What Is CD73? Its Role in Cancer and the Immune System

CD73 is an enzyme anchored to the outer surface of cells that converts adenosine monophosphate (AMP) into adenosine, a small signaling molecule with powerful effects on the immune system and blood vessels. Under normal circumstances, this conversion helps keep inflammation in check and protects tissues from damage. In cancer, though, tumors exploit CD73’s adenosine-generating activity to shield themselves from immune attack, making the enzyme one of the most actively pursued targets in cancer immunotherapy today.

What CD73 Actually Does

CD73, formally known as ecto-5′-nucleotidase, sits on the outside of many cell types, tethered to the cell membrane by a molecular anchor. Its primary job is straightforward: it strips a phosphate group from AMP, producing adenosine. This is the final step in a cascade that begins when cells release ATP, the universal energy currency, into the space around them. A partner enzyme called CD39 handles the first part, breaking ATP down into AMP. CD73 then finishes the job, turning AMP into adenosine.¹ That two-step relay, sometimes called the CD39-CD73 axis, controls how much adenosine accumulates around cells at any given moment.

Why does adenosine matter? Once produced, it binds to receptors on nearby cells, particularly immune cells, and generally tells them to calm down. In healthy tissue, this is protective. Inflammation that spirals out of control can damage organs, and adenosine acts as a brake. CD73 is especially prominent on regulatory T cells, the immune cells specifically tasked with suppressing overactive immune responses.² The enzyme is also found on endothelial cells lining blood vessels, where the adenosine it generates helps maintain the vascular barrier, the tight seal that keeps blood components from leaking into surrounding tissue.³

How Tumors Hijack CD73

Tumors are famously hostile environments: starved of oxygen, acidic, and crowded. That low-oxygen state, or hypoxia, turns out to be a direct trigger for CD73 production. In gastric cancer cells, researchers showed that the low-oxygen sensor HIF-1α physically binds to the CD73 gene’s promoter region and cranks up its expression. Block HIF-1α, and the hypoxia-driven surge in CD73 largely disappears.⁴ The result is a tumor microenvironment drenched in adenosine, a molecule that happens to be profoundly immunosuppressive when it accumulates at high concentrations.

Under normal conditions, adenosine signaling through specific receptors (called A2a and A2b) is a healthy feedback mechanism that protects tissues from collateral immune damage. Tumors co-opt this same pathway. The adenosine flooding the tumor microenvironment engages those receptors on incoming immune cells and essentially tells them to stand down, even though there is a clear threat to eliminate.⁵ It is one of the more elegant tricks in cancer biology: the tumor does not need to destroy immune cells directly. It just repurposes the body’s own peacekeeping signals.

The Immune Cells That Get Silenced

The immune cells hit hardest by CD73-generated adenosine are the ones that would otherwise be most dangerous to the tumor. CD8+ T cells, the cytotoxic killers that can recognize and destroy cancer cells, are among the primary casualties. Studies in mouse models of lung, melanoma, and breast cancer found that CD73-expressing immune-suppressive cells called monocytic myeloid-derived suppressor cells (MDSCs) build up inside tumors and produce enough adenosine to shut down CD8+ T cell activity.⁶ In pancreatic cancer models, knocking down the gene that encodes CD73 significantly reduced the proportion of these suppressor cells within tumors.⁷

Natural killer (NK) cells, another critical arm of anti-tumor immunity, face the same problem. Adenosine binding to receptors on NK cells blunts their ability to kill.⁸ The net effect is a tumor that has effectively erected an invisible fence: immune cells can migrate to it, recognize it as abnormal, but then find themselves functionally paralyzed once they arrive. This is why CD73 has attracted such intense interest from drug developers. The immune system often has enough firepower to destroy a tumor, it just cannot pull the trigger in an adenosine-rich environment.

