Sialic Acid-Binding Immunoglobulin-Like Lectin 9 as a Potential Therapeutic Target for Chronic Obstructive Pulmonary Disease
Chronic obstructive pulmonary disease (COPD) has emerged as a leading cause of morbidity and mortality worldwide, ranking as the third most common cause of death. This progressive respiratory disorder is characterized by airflow limitation and breathing difficulties, imposing a significant economic burden on healthcare systems. COPD encompasses chronic bronchitis and emphysema, with neutrophilic inflammation playing a central role in its pathogenesis. Despite advancements in understanding the disease, current therapeutic strategies, primarily bronchodilators and corticosteroids, offer limited benefits and are often associated with severe side effects. This underscores the urgent need for novel, effective, and safe anti-neutrophilic therapies to delay disease progression. Sialic acid-binding immunoglobulin-like lectin 9 (Siglec-9) has emerged as a promising therapeutic target due to its role in regulating neutrophil function and inflammation.
The Role of Neutrophils in COPD Pathogenesis
Neutrophils are the most abundant inflammatory cells in the innate immune system, playing a critical role in defending against microbial invasion. However, in COPD, excessive neutrophil recruitment and activation contribute to chronic inflammation, tissue damage, and disease progression. Cigarette smoke and environmental pollutants are primary risk factors for COPD, triggering the release of chemoattractants such as leukotriene B4 (LTB4) and interleukin-8 (IL-8). These mediators promote neutrophil adhesion to endothelial cells and migration into the airways and lung parenchyma.
In COPD patients, neutrophils exhibit prolonged survival due to dysregulation of apoptotic pathways. Pro-apoptotic genes such as Bac and CASP are downregulated, while anti-apoptotic genes like Bcl-2 and Mcl-1 are upregulated. Additionally, cigarette smoke impairs the phagocytosis of apoptotic neutrophils, leading to secondary necrosis and the release of proinflammatory signals. Neutrophils in COPD also release proteases such as neutrophil elastase, matrix metalloproteinases (MMPs), and reactive oxygen species (ROS), which contribute to airway remodeling, mucus hypersecretion, and emphysematous destruction.
Current Therapeutic Strategies and Limitations
The mainstay of COPD treatment includes bronchodilators and corticosteroids, which provide symptomatic relief but fail to significantly alter disease progression or reduce exacerbation frequency. Corticosteroids, while effective in managing eosinophilic inflammation in asthma, are largely ineffective in controlling neutrophilic inflammation in COPD. Moreover, their long-term use is associated with adverse effects such as osteoporosis, diabetes, and increased infection risk.
Emerging therapies targeting specific inflammatory mediators, such as tumor necrosis factor-alpha (TNF-α) inhibitors and interleukin-17 (IL-17) inhibitors, have shown limited success. This is likely due to the complexity of neutrophilic inflammation in COPD, which involves multiple redundant pathways. Broad-spectrum agents that directly suppress neutrophil activation and recruitment, such as phosphodiesterase 4 inhibitors and kinase inhibitors, are under investigation. However, concerns about immune system dysfunction and increased susceptibility to infections remain.
Siglecs: An Overview
Siglecs (sialic acid-binding immunoglobulin-like lectins) are a family of immunoregulatory receptors primarily expressed on hematopoietic cells. They recognize sialylated glycoconjugates and modulate immune responses through immunoreceptor tyrosine-based inhibitory motifs (ITIMs) in their cytoplasmic tails. Among the 15 Siglecs identified in humans, Siglec-9 is of particular interest due to its high expression on neutrophils and monocytes.
Siglec-9, encoded by the SIGLEC9 gene, functions as an inhibitory receptor. Its ITIMs recruit tyrosine phosphatases such as Src homology-2 domain-containing tyrosine phosphatase (SHP)-1 and SHP-2, which negatively regulate immune cell activation. In mice, Siglec-E is considered the functional ortholog of human Siglec-9, sharing approximately 50% to 80% sequence identity and similar expression patterns.
