I. Molecular Characteristics and Expression Regulation of Canine IL-8

Canine interleukin-8 (Canine IL-8) is an important member of the CXC chemokine family. As a potent pro-inflammatory cytokine, it plays a key role in immune defense and inflammatory responses in canines. Its encoding gene is located in a specific region of canine chromosomes, and its gene sequence has high homology with IL-8 of other mammals, especially with strong conservation in functional domain regions. Mature canine IL-8 is a small protein composed of 72 amino acids with a molecular weight of approximately 8 kDa, containing a typical CXC motif (cysteine-cysteine-other amino acids), a structural feature that enables it to specifically bind to chemokine receptors on the surface of target cells.

The expression of canine IL-8 is strictly transcriptionally regulated, with low basal expression in various tissues of dogs under normal physiological conditions, but its expression increases rapidly when stimulated by pathogen infection or tissue damage. Inflammatory stimulants such as lipopolysaccharide (LPS), tumor necrosis factor-α (TNF-α), and interleukin-1β (IL-1β) can promote the transcriptional initiation of the canine IL-8 gene by activating signaling pathways such as NF-κB and MAPK. Immune cells (such as macrophages and neutrophils) and tissue cells (such as epithelial cells and endothelial cells) are the main secretory cells of canine IL-8. Under inflammatory signal stimulation, these cells can synthesize and secrete large amounts of IL-8 within hours, exerting biological effects through autocrine and paracrine methods.

II. Biological Functions and Mechanisms of Action of Canine IL-8

(I) Chemotactic and Activating Effects on Immune Cells

Chemotactic activity is the core biological function of canine IL-8, which can specifically attract neutrophils to recruit to inflammatory sites through binding to highly expressed CXCR1 and CXCR2 receptors on the surface of neutrophils. After canine IL-8 binds to receptors, it can activate intracellular calcium signaling pathways, causing cytoskeletal reorganization and enabling neutrophils to produce directional migration. In vitro chemotaxis experiments have shown that the chemotactic effect of canine IL-8 on neutrophils is concentration-dependent, which can produce significant chemotactic effects within the nanomolar concentration range.

In addition to chemotaxis, canine IL-8 can also activate various functions of neutrophils, including promoting degranulation, respiratory burst, and phagocytic activity. Activated neutrophils can release antimicrobial substances such as reactive oxygen species and lysosomal enzymes, enhancing the ability to clear pathogens. At the same time, canine IL-8 can up-regulate the expression of adhesion molecules (such as integrins) on the surface of neutrophils, promoting their adhesion to vascular endothelial cells and creating conditions for neutrophils to cross the vascular wall to reach inflammatory tissues.

(II) Regulatory Functions in Inflammatory Responses

Canine IL-8 plays an important regulatory role in the initiation and amplification stages of inflammatory responses. In the early stage of inflammation, it serves as a "pioneer signal" to recruit neutrophils to damaged sites and initiate local immune responses; with the progression of inflammation, IL-8 can stimulate the secretion of other inflammatory factors (such as IL-6 and TNF-α), forming an inflammatory cascade reaction and enhancing immune defense capabilities. At the same time, canine IL-8 is also involved in the activation of vascular endothelial cells, inducing vasodilation and increased vascular permeability, promoting the exudation of inflammatory cells and plasma components to inflammatory sites, and forming typical characteristics of inflammatory responses.

In the immune regulatory network, canine IL-8 has complex interactions with other cytokines. It can enhance the antigen-presenting ability of macrophages and promote the proliferation and differentiation of T cells; at the same time, other cytokines can also reversely regulate the expression of IL-8, forming a fine inflammatory regulatory balance. This balance mechanism ensures that inflammatory responses can effectively clear pathogens while avoiding excessive inflammation causing damage to normal tissues.

III. The Role of Canine IL-8 in Physiological and Pathological Processes

(I) Immune Defense in Physiological State

Under normal physiological conditions, canine IL-8 exerts immune defense functions by participating in innate immune responses. When the canine body is invaded by pathogens such as bacteria and fungi, local tissue cells rapidly secrete IL-8 to recruit neutrophils to infection sites. Under the action of IL-8, neutrophils are activated to clear pathogens through phagocytosis and release antimicrobial peptides and other substances to inhibit pathogen spread. This process is particularly important in mucosal tissues such as skin, respiratory tract, and digestive tract that communicate with the outside world, forming the first line of defense for the canine body against foreign pathogen invasion.

Canine IL-8 also plays a positive role in wound repair. After tissue damage, the released IL-8 can attract neutrophils to gather at wound sites, clear necrotic tissues and foreign bodies, and promote the proliferation and migration of fibroblasts to accelerate wound healing. Studies have shown that appropriate levels of IL-8 expression are necessary for normal wound repair, and abnormal expression may lead to delayed wound healing or excessive scar formation.

