411Section 3: Inflammation

Inflammation: Concepts, what do we think of when we hear inflammation?

  • “Inflammation is the organism’s response to injury with the aims of inactivating/destroying invading organisms, removing irritants, and setting the stage for tissue repair” (Guzik et al., 2003).

  • It is described as a coordinated response to “danger” signals (Gomez et al., 2013).

  • It is crucial for containment and resorption of damaged tissue (Nocito et al., 2007).

  • Defined as a dynamic, protective response of vascularized tissue to injury (Theoharides et al., 2012).

  • It is a complex and dynamic process that restores anatomical continuity and function (Diegelmann & Evans, 2004).

  • Key takeaway: the body’s response to insult involves multiple overlapping stages, mediators, and cell types, and the patient’s response guides rehab and exercise progression.

Inflammation: Scope, Terminology, and Definitions

  • Inflammation can arise from any insult to tissues (mechanical, chemical irritants, cellular, vascular etc.). Protective, defensive, and healing mechanisms

  • Primary mechanism for survival

    • trigger inflammatory stimuli:

      • stress

      • trauma

      • tissue injury or death- release of chemical mediators of inflammation

  • It involves responses and healing stages that are broadly similar across insults, though durations vary and stages may overlap.

  • Understanding the stages helps guide exercise/activity/rehabilitation progression for patients/clients.

  • The core question: what makes inflammation beneficial vs. detrimental in a rehab context? timely resolution vs. chronicity.

Immunity: Innate vs Acquired

  • Innate (nonspecific) immunity: first line of defense

    • Anatomic barriers: skin, mucous membranes

    • Physiologic barriers: body temperature, low pH (e.g., stomach)

    • Phagocytic cells: granulocytes; involvement in inflammation (monocytes, macrophages)

    • Acquired (specific) immunity: activation of lymphocytes (WBC)

      • Develops after exposure to pathogens

      • Synthesis of antibodies to neutralize foreign antigens

  • Inflammation sits at the interface of innate and adaptive immunity, helping recruit cells and mediators that shape the immune response.

Inflammation: Acute, Sub-Acute, and Chronic (Definitions Revisited)

  • Acute inflammation: under pathologic conditions, increased blood flow and vascular permeability; accumulation of fluid, leukocytes, and inflammatory mediators (cytokines- cell metabolism changes/chemokines-attract other chemicals in cells toward the injured region).

  • Sub-acute/Proliferation (early healing): tissue proliferation and remodeling begin; humoral and cellular responses create a framework for repair. Chemical: Sodium, Hydrogen, Calcium

  • Chronic: tissue remodeling and proliferation continue; development of specific immune responses at the injury site; can lead to tissue remodeling and scar formation.

  • Overlaps among stages are common; durations vary depending on tissue, injury, and systemic factors.

  • During acute/sub-acute phases, soluble mediators recruit leukocytes via CAMs and chemoattractants, and regulate resident cells (fibroblasts, endothelial cells, tissue macrophages, mast cells).

  • Leukocytes involved include monocytes, lymphocytes, neutrophils, and eosinophils.

  • Some mediators trigger systemic effects (fever, hypotension, acute-phase protein synthesis, leukocytosis, cachexia).

Injury and Cellular Injury: Mechanisms and Triggers

  • Cell injury can arise from:

    • Ischemia: Hypoxia/anoxia causing metabolic disruption and acidosis (ischemia) ex: heart attack

    • Infectious agents: bacteria (toxins) and viruses (membrane disruption, DNA alteration)

    • Immune Reaction: Autoimmunity (hypersensitivity) ex: allergy

    • Genetic factors: chromosomal abnormalities, mutations

    • Nutritional deficiencies/overabundance

    • Physical Factors: Excess physical factors: trauma, extreme physical agents, mechanical factors, temp

    • Chemical factors:Toxins and free radicals cause redox damage

  • These factors initiate inflammatory cascades and tissue injury, prompting repair processes.

Acute Stage: Immediate Response and Mediators

  • Disruption of tissue integrity features:

    • Immediate Response

    • Release of chemical mediators OR

    • Vascular constriction followed by vasodilation and increased permeability

    • Platelet activation and fibrin clot formation to bond wound margins

      • bond clotting, thrombus, cytokines, IL-1, prostaglandin and growth factor, mast cells

    • Cytokines and growth factors drive cell signaling for repair

  • Early cellular actors include neutrophils and mast cells as primary mediators; macrophages arrive subsequently to continue cleanup and signaling.

