Siglec-F is a receptor found on the surface of mouse eosinophils that acts as a brake on these inflammatory white blood cells, triggering their death when activated and reducing their accumulation in tissues. Eosinophils are central players in allergic diseases and certain chronic inflammatory conditions, and Siglec-F has emerged as one of the most studied molecular handles for dialing down eosinophil-driven damage. Its closest functional counterpart in humans, called Siglec-8, has already been tested in clinical trials, making the biology of Siglec-F directly relevant to the development of new treatments for conditions like eosinophilic asthma, eosinophilic esophagitis, and related disorders.
What Siglec-F Recognizes
Siglec-F belongs to a family of receptors that read sugar structures on the surfaces of other cells and on secreted molecules. The specific sugar chain it prefers is called 6′-sulfo-sialyl Lewis X, a modified carbohydrate that carries both a sialic acid cap and a sulfate group.1Glycobiology. Mouse Siglec-F and human Siglec-8 are functionally convergent paralogs that are selectively expressed on eosinophils and recognize 6′-sulfo-sialyl Lewis X as a preferred glycan ligand That double modification matters: without the sulfate, Siglec-F binds far less effectively. The receptor sits on the eosinophil surface and samples its surroundings for these sugar structures, which are displayed on glycoproteins in the lungs and other mucosal tissues.2PubMed Central. Characterization of Expression of Glycan Ligands for Siglec-F in Normal Mouse Lungs The selectivity of this binding is what gives Siglec-F its specificity: it does not respond to every sugar-coated molecule in the body, just those carrying the right combination of sialic acid and sulfation.
How the Signal Gets Inside the Cell
Inside the eosinophil, the tail of the Siglec-F receptor contains a signaling motif (called an ITIM) that is typically associated with shutting down cell activity. In many receptors of this family, engagement of the ITIM recruits enzymes that dampen activation signals, push cells toward programmed death, or suppress the release of inflammatory mediators. Siglec-F was long assumed to work the same way. But research has shown that the picture is more complicated.
When researchers mutated the key signaling residues in the ITIM, expecting the receptor to lose its function, something surprising happened: Siglec-F could still enhance the release of inflammatory mediators from eosinophils stimulated with IL-33, a cytokine involved in allergic responses. Only when the entire cytoplasmic tail was deleted did this activity disappear.3PubMed Central. Siglec-F promotes IL-33-induced cytokine release from bone-marrow derived eosinophils independently of the ITIM and ITIM-like motif phosphorylation This means Siglec-F has signaling capabilities that do not depend on its classical inhibitory motif, and some of those capabilities are activating rather than inhibitory. The receptor is not a simple off switch for eosinophils. Depending on the context, it can promote cell death in some situations and amplify mediator release in others.
Natural Ligands in the Lung
One of the more revealing discoveries about Siglec-F is where its natural binding partners come from. In healthy mouse lungs, airway mucins, the thick glycoprotein molecules that make up mucus, carry the specific sugar decorations that Siglec-F recognizes. Proteomic work identified two mucins in particular, Muc5b and Muc4, as glycoprotein ligands for the receptor. When purified mucin preparations were incubated with eosinophils in the lab, the eosinophils died.4PubMed Central. Endogenous airway mucins carry glycans that bind Siglec-F and induce eosinophil apoptosis This suggests that the mucus layer in the lungs may serve a previously unrecognized immune function: keeping eosinophil numbers in check by presenting the very sugar structures that tell them to die.
The production of these sugar ligands in the airways depends on a specific enzyme, a sialyltransferase called ST3Gal-III. Mice lacking this enzyme showed reduced expression of Siglec-F ligands on their airway epithelium and, as a consequence, reduced eosinophil death in the lungs.5PubMed Central. Sialyltransferase ST3Gal-III regulates Siglec-F ligand formation and eosinophilic lung inflammation in mice Importantly, the expression of these ligands is not static. During allergic inflammation, cytokines like IL-4 and IL-13 boost the production of Siglec-F ligands on airway epithelial cells, as do innate immune triggers like fungal extracts and bacterial components.6PubMed Central. The role of lung epithelial ligands for Siglec-8 and Siglec-F in eosinophilic inflammation The upregulation during inflammation looks like a feedback mechanism: as eosinophils flood in, the tissue ramps up the very signals that should tell those eosinophils to self-destruct. When this feedback works properly, eosinophilic inflammation resolves. When it does not, chronic disease can follow.
