STANFORD, CA – April 24, 2024 – A protein primarily known for its vital role in stimulating red blood cell production, erythropoietin (EPO), has been revealed to possess a surprising and critical function in dampening the immune system’s response to cancer. This groundbreaking discovery, made by researchers at Stanford Medicine and published today in the prestigious journal Science, challenges decades of biological understanding and offers a compelling new target for cancer immunotherapies, particularly for tumors previously deemed resistant to treatment.
The research demonstrates that by blocking EPO’s activity, scientists were able to transform notoriously "cold," or immune-resistant, liver tumors in mice into "hot" tumors, teeming with an arsenal of cancer-fighting immune cells. When this intervention was combined with an existing immunotherapy designed to further activate these immune cells, the results were dramatic: complete regression of existing liver tumors in the majority of treated animals, which subsequently lived for the entire duration of the experiment. In stark contrast, control animals, receiving no such treatment, 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," exclaimed Dr. Edgar Engleman, a professor of pathology and medicine at Stanford, and senior author of the research. His palpable excitement underscores the profound implications of this finding, with the hope that treatments targeting this newly uncovered mechanism will swiftly advance to human trials.
Dr. David Kung-Chun Chiu, a basic life research scientist and the lead author of the study, spearheaded the intricate experimental work that led to this revelation. While the current findings are based on extensive mouse model studies, strong indicators suggest that EPO plays a remarkably similar, detrimental role in numerous types of human cancers, heralding a potential paradigm shift in oncology.
The Unforeseen Role: EPO’s Dual Identity
Erythropoietin, or EPO, has long been celebrated for its life-sustaining function: regulating erythropoiesis, the process by which the body produces red blood cells. This hormone, primarily synthesized by the kidneys, acts on stem cells in the bone marrow to ensure a steady supply of oxygen-carrying red blood cells, crucial for preventing anemia and maintaining overall physiological function. For decades, its role in human health and disease was largely confined to this hematopoietic pathway, with therapeutic applications in treating anemia, particularly in patients with chronic kidney disease or those undergoing chemotherapy.
However, the new research illuminates a previously unrecognized, darker side to EPO, positioning it as a potent orchestrator of immune suppression within the tumor microenvironment. This dual identity—a life-sustaining hormone in one context, a cancer-abetting agent in another—redefines our understanding of this ubiquitous protein and opens a novel avenue for therapeutic intervention.
A Chronology of Clues: From Red Blood Cells to Immune Resistance
The journey to this discovery was not linear, tracing back through years of clinical observations and scientific inquiries that, in hindsight, offered tantalizing clues about EPO’s broader influence.
The Enigma of EPO: A Long-Known Factor in Health
For nearly 40 years, EPO has been a cornerstone of hematology. Its discovery and subsequent pharmaceutical development revolutionized the treatment of anemia, significantly improving the quality of life for millions. The scientific community’s focus remained squarely on its erythropoietic functions, with extensive research detailing its receptor (EPOR) on red blood cell precursors and the signaling pathways it activates to promote their growth and differentiation. This well-established paradigm, however, inadvertently obscured its potential involvement in other biological processes, particularly within the complex landscape of cancer.
Early Warnings: EPO and Accelerated Tumor Growth
A pivotal moment in the historical narrative of EPO and cancer occurred more than a decade ago, though its full significance was not grasped until now. Clinical studies involving cancer patients suffering from anemia revealed a disturbing trend: the administration of exogenous EPO, intended to stimulate red blood cell formation and alleviate anemia, paradoxically correlated with accelerated tumor growth. The connection was so compelling 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 cancers.
This warning, while critical for patient safety, presented a perplexing biological puzzle. Why would a protein designed to promote red blood cell production also fuel tumor progression? Early hypotheses often revolved around tumor cells directly utilizing EPO for their own growth or survival, but a direct link to the immune system remained elusive.
Adding another layer to this mystery, researchers observed a clear correlation between patient prognosis and the levels of naturally occurring EPO and its receptor (EPOR) within tumors. "Those old reports showed clearly that the more EPO or EPOR there was in tumors, the worse off the patients were," Dr. Engleman recalled, highlighting the consistent, albeit unexplained, association. Yet, despite these strong correlative signals, the critical connection between EPO and cancer immunity remained undiscovered. "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 Genesis of the Study: Unpacking Anti-PD-1 Resistance
The current breakthrough emerged from a focused effort to understand the mechanisms underlying resistance to immunotherapy, particularly anti-PD-1 therapies. Dr. Chiu meticulously developed and studied genome editing techniques to create several sophisticated mouse models of liver cancer. These models were designed to accurately recapitulate specific mutations, histological features, and responses to approved therapies observed in various subtypes of human liver cancers. Tumor formation was initiated either by injecting a combination of DNA encoding liver cancer-associated proteins into the animals’ tail veins or by directly implanting liver cancer cells into their livers.
