By Billy Candra 
Stanford University School of Medicine researchers have made a groundbreaking discovery, revealing that erythropoietin (EPO), a protein identified nearly four decades ago for its crucial role in stimulating red blood cell production, plays an unexpected and critical function in suppressing the immune system’s ability to combat cancer. This paradigm-shifting insight, published online April 24 in the prestigious journal Science, uncovers a novel mechanism by which tumors evade immune surveillance and suggests powerful new therapeutic strategies to overcome resistance to existing immunotherapies.
The research demonstrates that inhibiting EPO’s activity can transform previously "cold" or immune-resistant liver tumors in mice into "hot" tumors, teeming with cancer-fighting immune cells. When this blockade was combined with an immunotherapy designed to further activate these immune cells, the treatment resulted in complete regression of existing liver tumors in a majority of the treated animals. Remarkably, these treated mice survived for the entire duration of the experiment, a stark contrast to control animals, which succumbed to their disease within a few weeks. This profound shift in tumor microenvironment and treatment efficacy signals a fundamental breakthrough in understanding the intricate interplay between cancer and the immune system.
The Dual Nature of Erythropoietin: From Hematopoiesis to Immunosuppression
Erythropoietin (EPO) has long been revered in medical science for its primary role in erythropoiesis, the process of red blood cell formation. Produced primarily by the kidneys, EPO is a glycoprotein hormone that regulates the production of red blood cells in the bone marrow, a function critical for oxygen transport throughout the body. Its therapeutic application in treating anemia, particularly in patients with chronic kidney disease or undergoing chemotherapy, has been a cornerstone of hematology for decades. Synthetic forms of EPO, such as epoetin alfa, have significantly improved the quality of life for millions by alleviating anemia-related fatigue and other symptoms.
However, the journey of EPO in oncology has been fraught with complexity. More than a decade ago, clinical observations began to raise red flags. Studies indicated that administering exogenous EPO to anemic cancer patients to stimulate red blood cell formation sometimes led to an unexpected and concerning acceleration of tumor growth. This alarming correlation prompted the U.S. Food and Drug Administration (FDA) in 2007 to mandate a black box warning on EPO-stimulating agents, cautioning against their use in patients with certain cancers due to the risk of tumor progression and reduced survival. Despite these clear clinical signals, the precise mechanism linking EPO to cancer aggression remained elusive, particularly its potential involvement in immune evasion. The prevailing scientific consensus continued to view EPO primarily through the lens of red blood cell growth.
"Research from more than a decade ago has shown that giving EPO to cancer patients with anemia to stimulate red blood cell formation accelerates the growth of the tumor," noted Dr. Edgar Engleman, MD, PhD, a professor of pathology and of medicine at Stanford and the senior author of the new Science paper. He further emphasized the long-standing puzzle: "The connection was so striking that in 2007 the Food and Drug Administration required a black box warning label on the drug cautioning against its use in people with cancers. Researchers also saw a clear correlation between patient prognosis and the levels of naturally occurring EPO and its receptor in the tumor. 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." This statement underscores the monumental shift in understanding that this new research represents, challenging deeply ingrained scientific paradigms.
Unraveling the Mechanism: "Cold" vs. "Hot" Tumors and the Hypoxic Link
The groundbreaking investigation, led by basic life research scientist Dr. David Kung-Chun Chiu, PhD, involved the meticulous development and study of advanced genome editing techniques to create several sophisticated mouse models of liver cancer. These models were carefully designed to recapitulate specific mutations, histological features, and responses to approved therapies observed in various subtypes of human liver cancers. Tumor formation was induced either by injecting a combination of DNA encoding proteins associated with liver cancer into the animals’ tail vein or by implanting liver cancer cells directly into the animals’ livers, providing robust platforms for studying tumor development and treatment response.
A primary focus of the research was to understand the varying efficacy of common immunotherapies, specifically those targeting the programmed cell death protein 1 (PD-1) pathway. Anti-PD-1 therapies, such as the commercially available Keytruda, are a class of immune checkpoint inhibitors that work by blocking the PD-1 protein on immune cells, primarily T cells. This blockade prevents cancer cells from "turning off" T cells, thereby unleashing the immune system to attack the tumor. While these therapies have revolutionized the treatment of several cancers, including melanoma, Hodgkin’s lymphoma, and certain types of lung cancer, transforming patient outcomes, a significant proportion of tumors, including most liver, pancreas, colon, breast, and prostate cancers, remain stubbornly resistant.
The researchers observed a phenomenon well-documented in human oncology: some combinations of genetic mutations led to the development of "cold" liver tumors. These tumors were largely ignored by the immune system, characterized by a scarcity of tumor-infiltrating T cells. Consequently, these cold tumors did not shrink when the animals were treated with anti-PD-1 therapy, highlighting a critical challenge in cancer treatment. In stark contrast, other genetic mutations resulted in "hot" or "inflamed" tumors, which were richly infiltrated with T cells. These hot tumors proved highly sensitive to anti-PD-1 treatment, which effectively triggered the T cells to mount a robust attack against the cancer.
The unexpected twist emerged when the team analyzed EPO levels. Cold tumors, those impervious to anti-PD-1 therapy, displayed significantly elevated levels of EPO compared to their hot counterparts. This observation sparked a new line of inquiry. The researchers hypothesized that this increase in EPO was likely a consequence of the oxygen-poor microenvironment—a condition known as hypoxia—prevalent within cold tumors. Hypoxia is a common feature of rapidly growing tumors, as their demand for oxygen often outstrips the blood supply. This low-oxygen state is known to induce cancer cells to produce various proteins, which, in turn, can ramp up the production of EPO in an attempt to stimulate more red blood cell formation to combat the local oxygen deficit.
