CD24 is a small protein anchored to the surface of many cell types that acts as a molecular identity badge, telling the immune system which cells to leave alone and which to attack. It sits at a fascinating crossroads in biology: the same molecule that helps immune cells distinguish “self” from “danger” is hijacked by tumors to dodge destruction, plays a role in autoimmune diseases like multiple sclerosis, and is now the target of several experimental therapies. Understanding CD24 means understanding one of the immune system’s most consequential switches and why flipping it in the wrong direction can lead to very different diseases.
What CD24 Actually Is
CD24 is a cell-surface receptor, tethered to the outer membrane of cells by a fatty molecular anchor called a GPI (glycophosphatidylinositol) link. The protein core itself is tiny, but it is coated in sugar chains that make it look much larger and give it its functional diversity.1PubMed. Analysis of the structure, evolution, and expression of CD24, an important regulator of cell fate Those sugar chains vary depending on the tissue, so CD24 on a brain cell looks chemically different from CD24 on a white blood cell, even though the underlying protein is the same.2Oxford Academic. Heat-stable antigen (CD24) as ligand for mouse P-selectin This structural flexibility lets the molecule perform different jobs in different contexts, which is why it shows up in discussions of cancer, inflammation, stem cell biology, and neurological development.
CD24 is expressed across a broad range of tissues. Immune cells such as B cells carry it, as do cells in the gut, kidneys, brain, and many other organs. Stem cells also express it, and tracking CD24 levels has proven useful for distinguishing between different states of cellular maturity in both mouse and human cells.3Nature Communications. CD24 tracks divergent pluripotent states in mouse and human cells This wide distribution is precisely what makes CD24 both powerful and complicated as a drug target: it does important work in healthy tissue, not just in disease.
How CD24 Calms the Immune System
One of CD24’s most important jobs is dampening inflammation. When cells are damaged or die, they release internal molecules called danger signals. These molecules normally live inside cells and never encounter the immune system, so when they spill out, the body treats them as alarms. CD24 binds to several of these danger signals and quiets the immune response they would otherwise trigger.4PubMed Central. CD24 and Siglec-10 selectively repress tissue damage-induced immune responses
The mechanism works through a partner receptor on immune cells called Siglec-10 (or Siglec-G in mice). CD24 grabs the danger molecules and presents them to Siglec-10, which then sends a “stand down” signal into the immune cell, blocking the activation of inflammatory pathways. Think of it as a bouncer at the door of the immune response: CD24 intercepts the alarm molecules and hands them off to Siglec-10, which decides the threat doesn’t warrant a full-scale reaction. This process selectively suppresses inflammation caused by tissue damage without interfering with the immune system’s ability to fight actual infections, a distinction that makes CD24 biologically elegant and therapeutically interesting.
The “Don’t Eat Me” Signal in Cancer
Tumors are masters of self-preservation, and CD24 gives them a powerful survival tool. Cancer cells frequently crank up their CD24 expression, essentially coating themselves in a molecular cloak that tells nearby immune cells, particularly macrophages, to back off. Macrophages are the immune system’s cleanup crew; they engulf and destroy abnormal cells in a process called phagocytosis. When a tumor cell displays high levels of CD24, it sends a “don’t eat me” signal through Siglec-10 on macrophages, suppressing their appetite for the cancer cell.5Cancer Discovery. CD24 Is a “Don’t Eat Me” Signal That Promotes Tumor Immune Escape
The evidence for this is direct. When researchers deleted CD24 from breast cancer cells and co-cultured them with macrophages, the macrophages started engulfing the cancer cells. Blocking CD24 with antibodies had the same effect, making cancer cells more vulnerable to destruction. This makes CD24 an “innate immune checkpoint,” a concept that parallels the better-known checkpoints like PD-1/PD-L1 and CD47/SIRPα that have already transformed cancer treatment.6PubMed Central. Overexpressed CD24 and CD47 Indicate a Worse Prognosis in Cervical Cancer
Beyond shielding tumors from macrophages, CD24 helps cancer cells spread. It serves as a ligand for P-selectin, a molecule on platelets. When tumor cells circulating in the bloodstream latch onto platelets via CD24, the platelet coat may shield them from immune detection during metastasis. In laboratory experiments, antibodies against CD24 blocked about two-thirds of platelet adhesion to prostate cancer cells, suggesting this interaction is a meaningful part of how some cancers colonize distant organs.
