STANFORD, CA – April 24, 2024 – In a groundbreaking discovery published today in the prestigious journal Science, researchers have unveiled a surprising, critical role for a protein identified nearly four decades ago. Erythropoietin (EPO), long celebrated for its ability to stimulate the production of red blood cells, has now been found to act as a formidable suppressor of the immune system’s response to cancer, offering a new frontier in the battle against various malignancies.
The findings demonstrate that blocking the activity of EPO can dramatically transform previously "cold," or immune-resistant, liver tumors in mice into "hot" tumors, teeming with cancer-fighting immune cells. When this strategic intervention was combined with an existing immunotherapy designed to further activate these newly mobilized immune cells, the results were nothing short of remarkable: complete regression of existing liver tumors was observed in the vast majority of treated animals, which subsequently lived for the entire duration of the experiment. In stark contrast, control animals succumbed to the disease within a mere few weeks.
"This is a fundamental breakthrough in our understanding of how the immune system is turned off and on in cancer," declared Dr. Edgar Engleman, MD, PhD, a professor of pathology and of medicine and the senior author of the research. His voice resonated with a blend of scientific rigor and profound excitement. "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."
The research, spearheaded by lead author Dr. David Kung-Chun Chiu, PhD, a basic life research scientist, not only recontextualizes a well-known biological agent but also opens compelling avenues for novel cancer therapies, particularly for those resistant to current immunotherapeutic approaches.
Main Facts: A Paradigm Shift in Cancer Immunology
The central revelation of this study is the unexpected dual nature of Erythropoietin (EPO). For decades, EPO has been synonymous with erythropoiesis – the process of red blood cell formation – a vital function in combating anemia and ensuring oxygen delivery throughout the body. Its therapeutic application in conditions like chronic kidney disease, where natural EPO production is impaired, has been a cornerstone of modern medicine. However, this new research reveals EPO’s insidious role within the tumor microenvironment: actively dampening the immune system’s ability to recognize and destroy cancer cells.
The mechanism uncovered is profound. Tumors that are typically unresponsive to immunotherapy, often termed "cold" due to their sparse immune cell infiltration, were found to produce elevated levels of EPO. This EPO then binds to receptors on specific immune cells called macrophages, effectively reprogramming them into an immunosuppressive state. These reprogrammed macrophages then act as bouncers, shooing away critical cancer-killing T cells and suppressing any nascent immune activity.
The Stanford-led team demonstrated that by interrupting this EPO signaling pathway – either by preventing tumor cells from producing EPO or by blocking its receptors on macrophages – these "cold" tumors undergo a dramatic transformation. They become "hot," characterized by a robust infiltration of T cells and other immune effector cells. This re-sensitization makes them vulnerable to existing immunotherapies, specifically anti-PD-1 treatments. The synergistic effect of blocking EPO signaling and administering anti-PD-1 therapy proved exceptionally potent, leading to complete and sustained tumor regression in preclinical models.
This discovery holds profound implications, as it suggests a broadly applicable strategy for converting a significant proportion of immune-resistant cancers into treatable ones. While the current study focused on liver cancer models in mice, strong correlative data from human cancer databases indicates that this EPO-mediated immune suppression is likely at play across numerous human cancer types, including kidney, breast, colon, and skin cancers. The urgency and optimism surrounding these findings are palpable, with a clear trajectory envisioned towards human clinical trials.
Chronology: From Red Blood Cells to Immune Suppression – An Evolving Understanding
The journey to this pivotal discovery spans nearly 40 years of scientific inquiry and a decade of perplexing clinical observations. EPO itself was first identified in the 1970s and 80s for its definitive role in stimulating red blood cell production, quickly becoming a cornerstone of treatment for anemia, particularly in patients with kidney disease. Its mechanism of action – binding to erythropoietin receptors (EPOR) on hematopoietic stem cells in the bone marrow – was well-established.
However, the story of EPO took a complex turn over a decade ago when its use in cancer patients with anemia began to raise red flags. Clinicians observed a disturbing trend: administering recombinant human EPO (rhEPO) to cancer patients to alleviate chemotherapy-induced anemia, while effective in boosting red blood cell counts, often accelerated tumor growth and worsened patient outcomes. The connection was so striking and concerning that, in 2007, the U.S. Food and Drug Administration (FDA) mandated a "black box" warning label on EPO-stimulating agents, cautioning against their use in people with certain cancers due to increased risks of tumor progression and reduced survival. This was a significant regulatory action, underscoring the seriousness of the observed correlation.
Despite this clear clinical signal, the precise biological mechanism linking EPO to accelerated tumor growth remained largely elusive. Researchers noted a correlation between higher levels of naturally occurring EPO and its receptor (EPOR) in tumors and poorer patient prognoses, but the underlying causality was not understood. "Those old reports showed clearly that the more EPO or EPOR there was in tumors, the worse off the patients were," Dr. Engleman recalled. "But the connection between EPO and cancer immunity was never made until now."
