By Lina carolina 
Stanford University Researchers Uncover Novel Mechanism by Which Cancer Evades Immune System, Paving Way for New Therapies
A protein, initially identified nearly four decades ago for its crucial role in stimulating the production of red blood cells, has been revealed to play a surprisingly critical and previously unrecognized function: dampening the immune system’s response to cancer. This groundbreaking discovery, emerging from research at Stanford University, has demonstrated that blocking this protein’s activity can transform formerly immune-resistant liver tumors in mice into "hot" tumors teeming with cancer-fighting immune cells. The findings, published in the prestigious journal Science, hold significant promise for the development of novel cancer therapies applicable to a wide range of malignancies.
The protein in question is erythropoietin, or EPO. While its well-established function as a growth factor for red blood cells has been understood since the early 1980s, its involvement in immune suppression within the tumor microenvironment was not suspected until recently. This paradigm shift in understanding opens a new avenue for combating cancers that have historically proven resistant to existing treatments, particularly immunotherapies.
Transforming "Cold" Tumors into "Hot" Battlegrounds
In the study, researchers successfully rendered formerly "cold," or immune-resistant, liver tumors in mice highly susceptible to immune attack. By blocking the activity of EPO, these tumors, which had previously been ignored by the immune system, became "hot," meaning they were infiltrated by a substantial number of cancer-fighting immune cells, specifically T cells. This transformation was not merely academic; when combined with an immunotherapy that further activates these T cells against the cancer, the treatment led to the complete regression of existing liver tumors in a majority of the treated mice. These animals survived for the entire duration of the experimental period, a stark contrast to control groups that succumbed to the disease within a few weeks.
Dr. Edgar Engleman, MD, PhD, a professor of pathology and of medicine at Stanford University and the senior author of the study, expressed profound excitement about the implications of this discovery. "This is a fundamental breakthrough in our understanding of how the immune system is turned off and on in cancer," Dr. Engleman stated. "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 lead author of the study is David Kung-Chun Chiu, PhD, a basic life research scientist. The research, which details the intricate molecular mechanisms at play, was published online on April 24th.
Extending Beyond Liver Cancer: Broad Applicability Suspected
While the initial research was conducted using mouse models of liver cancer, there are compelling indications that EPO’s immunosuppressive role is not confined to this specific organ. Strong evidence suggests that EPO plays a similar, detrimental role in a broad spectrum of human cancers.
The connection between EPO and cancer has a prior, albeit misunderstood, history. Research dating back more than a decade revealed that administering EPO to cancer patients suffering from anemia, to stimulate red blood cell formation, inadvertently accelerated tumor growth. This phenomenon was so concerning that in 2007, the U.S. Food and Drug Administration (FDA) mandated a black box warning label on EPO-based drugs, cautioning against their use in individuals with cancer. Furthermore, researchers observed a clear correlation between a patient’s prognosis and the levels of naturally occurring EPO and its receptor within their tumors; higher levels were consistently associated with poorer 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." This established identity of EPO as solely a red blood cell stimulant had, for decades, obscured its more sinister role in immune evasion.
Delving into the Mechanisms: Mouse Models and Immunotherapy
To investigate this phenomenon, Dr. Chiu developed and utilized several sophisticated mouse models of liver cancer. These models were designed to meticulously 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 DNA encoding proteins associated with liver cancer into the animals’ tail veins or by implanting pre-established liver cancer cells directly into their livers.
A key focus of the research was the impact of a common immunotherapy known as anti-PD-1 therapy. This treatment targets a molecule called PD-1, found on T cells, a critical component of the immune system. By binding to PD-1, cancer cells can effectively dampen the activity of T cells, thus evading immune destruction. Anti-PD-1 therapies, such as the widely marketed Keytruda, have revolutionized the treatment of several cancers, including melanoma, Hodgkin’s lymphoma, and certain types of lung cancer, leading to dramatic improvements in patient outcomes. However, a significant limitation of these therapies is their ineffectiveness against a large majority of tumors, including many common forms of liver, pancreas, colon, breast, and prostate cancers.
The Stanford researchers observed that, mirroring human liver cancers, certain combinations of mutations in their mouse models led to the development of liver tumors that were largely ignored by the immune system. These "cold" tumors were characterized by a scarcity of T cells, rendering them immune privileged and unresponsive to anti-PD-1 treatment. In contrast, other mutations resulted in "hot" or "inflamed" tumors, which were densely populated with T cells and highly sensitive to anti-PD-1 therapy, which successfully activated the T cells to launch an attack against the cancer.