CD73 Does More Than Suppress Immunity

Immune evasion is the headline story for CD73 in cancer, but it is not the only one. The enzyme also helps tumors build blood vessels, a process called angiogenesis. Research has demonstrated that CD73 on tumor cells increases the production of vascular endothelial growth factor (VEGF), one of the main signals that stimulate new vessel growth. Meanwhile, CD73 on host cells (the patient’s own non-cancer cells) is needed for the blood vessel cells themselves to form tubes and migrate. Notably, these pro-angiogenic effects depend on both the enzyme’s adenosine-producing activity and on separate, non-enzymatic functions of the protein.⁹ Experiments in melanoma-bearing mice lacking CD73 showed significantly reduced blood vessel density in tumors, and activating specific adenosine receptors could partially or fully restore vessel formation depending on which receptor was stimulated.¹⁰

CD73 also appears to play a role in metastasis through mechanisms that go beyond adenosine production. In melanoma cell lines, the protein has been shown to facilitate cell migration and invasion, partly by acting as a co-receptor that helps cells interact with the extracellular matrix, the scaffolding between cells that migrating cancer cells must navigate.¹¹ And in neuroblastoma, CD73 has been linked to the maintenance of cancer stem cell properties, including the ability to self-renew and resist differentiation, traits associated with treatment resistance and relapse.¹²

CD73 as a Prognostic Marker

Given its involvement in so many tumor-promoting processes, researchers have asked whether measuring CD73 levels can predict how well or poorly a patient is likely to do. The answer is yes, but with an important twist: the direction of the prediction depends on the cancer type. A large evaluation spanning multiple cancer types found that, across the board, high CD73 expression was associated with shorter overall survival. But when the researchers broke the data down by specific cancers, the picture grew more complicated. In breast and ovarian cancer, high CD73 correlated with worse outcomes. In lung and gastric cancer, the opposite was true: high CD73 was actually associated with better survival.¹³

That counterintuitive finding is a reminder that the tumor microenvironment is not uniform across cancer types. In some contexts, CD73 expression on tumor cells might reflect a broader immune infiltration pattern that happens to coincide with better prognosis, even though the enzyme itself is immunosuppressive. In other words, CD73 levels are a useful signal, but interpreting them requires knowing the specific cancer being studied.

In metastatic melanoma, researchers have moved beyond tissue-level expression and measured CD73 enzyme activity in the blood. One study of patients treated with the immunotherapy drug nivolumab found that those with high soluble CD73 enzyme activity before starting treatment had a median progression-free survival of just 2.6 months, compared with 14.2 months for patients with lower activity. In multivariate analysis, circulating CD73 activity emerged as the single strongest prognostic factor for both overall and progression-free survival.¹⁴ A separate, larger study of metastatic melanoma patients confirmed that high serum CD73 activity was an independent prognostic factor alongside established markers like serum lactate dehydrogenase and brain metastases.¹⁵ These findings raise the possibility that a blood test for CD73 activity could one day help clinicians decide which patients are most likely to benefit from immunotherapy.

Targeting CD73 With Drugs

The therapeutic logic is simple: if tumors use CD73-generated adenosine to hide from the immune system, blocking CD73 should strip that camouflage and let immune cells do their jobs. Two broad strategies are in development: antibodies that bind directly to the CD73 protein and small molecules that inhibit its enzymatic activity.

Oleclumab is the most clinically advanced anti-CD73 antibody. It works by locking onto CD73, physically blocking its ability to produce adenosine and also triggering the cell to pull the enzyme off its surface through internalization. In the first-in-human trial, patients receiving oleclumab showed sustained drops in both free soluble CD73 in the blood and CD73 expression on tumor cells, accompanied by reduced CD73 enzyme activity.¹⁶ Clinical trials have since tested oleclumab in combination with the PD-L1 inhibitor durvalumab and chemotherapy in settings like metastatic pancreatic cancer.¹⁷

Small-molecule CD73 inhibitors represent a parallel track. These are chemically simpler compounds that can potentially be taken orally and may reach tissue compartments that antibodies struggle to penetrate. One novel approach targets the pathway upstream of CD73 itself: a compound called ST80 disrupts the interaction between CD73 and a protein called OTUD4 that stabilizes it. In laboratory models of triple-negative breast cancer, lung cancer, ovarian cancer, and colorectal cancer, ST80 reduced adenosine production in the tumor microenvironment, boosted CD8+ T cell activity, and increased levels of the anti-tumor signaling molecule interferon-gamma.¹⁸

Why Combination Therapy Matters

CD73 blockers are unlikely to work well as standalone treatments for most patients. The reason is that adenosine is just one of several immunosuppressive mechanisms tumors deploy. Blocking CD73 may relieve the adenosine brake, but if the tumor is also using checkpoint molecules like PD-1 or CTLA-4 to keep T cells exhausted, lifting adenosine suppression alone may not be enough. This is why combination approaches have attracted so much attention.