CD24 as a Cancer Stem Cell Marker
CD24 isn’t only a defensive shield for tumors. It also marks a particularly dangerous subpopulation of cancer cells known as cancer stem cells. These are the cells thought to seed new tumors, resist chemotherapy, and drive relapse after treatment. CD24 has been identified as a surface marker on cancer stem cells across several tumor types, and its functional role in these cells extends beyond mere identification: it participates in the signaling that drives tumor initiation and progression.7PubMed. CD24 blockade as a novel strategy for cancer treatment 8PubMed Central. Targeting CD24 as a novel immunotherapy for solid cancers
The clinical relevance of CD24 expression has been studied most thoroughly in breast cancer. A large study found that patients with high CD24 expression had worse survival across multiple measures: overall survival, disease-free survival, and distant metastasis-free survival. High CD24 was also correlated with lymph node spread and more advanced disease stage. The prognostic hit was especially pronounced in certain breast cancer subtypes. In triple-negative breast cancer, for instance, high CD24 roughly doubled the risk of death compared to low CD24.9PLOS ONE. CD24 Overexpression Is Associated with Poor Prognosis in Luminal A and Triple-Negative Breast Cancer
More recently, researchers have explored whether CD24 on circulating tumor cells (cancer cells that have broken free from the primary tumor and entered the bloodstream) could serve as a real-time biomarker. In one study of breast cancer patients, about 60% of circulating tumor cells tested positive for CD24, and patients with higher numbers of CD24-positive circulating cells had significantly lower survival rates.10PubMed Central. Correlation between the expression of CD24 on circulating tumor cells and prognosis in breast cancer If validated in larger studies, this could eventually offer clinicians a blood-based way to track disease aggression without repeated biopsies.
Beyond Cancer: CD24 in Autoimmune Disease
Because CD24 normally keeps inflammation in check, genetic variations that alter its function can tip the balance toward autoimmunity. A common variant in the CD24 gene, called the Ala57Val polymorphism, has been studied extensively in multiple sclerosis and systemic lupus erythematosus. A meta-analysis pooling data across multiple studies found that people carrying two copies of the Val variant had more than double the risk of developing multiple sclerosis, with this association strongest in people of European descent.11PubMed Central. Association between the CD24 Ala57Val polymorphism and risk for multiple sclerosis and systemic lupus erythematosus: a meta-analysis The Val variant is thought to accelerate CD24 protein breakdown, leaving less of it available to restrain inflammatory responses. With the brake partially released, the immune system may be more prone to attacking the body’s own tissues.
CD24 also plays a recognized role in normal brain development. It is involved in neuronal migration, the extension of nerve fibers, and the generation of new neurons, meaning its influence on the nervous system goes beyond autoimmune attack. In the context of multiple sclerosis, this dual relevance is particularly striking: the same molecule that shapes brain development also helps control the immune responses that, when they go wrong, damage the brain’s insulating myelin sheaths.12PubMed. The CD24 surface antigen in neural development and disease
Anti-CD24 Antibodies for Cancer Treatment
The clearest therapeutic strategy is straightforward: block CD24 on tumor cells so the immune system can see and destroy them. Several monoclonal antibodies targeting CD24 are in preclinical development, and the results so far are striking. An antibody called SWA11 significantly slowed the growth of lung and ovarian cancer in mouse models, primarily by reducing tumor cell proliferation rather than directly killing cells. Combining SWA11 with the chemotherapy drug gemcitabine produced even stronger tumor growth inhibition than either treatment alone.13British Journal of Cancer. Antibody targeting of CD24 efficiently retards growth and influences cytokine milieu in experimental carcinomas
A more recent antibody, IMM47, has shown especially potent activity in mouse models. At a moderate dose, IMM47 eliminated all tumors in one group of treated mice, and when those mice were later re-injected with the same cancer cells, the tumors failed to regrow, suggesting that the treatment had trained the immune system to remember and reject the cancer. In head-to-head comparisons with PD-1 checkpoint inhibitors that are already approved for human use, IMM47 outperformed them as a single agent. Combining IMM47 with PD-1 antibodies like Keytruda produced complete responses in all mice in some experiments.14PubMed Central. IMM47, a humanized monoclonal antibody that targets CD24, exhibits exceptional anti-tumor efficacy by blocking the CD24/Siglec-10 interaction and can be used as monotherapy or in combination with anti-PD1 antibodies for cancer immunotherapy These are preclinical results in mice, and the usual caveats apply: many cancer drugs that work in mice fail in humans. But the consistency of the data across multiple models is encouraging.