The research team’s initial interest centered on the effectiveness of a common immunotherapy targeting programmed cell death protein 1 (PD-1), a molecule found on immune cells called T cells. Anti-PD-1 therapies, such as the commercially available Keytruda, work by blocking PD-1, thereby preventing cancer cells from dampening the activity of T cells. These therapies have revolutionized the treatment of several human cancers, including melanoma, Hodgkin’s lymphoma, and certain lung cancers, transforming patient outcomes for many. However, a significant majority of tumors, including most liver, pancreas, colon, breast, and prostate cancers, remain stubbornly resistant to anti-PD-1 treatment.
The Serendipitous Discovery: From Cold Tumors to EPO Levels
Consistent with observations in human liver cancers, the researchers found that certain combinations of genetic mutations in their mouse models led to the development of liver tumors that were largely ignored by the immune system. These "immune privileged," or "cold," tumors did not shrink when the animals were treated with anti-PD-1 because they contained very few T cells capable of mounting an attack. In stark contrast, other mutations resulted in "hot," or "inflamed," tumors that were replete with T cells and, consequently, highly sensitive to anti-PD-1 treatment, which successfully triggered the T cells to eliminate the cancer.
It was during this comparative analysis that the unexpected connection began to emerge. The cold tumors, characterized by their immune resistance, displayed markedly elevated levels of EPO compared to their hot counterparts. This increase, the researchers hypothesized, was likely a consequence of the oxygen-poor microenvironment—a condition known as hypoxia—prevalent within cold tumors. Hypoxia is a well-known inducer of specific proteins in cancer cells, which, in turn, ramp up the production of EPO. The conventional wisdom dictated that this surge in EPO was merely an attempt to produce more red blood cells to combat the low oxygen levels within the tumor, a cellular survival mechanism.
"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 moment of realization marked a critical turning point, prompting the team to investigate EPO’s role beyond its established function.
Supporting Data: Unveiling the Immunosuppressive Pathway
The team’s curiosity ignited, they embarked on a series of exhaustive experiments and analyses, building a robust body of evidence that firmly established EPO’s direct involvement in immune suppression.
Confirming Human Cancer Correlation
First, the researchers delved into existing human cancer databases. Their analysis confirmed that elevated levels of EPO were indeed correlated with poorer survival rates in patients across a spectrum of cancers, including those of the liver, kidney, breast, colon, and skin. This re-affirmation of earlier clinical observations, now viewed through the lens of potential immune evasion, strengthened their hypothesis.
Experimental Validation: Manipulating EPO and Tumor Fate
The next step involved direct manipulation of EPO production within the mouse tumors. The results were startling. Mutations that had previously led to the development of cold, immune-resistant tumors now caused hot, inflamed tumors when the tumor cells were genetically modified to be unable to produce EPO. Conversely, hot tumors, which had previously been successfully eradicated by the immune system, thrived and grew aggressively when they were engineered to secrete elevated levels of EPO. These reciprocal experiments provided compelling functional evidence that EPO production by tumor cells was a direct determinant of the tumor’s immune status.
The Macrophage Mastermind: EPO’s Cellular Target
To pinpoint the exact cellular mechanism, the researchers conducted further detailed investigations. They discovered that in cold tumors, the tumor cells actively produce and secrete EPO into the surrounding microenvironment. This secreted EPO then binds to specific receptors (EPOR) located on the surface of immune cells called macrophages, which are abundant within tumors.
Upon binding to EPO, these macrophages undergo a critical transformation: they switch to an immunosuppressive role. In this altered state, these "immunosuppressive macrophages" actively deter cancer-killing T cells from entering the tumor microenvironment and, furthermore, dampen the activity of any T cells that manage to infiltrate. This EPO-mediated crosstalk between tumor cells and macrophages creates a protective shield around the cancer, effectively shutting down the anti-tumor immune response.
The Power of Combination Therapy: Synergistic Eradication
The ultimate test of this newly identified pathway involved studying the combinatorial effect of simultaneously blocking the EPO signaling pathway and the anti-PD-1 pathway. The results were nothing short of remarkable.
In these crucial experiments, none of the mice with cold liver tumors treated with control agents or with anti-PD-1 alone survived beyond eight weeks after tumor induction. This stark outcome highlighted the recalcitrant nature of these immune-resistant tumors.