The Hypoxia-EPO Axis: A Newly Discovered Link
A critical and unexpected finding emerged when the researchers analyzed the EPO levels in these different tumor types. They discovered that the cold tumors exhibited significantly elevated levels of EPO compared to their hot counterparts. This increase was strongly linked to the oxygen-poor microenvironment, a condition known as hypoxia, which is prevalent in many cold tumors. Hypoxia, a state of low oxygen, triggers cancer cells to produce specific proteins that, in turn, upregulate the production of EPO. The rationale for this upregulation, from the cancer cell’s perspective, is to stimulate the production of more red blood cells, thereby attempting to alleviate the oxygen deprivation.
"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." This illustrates how deeply entrenched prior understanding can sometimes blind researchers to novel interpretations.
Experimental Validation: Manipulating EPO Production
Driven by this hypothesis, the researchers turned to existing databases to confirm their suspicions. They found a consistent correlation between elevated EPO levels and poorer survival rates in patients with cancers of the liver, kidney, breast, colon, and skin, reinforcing the link between EPO and adverse cancer outcomes.
To experimentally validate their findings, they ingeniously manipulated the ability of tumor cells to produce EPO. The results were striking and confirmed their hypothesis. When mutations that typically led to the development of cold tumors were modified to prevent EPO production, these tumors transformed into hot tumors. Conversely, hot tumors that had previously been effectively eradicated by the immune system thrived and grew when engineered to produce elevated levels of EPO. This demonstrated a direct causal relationship between EPO production and the tumor’s ability to evade immune surveillance.
Unmasking the Macrophage’s Role in Immune Suppression
Further meticulous research revealed the specific mechanism by which EPO exerts its immunosuppressive effects. In cold tumors, cancer cells secrete EPO, which then binds to receptors on the surface of immune cells known as macrophages. Macrophages, which can adopt diverse roles, are essentially reprogrammed by EPO to adopt an immunosuppressive function. In this state, they actively deter cancer-killing T cells from infiltrating the tumor microenvironment and suppress the activity of those T cells that do manage to enter. This EPO-moderated crosstalk between tumor cells and macrophages is a pivotal element in the tumor’s immune evasion strategy.
The critical importance of this EPO-mediated interaction became vividly apparent when the researchers investigated the combined effect of simultaneously blocking the EPO signaling pathway and the anti-PD-1 pathway. In experiments involving mice with cold liver tumors, those treated with either a control substance or anti-PD-1 alone showed limited survival, with none living beyond eight weeks after tumor induction. In stark contrast, mice whose macrophages were engineered to be unable to produce the EPO receptor exhibited significantly improved outcomes, with 40% surviving for 18 weeks, the duration of the experiment. When anti-PD-1 treatment was administered to mice lacking the EPO receptor, a remarkable 100% of animals survived for the entire experimental period.
"It’s simple," Dr. Engleman concluded. "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 underscores the dual benefit of targeting EPO: not only does it hinder tumor growth, but it also re-sensitizes previously resistant tumors to established immunotherapies.
Future Directions: Translating Discovery to Therapy
The implications of this research are profound, offering a tangible path toward developing novel and more effective cancer therapies. Dr. Engleman and his team are actively engaged in designing treatments that target EPO signaling in human cancers. One potential approach involves non-specifically targeting the EPO protein itself. While this might lead to anemia, a manageable side effect in the context of a potentially life-saving cancer therapy, it remains a consideration. A more targeted strategy involves selectively blocking the EPO receptors on the surface of macrophages within the tumor microenvironment, aiming to disrupt the immunosuppressive signaling without affecting systemic EPO function.
"I continue to be amazed by this finding," Dr. Engleman stated. "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 researchers are optimistic that this newly uncovered mechanism of immune suppression by EPO will lead to breakthrough treatments for cancers that are currently difficult to manage.
The collaborative nature of this research is highlighted by contributions from the New York Blood Center and the pharmaceutical company ImmunEdge Inc. The study received vital funding from the National Institutes of Health, with grants R01CA262361, P01CA244114, U54CA2745115, and P01HL149626 playing a crucial role in enabling this significant scientific advancement. Notably, Dr. Chiu is a cofounder of ImmunEdge Inc., and Dr. Engleman is a founder, shareholder, and board member of the same company. Both are affiliated with Stanford University as inventors of a patent related to EPO receptor agonists and antagonists (PCT/US2023/063997), underscoring their direct involvement in the translation of this discovery into potential therapeutic applications. This synergy between academic research and commercial development is often critical for accelerating the journey from laboratory bench to patient bedside.