"Hypoxia in tumors has been studied for decades," Dr. Engleman commented. "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." This candid admission underscores the novelty and unexpected nature of the discovery, redirecting scientific attention to a previously overlooked role for EPO in the tumor microenvironment.
To further validate their findings, the researchers cross-referenced their data with existing public databases, confirming a clear correlation: elevated levels of EPO were consistently associated with poorer survival rates in human patients with a wide array of cancers, including those of the liver, kidney, breast, colon, and skin. This strong correlative evidence bolstered the hypothesis that EPO’s role extended beyond mere red blood cell stimulation and was directly implicated in tumor progression and immune evasion in humans.
The Critical Role of Macrophages: EPO’s Immunosuppressive Pathway
Armed with this compelling evidence, the researchers began to experimentally manipulate EPO production in their mouse models. The results were striking: mutations that typically led to the development of cold tumors instead caused hot tumors when the tumor cells were genetically modified to be incapable of producing EPO. Conversely, hot tumors that had previously been successfully eradicated by the immune system thrived when they were engineered to produce elevated levels of EPO. These reciprocal experiments provided robust functional proof that EPO was not merely correlated with tumor behavior but was actively orchestrating it.
Further exhaustive research meticulously unraveled the precise cellular and molecular mechanism. In cold tumors, the tumor cells themselves produce and secrete EPO. This secreted EPO then binds to specific receptors located on the surface of immune cells known as macrophages. Macrophages are versatile immune cells that play a complex, often dual, role in the tumor microenvironment. They can either be tumor-suppressing (M1-like) or tumor-promoting (M2-like). The binding of EPO to its receptors on macrophages was found to trigger a crucial switch: these macrophages transitioned into an immunosuppressive state. In this altered state, they actively "shooed away" or excluded cancer-killing T cells from the tumor microenvironment and simultaneously tamped down the activity of any T cells that managed to infiltrate, effectively neutralizing the immune response.
This EPO-moderated crosstalk between tumor cells and macrophages proved to be of paramount importance, particularly when the researchers investigated the combinatorial effect of simultaneously blocking both the EPO signaling pathway and the anti-PD-1 pathway. In these pivotal experiments, mice with cold liver tumors treated with either a control substance or anti-PD-1 alone showed dismal survival, with none living beyond eight weeks after tumor induction. However, a significant improvement was observed in mice where macrophages were genetically modified to be unable to make the EPO receptor: 40% of these animals lived for 18 weeks after tumor induction, the point at which the experiment was terminated. The most dramatic outcome occurred when anti-PD-1 treatment was administered to mice lacking the EPO receptor on their macrophages – all of these animals survived for the entire duration of the experiment, achieving complete tumor regression.
"It’s simple," Dr. Engleman stated, encapsulating the profound simplicity of their findings. "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 declarative statement highlights the potential for a new class of therapies that could fundamentally alter the tumor microenvironment, making previously resistant cancers vulnerable to existing immunotherapies.
Future Directions and Therapeutic Implications
The implications of this discovery are vast and immediate. Dr. Engleman and his colleagues are already actively engaged in designing new therapeutic strategies aimed at targeting EPO signaling in human cancers. Several approaches are being considered. One potential strategy involves non-specifically targeting the EPO protein itself. While this could potentially lead to side effects such as anemia, Dr. Engleman speculates that for an effective cancer therapy, this might be an acceptable trade-off, given the severity of the disease.
A more refined and potentially safer approach, however, focuses on selectively blocking the EPO receptors specifically on the surfaces of macrophages within the cancer microenvironment. This targeted intervention could prevent macrophages from adopting their immunosuppressive role without broadly interfering with EPO’s essential function in red blood cell production. Such a precision approach would minimize systemic side effects while maximizing therapeutic impact.
The potential reach of this discovery extends to a broad spectrum of human cancers, particularly those currently resistant to immune checkpoint inhibitors. By reprogramming the tumor microenvironment from "cold" to "hot," EPO-targeting therapies could unlock the full potential of existing immunotherapies, offering hope to millions of patients for whom current treatments are insufficient. The collaborative nature of the research, involving contributions from the New York Blood Center and the pharmaceutical company ImmunEdge Inc., underscores the interdisciplinary effort behind this breakthrough. The study received substantial funding from the National Institutes of Health (grants R01CA262361, P01CA244114, U54CA2745115, and P01HL149626), highlighting the recognized importance of this area of research.
As is standard practice in cutting-edge biomedical research, the authors have disclosed potential conflicts of interest. Dr. Chiu is a cofounder of ImmunEdge Inc., and Dr. Engleman is a founder, shareholder, and board member of ImmunEdge Inc. Both Dr. Chiu and Dr. Engleman are Stanford-affiliated inventors of a patent application (PCT/US2023/063997) entitled "EPO receptor agonists and antagonists," reflecting the direct translational potential of their work.
"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." This sentiment encapsulates the cautious yet profound optimism surrounding a discovery that not only fundamentally rewrites our understanding of cancer immunology but also paves a clear path toward innovative, life-saving treatments for a multitude of human cancers. The journey from a red blood cell stimulant to a critical immune modulator marks a significant chapter in the ongoing fight against cancer.
