Modeling Principle

The SD rat adjuvant-induced arthritis model (AA) is a widely used classic and standardized immunogenic arthritis animal model in translational research on rheumatoid arthritis (RA), mainly applied in RA pathogenesis research, pharmacodynamic evaluation of anti-inflammatory/antirheumatic drugs (DMARDs, NSAIDs, glucocorticoids, biologics) and bone destruction protection strategies. Modeling principle: a single intradermal injection of complete Freund's adjuvant (CFA, containing heat-killed Mycobacterium tuberculosis H37Ra) at the rat tail base induces cross-reactive immune responses against self cartilage proteoglycans/type Ⅱ collagen, activates T cells (Th1/Th17) and releases abundant pro-inflammatory cytokines (TNF-α, IL-1β, IL-6, IL-17), triggering primary local inflammation (injection side) and secondary symmetrical polyarthritis (contralateral and four limbs). Synovial hyperplasia, pannus formation, cartilage degradation and bone erosion occur, accompanied by systemic inflammatory responses (body weight loss, fever, splenomegaly). SD rats are highly susceptible to adjuvant-induced arthritis with simple modeling, high success rate, stable onset window (secondary arthritis typically appearing 10–14 days after primary injection) and typical arthritis phenotype, serving as a common in-vivo model for evaluating novel RA drugs, natural anti-inflammatory products, immunomodulators and bone-protective strategies. The model combines both cellular and humoral immune mechanisms, and the secondary polyarthritis closely resembles the symmetrical peripheral joint involvement of human RA, widely applied in drug screening, mechanism validation and bone destruction protection research.

Model Validation Criteria

Quantitative Criteria for Clinical and Serological Validation

1. Secondary arthritis incidence: AA model group achieves incidence ≥90%, average arthritis index of affected rats ≥8 (total score 16) with symmetrical polyarthritis; 2. Joint swelling: posterior ankle joint swelling and paw volume of model group significantly increased compared with normal control (P<0.05), swelling persists ≥10 days; 3. Serology: serum TNF-α, IL-1β, IL-6 and IL-17 levels of model group significantly higher than normal control, rheumatoid factor (RF) positive rate elevated; 4. Positive drug validation: arthritis score and incidence of MTX or dexamethasone positive control group significantly reduced compared with model group (P<0.05), confirming sensitive and effective pharmacodynamic evaluation system, and the AA model is judged successfully established.

Imaging and Histopathological Gold-Standard Indexes

1. X-ray/Micro-CT imaging: model group hind limb joints show joint space narrowing, periarticular soft tissue swelling, bone erosion/destruction foci, periosteal reaction and osteoporosis signs; semi-quantitative score (joint destruction score 0–4) significantly elevated compared with normal control; 2. HE staining: synovial hyperplasia and thickening, pannus formation, massive inflammatory cell infiltration, joint cavity exudation, cartilage surface erosion and bone destruction; 3. Safranin O-fast green staining: loss of cartilage matrix proteoglycan, cartilage thinning and fissure formation; 4. TRAP staining: significantly increased osteoclast number at synovium-bone junction, indicating activated bone resorption; 5. Immunohistochemistry: increased TNF-α/IL-17 positive cells in synovial tissue. These pathological changes are highly consistent with human RA synovitis-pannus-cartilage destruction-bone erosion features, jointly validating the pathological establishment of the model.

Academic Evidence for Model Validity

Core validation evidence for SD rat AA model: ① single CFA injection induces high-incidence, typical-phenotype secondary polyarthritis with simple modeling and low cost; ② combines both cellular and humoral immune mechanisms, and secondary symmetrical polyarthritis closely resembles the peripheral joint involvement of human RA; ③ multi-dimensional indexes including arthritis score, joint swelling, inflammatory factors, RF, imaging and pathology are reproducible and cross-validated; ④ positive drug control system validated effective, confirming model sensitivity to drug intervention; ⑤ model evaluates prophylactic/therapeutic administration strategies and bone-protective effects. Combined verification of above indexes confirms model reliability and reproducibility, serving as a widely used in-vivo model for RA drug screening and mechanism research.

Model Advantages

The SD rat AA model features simple modeling (single CFA tail base injection), high success rate (≥90%), low cost and relatively short cycle (approximately 4–5 weeks), suitable for large-scale drug screening. Secondary symmetrical polyarthritis closely resembles peripheral joint involvement of human RA with both cellular and humoral immune mechanisms. Multi-dimensional indicators including incidence, arthritis score, joint swelling, paw volume, inflammatory factors, RF, imaging and pathology can be quantitatively evaluated with complete evidence chain. Both prophylactic and therapeutic administration protocols are supported, compatible with pharmacodynamic evaluation of chemical drugs, NSAIDs, DMARDs, biologics, natural anti-inflammatory products and bone-protective strategies. Rats have moderate body size with diverse administration routes, convenient blood sampling and sufficient pathological sampling. Results are readily accepted by rheumatology and pharmacology SCI journals, suitable for NSFC projects, postgraduate dissertations and research proposals.

Research Applications

The SD rat AA model is mainly applied to dissect the pathological mechanism of rheumatoid arthritis immune-inflammatory network, synovial hyperplasia, pannus formation, cartilage degradation and bone erosion; to perform in-vivo pharmacodynamic evaluation of anti-inflammatory/antirheumatic drugs (NSAIDs, DMARDs, glucocorticoids), biologics (anti-TNF-α, anti-IL-6R), targeted small molecules and natural anti-inflammatory products; to optimize prophylactic/therapeutic administration protocols and investigate synergistic effects of combination therapy; to excavate novel therapeutic targets (RANKL, IL-17, JAK/STAT, etc.) and regulatory networks of RA. It extensively serves basic rheumatology research, anti-RA new drug development, NSFC projects, postgraduate dissertations and SCI manuscript methodology construction, acting as a commonly used standardized in-vivo model for RA drug screening and translational medicine research.

SD rat adjuvant-induced arthritis model, complete Freund's adjuvant-induced arthritis, rheumatoid arthritis animal model, immunogenic arthritis, arthritis index score, secondary inflammatory response, synovial hyperplasia and bone destruction, anti-inflammatory antirheumatic drug efficacy evaluation