  • This phase is punctuated by protection, containment, and initial cleanup of necrotic tissue.

  • Chemical responses with sodium, calcium, and hydrogen

  • Osmotic maintenance, fluid follows solutes=swelling

  • Key clinical signs reflect this phase: swelling, redness, heat, pain, loss of function, muscle guarding/spasm, and reflexive immobilization safeguarding the region.

Inflammation: Soluble Mediators and Cellular Communication

  • Five major categories of soluble mediators:

    • Inflammatory lipid metabolites derived from plasma membrane phospholipids (eicosanoids): prostaglandins, thromboxanes, leukotrienes, lipoxins; include derivatives of EPA/DHA.

    • Platelet-activating factor (PAF).

    • Other derivatives of arachidonic acid (eicosanoids).

    • Specialized Pro-Resolving Mediators (SPMs): resolvins, protectins, maresins (immunoresolvents that assist resolution rather than suppression) - lead to conversion of M1 to M2 factor, once concentration changes, they make the change

    • Three cascades of soluble proteases/substrates: clotting, complement, and kinin cascades; collectively generate pro-inflammatory peptides.

  • Nitric oxide (NO): IMPORTANT-vasodilation-prostiglandin-chemokines (attract other cells to the area)-potent endogenous vasodilator with an increasingly understood role in inflammation; contributes to vascular tone and leukocyte recruitment.

  • Cytokines: Cell-derived- Signalling proteins, similar to hormones and neurotransmitters. What is the concentration of chemicals within this damaged cell? Cell-derived polypeptides that orchestrate the inflammatory response, determine cellular infiltrate composition, state of cellular activation, and systemic responses. They can act autocrine (chemical binds with cell), paracrine(chemicals are produced by specific, local cell, or endocrine (bloodstream) -like. locally at the tissues, liver, CNS

  • Arachidonic acid derivatives and cytokines are central to orchestrating local and systemic inflammatory responses. ex: Prostanoids, prostaglandins, thromboxanes

Arachidonic Acid Pathway and Cyclooxygenase/Lipoxygenase Routes

  • Prostinoids: yield biological processes in inflammation and cellular proliferation and division

    • Oxygenated fatty acids-carbon

    • classified- denoted “PG”, “TX”

Arachidonic acid

  • produces prostaglandins when it’s cleaved from the plasma membrane

    • rate-limiting step: “delay” when release of arachidonic acid from membrane-bound phospholipids

      • catalyzed by a COX enzyme to form a prostaglandin or thromboxane

  • (AA) release and metabolism: stimulates COX enzymes

    • Phospholipase A2 (PLA2) liberates AA from membrane phospholipids.

    • AA is metabolized via two primary enzyme cascades:

    • Cyclooxygenase enzymes (COX) pathway: produced constantly in cell depending on stress or relaxation, determined by the concentration of certain COX enzymes

      • COX-1 (constitutive) and COX-2 (inducible) convert AA to Prostaglandin G2 (PGG2) and then Prostaglandin H2 (PGH2).

      • Tissue-specific synthases convert PGH2 to specific prostaglandins and thromboxanes: PGE2, PGI2, PGD2, PGF2α, TXA2.

      • COX-1: mediates normal cell activity and maintains homeostasis (e.g., protective gastric mucosa (stomach) via prostaglandins, renal function (kidneys), and platelet activity by synthesizing prostaglandins and thromboxanes).

      • COX-2 is induced in injured/stressed cells- healing by (cytokines) to produce prostaglandins that propagate inflammation and promote healing; also linked to VEGF-mediated angiogenesis and tissue repair. Emergency enzyme to protect from further injury

      • COX-3 is exclusively CNS

        • mediate signalling to cells in the CNS

          • functions centrally, not locally

          • Acetaminophen- reduces pain through inflammatory/healing reaction, if not recognized in the CNS- allows tissues to heal by masking pain, allowing PNS to heal the injured tissue

        • insends and steroids inhibit the arachidonic acid pathway

    • Lipoxygenase (LOX) pathway:

      • 5-LOX leads to leukotriene synthesis (e.g., LTB4, LTC4, LTD4, LTE4), contributing to chemotaxis, vascular permeability, and bronchoconstriction.

      • 12-LOX and 15-LOX yield other LOX-derived mediators that participate in resolution and remodeling.

Eicosanoids- how are they created?