Engleman further elaborated on the intellectual challenge this presented: "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." The entrenched understanding of EPO’s primary function likely created a blind spot, making it difficult for researchers to consider alternative, non-hematopoietic roles, particularly one as counterintuitive as immune suppression in cancer.
The current breakthrough began with Dr. Chiu’s meticulous work in developing sophisticated mouse models of liver cancer. Utilizing advanced genome editing techniques, Chiu engineered various models that faithfully recapitulated specific genetic mutations, histological features, and responses to approved therapies observed in 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 directly implanting liver cancer cells into the animals’ livers. These models provided the robust experimental platform necessary to dissect complex tumor-immune interactions.
The research team’s initial focus was on understanding the efficacy of a common immunotherapy targeting programmed cell death protein 1 (PD-1) on immune cells, specifically T cells. Anti-PD-1 therapies, such as Keytruda, work by blocking PD-1, thereby releasing the brakes on T cells and allowing them to attack cancer cells. While these therapies have revolutionized the treatment of cancers like melanoma, Hodgkin’s lymphoma, and some lung cancers, a large majority of tumors—including most liver, pancreas, colon, breast, and prostate cancers—remain stubbornly resistant. This resistance often stems from these tumors being "cold," lacking sufficient T cell infiltration.
It was within this context that the team made their pivotal observation. Consistent with human liver cancers, some mouse models developed "cold" liver tumors that were largely ignored by the immune system, rendering them immune-privileged. These tumors showed little to no shrinkage when treated with anti-PD-1, precisely because few T cells were present within the tumor microenvironment to be activated. Conversely, other mutations led to "hot" or "inflamed" tumors, replete with T cells and highly sensitive to anti-PD-1 treatment.
The "unexpected" finding arrived when researchers analyzed the molecular profiles of these distinct tumor types. The "cold" tumors displayed significantly elevated levels of EPO compared to their "hot" counterparts. This increase was attributed to hypoxia – an oxygen-poor microenvironment prevalent in many rapidly growing tumors. Hypoxia is known to induce the production of various proteins in cancer cells, which, in turn, ramp up EPO production as a physiological response to stimulate more red blood cells and combat low oxygen levels. "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." This realization marked the turning point, prompting the team to investigate EPO’s role beyond its established function.
Supporting Data: Experimental Validation and Mechanistic Elucidation
The journey from initial observation to a fully elucidated mechanism involved a rigorous series of experiments and data analysis. The researchers first sought to validate the correlation between EPO levels and patient prognosis in a broader context. By leveraging existing public databases, they confirmed that elevated levels of EPO were indeed consistently correlated with poorer survival rates in human patients across a range of cancers, including those of the liver, kidney, breast, colon, and skin. This strong epidemiological evidence bolstered their hypothesis that EPO’s role in cancer progression was not limited to their mouse liver models.
The next crucial step involved direct manipulation of EPO production within the tumor cells themselves. The results were striking and definitive. Mouse models that typically developed "cold" tumors, characterized by immune resistance, instead produced "hot" tumors when they were genetically modified to be unable to make EPO. This demonstrated a direct causal link: the absence of tumor-derived EPO was sufficient to transform an immune-evading tumor into an immune-responsive one. Conversely, when "hot" tumors, which were previously susceptible to immune attack, were engineered to produce elevated levels of EPO, they thrived and grew aggressively, successfully evading the immune system that had once kept them in check. These reciprocal experiments provided compelling evidence for EPO’s direct involvement in dictating the immune status of tumors.
To uncover the precise cellular and molecular mechanism, the team delved deeper. Their exhaustive research revealed a sophisticated crosstalk between tumor cells and specific immune cells. In "cold" tumors, the cancer cells themselves synthesize and secrete EPO. This secreted EPO then acts as a signaling molecule, binding to its receptors (EPOR) present on the surface of immune cells known as macrophages. Macrophages are versatile immune cells that can adopt various functional states. Upon binding EPO, these macrophages undergo a phenotypic switch, transitioning into an immunosuppressive role. In this state, they actively "shoo away" cancer-killing T cells, creating an inhospitable environment for anti-tumor immunity, and further dampen any existing T cell activity, effectively creating an immune "desert" within the tumor.
The critical importance of this EPO-moderated crosstalk between tumor cells and macrophages became unequivocally clear in experiments designed to study the combinatorial effect of simultaneously blocking the EPO signaling pathway and the anti-PD-1 pathway. These experiments represented the pinnacle of their therapeutic investigation.
In the control groups, mice bearing "cold" liver tumors and treated with either a placebo or anti-PD-1 therapy alone showed dismal outcomes, with none surviving beyond eight weeks after tumor induction. This stark reality underscored the inherent resistance of "cold" tumors to conventional immunotherapy.
