of Spatial Genomics: Pinpointing Cellular Interactions
To achieve this detailed understanding, the Johns Hopkins team employed a two-pronged approach. Initially, they performed single-cell RNA sequencing on tissue samples from 33 diverse brain tumors, ranging from low-grade to high-grade malignancies. This analysis helped them identify two specific populations of MDSCs within IDH-wildtype glioblastomas, a subtype known for its particular aggressiveness.
The subsequent and more advanced stage of their research involved spatial transcriptomics. This sophisticated technology allows researchers to examine gene expression patterns at a cellular level within the context of their physical location in the tissue. By analyzing over 750,000 immune cells and more than 350,000 tumor and associated cells from these samples, they were able to definitively pinpoint the spatial relationship between MDSCs and GSCs. The data unequivocally showed these two cell types were not randomly distributed but were in close proximity, forming a symbiotic unit.
A Symbiotic Exchange: Fueling Tumor Growth and Immune Evasion
The findings suggest a sophisticated and mutually beneficial relationship between GSCs and MDSCs, a dynamic that significantly contributes to glioblastoma’s destructive capabilities. The research details a clear "feed-forward" mechanism where GSCs actively recruit and nurture MDSCs, and in turn, MDSCs create an immunosuppressive environment that shields the tumor from immune attack and promotes its unchecked growth.
Specifically, GSCs were found to secrete chemical signals, known as chemokines, which act as attractants for MDSCs. Furthermore, GSCs provide growth and activation factors essential for the survival and function of MDSCs. This initial step establishes the presence of these immunosuppressive cells within the tumor’s core.
In a reciprocal fashion, the MDSCs, once established and activated, produce growth factors that directly fuel the proliferation and survival of the glioblastoma stem cells. This creates a vicious cycle where each cell type supports and enhances the other, leading to increased tumor burden and enhanced aggressiveness.
"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," explained Christina Jackson, M.D., 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 at Johns Hopkins when this research was conducted. "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."
Key Molecular Mediators of the Symbiosis
The research team delved deeper to identify the specific molecular players involved in this intricate cellular dialogue. They discovered that two key chemokines produced by GSCs, Interleukin-6 (IL-6) and Interleukin-8 (IL-8), are critical in attracting and activating MDSCs. These molecules are well-known for their roles in inflammatory responses, and importantly, MDSCs possess receptors specifically 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. This precise targeting of MDSCs by GSCs highlights the sophisticated evolutionary strategies employed by these aggressive tumors.
On the other side of this cellular partnership, the study identified Fibroblast Growth Factor 11 (FGF11) as a critical growth factor secreted by MDSCs to nourish the GSCs. Remarkably, FGF11 had not previously been implicated in the progression of brain cancers or other malignancies, underscoring the novelty of this finding and its potential as a new therapeutic target.
Corroborating Evidence and Prognostic Implications
Further strengthening their findings, the researchers observed a significant difference in the presence of MDSCs and GSCs in less aggressive brain tumors. Tumors exhibiting a mutation in the IDH1 gene, which are generally less aggressive and associated with better prognoses, showed a marked scarcity of MDSCs and far fewer cancer stem cells.
This observation prompted a broader investigation into the correlation between MDSC infiltration and patient survival across all brain cancers. Utilizing the comprehensive National Cancer Institute’s Cancer Genome Atlas (TCGA) database, which contains a vast repository of cancer samples and associated clinical data, the team found a strong and consistent correlation. Tumors with fewer cancer stem cells and lower levels of MDSC infiltration were associated with significantly better patient outcomes. This finding underscores the critical role of this symbiotic relationship in determining the overall aggressiveness and prognosis of brain tumors.
Implications for Future Therapies and Ongoing Research
The identification of this specific GSC-MDSC axis and the molecular mediators involved has profound implications for the development of novel therapeutic strategies against glioblastoma. The research suggests that targeting either the GSCs’ ability to recruit and activate MDSCs or the MDSCs’ ability to support GSCs could represent effective ways to disrupt tumor growth.
"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 avenue is already under investigation. Jamie Spangler, Ph.D., an associate professor of biomedical engineering at Johns Hopkins, has developed an investigational bispecific antibody. This antibody is designed to bind to the receptors for both IL-6 and IL-8, effectively blocking the signaling pathways that attract and activate MDSCs to the tumor site. This represents a tangible step towards translating these fundamental research findings into clinical applications.
Collaborative Efforts and Funding Acknowledgements
This extensive research was a testament to significant collaborative efforts. The study’s co-authors include a distinguished group of researchers from Johns Hopkins: 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. Additional contributions were made by researchers from the Stanford University School of Medicine in California.
The project was generously supported by a multitude of funding sources, including grants from the National Institutes of Health (specifically #F32NS108580, #R01HG010889, R01HG009518, RA37CA230400, U07CA230691), 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.
Several researchers involved disclosed potential conflicts of interest, which were managed by The Johns Hopkins University in accordance with its conflict-of-interest policies. These disclosures included consulting roles, grant support, and stock ownership in various biotechnology and pharmaceutical companies. These disclosures are a standard and crucial part of scientific reporting, ensuring transparency and ethical conduct in research.
The comprehensive understanding of the intricate interplay between glioblastoma stem cells and myeloid-derived suppressor cells represents a significant leap forward in the fight against this devastating disease. By illuminating these previously hidden symbiotic relationships, the research provides a critical roadmap for developing more effective and targeted immunotherapies, offering renewed hope for patients battling aggressive brain tumors.