Siglec-9 in Neutrophil Apoptosis and Autophagy
Siglec-9 plays a critical role in regulating neutrophil apoptosis and autophagy. Ligation of Siglec-9 by specific antibodies or synthetic ligands induces caspase-dependent apoptosis in neutrophils. In the presence of proinflammatory cytokines such as granulocyte-macrophage colony-stimulating factor (GM-CSF), interferon-alpha (IFN-α), and interferon-gamma (IFN-γ), Siglec-9 ligation shifts to caspase-independent autophagic-like cell death, which is more cytotoxic. Both pathways are dependent on reactive oxygen species (ROS) generation.
Intravenous immunoglobulin (IVIg) preparations containing natural anti-Siglec-9 autoantibodies have been shown to accelerate neutrophil death in a cytokine-rich microenvironment. This effect involves ROS and can lead to both beneficial suppression of neutrophilia and undesired neutropenia. However, IVIg-induced neutropenia is reversible and has not been associated with severe infectious complications.
Siglec-E in Neutrophil Recruitment
In murine models, Siglec-E has been identified as a negative regulator of neutrophil recruitment to the lungs. It inhibits CD11b β2-integrin-dependent neutrophil migration by reducing the phosphorylation of Syk-Tyr317 and p38 mitogen-activated protein kinase (MAPK), which are essential for lung neutrophil recruitment. Siglec-E also suppresses ROS production by neutrophils, further dampening inflammatory responses. These findings suggest that targeting Siglec-9 or its ortholog Siglec-E could provide a novel therapeutic approach for neutrophil-driven inflammatory diseases such as COPD.
Natural Ligands of Siglec-9 and Siglec-E
The natural ligands of Siglec-9 and Siglec-E have not been fully characterized, but recent studies have identified several potential candidates. Group B Streptococcus (GBS) exploits sialic acid mimicry to bind Siglec-9 and evade immune surveillance. Soluble Siglec-9 (sSiglec-9) can competitively inhibit this interaction, providing antibacterial benefits. High-molecular-weight hyaluronan (HMW-HA) also binds Siglec-9 in a sialic acid-independent manner, inducing neutrophil dysfunction and apoptosis.
In the context of cancer, sialylated ligands such as mucin 1 (MUC1) and lectin galactoside-binding soluble 3 binding protein (LGALS3BP) interact with Siglec-9 to modulate immune responses. These interactions highlight the potential of Siglec-9 as a therapeutic target in both infectious and neoplastic diseases.
Siglec-9 in COPD: Clinical Evidence
Increased expression of Siglec-9 and sSiglec-9 has been observed in COPD patients, particularly in response to cigarette smoke extract and lipopolysaccharide (LPS) exposure. Dexamethasone, a commonly used corticosteroid, enhances neutrophil Siglec-9 expression but does not affect sSiglec-9 levels. This suggests that corticosteroids may exert their anti-inflammatory effects, in part, by upregulating Siglec-9.
Polymorphisms in the SIGLEC9 gene have been associated with COPD phenotypes, including exacerbation frequency and emphysema severity. For example, the GA haplotype (rs2075803 = G and rs2258983 = A) is linked to increased TNF-α production and higher exacerbation risk. Another polymorphism (rs16988910) has been associated with emphysematous destruction in African-American populations. These findings underscore the importance of Siglec-9 in modulating innate immune responses and its potential as a biomarker and therapeutic target in COPD.
Conclusion
COPD remains a major global health challenge, with neutrophilic inflammation playing a central role in its pathogenesis. Current therapies offer limited benefits and are often associated with significant side effects, highlighting the need for novel therapeutic strategies. Siglec-9, a neutrophil-specific inhibitory receptor, has emerged as a promising target due to its role in regulating neutrophil apoptosis, autophagy, and recruitment. Targeting Siglec-9 or its ortholog Siglec-E could provide a novel approach to controlling neutrophilic inflammation in COPD and related diseases. Future research should focus on developing Siglec-9-based therapies, such as synthetic ligands, specific antibodies, and small molecule compounds, to improve outcomes for COPD patients.
doi.org/10.1097/CM9.0000000000001381
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