  • regulate a broad set of inflammatory processes to cellular injury

    • PGE2 mediates local inflammatory signs, the PRIMARY ACTIVATOR (erythema, edema) via increased capillary permeability and vasodilation, and sensitizes nociceptors. Causes the pain- generator potential (lowers pain threshold)- action potential

      • can conversely inhibit inflammation, angiogenesis, cell proliferation (cell growth), satellite cells activation, Pkt pathway, protein kinase, fever, inflammation, increases temp (fever)

    • Prostaglandins contribute to fever (pyrogenic effects) and can influence pain thresholds.

    • Thromboxanes/Thrombus formation

      • promote platelet aggregation and clot formation, stabilizing wound margins

  • Integration: prostaglandins amplify inflammatory signaling, pain, and vascular changes to notify healing; leukotrienes and other LOX products contribute to leukocyte recruitment and vascular permeability; SPMs promote resolution when appropriate.

Transmembrane Eicosanoid Receptors

  • Thromboxanes/Thrombus formation

    • promote platelet aggregation and clot formation, stabilizing wound margins

    • smooth muscle contraction-vasoconstriction-calcium

    • signals body damage

    • cell proliferation, gene activation, differentiation, mitosis, kinase pathway (exercise-related)

  • Prostanoids:

  • What’s the difference between arachidonic acids, thromboxanes, prostaglandins, and eicosanoids?

  • PDF2A: Mediates cell growth and uterine contraction

    • hypertrophic cell growth in muscles, seen in resistance training

    • Introduction of interleukin synthesis- cytokines

    • luteolysis

    • cell signaling to divide

    • transcribes DNA and RNA -increases protein synthesis

  • PGE2:

    • regulates and stimulates a variety of cytokines (TNF-α and IL-6) and nitric oxide

      • vasodilation

      • pro-inflammatory compound, always around, in strong concentrations

    • Induces relaxation in arterial smooth muscle - vasodilation-digestion

    • Diverse effects on signal transduction - stimulate or attract more pro-inflammatory or inhibit less pro-inflammatory

    • PGE2 blocked- tissues never matured

    • stimulates satellite cells - muscle and other tissue growth

  • Prostacyclin (PGI2)

    • inhibits platelet aggregation - vasodilatation

    • counteracts thromboxane - vasoconstrictors, mast cells

  • PGD2: activates T-helper immune cells

    • inhibits platelet aggregation

    • prostiglandin mediator for: sleep, cell survival, allergic responses

    • negative feedback mechanisms in the inflammatory reaction to limit the response feedback

  • Second Messenger Role

    • reorganization of the actin cytoskeleton

    • cascade into protein synthesis

    • Stabilization of the plasma membrane

Cytokines in Acute and Chronic Inflammation

  • Glycoprotein (sugar proteins) - neuro-endocrine loop - mediators of acute inflammation - TNFa and IL-1 are the most potent inflammatory molecules known

    • interleukin-1,6,11,8

    • tumor necrosis factor alpha

    • granulocytes

  • IL-1: release of histamine from mast cells- vasodilatation and vascular permeability

    • mononuclear phagocytes

    • macrophages

    • fibroblasts

    • keratinocytes

    • T and B lymphocytes

  • TNF - Tumor Necrosis Factor

Stages of Inflammation and Healing: Timeline and Characteristics-each stage preps the other stage

vasoconstriction is short-lived, activating cells (platelets)

  • Acute Stage (roughly the first 4–6 days): pressure on nerve tissue- hemostasis

    • Vascular changes predominate in the first 48 hours: capillary permeability increases, exudate accumulates, and edema develops.

    • Exudation brings leukocytes (neutrophils) and mediators to the injury site. Some chemical mediators

    • Tissue tension increases due to edema; muscle guarding and protective immobilization occur.

    • Clot formation (platelets/fibrinogen) helps contain the injury and begin the binding of wound margins. almost immediately

    • Phagocytosis of necrotic tissue begins (monocytes -> macrophages later (roll over tissues- lots of receptors), to clean debris. don’t want to build new tissue on top of bad tissue, so needs to be cleaned

    • Early angiogenesis (new capillary beds) forms to support healing.

    • Clinical signs: more pressure in nerves, swelling, redness, heat, pain (PGE2), loss of function; systemic signs may include fever in some cases.

