A preclinical study conducted by researchers at The University of Texas MD Anderson Cancer Center found engineered natural killer (NK) cells with the TFG-[beta] receptor deleted could overcome immune suppression and effectively eliminate glioblastoma stem cells (GSC).
"There is tremendous interest in utilizing immunotherapy to improve treatments for patients with glioblastoma, but there has been limited success to date," said senior author Katy Rezvani, MD, PhD, Professor of Stem Cell Transplantation & Cellular Therapy. "We are encouraged by our findings that support a combinatorial approach of NK cell-based immunotherapy together with disruption of the TGF-[beta] signaling axis to overcome the immune defenses of GSCs in the brain."
Glioblastoma multiforme (GBM), or grade IV astrocytoma, is the most common and aggressive type of primary brain tumor. GSCs are resistant to all standard therapies and largely responsible for disease recurrence in patients. Despite current treatment with resection, radiotherapy and temozolomide, the outcome for brain tumors is poor with a reported median survival of 14.6 months and a 2-year survival of 26.5 percent as the tumor invariably relapses.
The dismal outcome for this type of tumor has stimulated keen interest in immunotherapy as a means to circumvent one or more of the factors that have limited the impact of available treatments: 1) rapid growth rate of these aggressive tumors; 2) their molecular heterogeneity and propensity to invade critical brain structures, and 3) the tumor regenerative power of a small subset of GSCs responsible for resistance and recurrence.
The research was focused first on understanding how NK cells acted against GSCs in the brain and, secondly, devising strategies to overcome the immune suppression mechanisms employed by GSCs to evade NK cell recognition, said Rezvani.
NK Cell Capabilities
Previously published data suggested that NK cells may be capable of infiltrating GBM, but it was unclear whether the stem cells would indeed be susceptible to NK-cell killing, Rezvani explained.
Mass cytometry and single-cell RNA sequencing of primary tumor samples confirmed the presence of NK cells in the tumor tissue, but revealed that glioblastoma-infiltrating NK cells acquired an altered phenotype associated with impaired lytic function relative to matched peripheral blood NK cells from glioblastoma patients or healthy donors. Profiling those NK cells led the researchers to discover that TGF-[beta], produced by the stem cells, was responsible for shutting down the NK cells.
"GSCs proved highly susceptible to NK-mediated killing in vitro, but evaded NK cell recognition via a mechanism requiring direct [alpha]v integrin-mediated cell-cell contact, leading to the release and activation of TGF-[beta] by the GCSs," Rezvani noted.
Genetic Engineering & Immune Suppression
Knowing that TGF-[beta] produced by GSCs was responsible for blocking NK cell activity, the researchers next investigated strategies to block TGF-[beta] signaling and restore the anti-tumor activity of NK cells. They treated GSC-engrafted mice with allogeneic NK cells and inhibitors of integrin or TGF-[beta] signaling, which prevented GSC-induced NK cell dysfunction and tumor growth. More effective, however, was genetically engineering NK cells to remove the TGF-[beta] receptor completely.
"We were able to overcome the immunosuppressive environment in the brain by using CRISPR gene engineering to render NK cells resistant to TGF-[beta]. The engineered NK cells were then able to eliminate the tumor-regenerating GSCs," said Rezvani.
Treatment with these gene-edited NK cells resulted in a significant improvement in overall survival relative to untreated controls or treatment with unedited NK cells, she added.
Advancing NK Cell Therapies
Rezvani and her team are currently awaiting FDA approval for a Phase I clinical trial with MD Anderson's neuro-oncologist Shiao-Pei Weathers, MD, as the principal investigator of the future study to evaluate the safety of this experimental approach, as well as efficacy and overall survival, as a novel treatment for glioblastoma.
"Our approach can apply to other types of solid tumors where TGF-[beta] plays an important role in immune invasion," said Rezvani. "Using the powerful CRISPR platform, we can also look at targeting other pathways to enhance immunotherapy for solid tumors such as pancreatic, kidney, breast or ovarian cancer."
Rezvani and her research team have worked to advance NK cells as a cancer therapy with the support of MD Anderson's Moon Shots Program, a collaborative effort to rapidly develop scientific discoveries into meaningful clinical advances that save patients' lives. The current work was supported by the adoptive cell therapy platform and the Glioblastoma Moon Shot, in collaboration with Frederick Lang, MD, Chair of Neurosurgery, and Amy Heimberger, MD, at Northwestern University Feinberg School of Medicine.
Amy Gallagher is a contributing writer.