Researchers have also built antibody-drug conjugates, or ADCs, that use a CD24-targeting antibody to deliver a toxic payload directly to cancer cells. In osteosarcoma (bone cancer) models, a CD24 ADC carrying the toxin mertansine induced complete and lasting tumor regression, while sparing normal bone-forming cells.15Advanced Therapeutics. Therapeutic Targeting of Osteosarcoma and Lung Metastases in Preclinical Models Using a CD24 Antibody‐Drug Conjugate This selectivity matters because one of the major concerns with targeting CD24 is that healthy tissues express it too. An ADC that homes in on CD24-overexpressing tumor cells without damaging CD24-expressing normal cells would sidestep a significant safety hurdle.
Dual Checkpoint Blockade and Bispecific Approaches
CD24 is not the only “don’t eat me” signal tumors use. CD47, which engages its own receptor on macrophages (SIRPα), serves a nearly identical purpose. Many tumors express both CD24 and CD47, creating a double shield against phagocytosis. This redundancy is one reason why blocking just one pathway may not be enough: knock down one shield and the other compensates.
To tackle this, researchers have designed a bispecific antibody fusion protein called PPAB001 that blocks both CD47 and CD24 simultaneously. In mouse models using breast and ovarian cancer cells, PPAB001 promoted macrophage engulfment of tumor cells more effectively than antibodies targeting either checkpoint alone, and produced stronger tumor growth inhibition.16Molecular Therapy Oncology. Dual blockade of CD47 and CD24 signaling using a novel bispecific antibody fusion protein enhances macrophage immunotherapy This dual-blockade concept mirrors the logic behind combination checkpoint inhibitor therapy that has already improved outcomes in several human cancers. Whether CD24/CD47 dual blockade can replicate that success in patients remains to be tested.
CD24Fc: Harnessing CD24 to Treat Inflammation
While the cancer therapies aim to block or destroy CD24, a different class of treatments does the opposite: it delivers extra CD24 to calm an overactive immune system. CD24Fc is a recombinant fusion protein that pairs the active portion of CD24 with a fragment of an antibody to make it more stable in the body. The idea is to mimic and amplify CD24’s natural ability to suppress inflammation caused by danger signals.
CD24Fc has been tested in the context of graft-versus-host disease, a serious complication of bone marrow transplants where donor immune cells attack the recipient’s body. In experimental models, CD24Fc reduced graft-versus-host disease while still allowing the transplanted immune cells to attack residual tumor cells, a difficult balance that most immunosuppressants struggle to achieve.17PubMed Central. Siglec-G represses DAMP-mediated effects on T cells CD24Fc has also been explored in SIV-infected primates (the monkey equivalent of HIV), where researchers tested whether dampening chronic inflammation could slow progression to AIDS.18PubMed. CD24 and Fc fusion protein protects SIVmac239-infected Chinese rhesus macaque against progression to AIDS
The broader implication is that CD24-based anti-inflammatory strategies could apply to any condition driven by excessive danger-signal inflammation. Sepsis, acute respiratory distress syndrome, and chronic inflammatory diseases are all candidates.19PubMed Central. Insights into CD24 and Exosome Physiology and Potential Role in View of Recent Advances in COVID-19 Therapeutics: A Narrative Review The challenge is figuring out the right dose and timing: too much immune suppression could open the door to infections or impair the body’s ability to detect cancer.
