4.9.4 Inflammatory, Immunological, and Pain Models
Inflammatory and immune-mediated disease models range from acute, hours-long assays suited to high-throughput screening of anti-inflammatory candidates to...
Inflammatory and immune-mediated disease models range from acute, hours-long assays suited to high-throughput screening of anti-inflammatory candidates to chronic, weeks-long models capturing the autoimmune and tissue-destructive character of diseases such as rheumatoid arthritis.
Rheumatoid Arthritis — Complete Freund's Adjuvant (CFA)-Induced Model
Rheumatoid arthritis is a chronic autoimmune disease characterised by synovial inflammation and progressive joint destruction. The adjuvant-induced arthritis model, typically performed in Wistar or Lewis rats, involves a single sub-plantar or intradermal injection of Complete Freund's Adjuvant (heat-killed Mycobacterium tuberculosis suspended in mineral oil), which triggers a delayed-type hypersensitivity response producing polyarthritis affecting both the injected and, characteristically, distal untreated paws within ten to fourteen days, reflecting a systemic autoimmune process rather than a purely local inflammatory reaction. Disease severity is quantified using paw volume (plethysmometry), an arthritic scoring system grading redness, swelling, and joint rigidity across all four limbs, radiographic assessment of bone erosion, and histopathological synovial inflammation scoring. Methotrexate, the cornerstone disease-modifying antirheumatic drug in human rheumatoid arthritis therapy, together with NSAIDs such as indomethacin, serve as standard reference comparators. This model's principal strength is its capture of the systemic, immune-mediated character of rheumatoid arthritis; collagen-induced arthritis, using type II collagen immunisation, is a mechanistically complementary alternative model increasingly favoured for its closer resemblance to the autoantibody-driven pathology of the human disease.
Acute Inflammation — Carrageenan-Induced Paw Oedema Model
Acute inflammation is most commonly modelled using sub-plantar injection of 0.1 mL of 1% carrageenan (a sulphated polysaccharide extracted from red seaweed) into the hind paw of rats, which triggers a well-characterised, biphasic inflammatory response: an early phase (0–2 hours) mediated by histamine, serotonin, and bradykinin, followed by a later phase (2–6 hours) mediated predominantly by prostaglandins and neutrophil infiltration. Paw swelling is quantified by plethysmometry (water or mercury displacement) at hourly intervals up to four to six hours post-injection, and percentage inhibition of oedema relative to vehicle-treated controls is calculated for each treatment group. Indomethacin or diclofenac serve as standard NSAID reference comparators. This model's speed (a complete experiment within a single day), reproducibility, and mechanistic tractability (the biphasic time course allows differentiation between early mediator-blocking and late prostaglandin-blocking drug mechanisms) make it the most widely used first-line screen for candidate anti-inflammatory agents, although it captures only acute, and not chronic or autoimmune, inflammatory processes.
Analgesia — Tail-Flick and Hot-Plate Models
Nociceptive (pain) pharmacology is assessed using thermal reflex-withdrawal assays that measure the latency between a defined noxious thermal stimulus and a reflexive withdrawal response. In the tail-flick test, radiant heat or a focused heat source is applied to the tail, and the latency to tail withdrawal is recorded, reflecting a predominantly spinal reflex circuit. In the hot-plate test, an animal is placed on a surface maintained at a fixed noxious temperature (commonly 52–55°C), and the latency to a supraspinally integrated response (paw licking or jumping) is recorded, providing complementary information about centrally mediated analgesia. Increased latency following drug treatment indicates analgesic activity, with morphine as the standard opioid reference and a defined cut-off time enforced in both assays to prevent thermal tissue injury. Chemical nociceptive models, including the formalin test (biphasic paw-licking response to subcutaneous formalin injection, distinguishing acute nociceptive and inflammatory pain phases) and the acetic-acid writhing test (counting abdominal writhing responses following intraperitoneal acetic acid, sensitive to both opioid and NSAID analgesics), provide further mechanistically distinct assessment of analgesic efficacy.
Fever — Yeast-Induced Pyrexia Model
Fever is a regulated elevation of core body temperature mediated by hypothalamic prostaglandin E2 synthesis in response to circulating pyrogenic cytokines. The yeast-induced pyrexia model produces this response experimentally by subcutaneous injection of a 15–20% aqueous suspension of brewer's yeast, which triggers an inflammatory cytokine cascade and a measurable rise in rectal temperature peaking approximately 18–19 hours post-injection. Antipyretic efficacy is assessed by comparing rectal temperature (measured with a digital or thermocouple thermometer) in treated versus vehicle-control animals at defined time points following drug administration at the temperature peak, with paracetamol or aspirin as standard reference antipyretics. The model's principal strength is its mechanistic parallel to genuine cytokine-mediated fever, as opposed to models of hyperthermia produced by direct heat exposure, which do not involve the same central prostaglandin-mediated regulatory pathway and are therefore not appropriate substitutes for antipyretic drug screening.