    • Glycocalic: identifies invading cells, produces cytokines in response to other chemicals to help

      • Macrophages (M1 and M2s)

      • Neutrophils

      • Mast cells

      • WBC

    • Tissue tension and movement restriction move to healing

  • Sub-Acute Stage - 10 to 17 days (Proliferation): lay down a lot of tissue

    • muscle: 5-8 days, tendon/ligament: 3-5 weeks

      • Lays down very immature and weak tissue, vascularization, and capillary production

        • Characterized by rapid collagen synthesis and deposition, growth factor activation (myoblast/fibroblast proliferation). Replace damaged sarcomeres

        • Dense fibroblast activity by day 4 post-injury; collagen synthesis leads to scar formation and tissue stiffness.

        • Satellite cells activate with myoblast proliferation to restore muscle fibers.

        • Remodeling and growth continue aggressively up to about 21 days if no setbacks occur.

        • Clinically, inflammatory signs progressively diminish as healing progresses. reduction in acute effects, less swelling, heat, and an increase in mobility

  • Chronic Stage Remodeling (Maturation-breaking down tissue, and Remodeling-building the tissue): 8-10 weeks

    • Contraction modelling

      • Myoblast/fibroblast differentiation and fusion produce immature myofibers; still weak, maturation continues as collagen fibers form organized fibrils.

      • Remodeling aligns collagen fibers in response to mechanical load; stresses can be physiologically beneficial or harmful.

      • Scar retraction occurs through myofibroblast activity and matrix remodeling; immature collagen from hydrogen bonds initially, making it susceptible to remodeling.

      • Proper remodeling requires gentle and persistent mechanical loading to guide tissue organization and thickening (hydrogen to covalent bonds); potential remodeling duration ranges into weeks and months.

  • Chronic Re-modeling 14 weeks: ensure that tissues do not become restricted by poorly remodeled scars; need to stabilize the area, if stresses are excessive, remodeling may be interrupted, and inflammation can re-emerge.

    • High-density connective tissue takes longer to remodel; immature tissue is prone to adhesions and limited ROM.

    • no signs of inflammation, very close to functional

    • RegenX

  • Chronic Perpetual Stage: a long-standing, recurrent inflammatory state due to repeated irritants or overload during healing. only so much scar tissue until it never heals, and it’s “old Play-Doh”

    • Excessive stress, once not fully healed, can risk a return to the acute stage, only 3-5 times

    • Similar to sub-acute - trying to generate integrity again - protioliticy activity

      • Fibroblast proliferation persists; immature collagen and myofibers degrade, and new proteolytic activity continues.

      • Prostaglandins from injured tissue and sustained proteolysis lead to ongoing tissue weakness and potential functional limitation.

      • Clinically: increasing pain, swelling, and muscle guarding that persist beyond typical recovery; stiffness increases after rest and ROM reduces; risks of adhesions and functional loss rise.

  • Practical rehab implication: improper or excessive loading during remodeling can perpetuate inflammation and create restrictive scar tissue that is difficult to remodel later.

Cellular Players in Acute Inflammation

  • Mast cells (in damaged region): Granules-glyco amino glacand- collagen and elastic-based tissues

    • Activate and release proteoglycans (hyaluronic acid, chondroitin sulfate), high negative charge=lots of sodium = attracts water = swelling, but opens space for healing and keeps unhealthy cells and growth factors inside. to form a gel-like matrix/boundary/contained region that encapsulates the region, aiding containment of debris and mediators.

    • Activation leads to the release of mediators that promote vasodilation, permeability, and leukocyte recruitment.

    • Maturation is influenced by surface receptor signaling and local cytokines (TNF, cytokine: all healing processes = IL-1 “blocker”).

    • Secreted mediators include cytokines/chemokines, histamine, serotonin (metabolic activity of cells), kinins (bradykinin), proteases, CRH-corticoreleasing hormone- stimulates cortisol, blocks insulin, and eicosanoids; these drive downstream mitogenic pathways.

Macrophages

  • (two major states, M0-resting macrophages (inactivated), M1 and M2; also ED1/ED2 markers):

  • Helps a lot with the acute stage- produces growth factors, which also produce chemicals that shut off inflammation

    • Phagocytose (engulf) chemical invaders

      • Monocytes: immature macrophages, arrive ~6 hrs after the inflammatory response, probably less

      • Mature to macrophages: like neutrophils, but stronger killing

      • Secretes chemical mediators

      • coordinate response with other cells and growth factors

      • An intermediary between innate and acquired immune response

        • M1/ED1 (pro-inflammatory): synthesize and release chemokines (e.g., NO), cytokines (e.g., IL-1, IL-6, IL-12), and prostanoids that promote inflammation.

        • M2/ED2 (anti-inflammatory): promotes resolution and tissue repair; secretes growth factors that induce neovascularization (producing more capillaries) and granulation tissue (healing tissue foundation) formation.