However, the results for the intervention groups were transformative. In mice where macrophages were genetically modified to be unable to make the EPO receptor – thereby disrupting the EPO signaling pathway – a significant improvement in survival was observed. Forty percent of these animals lived for the full 18-week duration of the experiment, at which point it was terminated. This indicated that merely disarming the macrophages’ ability to respond to EPO was enough to allow the immune system to exert some control over the tumor.
The most dramatic outcome occurred when this disruption of EPO signaling (macrophages lacking the EPO receptor) was combined with anti-PD-1 treatment. In this cohort, an astounding 100% of the animals lived for the entire duration of the experiment. This synergistic effect demonstrated that removing EPO-mediated immune suppression not only made the tumors "hot" but also primed them perfectly for the enhanced immune activation provided by anti-PD-1 therapy.
"It’s simple," Dr. Engleman stated with conviction, summarizing the profound implications of these 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 direct and powerful outcome provides a strong rationale for translating these findings into clinical applications.
Official Responses: Optimism and Collaborative Momentum
The scientific community and the researchers involved are brimming with optimism regarding the potential impact of this discovery. Dr. Engleman’s enthusiasm is evident, emphasizing the "fundamental breakthrough" and expressing a fervent hope for rapid translation to human trials. This sentiment reflects the significant unmet need for effective treatments for immune-resistant cancers.
The research was a collaborative endeavor, with contributions from institutions beyond Stanford. Researchers from the New York Blood Center played a role in the study, lending their expertise to various aspects of the complex biological investigation. Furthermore, the pharmaceutical company ImmunEdge Inc. also contributed to the research, indicating a direct pathway from academic discovery to potential therapeutic development.
Financial backing for such extensive and complex research is crucial. The study received substantial funding from multiple grants from the National Institutes of Health (NIH), specifically R01CA262361, P01CA244114, U54CA2745115, and P01HL149626. This federal support underscores the perceived significance and potential impact of the research from a national health perspective.
The lead author, Dr. David Kung-Chun Chiu, is a cofounder of ImmunEdge Inc., highlighting his commitment to translating these scientific insights into clinical solutions. Similarly, Dr. Edgar Engleman is a founder, shareholder, and board member of ImmunEdge Inc. Both Dr. Chiu and Dr. Engleman are also Stanford-affiliated inventors of a patent application (PCT/US2023/063997) titled "EPO receptor agonists and antagonists." This commercial and intellectual property involvement signals a clear intent to develop and advance therapies based on this groundbreaking research, bridging the gap between bench science and patient care.
Implications: A New Horizon for Cancer Therapy
The implications of this discovery are vast and potentially transformative for cancer treatment. The most immediate and exciting prospect is the development of novel therapeutic strategies to target EPO signaling in human cancers. Dr. Engleman and his colleagues are already actively engaged in designing such treatments.
One potential approach involves non-specifically targeting the EPO protein itself. While this could effectively lower EPO levels and thus reduce immune suppression, it comes with a known side effect: anemia, given EPO’s primary role in red blood cell production. However, Dr. Engleman speculates that for patients battling aggressive, immune-resistant cancers, this might be an "acceptable trade-off for an effective cancer therapy." The severity of the disease and the potential for life-saving benefits could outweigh the manageable side effect of anemia, which can often be addressed with supportive care.
A more refined and potentially less toxic alternative approach involves selectively blocking the EPO receptors specifically on the surfaces of macrophages within the tumor microenvironment. This targeted strategy would aim to disarm the immunosuppressive macrophages without interfering with EPO’s vital role in red blood cell production elsewhere in the body. Such precision medicine could offer a powerful way to reprogram the tumor microenvironment without inducing systemic side effects. Developing such selective inhibitors will be a key area of focus for future drug development.
Beyond the immediate therapeutic implications, this research significantly deepens our fundamental understanding of tumor biology and immune evasion. It highlights how cancer cells cleverly co-opt normal physiological pathways, like the hypoxia-induced EPO production, for their own survival and immune escape. This paradigm shift in understanding EPO’s role mandates a re-evaluation of how we perceive and target the tumor microenvironment. It suggests that factors previously considered benign or even beneficial (like EPO in the context of anemia) can, in the warped reality of cancer, become critical enablers of disease progression.
While the current findings are incredibly promising, Dr. Engleman injects a note of scientific caution, acknowledging the inherent complexity of cancer: "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." Cancer is a heterogeneous disease, and individual patient responses will undoubtedly vary. However, the breadth of cancers where elevated EPO levels correlate with poor prognosis suggests a wide applicability for this therapeutic strategy.
The long-term vision is clear: to integrate anti-EPO receptor therapies or EPO-blocking agents into existing immunotherapy regimens, particularly for those "cold" tumors that currently defy treatment. By unlocking the immune system’s full potential, this discovery holds the promise of transforming patient outcomes, offering hope to millions facing challenging cancer diagnoses, and ushering in a new era of more effective and intelligent cancer therapies. The journey from a decades-old protein to a novel cancer fighter underscores the enduring power of scientific curiosity and the unexpected pathways to medical breakthroughs.
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