However, a dramatic improvement was observed in mice where the EPO signaling pathway was disrupted. Specifically, 40% of mice whose macrophages were genetically unable to produce the EPO receptor lived for 18 weeks after tumor induction, at which point the experiment was terminated. This indicated that merely disarming the macrophages’ ability to respond to EPO significantly prolonged survival.
The most striking outcome occurred when anti-PD-1 treatment was administered to mice lacking the EPO receptor on their macrophages: all animals lived for the duration of the experiment, achieving complete tumor regression. This synergistic effect underscored the critical role of EPO-mediated immunosuppression in rendering tumors resistant to current immunotherapies and demonstrated that overcoming this barrier could unlock profound therapeutic benefits.
"It’s simple," Dr. Engleman summarized, emphasizing the clarity of the 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 statement encapsulates the elegance and power of the discovery.
Official Responses and Future Outlook
The findings have been met with significant enthusiasm within the scientific community, particularly among those focused on overcoming immunotherapy resistance. Dr. Engleman’s repeated expressions of excitement and optimism reflect the potential for this discovery to translate into tangible benefits for cancer patients. The historical context of the FDA’s black box warning, now viewed through this new immunological lens, underscores the deep clinical relevance and the urgent need to revisit EPO’s role in cancer management.
The initial clinical observations that led to the FDA warning, though initially perplexing, now find a coherent explanation. The acceleration of tumor growth previously observed with EPO administration was likely not due to direct tumor cell proliferation alone, but rather a potent suppression of the patient’s anti-cancer immune response, allowing existing tumors to flourish unchecked. This revelation mandates a re-evaluation of how EPO is used in patients with cancer-associated anemia and opens the door for novel therapeutic strategies that target this pathway.
Implications: Paving the Way for New Cancer Therapies
The implications of this discovery are vast, extending far beyond the liver cancer models studied. The strong indications that EPO plays a similar immunosuppressive role in many types of human cancers—including kidney, breast, colon, and skin cancers, as evidenced by patient prognosis data—suggests a broad applicability for future therapies.
Translational Potential: From Bench to Bedside
Dr. Engleman and his colleagues are already actively engaged in designing new treatment strategies that specifically target EPO signaling in human cancers. The rapid progression from basic discovery to therapeutic design highlights the urgency and clinical promise of this research.
Two primary approaches are being considered:
- Non-specific EPO targeting: This involves lowering overall EPO levels. While potentially effective, this strategy could induce anemia, given EPO’s primary role in red blood cell production. However, Dr. Engleman speculates that for an effective cancer therapy, this might be an acceptable trade-off, carefully managed with supportive care.
- Selective blockade of EPO receptors on macrophages: This approach offers greater precision, aiming to specifically disarm the immunosuppressive macrophages within the tumor microenvironment without broadly affecting EPO’s systemic red blood cell functions. This more targeted strategy could potentially mitigate side effects like anemia.
Challenging Dogma and Redefining Immunotherapy
This discovery represents a significant challenge to long-held biological dogma. A protein so deeply understood and therapeutically utilized for its hematopoietic function has now been unmasked as a critical player in cancer immunology. This underscores the intricate and often unexpected ways in which cellular pathways can be hijacked by cancer.
Furthermore, this research offers a compelling pathway to overcome resistance to existing immunotherapies. For the vast majority of "cold" tumors that currently do not respond to anti-PD-1 treatments, blocking EPO signaling could act as a crucial sensitizer, effectively turning these resistant tumors into responsive ones. This could dramatically expand the number of patients who can benefit from the life-saving potential of immunotherapy.
Broader Impact
The potential economic and societal impact of this research is substantial. New drug development based on these findings could lead to improved patient outcomes, extended survival, and potentially even cures for cancers that are currently untreatable with immunotherapy. Reducing the burden of cancer through more effective treatments would have profound positive effects on public health worldwide.
As Dr. Engleman eloquently states, "I continue to be amazed by this finding. 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 received funding from the National Institutes of Health (grants R01CA262361, P01CA244114, U54CA2745115, and P01HL149626), underscoring the significance recognized by major scientific funding bodies. Researchers from the New York Blood Center and the pharmaceutical company ImmunEdge Inc. also contributed to the research. It is important to note the affiliations of the lead researchers: 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," highlighting the direct translational potential of their scientific endeavors.
This groundbreaking research marks a pivotal moment in oncology, offering not just a deeper understanding of cancer immunity but also a clear path forward for developing desperately needed new treatments for millions of patients worldwide. The long-hidden role of EPO, once primarily a friend to red blood cells, now stands revealed as a formidable foe to anti-cancer immunity, awaiting its therapeutic neutralization.
About the Author
