By Lina carolina 
A groundbreaking discovery, poised to reshape our understanding of cancer immunology, has revealed that erythropoietin (EPO), a hormone long recognized for its vital role in stimulating red blood cell production, is a key player in suppressing the immune system’s ability to combat cancerous tumors. This revelation, emerging from meticulous research, demonstrates that by blocking EPO’s activity, formerly "cold" or immune-resistant liver tumors in mice were transformed into "hot" tumors teeming with cancer-fighting immune cells. When this blockade was coupled with an existing immunotherapy that further energizes these immune cells, the combined treatment resulted in the complete regression of existing liver tumors in the vast majority of test subjects. These treated animals survived for the entire duration of the experimental period, a stark contrast to control animals, which succumbed to the disease within weeks.
This paradigm-shifting finding, published online on April 24th in the prestigious journal Science, was spearheaded by Edgar Engleman, MD, PhD, a professor of pathology and medicine, and led by basic life research scientist David Kung-Chun Chiu, PhD. Dr. Engleman expressed profound enthusiasm for the discovery, stating, "This is a fundamental breakthrough in our understanding of how the immune system is turned off and on in cancer. I could not be more excited about this discovery, and I hope treatments that target the mechanism we uncovered will quickly move forward to human trials."
From Anemia Treatment to Cancer Immunosuppression: The Unexpected Role of EPO
The journey to this discovery began with decades of established knowledge about EPO. First identified and characterized in the early 1970s for its critical function in erythropoiesis – the process of red blood cell formation – EPO was soon recognized as a therapeutic target for anemia, a condition characterized by a deficiency of red blood cells. Pharmaceutical companies developed recombinant EPO, which revolutionized the treatment of anemia, particularly in patients with chronic kidney disease or undergoing chemotherapy.
However, an unsettling pattern began to emerge in the early 2000s. Observational studies and clinical trials investigating the use of EPO in cancer patients with anemia revealed an unexpected and concerning correlation: the administration of EPO appeared to accelerate tumor growth and worsen patient prognosis. This observation was significant enough to prompt the U.S. Food and Drug Administration (FDA) to issue a black box warning in 2007, cautioning against the use of EPO in cancer patients. Researchers noted a clear link between higher levels of naturally occurring EPO and its receptor (EPOR) within tumors and poorer patient outcomes. "Those old reports showed clearly that the more EPO or EPOR there was in tumors, the worse off the patients were," Dr. Engleman explained. "But the connection between EPO and cancer immunity was never made until now. In fact, it took a long time and a lot of experiments to convince us that EPO plays a fundamental role in blocking the immune response to cancer, because EPO is so well established as a red blood cell growth factor."
Unraveling the Mechanism: Mouse Models and Immunotherapy
The recent research, initiated by Dr. Chiu’s expertise in genome editing, utilized sophisticated mouse models of liver cancer. These models were meticulously engineered to replicate specific mutations, histological features, and responses to existing therapies seen in various subtypes of human liver cancers. Tumor formation was induced either by injecting DNA encoding key cancer-associated proteins or by implanting established liver cancer cells directly into the animals’ livers.
A central focus of the study was the interaction between EPO and a widely used class of cancer immunotherapies known as checkpoint inhibitors, specifically those targeting the PD-1 molecule. PD-1 is a protein found on T cells, a crucial component of the immune system responsible for identifying and destroying cancerous cells. When cancer cells express ligands that bind to PD-1, they effectively "put the brakes" on T cell activity, allowing the tumor to evade immune surveillance. Anti-PD-1 therapies, such as the commercially available Keytruda, work by blocking this interaction, thereby unleashing T cells to attack the cancer. While these therapies have transformed outcomes for patients with certain cancers like melanoma, Hodgkin’s lymphoma, and some lung cancers, a significant challenge remains: many common cancers, including liver, pancreas, colon, breast, and prostate cancers, are inherently resistant to anti-PD-1 treatment.
The researchers observed that, mirroring human liver cancers, some of the engineered mouse models developed tumors that were largely ignored by the immune system. These "cold" tumors were characterized by a scarcity of T cells and consequently showed no response to anti-PD-1 therapy. In contrast, other tumor models, harboring different genetic mutations, developed "hot" or inflamed tumors that were rich in T cells and highly susceptible to anti-PD-1 treatment, which successfully triggered an immune attack against the cancer.
