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Over the past decade, the landscape of cancer immunotherapy has been revolutionized by the discovery and clinical application of immune checkpoint inhibitors. While pathways such as PD-1/PD-L1 and CTLA-4 have dominated clinical research and search engine trends, the scientific community is increasingly focusing on the innate immune system's role in tumor evasion. Among the most actively discussed and highly searched emerging targets is CD24, a heavily glycosylated cell surface protein. Recently vaulted to the forefront of oncological research, CD24 has been identified as a potent "don't eat me" signal, positioning it alongside established innate checkpoints like CD47. As researchers delve deeper into the complexities of the tumor microenvironment, understanding the molecular virology, cellular interactions, and therapeutic potential of CD24 is essential for advancing the next generation of targeted immunotherapies.
CD24, historically known as heat stable antigen (HSA), is a small, heavily glycosylated mucin-like cell surface protein. Unlike typical transmembrane proteins, CD24 is tethered to the outer leaflet of the plasma membrane via a glycosylphosphatidylinositol (GPI) anchor. The protein core itself is remarkably short—consisting of only about 30 amino acids in humans—but it undergoes extensive post-translational modifications. N-linked and O-linked glycosylations constitute the vast majority of the molecule's mass, creating a dense carbohydrate shield. This diverse glycosylation pattern is highly tissue-specific and allows CD24 to interact with a wide variety of ligands, functioning as a highly adaptable adhesion molecule and signaling mediator.
In normal human physiology, CD24 expression is tightly restricted. It is prominently featured during embryogenesis and plays a crucial role in the development of the central nervous system, regulating neurogenesis and cellular migration. In the adult immune system, CD24 is primarily utilized as a differential marker for B-cell maturation and is expressed on certain subpopulations of T cells, neutrophils, and specialized epithelial cells. Under homeostatic conditions, CD24 functions to regulate immune homeostasis, preventing catastrophic autoimmune responses by interacting with specific inhibitory receptors on immune effectors. However, this protective mechanism is frequently hijacked by malignant tissues to establish immune tolerance.
The surge in current scientific interest surrounding CD24 is largely attributed to the elucidation of its interaction with Sialic acid-binding Ig-like lectin 10 (Siglec-10). Siglec-10 is an inhibitory receptor predominantly expressed on the surface of tumor-associated macrophages (TAMs), dendritic cells, and a subset of natural killer (NK) cells. Recent breakthrough studies have demonstrated that many solid tumors aggressively upregulate CD24 to exploit this interaction, effectively neutralizing the body's primary phagocytic defenders.
When highly sialylated CD24 on the surface of a cancer cell binds to Siglec-10 on a macrophage, it initiates a potent intracellular signaling cascade within the immune cell. This binding triggers the phosphorylation of immunoreceptor tyrosine-based inhibitory motifs (ITIMs) located on the intracellular tail of Siglec-10. This, in turn, recruits and activates the protein tyrosine phosphatases SHP-1 and SHP-2. The activation of these phosphatases aggressively dampens the cytoskeletal rearrangements and actin polymerization required for engulfment, effectively transmitting a "don't eat me" signal. This mechanism is highly analogous to the CD47-SIRPα pathway but appears to operate independently and often dominates in specific cancer subtypes, such as ovarian and breast cancers, where CD47 blockade alone has shown limited clinical efficacy.
Figure 1. Schematic of CD24-Siglec-10 signaling in cancer immunotherapy
(Source: Gu Y, et al. 2023)
The clinical relevance of CD24 is underscored by its widespread overexpression across a broad spectrum of aggressive solid malignancies. High levels of CD24 have been consistently documented in ovarian cancer, triple-negative breast cancer (TNBC), hepatocellular carcinoma, pancreatic ductal adenocarcinoma, and non-small cell lung cancer. In the era of precision medicine, CD24 is increasingly recognized not just as a therapeutic target, but as a critical prognostic biomarker.
