By Lina carolina 
A protein identified nearly 40 years ago for its ability to stimulate the production of red blood cells has been found to play a surprising and critical role in dampening the immune system’s response to cancer. This groundbreaking discovery, emerging from extensive research, suggests a novel therapeutic avenue that could transform the treatment of numerous cancer types.
The protein, erythropoietin (EPO), a hormone primarily recognized for its function in erythropoiesis – the process of red blood cell formation – has been revealed to possess a dual nature. While its role in combating anemia is well-established, new findings indicate that it actively suppresses anti-cancer immunity, effectively shielding tumors from the body’s natural defenses.
Unlocking "Cold" Tumors: A Paradigm Shift in Cancer Immunology
The research, published online on April 24th in the prestigious journal Science, demonstrates a pivotal mechanism by which cancer cells can evade immune surveillance. By blocking the activity of EPO, researchers were able to convert formerly "cold" or immune-resistant liver tumors in mice into "hot" tumors, teeming with cancer-fighting immune cells. This transformation, when combined with existing immunotherapy, led to the complete regression of established liver tumors in a significant majority of the experimental subjects. Animals treated with this dual approach survived for the entire duration of the experiment, a stark contrast to control groups that succumbed to the disease within weeks.
"This is a fundamental breakthrough in our understanding of how the immune system is turned off and on in cancer," stated Edgar Engleman, MD, PhD, a professor of pathology and medicine at Stanford University, and the senior author of the study. "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."
David Kung-Chun Chiu, PhD, a basic life research scientist and the lead author of the study, spearheaded the development of sophisticated mouse models that were instrumental in unraveling this complex biological interplay. These models, engineered to recapitulate specific mutations, histological features, and therapeutic responses seen in human liver cancers, provided a crucial platform for the investigation.
A Long-Underestimated Hormone’s Role in Cancer Immunity
The discovery that EPO actively suppresses anti-cancer immunity marks a significant departure from decades of established understanding. For years, EPO was solely associated with its hematopoietic function, crucial for delivering oxygen throughout the body. However, the research team meticulously pieced together a compelling narrative of EPO’s involvement in immune evasion.
Early indications of EPO’s detrimental impact in cancer emerged over a decade ago. Clinical observations revealed that administering EPO to cancer patients suffering from anemia, to stimulate red blood cell production, paradoxically accelerated tumor growth. This correlation was so pronounced that in 2007, the U.S. Food and Drug Administration (FDA) mandated a black box warning on EPO-based drugs, cautioning against their use in individuals with cancer. Further epidemiological studies identified a clear link between higher levels of naturally occurring EPO and its receptor (EPOR) within tumors and poorer patient prognoses.
"Those old reports showed clearly that the more EPO or EPOR there was in tumors, the worse off the patients were," explained Dr. Engleman. "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."
Deconstructing the Tumor Microenvironment: From Hypoxia to Immune Suppression
The research team employed advanced genome editing techniques to meticulously create several mouse models of liver cancer. These models were designed to mimic the diverse landscape of human liver malignancies, allowing researchers to study tumor development and their responses to various therapeutic interventions. Tumor formation was induced through either the injection of DNA encoding cancer-associated proteins or the implantation of liver cancer cells directly into the animals’ livers.
A key focus of the study was the impact of a widely used immunotherapy targeting the PD-1 molecule on T cells, a critical component of the adaptive immune system. The PD-1 pathway acts as a brake on T cell activity, preventing them from attacking cancer cells. Therapies designed to block this pathway, such as the commercially available Keytruda, have revolutionized the treatment of certain cancers like melanoma and some lung cancers, leading to remarkable patient outcomes. However, a substantial proportion of tumors, including many liver, pancreatic, colon, breast, and prostate cancers, remain resistant to these therapies.