Allergic Airway Inflammation
The most extensively studied disease context for Siglec-F is allergic airway inflammation, often modeled in mice by repeated exposure to ovalbumin (a chicken egg protein used as a standard allergen in immunology research). Giving these allergen-challenged mice an antibody that binds Siglec-F produced substantial reductions in eosinophil counts in the tissue surrounding the airways. Beyond lowering eosinophil numbers, the antibody also reduced a hallmark of chronic asthma: subepithelial fibrosis, the thickening and scarring of tissue beneath the airway lining. Lung collagen levels dropped, and the number of cells producing TGF-beta, a fibrosis-promoting growth factor, decreased as well.7PubMed Central. Anti-Siglec-F antibody reduces allergen-induced eosinophilic inflammation and airway remodeling
Studies in mice genetically lacking Siglec-F offered a complementary perspective. When these knockout mice were allergen-challenged, they developed worse disease than normal mice: more peribronchial fibrosis, higher collagen deposition, and elevated levels of fibronectin, an extracellular matrix protein associated with tissue remodeling.8PubMed Central. Chronic OVA allergen challenged Siglec-F deficient mice have increased mucus, remodeling, and epithelial Siglec-F ligands which are up-regulated by IL-4 and IL-13 In other words, removing the Siglec-F brake made eosinophilic inflammation worse, while engaging the brake with an antibody made it better. These gain-of-function and loss-of-function experiments together build a convincing case that Siglec-F is genuinely restraining eosinophil-driven tissue damage in the lungs.
Combining anti-Siglec-F treatment with other interventions has also shown promise. In one study, mice treated simultaneously with antibodies against both Siglec-F and IL-33 (a cytokine that amplifies allergic responses) showed reductions in both eosinophil and neutrophil counts, with individual treatments cutting allergen-driven tissue infiltration by about half.9PubMed Central. Additive anti-allergic effects of anti-interleukin-33 and anti-Siglec-F treatments in a murine model of allergic asthma The fact that the effects were additive hints that Siglec-F targeting and IL-33 blockade work through partly independent pathways, raising the possibility that combination strategies could outperform either approach alone.
Eosinophilic Disease in the Gut
Eosinophilic inflammation is not confined to the airways. Eosinophilic esophagitis, a condition where eosinophils accumulate in the esophageal lining and cause swallowing difficulty, has become increasingly recognized over the past two decades. In a mouse model of this disease, administering an anti-Siglec-F antibody reduced esophageal eosinophil counts from roughly 320 per square millimeter in untreated allergen-challenged mice to about 96 per square millimeter, bringing levels close to those seen in non-challenged animals. The antibody also reduced angiogenesis, the growth of new blood vessels in the inflamed tissue.10PubMed Central. Siglec-F Inhibition Reduces Esophageal Eosinophilia and Angiogenesis in a Mouse Model of Eosinophilic Esophagitis
The relevance extends beyond the esophagus. Mice engineered to overproduce IL-5 (a growth factor for eosinophils) develop massive eosinophil accumulation throughout the body. A single dose of anti-Siglec-F antibody in these hypereosinophilic mice produced rapid drops in blood eosinophil counts, with corresponding reductions in tissue eosinophil burden. The mechanism was directly apoptotic: the antibody was triggering eosinophil death both in the blood and in the tissues.11PubMed Central. Siglec-F antibody administration to mice selectively reduces blood and tissue eosinophils This systemic effectiveness matters because many eosinophilic diseases affect multiple organs simultaneously, and a treatment that works only in one tissue compartment would have limited clinical utility.
From Mouse Receptor to Human Drug Target
Siglec-F exists only in mice. Humans carry a different but functionally similar receptor called Siglec-8. The two proteins recognize the same preferred sugar ligand, are both expressed primarily on eosinophils, and both trigger eosinophil death when engaged. On human eosinophils, Siglec-8 engagement causes apoptosis, while on human mast cells (which also express Siglec-8), engagement inhibits the release of allergic mediators without killing the cell.12PubMed Central. Siglec-8 on human eosinophils and mast cells, and Siglec-F on murine eosinophils, are functionally related inhibitory receptors This dual activity on two different cell types makes Siglec-8 an unusually appealing drug target: a single antibody could potentially reduce eosinophil numbers while also calming mast cell activity.
The therapeutic concept has already been tested in people. An anti-Siglec-8 antibody called AK002 (lirentelimab) was evaluated in a randomized trial of patients with eosinophilic gastritis and duodenitis. The results were striking: patients receiving the antibody saw an average 86% reduction in gastrointestinal eosinophil counts, compared with a 9% increase in the placebo group. About 63% of treated patients met the criteria for a treatment response, versus 5% on placebo. Symptom scores improved by roughly 48% with the antibody and 22% with placebo.13PubMed Central. Anti-Siglec-8 Antibody for Eosinophilic Gastritis and Duodenitis These findings provided the first clinical proof that the Siglec-F/Siglec-8 biology translates from mouse models into meaningful human benefit. The earlier mouse work on anti-Siglec-F antibodies promoting eosinophil apoptosis in vivo was directly predictive of what happened in patients.14PubMed Central. Siglec-8 as a drugable target to treat eosinophil and mast cell-associated conditions
Subsequent clinical development of lirentelimab has been more mixed, however. Later-phase trials in eosinophilic esophagitis and eosinophilic gastritis did not consistently meet their primary endpoints for symptom improvement, despite showing clear eosinophil depletion. The disconnect between tissue eosinophil reduction and symptom relief has raised questions about whether eosinophils alone drive the symptoms patients experience, or whether other cell types and pathways also need to be addressed. This is an active area of investigation and does not necessarily undermine the biology of Siglec-8 engagement; rather, it highlights that eosinophilic diseases involve more than just eosinophils.
