Neutrophils are key to harnessing anti-tumor immune response from radiation therapy, study finds

Radiation therapy is one of three core modalities to treat cancer. Researchers found that radiation therapy targeted against a tumor can act as a “cancer vaccine” by causing neutrophil-mediated tumor cell death that alerts the immune system to fight the cancer cells at other anatomical sites. The so-called “abscopal effect,” in which radiation therapy delivered … Continue reading “Neutrophils are key to harnessing anti-tumor immune response from radiation therapy, study finds”

Radiation therapy is one of three core modalities to treat cancer. Researchers found that radiation therapy targeted against a tumor can act as a “cancer vaccine” by causing neutrophil-mediated tumor cell death that alerts the immune system to fight the cancer cells at other anatomical sites.

The so-called “abscopal effect,” in which radiation therapy delivered to a primary site of cancer also results in shrinkage or elimination of cancer cells in non-irradiated metastatic sites has been observed for decades. “The abscopal effect is only seen sporadically, but when it does happen, the effect induces a long-lasting, anti-tumor response in patients,” said senior author Dr. Raquibul Hannan, Assistant Professor of Radiation Oncology and a member of the Harold C. Simmons Comprehensive Cancer Center. “Our study in mice was designed to understand this phenomenon and identify strategies to enhance it.”

Findings from the study are reported in the Proceedings of the National Academy of Science. Researchers discovered what happens inside the tumor after radiation and how these events not only kill tumor cells but also lead to a whole-body anti-tumor response.

Study results show that neutrophils, the most abundant white blood cell in the body, are key players in the radiation-induced anti-tumor immune response. In the absence of radiotherapy, cancer cells transform neutrophils into tumor-associated neutrophils or TANs to help promote cancer cell growth. Radiation therapy, in addition to destroying TANs, recruits new neutrophils into the tumor. The radiation-induced neutrophils (RT-Ns) attack the tumor cells by producing molecules that damage them. The study further demonstrates how RT-Ns are also key players in generating a downstream tumor-specific, T cell-mediated anti-tumor immune response.

“To our knowledge this is the first study to identify RT-Ns and to demonstrate their anti-tumor activity via both innate and adaptive immune responses,” said Dr. Tsuguhide Takeshima, Instructor in Radiation Oncology and the lead author on the report.

Importantly, the researchers were able to discover a way to enhance the tumor killing capacity of the RT-Ns by administering G-CSF (Granulocyte-colony stimulating factor), a naturally occurring protein (cytokine) in the body that stimulates bone marrow to produce more white blood cells including neutrophils. In the clinic, G-CSF is widely used to treat blood cell deficiencies in patients receiving chemotherapy. Researchers found that the combination of G-CSF and RT-Ns potentiated the anti-tumor immune response, presumably by inducing a more robust neutrophil response.

“We think these are exciting finding that should be easily translatable to the clinic since G-CSF is routinely used to treat neutropenia,” said Dr. Ellen Vitetta, Professor of Immunology and Microbiology, and holder of The Scheryle Simmons Patigian Distinguished Chair in Cancer Immunobiology, and a coauthor on the publication.

“These results provide support for evaluating the combined use of radiation therapy and G-CSF in pre-clinical and clinical settings” said Dr. Hannan. “Our long-term goal is to eliminate the sporadic nature of the abscopal effect of radiation therapy and dependably induce the response every time.” Dr. Hannan is the principal investigator on three ongoing clinical trials at UT Southwestern Medical Center that strategically combine radiation therapy with immunotherapy for cancer patients.

New strategy identified for treating acute myeloid leukemia

“AML is a devastating form of cancer; the five-year survival rate is only 30 percent, and it is even worse for the older patients who have a higher risk of developing the disease,” says David Scadden, MD, director of the MGH Center for Regenerative Medicine (MGH-CRM), co-director of the Harvard Stem Cell Institute (HSCI), and senior author of the Cell paper. “New therapies for AML are extremely limited — we are still using the protocols developed back in the 1970s — so we desperately need to find new treatments.”

In AML, the normal process by which myeloid stem cells differentiate into a specific group of mature white blood cells is halted, leading to the proliferation of immature, abnormal cells that crowd out and suppress the development of normal blood cells. A wide range of genetic changes occurs in AML, but the authors proposed that the effects on differentiation had to funnel through a few shared molecular events. Using a method created by lead author David Sykes, MD, PhD, of the MGH-CRM and HSCI, the team discovered that a single dysfunctional point in the pathway common to most forms of AML could be a treatment target.