(II) Inflammatory Mediation in Pathological State

Under pathological conditions, abnormally high expression of canine IL-8 is closely related to the occurrence and development of various inflammatory diseases. When the body is subjected to continuous inflammatory stimulation or immune regulation imbalance, canine IL-8 is continuously secreted in large quantities, leading to excessive aggregation and activation of neutrophils at inflammatory sites, releasing a large number of inflammatory mediators and reactive oxygen species, resulting in tissue damage. This excessive inflammatory response is the common pathological basis of many chronic inflammatory diseases in canines.

In skin inflammation, canine IL-8 can be secreted by damaged keratinocytes and infiltrated immune cells, promoting neutrophils to migrate to skin inflammatory sites and aggravating skin erythema, edema, and pruritus symptoms. During respiratory tract infections, IL-8 secreted by bronchial epithelial cells can mediate the aggregation of neutrophils in the airway, causing airway inflammation and increased mucus secretion, leading to symptoms such as cough and dyspnea. In addition, canine IL-8 is also involved in pathological processes through similar mechanisms in chronic inflammatory diseases such as arthritis and enteritis.

IV. Association Between Canine IL-8 and Common Canine Diseases

(I) Inflammatory Skin Diseases

Canine atopic dermatitis is one of the skin diseases most closely related to canine IL-8. Clinical studies have found that the mRNA and protein levels of IL-8 in the skin tissues of affected dogs are significantly higher than those of healthy dogs, and the expression level is positively correlated with the severity of skin inflammation. In lesioned skin, activated T cells and mast cells can secrete large amounts of IL-8, recruiting neutrophils and eosinophils to infiltrate and form an inflammatory microenvironment. Immunohistochemical analysis shows that IL-8 is mainly distributed in epidermal keratinocytes and dermal infiltrating cells, and its expression fluctuates with disease activity.

Pyoderma is a common bacterial skin infection in dogs. Studies have confirmed that canine IL-8 plays a key role in the pathogenesis of pyoderma. After pathogenic bacteria infection, macrophages in skin tissues quickly release IL-8 to initiate neutrophil recruitment. Although this process helps clear bacteria, excessive IL-8-mediated inflammatory response can lead to skin suppuration and ulceration. Detecting IL-8 levels in lesioned skin secretions can be used as an indicator to evaluate the inflammatory degree and treatment effect of pyoderma.

(II) Respiratory and Gastrointestinal Inflammatory Diseases

In canine bacterial pneumonia, the concentration of IL-8 in bronchoalveolar lavage fluid increases significantly, which is positively correlated with the number of neutrophils. IL-8 can mediate the aggregation of neutrophils in infected lung tissues to participate in pathogen clearance, but continuous high expression can cause lung tissue damage and pulmonary edema. Clinical studies have shown that the decrease rate of IL-8 levels during treatment of dogs with pneumonia is closely related to the degree of recovery, which can be used as a biological marker to judge prognosis.

In canine acute gastroenteritis, IL-8 released by epithelial cells after intestinal mucosal damage can mediate the migration of neutrophils to intestinal inflammatory sites. This process helps clear intestinal pathogens but may also aggravate intestinal mucosal damage. Detecting IL-8 levels in feces or serum of affected dogs is helpful for evaluating the activity of intestinal inflammation and guiding the adjustment of clinical treatment plans.

V. Detection Methods and Clinical Application Value of Canine IL-8

(I) Main Detection Technologies and Methods

Current methods for detecting canine IL-8 mainly include enzyme-linked immunosorbent assay (ELISA), real-time quantitative polymerase chain reaction (qPCR), and immunohistochemical technology. Based on the principle of antigen-antibody specific binding, ELISA can quantitatively detect IL-8 protein levels in serum, tissue homogenates, or cell culture media, with the advantages of high sensitivity, strong specificity, and simple operation, making it the most commonly used detection method in clinical and scientific research.

qPCR technology realizes quantitative analysis of IL-8 mRNA expression levels by amplifying specific fragments of the IL-8 gene, which is suitable for detecting gene expression changes in tissue samples or cells and can reflect the expression regulation of IL-8 in the early stage. Immunohistochemical technology can localize the distribution of IL-8 in tissues, clarify its main source cells, and provide morphological evidence for studying the local role of IL-8. These detection methods have their own advantages, and combined application can comprehensively evaluate the expression characteristics of canine IL-8 in diseases.

(II) Clinical Application Potential

As an inflammatory marker, canine IL-8 has broad application prospects in veterinary clinics. In disease diagnosis, detecting IL-8 levels in body fluids can assist in identifying the type and cause of inflammatory diseases. For example, in dogs with fever of unknown origin, elevated IL-8 suggests a high possibility of bacterial infection. In efficacy evaluation, dynamic changes in IL-8 levels during treatment can timely reflect the effectiveness of treatment plans and guide treatment adjustments.

In prognostic judgment, persistently high IL-8 levels in acute inflammatory diseases often indicate a poor prognosis, while a rapid decrease after treatment indicates a good recovery. In addition, canine IL-8 can be used as a potential target for drug development, and antagonists targeting IL-8 or its receptors may provide new treatments for chronic inflammatory diseases in canines. Currently, related small molecule inhibitors and monoclonal antibodies have entered preclinical research stages.

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