Siglec-F Beyond Eosinophils
Although Siglec-F is most strongly associated with eosinophils, it is not exclusively found on them. Mouse alveolar macrophages, the immune cells that reside in the air spaces of the lungs and clear debris and pathogens, also express Siglec-F. Peritoneal macrophages, by contrast, do not.15PubMed Central. Expression and preliminary functional analysis of Siglec-F on mouse macrophages Interestingly, engaging Siglec-F on these alveolar macrophages did not seem to affect their ability to engulf particles, whether the macrophages were resting or stimulated with inflammatory signals like TNF-alpha or bacterial lipopolysaccharide. So while Siglec-F is present on alveolar macrophages, its functional significance there remains unclear.
A more provocative finding has emerged from cancer research. In mouse models of lung tumors, a population of Siglec-F-expressing cells within the tumor microenvironment turned out to be not eosinophils but neutrophils. Specifically, these were mature, long-lived neutrophils that upregulated Siglec-F in the tumor setting. Single-cell transcriptomic analysis confirmed that these Siglec-F-high cells in tumor tissue were indeed neutrophils by gene expression profile, despite carrying a marker traditionally used to identify eosinophils.16PubMed Central. Tumor-Promoting Ly-6G+ SiglecFhigh Cells Are Mature and Long-Lived Neutrophils These tumor-associated neutrophils appeared to promote tumor growth. The finding complicates the simple narrative that Siglec-F equals eosinophil, and it raises questions about what Siglec-F does when expressed on a cell type whose biology is fundamentally different from an eosinophil’s. For researchers working with mouse lung tumors, it also serves as a practical caution: using Siglec-F as a marker to identify eosinophils by flow cytometry can be misleading in the tumor microenvironment.
Why Siglec Genes Evolve So Fast
One of the more intriguing aspects of the Siglec family is how rapidly its genes are changing. Large-scale sequencing of the CD33-related Siglec gene cluster across five mammalian species revealed that these genes are evolving much faster than their neighbors in the genome. The region containing Siglec genes showed lower sequence conservation between species compared to adjacent genes. The evolutionary mechanisms at work were varied and aggressive: gene duplications, gene deletions (including the human-specific loss of a functional primate Siglec called Siglec-13), shuffling of exons between genes, and accelerated accumulation of mutations specifically in the sugar-binding domain.17PubMed Central. Large-scale sequencing of the CD33-related Siglec gene cluster in five mammalian species reveals rapid evolution by multiple mechanisms
The leading explanation for this rapid evolution is an arms race between hosts and pathogens. Many disease-causing bacteria and viruses coat themselves in sialic acids, the same class of sugars that Siglec receptors recognize. A pathogen that can engage an inhibitory Siglec on an immune cell could potentially suppress the immune response to its own advantage. Under this pressure, the host’s Siglec genes would be pushed to change their binding properties, and pathogens would counter-adapt, creating a cycle of reciprocal evolution. This is why Siglec-F in mice and Siglec-8 in humans are not direct orthologs descended from a single ancestral gene, but rather functional paralogs: different genes that have converged on a similar biological role through independent evolutionary paths. The practical consequence for researchers is that findings from Siglec-F in mice inform but do not perfectly predict the behavior of Siglec-8 in humans, a gap that always needs to be kept in mind when translating mouse data to clinical settings.
Broader Siglec Family and Cross-Reactivity Across Immune Cells
Siglec-F and Siglec-8 are part of a larger family of about fifteen Siglec receptors in humans, each with different expression patterns across immune cell types. Other family members are found on B cells, neutrophils, monocytes, and natural killer cells. Because many of these receptors carry the same type of inhibitory signaling motif as Siglec-F, there is substantial interest in whether they can all be harnessed therapeutically. Antibody engagement of Siglec-9, for example, induces neutrophil death, and targeting CD22 (another Siglec family member) depletes B cells.18PubMed Central. Siglecs as targets for therapy in immune-cell-mediated disease Each family member offers a potential point of intervention for a different immune cell population.
The breadth of the family also introduces risks. Targeting one Siglec could have off-target effects if the receptor is expressed on cells beyond the intended population, or if the sugar ligands it recognizes are shared with other Siglecs. The finding that Siglec-F shows up on tumor-associated neutrophils and alveolar macrophages in addition to eosinophils is a concrete example of this complexity. For the development of Siglec-8-directed therapies in humans, understanding these expression boundaries in detail will be essential to predicting both the benefits and the unintended consequences of long-term treatment. Early clinical trials have shown that eosinophil depletion itself appears well tolerated, as eosinophils are not essential for day-to-day immune defense in the way that neutrophils or T cells are. But the longer-term effects of sustained Siglec-8 engagement on mast cells, and any cells that might upregulate the receptor under pathological conditions, remain open questions that ongoing studies are working to answer.