Within the mouse models, the researchers observed that certain genetic mutations resulted in the development of "cold" tumors. These tumors were largely ignored by the immune system, characterized by a scarcity of T cells and a resulting insensitivity to anti-PD-1 treatment. Conversely, other mutations led to the formation of "hot" or "inflamed" tumors, which were infiltrated with T cells and demonstrated a strong response to anti-PD-1 therapy.
The unexpected finding came when the researchers analyzed the EPO levels in these different tumor types. They discovered that cold tumors exhibited significantly elevated levels of EPO compared to their hot counterparts. This increase was strongly correlated with the hypoxic microenvironment – a state of low oxygen – which is frequently prevalent in cold tumors. Hypoxia triggers the production of specific proteins within cancer cells that, in turn, stimulate EPO synthesis. The underlying rationale for this response, from the cancer cell’s perspective, is to enhance red blood cell production to counteract the oxygen deprivation.
"Hypoxia in tumors has been studied for decades," Dr. Engleman noted. "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."
The EPO-Macrophage Axis: A Newly Discovered Immune Checkpoint
To validate their hypothesis, the researchers delved into existing scientific databases, confirming a consistent correlation between elevated EPO levels and poorer survival rates in patients with liver, kidney, breast, colon, and skin cancers. Their subsequent experimental manipulations, focusing on the tumor cells’ ability to produce EPO, yielded striking results.
When mutations that typically led to cold tumors were modified to disable EPO production, the tumors transformed into hot tumors. Conversely, hot tumors, which were previously susceptible to immune attack, thrived when engineered to overproduce EPO. This demonstrated a direct causal link between EPO production and tumor immune evasion.
Further exhaustive research elucidated the precise mechanism: in cold tumors, cancer cells secrete EPO, which then binds to receptors on macrophages, a type of immune cell. This binding event prompts macrophages to adopt an immunosuppressive role, actively repelling cancer-killing T cells and suppressing their activity. This EPO-mediated crosstalk between tumor cells and macrophages represents a newly identified immune checkpoint, a crucial regulatory mechanism that governs the immune response.
The critical importance of this EPO-moderated communication was starkly evident when the researchers investigated the combined effect of blocking both the EPO signaling pathway and the anti-PD-1 pathway. In mice with cold liver tumors, treatment with either a control or anti-PD-1 therapy alone resulted in survival of no more than eight weeks. However, when macrophages were rendered unable to produce the EPO receptor, 40% of the mice survived for 18 weeks, the termination point of the experiment. When anti-PD-1 treatment was administered to mice lacking the EPO receptor, all animals survived for the entire duration of the study, signifying a complete eradication of the tumors.
"It’s simple," Dr. Engleman emphasized. "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."
Implications for Future Cancer Therapies
The implications of this discovery are far-reaching, offering a promising new strategy for tackling a wide spectrum of cancers that are currently resistant to immunotherapy. The research team is actively engaged in designing treatments that target EPO signaling in human cancers.
Two primary therapeutic approaches are being considered. One involves non-specifically targeting the EPO protein itself. While this could potentially lead to anemia, a side effect already associated with EPO-based therapies, Dr. Engleman speculates that this might be an acceptable trade-off for an effective cancer treatment. The alternative, and perhaps more targeted, approach is to selectively block the EPO receptors on the surface of macrophages within the tumor microenvironment. This strategy aims to disrupt the immunosuppressive signaling without affecting EPO’s primary role in red blood cell production.
"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 research was a collaborative effort, with contributions from the New York Blood Center and the pharmaceutical company ImmunEdge Inc. The study received funding from the National Institutes of Health, with specific grants including R01CA262361, P01CA244114, U54CA2745115, and P01HL149626.
Dr. Chiu is a co-founder of ImmunEdge Inc., and Dr. Engleman is a founder, shareholder, and board member of the company. Both are listed as inventors on a patent application (PCT/US2023/063997) related to EPO receptor agonists and antagonists, underscoring their deep involvement in translating this fundamental scientific breakthrough into potential clinical applications. This pioneering work not only reshapes our understanding of cancer immunology but also ignites hope for more effective and durable cancer treatments in the future.