EXO-CD24 and the COVID-19 Experience
During the COVID-19 pandemic, an inhaled formulation called EXO-CD24 gained attention as a treatment for the inflammatory lung damage that made severe cases so deadly. EXO-CD24 uses tiny cell-derived particles called exosomes that are engineered to overexpress CD24 on their surface. Inhaled directly into the lungs, they deliver the molecule right where the runaway inflammation is happening.
In a clinical study of patients with mild-to-moderate COVID-19-related respiratory distress, about 84% showed improved breathing signs within a week, and inflammatory markers like C-reactive protein and ferritin dropped significantly. Levels of the anti-inflammatory cytokine IL-10 went up, suggesting the treatment was actively rebalancing the immune response rather than simply suppressing it. No adverse events were attributed to the treatment itself.20PubMed Central. The safety and potential efficacy of exosomes overexpressing CD24 (EXO-CD24) in mild-moderate COVID-19 related ARDS
A separate study comparing EXO-CD24-treated patients with matched controls found no deaths in the treated group versus a roughly 13% mortality rate among controls. The survival benefit persisted over extended follow-up, and treated patients reported better quality of life including physical function and mobility.21PubMed Central. Reduced Mortality in COVID-19 Patients Treated With Inhaled Extracellular Vesicles Expressing CD24 These results are promising, but neither study was a large randomized controlled trial, so the strength of the evidence is still limited. If EXO-CD24 does hold up in rigorous trials, its utility would extend well beyond COVID-19 to other conditions where inflammatory lung damage is the primary killer.
Why Targeting CD24 Is Harder Than It Looks
For all the excitement around CD24 as a therapeutic target, several obstacles have slowed its path to the clinic. The most fundamental is that CD24 is expressed on many healthy tissues. An antibody that strips CD24 from tumor cells will also encounter CD24 on normal B cells, gut lining cells, kidney cells, and neurons. Strategies like antibody-drug conjugates that rely on higher CD24 expression on tumors than on normal tissue are one workaround, and early preclinical data on a targeted degrader that uses a separate receptor to concentrate its activity at tumor sites showed a favorable safety profile with minimal off-target damage to healthy tissue.22PubMed. Targeted degradation of CD24 by a transferrin receptor-engaging bispecific degrader enhances antitumor immunity But confirming safety in humans will require careful dose-finding studies.
Another complication is tumor heterogeneity. Not every cancer cell in a tumor expresses CD24 at the same level, and some tumors barely express it at all. A treatment that depends on high CD24 expression will miss low-expressing cells, which could survive treatment and repopulate the tumor. Redundancy among innate immune checkpoints compounds this problem: as noted earlier, CD47 provides a parallel escape route, and other “don’t eat me” signals exist beyond those two. Blocking one pathway may simply select for cancer cells that lean on another.23PubMed Central. CD24 as an innate immune checkpoint in solid tumors: biology, biomarker stratification, and therapeutic translation
Finally, there is no widely validated biomarker framework to tell clinicians which patients are most likely to benefit from CD24-targeted therapies. In checkpoint immunotherapy for PD-1/PD-L1, PD-L1 expression levels on tumors serve as an imperfect but useful guide for selecting patients. A comparable tool for CD24 does not yet exist in clinical practice. Work on measuring CD24 on circulating tumor cells is a step in that direction, but it remains experimental.
CD24’s Dual Nature and the Emerging Landscape
What makes CD24 unusual in the therapeutic landscape is its dual identity. In cancer, the goal is to block or eliminate it so that tumors lose their immune camouflage. In inflammatory and autoimmune diseases, the goal is to boost it so the immune system calms down. Very few molecular targets offer this two-directional therapeutic logic. The same molecule that researchers want to strip off a breast cancer cell is one they want to deliver more of into an inflamed lung.
This duality also means that safety monitoring for CD24-targeted cancer therapies will need to watch for inflammatory side effects (since removing CD24’s brake on inflammation could trigger autoimmune-like reactions), while CD24-boosting therapies like EXO-CD24 will need to ensure they are not inadvertently helping any undetected tumors hide. Neither concern is a dealbreaker, but both add complexity that simpler drug targets don’t carry. The field is still early, with most data from animal models or small human studies, and the coming years of clinical trials will determine whether CD24’s biological elegance translates into real treatment options for patients.