In preclinical mouse work, combining anti-CD73 antibodies with anti-PD-1 therapy produced striking results: complete tumor rejection in all treated mice in one colon cancer model. Anti-CD73 also enhanced the activity of both anti-CTLA-4 and anti-PD-1 antibodies across colon, prostate, and breast tumor models.¹⁹ The rationale extends to small-molecule inhibitors too: incorporating CD73 inhibitors can augment immune therapies targeting other checkpoints.²⁰ Think of it as removing multiple roadblocks on the same highway. Removing one obstacle helps. Removing several at once lets traffic flow much more freely.

The question that clinical trials are now trying to answer is whether these dramatic preclinical results translate to meaningful patient benefit. Mouse tumors and human tumors do not always respond the same way. The adenosine pathway is deeply intertwined with the tumor microenvironment’s metabolic state, its oxygen levels, its immune cell composition, and the particular cancer type. What works spectacularly in a mouse colon cancer model may produce only modest improvement in human pancreatic cancer. The clinical trial data published so far is encouraging enough to justify continued investment, but the field has not yet produced a clear-cut practice-changing result for any single cancer type.

How CD73 Expression Is Controlled

Understanding what controls CD73 levels may ultimately matter as much as blocking the enzyme itself, because it opens the door to predicting which patients will have high CD73 activity and may therefore benefit most from targeting it. Beyond hypoxia-driven upregulation, CD73 expression is also governed by epigenetic mechanisms, the chemical modifications that sit on top of DNA and determine whether a gene is active or silent.

In melanoma, researchers found that the CD73 gene (called NT5E) is frequently silenced by DNA methylation, a chemical tag that shuts down gene transcription. The gene was methylated and silenced in multiple melanoma cell lines but unmethylated and active in normal melanocytes. Treatment with a demethylating agent reactivated CD73 expression. In clinical melanoma samples, methylation of the CD73 gene occurred in both primary and metastatic tumors and correlated with lower CD73 mRNA levels.²¹ This epigenetic silencing adds complexity to the prognostic picture described earlier. In some melanoma cases, low CD73 on tumor cells may reflect epigenetic shutdown rather than a fundamentally less aggressive cancer. And the association with metastatic site specificity suggests that CD73 methylation patterns might help explain why melanoma preferentially spreads to certain organs over others.

CD73 Beyond the Tumor Itself

One aspect that makes CD73 research complicated is that the enzyme is not unique to cancer cells. It sits on many healthy cell types, and its normal functions are genuinely important. On blood vessel linings, CD73-produced adenosine maintains the vascular barrier. Experiments with CD73-deficient mice showed that the enzyme controls blood vessel permeability in selected lymphoid organs under normal conditions.²² When CD73 was depleted from human endothelial cells in the lab, the cells became elongated, developed stress fibers, and became leakier, mimicking the effects of inflammatory signaling.²³

These normal protective roles raise a practical concern for drug development. If you systemically block CD73, you could potentially increase vascular leakage or trigger unwanted inflammation in healthy tissues. In one mouse study, interferon-beta protected lungs from vascular damage after intestinal injury, and this protection was completely dependent on CD73. Mice lacking CD73 got no benefit from the interferon treatment and suffered worse lung damage than normal mice.³ Results like these underscore the need for therapies that can target CD73 preferentially within the tumor microenvironment rather than shutting it down everywhere in the body. Some approaches aim for this selectivity by using antibodies that concentrate at the tumor site or by targeting upstream regulators specific to the tumor’s biology, like the OTUD4-CD73 interaction mentioned earlier. Whether current therapies are selective enough to avoid significant on-target, off-tumor toxicity is one of the questions that ongoing clinical trials are designed to answer.

Radiation therapy adds another dimension. In irradiated mouse lungs, wild-type animals accumulated high adenosine levels and low AMP levels, while CD73-deficient mice showed the reverse: low adenosine and high AMP. This confirmed that the adenosine surge after radiation comes from CD73 converting AMP.²⁴ In that context, CD73-generated adenosine contributed to radiation-induced lung fibrosis, a serious complication of chest radiation.²⁵ The implication is provocative: blocking CD73 might not only improve immune-mediated tumor killing but could also reduce certain radiation side effects. That kind of dual benefit, if it holds up in clinical testing, would make CD73 an unusually versatile therapeutic target.