        • Resting macrophages exist as M0; upon activation, they become M1 (pro-inflammatory) to clear debris and pathogens, then transition to M2 to support tissue repair and remodeling.

  • Neutrophils and eosinophils:

    • Early responders to infection or tissue injury; perform phagocytosis and microbial killing; neutrophils are among the first cells in acute inflammation.

  • Platelets and clotting factors come before macrophages:

    • Form initial fibrin-rich clots to bond wound margins and limit bleeding; also release mediators that contribute to inflammation and healing.

  • Leukocytes (monocytes, T cells, B cells):

    • Recruited by chemokines and cytokines, participate in phagocytosis, antigen presentation, and adaptive immune responses as inflammation evolves.

  • Phagocytosis sequence (macrophage/neutrophil activity):

    • Bacterium binds receptors on phagocytes → formation of phagosome → phagolysosome formation → degradation of debris → exocytosis of debris and mediators.

Clinical Signs of Inflammation and Their Mediators

  • Classic signs connected to inflammatory mediators (PGE2 and others):

    • Rubor (redness), Tumor (swelling), Dolor (pain), Calor (heat), and Fever (systemic elevation of body temperature).

  • Localized inflammatory cascade includes increased vascular permeability, edema, and immune cell infiltration; systemic responses can include fever and acute-phase protein production.

Practical and Real-World Implications for Exercise and Rehabilitation

  • The patient/client response is the best guide for progression of exercise/activity and rehabilitation after injury or surgery.

  • Early stages require controlled loading to avoid re-aggravation of injury while promoting healing; overly aggressive loading can perpetuate inflammation and lead to chronic pathology.

  • In remodeling, balanced, progressive loading helps align collagen fibers and improve tissue strength without causing overuse injuries.

  • Awareness of chronic perpetual inflammation can help clinicians design strategies to reduce irritants and modulate loading, improving long-term function.

Stage Durations: Summary

  • Acute stage duration: oxidation reduction reaction- nerve endings are highly-sensitized (pressure/edema) 24 hours or even less, but A LOT of damage 4−6 days4-6\text{ days} (total acute duration typically ~4–6 days; begins at injury)-vascular changes-permiability- edema (sodium-swelling)- muscle guarding: PGE 2-spasms, clot formation due to damage to capillaries

    • phagocytosis, macrophages, early fibroblast activity, capillary beds

      • swelling/stiffness, redness, heat, pain, loss of function, muscle guarding, and spasm

  • Sub-acute (proliferation) duration: 10−17 days10-17\text{ days} (cumulative duration ~14–23 days; may extend up to ~2–3 weeks)

  • Chronic (maturation/remodeling) duration: 56−70 days56-70\text{ days} (cumulative total ~70–93 days; about 10–13 weeks)

Additional Notes and Cross-References

  • The material draws on multiple sources to define inflammation and its stages, including Guzik et al. (2003), Gomez et al. (2013), Nocito et al. (2007), Theoharides et al. (2012), Diegelmann & Evans (2004).

  • The content highlights the integration of immune signaling with tissue repair and the importance of mediators (cytokines, prostaglandins, leukotrienes, NO) in shaping the healing trajectory.

  • The content emphasizes the interplay between local tissue responses and systemic symptoms, and how rehabilitation protocols should respond to the evolving inflammatory landscape.

Quick Reference: Key Terms and Concepts

  • Inflammation: protective, dynamic response to injury involving vascular, cellular, and molecular components.

  • Innate immunity: first-line defense; non-specific.

  • Acquired immunity: specific response with antibodies.

  • Acute mediators: histamine, bradykinin, leukotrienes, prostaglandins, NO.

  • Arachidonic acid derivatives: prostaglandins (PGE2, PGI2, etc.), thromboxanes (TXA2), leukotrienes (LTB4, LTC4, LTD4, LTE4), lipoxins; SPMS promote resolution.

  • COX enzymes: COX-1 (homeostasis), COX-2 (inflammation), COX-3 (possible CNS role; acetaminophen selectivity).

  • LOX enzymes: 5-LOX, 12-LOX, 15-LOX; leukotrienes and other products.

  • M1/M2 macrophages: pro-inflammatory vs anti-inflammatory/repair roles.

  • Mast cells: early mediators of vascular changes and leukocyte recruitment; release proteoglycans and mediators, granuels -

  • Phagocytosis sequence: phagosome → phagolysosome → degradation → exocytosis.