Extensive retrospective clinical analyses have repeatedly demonstrated a strong inverse correlation between CD24 expression levels and patient survival outcomes. Tumors exhibiting robust CD24 expression are characteristically associated with higher histological grades, increased rates of lymph node metastasis, and an enhanced capacity for invasive growth. Furthermore, CD24 is frequently identified as a definitive marker for cancer stem cells (CSCs)—a small subpopulation of highly tumorigenic cells responsible for tumor initiation, therapy resistance, and disease relapse. By protecting these critical CSCs from macrophage-mediated clearance, CD24 essentially safeguards the root of the malignancy, explaining its strong association with poor clinical prognoses and chemoresistance.
A rapidly expanding subset of Google searches and contemporary oncological discussions focuses on the role of extracellular vesicles, particularly exosomes, in tumor progression. CD24 is not confined to the primary tumor cell membrane; it is frequently and selectively enriched in tumor-derived exosomes. These nanometer-sized lipid vesicles are continuously secreted by cancer cells into the systemic circulation, carrying their molecular cargo—including functional CD24—to distant anatomical sites.
Within the tumor microenvironment (TME), CD24-positive exosomes act as long-range signaling mediators. They can fuse with or be engulfed by circulating monocytes and tissue-resident macrophages, systemically disseminating the "don't eat me" signal and inducing a widespread immunosuppressive phenotype. Furthermore, these exosomes play a pivotal role in the formation of the pre-metastatic niche. By interacting with the endothelium and modifying the localized immune landscape of distant organs, CD24-rich exosomes prepare a hospitable environment for circulating tumor cells to seed and proliferate. Targeting exosomal CD24, therefore, represents a dual therapeutic opportunity: stripping the primary tumor of its local defense and neutralizing its systemic metastatic infrastructure.
Figure 2. Negative regulation on the NF-κB pathway by the CD24-Siglec-10 axis
(Source: Grigoropoulos I, et al. 2024)
The compelling preclinical data surrounding the CD24-Siglec-10 axis has catalyzed the rapid development of novel immunotherapeutics designed to disrupt this pathway. The most prominent approach involves the engineering of high-affinity monoclonal antibodies that specifically bind to either CD24 or Siglec-10, thereby sterically hindering their interaction. In preclinical models, the administration of CD24-blocking antibodies has been shown to successfully reverse macrophage paralysis, resulting in robust, phagocytosis-mediated tumor clearance.
Beyond simple blockade, the highly specific expression of CD24 on certain cancer cells makes it an excellent candidate for targeted drug delivery systems. Antibody-drug conjugates (ADCs) utilizing anti-CD24 antibodies to deliver potent cytotoxic payloads directly into the tumor are currently undergoing rigorous preclinical evaluation. Additionally, the development of Chimeric Antigen Receptor (CAR) macrophage and CAR-T cell therapies engineered to recognize CD24-positive malignancies represents the cutting edge of adoptive cell transfer technologies.
Despite this immense promise, translating CD24-targeted therapies into the clinic faces distinct challenges. The heavy and highly variable glycosylation of CD24 across different tissues demands meticulously designed antibodies capable of recognizing tumor-specific glycoforms to prevent off-target toxicities in healthy tissues. Furthermore, as tumors are highly heterogeneous, the future of CD24 therapy likely lies in rational combination strategies. Synergistically pairing CD24 blockade with adaptive immune checkpoint inhibitors (like anti-PD-L1) or traditional chemoradiation could simultaneously unleash both the innate and adaptive arms of the immune system, offering a comprehensive strategy to eradicate refractory solid tumors.
References
| Target | Cat. No. | Product Name | Expression System | Tag/Conjugate | Application | |
| CD24 | DAG-WT1239 | Recombinant Human CD24 VLP | HEK293 cells | N/A | ELISA, SPR | Inquiry |
| DAG-WT1240 | Recombinant Cynomolgus CD24 VLP | HEK293 cells | N/A | ELISA, SPR | Inquiry |
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