Osteoarthritis — Monoiodoacetate (MIA)-Induced Model
Osteoarthritis is a degenerative joint disease characterised by progressive articular cartilage breakdown, subchondral bone remodelling, and chronic joint pain. The monoiodoacetate model produces this pathology by a single intra-articular injection of sodium monoiodoacetate into the knee joint, which inhibits chondrocyte glycolysis (via inhibition of glyceraldehyde-3-phosphate dehydrogenase), causing chondrocyte death and cartilage degeneration that closely mimics the histopathological features of human osteoarthritis within two to three weeks. Disease severity and drug efficacy are assessed using weight-bearing asymmetry (via an incapacitance meter comparing load distribution between the injected and contralateral limb), von Frey mechanical allodynia testing, and histopathological cartilage (Mankin) scoring. NSAIDs and, increasingly, nerve growth factor-targeted biologics serve as reference comparators, reflecting both the inflammatory and neuropathic components of osteoarthritic pain. The model's speed and reproducibility make it the most widely used preclinical osteoarthritis model, although the acute chemical induction mechanism differs from the slow, mechanically driven degeneration underlying most human osteoarthritis, motivating the use of surgical (destabilisation of the medial meniscus) models in more mechanistically focused research.
Autoimmune Disease — Experimental Autoimmune Encephalomyelitis (EAE) Model
Multiple sclerosis and related CNS autoimmune demyelinating diseases are modelled using Experimental Autoimmune Encephalomyelitis, induced by immunising susceptible mouse strains (commonly C57BL/6) or rats with myelin-derived peptides (such as myelin oligodendrocyte glycoprotein, MOG35-55) emulsified in Complete Freund's Adjuvant, together with pertussis toxin to enhance blood-brain-barrier permeability and immune cell infiltration into the CNS. Immunised animals develop an ascending paralysis over one to three weeks, scored using a standardised 0–5 clinical scale ranging from a limp tail to complete hindlimb and forelimb paralysis, accompanied by histopathological CNS demyelination and inflammatory infiltrate. Corticosteroids and, increasingly, disease-modifying biologics targeting specific immune cell populations (such as natalizumab, an alpha-4 integrin antibody) serve as reference comparators. EAE is the principal preclinical model underlying the development of virtually all currently approved multiple sclerosis therapeutics, though its relapsing-remitting or chronic-progressive character varies considerably between mouse strains and immunisation protocols, requiring careful strain selection matched to the specific disease phenotype under investigation.
Immunosuppression — Cyclophosphamide-Induced Model
Drug-induced immunosuppression is modelled using cyclophosphamide, an alkylating agent that preferentially depletes rapidly dividing immune cell populations, particularly in the bone marrow and lymphoid organs. Administration of cyclophosphamide (commonly 50–200 mg/kg intraperitoneally, as a single dose or short repeated regimen) produces a reproducible, dose-dependent leucopenia and immune dysfunction that is used both to study the pathophysiology of immunosuppression itself and, more commonly, as a standard experimental background against which candidate immunostimulant or immunomodulatory agents (including many phytochemical and Ayurvedic formulations of research interest) are evaluated for their ability to restore immune parameters. Endpoints include total and differential white blood cell counts, relative spleen and thymus weight, humoral immune response (antibody titre following a test antigen such as sheep red blood cells), and cell-mediated immune response (delayed-type hypersensitivity reaction). Levamisole is a commonly used reference immunostimulant. This model is valued for its simplicity and reproducibility, and for directly modelling a clinically important adverse effect of cancer chemotherapy.
Sepsis — Caecal Ligation and Puncture (CLP) Model
Sepsis, a dysregulated systemic inflammatory response to infection carrying high clinical mortality, is modelled surgically using the caecal ligation and puncture procedure, in which the caecum is exteriorised, ligated below the ileocaecal valve, and punctured to allow controlled leakage of faecal material into the peritoneal cavity, producing a polymicrobial infection and systemic inflammatory response that closely mirrors the clinical progression of human intra-abdominal sepsis. A simpler, non-surgical alternative, the lipopolysaccharide (LPS)-induced endotoxaemia model, produces a systemic inflammatory response by intraperitoneal LPS injection alone, offering greater procedural reproducibility at the cost of omitting the genuine polymicrobial infectious component. Endpoints in both models include survival rate over 72–96 hours, circulating pro-inflammatory cytokines (TNF-alpha, IL-6), organ-function biomarkers reflecting multi-organ dysfunction, and bacterial colony counts in blood and peritoneal fluid. Broad-spectrum antibiotics combined with fluid resuscitation represent the clinical standard of care and serve as the benchmark against which novel sepsis-directed immunomodulatory therapies are compared; the CLP model is widely regarded as the preclinical gold standard for sepsis research given its close mechanistic parallel to human disease.