The EPO Connection: Hypoxia and Macrophage Modulation
A key observation that began to connect the dots was the finding that the cold tumors exhibited significantly elevated levels of EPO compared to their hot counterparts. This increase was strongly linked to the hypoxic microenvironment prevalent in many cold tumors. Hypoxia, or low oxygen levels, is a common characteristic of rapidly growing tumors that outstrip their blood supply. This oxygen-deprived state triggers cancer cells to produce specific proteins that, in turn, stimulate EPO production. The rationale behind this mechanism is to promote the generation of more red blood cells, thereby enhancing oxygen delivery to the tumor. "Hypoxia in tumors has been studied for decades," Dr. Engleman remarked. "It just didn’t dawn on anyone, including me, that EPO could be doing anything in this context other than serving as a red blood cell growth factor."
To validate this hypothesis, the researchers delved into existing human cancer databases. They confirmed a strong correlation between elevated EPO levels and poorer survival rates across a range of cancers, including liver, kidney, breast, colon, and skin. The team then proceeded to experimentally manipulate the ability of tumor cells to produce EPO in their mouse models. The results were striking and directly contradicted the long-held understanding of EPO’s function. When tumors were genetically modified to prevent EPO production, those that were previously cold transformed into hot tumors, attracting a significant influx of immune cells. Conversely, hot tumors that had previously been effectively cleared by the immune system began to thrive when engineered to produce elevated levels of EPO.
Further in-depth investigations revealed the precise mechanism by which EPO exerts its immunosuppressive effects. In cold tumors, secreted EPO binds to receptors on macrophages, a type of immune cell. Normally, macrophages play diverse roles, including engulfing cellular debris and pathogens, and initiating inflammatory responses. However, in the presence of EPO, these macrophages are reprogrammed to adopt an immunosuppressive phenotype. They actively deter cancer-killing T cells from infiltrating the tumor microenvironment and dampen the activity of those T cells that do manage to enter. This EPO-mediated "crosstalk" between tumor cells and macrophages emerged as a critical barrier to effective anti-tumor immunity.
A Potent Combination Therapy: Blocking EPO Signaling and PD-1
The researchers then tested the impact of simultaneously blocking both the EPO signaling pathway and the PD-1 pathway in their mouse models. The results were nothing short of transformative. Mice with cold liver tumors that received either control treatment or anti-PD-1 therapy alone survived for no more than eight weeks post-tumor induction. In stark contrast, when macrophages were engineered to be unable to express the EPO receptor, 40% of the mice survived for 18 weeks, the experimental endpoint. Even more dramatically, when anti-PD-1 treatment was administered to mice lacking the EPO receptor, all animals survived for the entire duration of the experiment.
"It’s simple," Dr. Engleman stated. "If you remove this EPO signaling, either by lowering the hormone levels or by blocking the receptors on the macrophages, you don’t just get a reduction in tumor growth, you get tumor regression along with sensitivity to anti-PD-1 treatment." This finding underscores the critical, dual role of EPO in both promoting tumor growth by creating a more favorable environment for cancer cells and actively suppressing the immune system’s ability to eliminate them.
Future Directions and Broader Implications
The implications of this discovery are far-reaching. The research team is now actively engaged in designing therapeutic strategies that target EPO signaling in human cancers. One potential approach involves non-specific targeting of the EPO protein itself. While this might lead to anemia, a side effect that Dr. Engleman speculates could be an acceptable trade-off for a highly effective cancer therapy, the researchers are also exploring more targeted interventions. A promising alternative is the selective blockade of EPO receptors specifically on the surface of macrophages within the tumor microenvironment, thereby mitigating the risk of systemic side effects.
The potential applicability of this finding extends beyond liver cancer. Given the strong correlation between EPO levels and poor prognosis observed across multiple human cancer types, it is highly probable that this newly elucidated immunosuppressive mechanism plays a significant role in the resistance of many other cancers to current immunotherapies. This research opens up new avenues for developing combination therapies that could overcome resistance and improve outcomes for a much broader spectrum of cancer patients.
"I continue to be amazed by this finding," Dr. Engleman concluded. "Not every tumor is going to respond in the same way, but I’m very optimistic that this discovery will lead to powerful new cancer therapies."
The study was a collaborative effort, with contributions from researchers at the New York Blood Center and the pharmaceutical company ImmunEdge Inc. Funding for this pivotal research was provided by the National Institutes of Health, including grants R01CA262361, P01CA244114, U54CA2745115, and P01HL149626. Notably, Dr. Chiu is a co-founder of ImmunEdge Inc., and Dr. Engleman is a founder, shareholder, and board member of the same company. Both are listed as inventors on a pending patent application (PCT/US2023/063997) related to EPO receptor agonists and antagonists, highlighting their deep involvement in translating this fundamental scientific breakthrough into clinical applications.