Previous studies had shown that the expression of a transcription factor called HoxA9 — which must be shut down for normal myeloid cell differentiation to proceed — is actually maintained in 70 percent of patients with AML. Since no inhibitors of HoxA9 had been identified, the researchers pursued a novel approach to screening potential inhibitors based not on their interaction with a particular molecular target but on whether they could overcome the differentiation blockade characteristic of AML cells.

They first set up a cellular model of AML by inducing HoxA9 overexpression in mouse myeloid cells genetically engineered to glow green if they reached maturity. The team then screened more than 330,000 small molecules to find which would produce the green signal in the cells, indicating that the HoxA9-induced differentiation blockade had been overcome. Only 12 compounds produced the desired result, 11 of which were found to act by suppressing a metabolic enzyme called DHODH, which was not previously known to have a role in myeloid differentiation. Further experiments showed that DHODH inhibition could induce differentiation in both mouse and human AML cells.

The team then tested a known DHODH inhibitor in several mouse models of AML and identified a dosing schedule that reduced levels of leukemic cells and prolonged survival with none of the adverse effects of normal chemotherapy. While six weeks of treatment did not prevent eventual relapse, treatment for up to 10 weeks appears to have led to long-term remission, including a reduction of the leukemia stem cells that can lead to relapse. Similar results were seen in mice into which human leukemia cells had been implanted.

“Drug companies tend to be skeptical of the kind of functional screening we used to identify DHODH as a target, because it can be complicated and imprecise. We think that with modern tools, we may be able to improve target identification, so applying this approach to a broader range of cancers may be justified,” says Scadden, who is chair and professor of Stem Cell and Regenerative Biology and Jordan Professor of Medicine at Harvard University. Additional investigation of the mechanism underlying DHODH inhibition should allow development of protocols for human clinical trials.

Unexpected cause of mutation in cancer identified

Reporting in the scientific journal Nature Communications, University of Minnesota researcher Reuben Harris and colleagues found that an enzyme known as APOBEC3H-I is the most likely cause of these previously unexplained mutations. Harris is a professor in the University’s College of Biological Sciences, a Howard Hughes Medical Institute Investigator and a member of the Masonic Cancer Center.

In breast cancer the solution became apparent in tumors lacking a related enzyme called APOBEC3B. All breast tumors with an APOBEC mutation footprint have APOBEC3B and, if they lack this enzyme due to a naturally occurring deletion, they invariably have APOBEC3H-I. The mutational contribution of APOBEC3H-I was also clear in lung cancer, in addition to expected mutational footprints from tobacco smoke and aging.

These findings are important because they provide a molecular explanation for a major source of mutation in many different types of cancer. The results point the way to fine-tuning the treatments of these cancers by inhibiting these enzymes in order to limit the development of mutations that undermine many current cancer therapies.

“Our results encourage the development of new cancer treatments that work by combining existing therapies and an APOBEC inhibitor to stop tumor cells from evading therapy by developing resistance mutations,” Harris said.

The findings are based on the fact that cancer begins when the genetic material inside a cell mutates, causing the cell to change and indiscriminately multiply. The processes that cause these mutations — say, ultraviolet light in the case of skin cancer or smoking in the case of lung cancer — leave a characteristic signature or footprint. The second-most common mutation type (behind aging) is that caused by the APOBEC enzymes. These enzymes are normally beneficial by helping to kill viruses, but the same DNA-mutating trait that makes them good at this job can also give them the ability to contribute mutations to tumor evolution if they become too abundant and/or misregulated.

Past research led by Harris showed that APOBEC3B is a source of mutations in breast cancers with APOBEC footprint. However, Harris and others also found that some breast tumors still have an APOBEC footprint but no APOBEC3B. To find out why, Harris, the lead author Gabriel Starrett, and collaborators from the University of Minnesota Masonic Cancer Center, the Howard Hughes Medical Institute, and the University of Saskatchewan began looking at variants of another APOBEC family member known as APOBEC3H. They were surprised to find that what was thought to be a relatively unstable, inactive variant known as APOBEC3H-I is likely the culprit. This protein is found in tumor cells with APOBEC signatures, and it is also much more active than expected from prior studies. APOBEC3H-I can access the nuclear DNA of the cell, providing a molecular explanation for its role in cancer mutagenesis.

Armed with the knowledge of APOBEC3H-I’s role in cancer, Harris is now looking to learn more about how this enzyme actually induces mutations. “We would like to better understand the underlying molecular mechanism,” he said. “As with any new process, a better understanding of the molecular details will provide additional angles for future therapies.”