Inflammation and repair

Inflammation is the body’s response to injurious agents such as pathogenic microbes, physical agents, chemical agents, or immune reactions. Based on duration, inflammation is classified as acute or chronic.

Acute inflammation usually lasts less than 2 weeks. It is the early, rapid, and short-lived response to injury. Chronic inflammation lasts longer and occurs when the inciting stimulus persists, or when the nature of the stimulus (for example, Mycobacteria and fungi) tends to produce a prolonged inflammatory response. Neutrophils are the hallmark of acute inflammation, while lymphocytes are the hallmark of chronic inflammation.

Cardinal signs of inflammation: There are 5 cardinal signs of inflammation - rubor or redness/ erythema; tumor or swelling; calor or heat; dolor or pain and functio laesa or loss of function.

Vascular events in acute inflammation: Transient arteriolar vasoconstriction occurs immediately as the first response in acute inflammation. Within a few minutes, this is followed by progressive vasodilation, mainly of arterioles, venules, and capillaries. Vasodilation leads to local warmth, edema, and increased capillary hydrostatic pressure. Next, microcirculation slows (stasis), which increases blood viscosity. Stasis is followed by transmigration of leukocytes from the capillary into the interstitium.

There is increased vascular permeability, which allows more fluid and cells to leak out of capillaries. Causes of increased vascular permeability are as follows:

1. Contraction of endothelial cells of venules due to action of histamine, bradykinin etc. It results in the formation of gaps in the endothelial lining.

2. Cytokines such as IL-1 and TNF alpha cause retraction of the intercellular junctions between endothelial cells of venules.

3. Toxins may directly cause endothelial damage and necrosis.

4. Attachment and activation of leukocytes to the endothelium may cause endothelial damage as a result of leukocyte derived enzymes and free radicals. It is seen mainly in venules and capillaries.

5. Newly formed blood vessels during the repair process are initially leaky.

Exudation of lymphocytes: The following steps are seen in the exudation of leukocytes in inflammation.

1. Margination: Vasodilation is followed by stasis of local blood flow. Stasis and increased transudation of plasma lead to hemoconcentration. This disrupts the normal axial flow of blood. As a result, blood cells move toward the periphery (margination), where they come into contact with the vessel wall.

2. Rolling and adhesion: Neutrophils first roll along the vessel wall (the “rolling phase”). During this time, transient bonds form between neutrophils and endothelial cells. Rolling is followed by firmer attachment (the “adhesion phase”). Both phases depend on specific factors on neutrophils and endothelial cells; dysfunction of these factors can lead to pathologic states.

3. Transmigration: After adhesion to the endothelium, neutrophils transmigrate into tissues. They do this by forming pseudopods and breaking the basement membrane with collagenases. At the same time, RBCs can also escape by diapedesis. After 24-48 hours, neutrophils are replaced by macrophages.

4. Chemotaxis: Chemotaxis is the directional migration of cells along a chemical gradient. In inflammation, chemokines guide leukocytes to the focus of inflammation. Potent chemokines (chemotactic factors) for leukocytes include leukotriene B4/LTB4, complements C5a and C3a, IL8, and bacterial formylated peptides. Other important chemokines include eotaxin for eosinophils, MCP 1 for monocytes, and platelet factor 4 for neutrophils, monocytes, and eosinophils.

5. Phagocytosis: Phagocytosis is the process by which a cell engulfs other cells or particles. Professional phagocytes include monocytes, macrophages, neutrophils, dendritic cells, osteoclasts, and eosinophils. In addition, fibroblasts, epithelial cells, and endothelial cells can also perform phagocytosis when activated.

Phagocytosis occurs in the following three steps:

1. Recognition and attachment: Macrophages express receptors such as the mannose receptor and scavenger receptor, which help identify microorganisms. Opsonins are molecules that coat microorganisms and are recognized by phagocytes through specific opsonin receptors. Opsonin-coated microbes are phagocytosed. IgG, C3b, and lectins act as opsonins.

2. Engulfment: Phagocytes extend pseudopods around a particle with the help of actin filaments. This is followed by formation of a phagosome. The phagosome then fuses with the lysosome to form a phagolysosome.

3. Killing and degradation: After engulfment, microbes are killed and degraded by the phagocyte in the following ways:

   1. Intracellular mechanisms: Microbes are killed by both oxidative and non-oxidative mechanisms. Oxidative intracellular killing depends on free radicals such as superoxide, H2O2, OH-, and HOCl. The enzyme NADPH oxidase is present in the phagosome and converts oxygen to the superoxide free radical. Superoxide is then converted to H2O2. Respiratory burst is the phase of increased oxygen consumption by the phagocyte during free radical production (oxygen is required for this process).

      1. Oxidative MPO dependent mechanism: The enzyme MPO (myeloperoxidase) is present in the azurophilic granules of phagocytes. MPO converts H2O2, in the presence of halides like Cl-, I- or Br-, to form HOCl, HOI, and HOBr respectively. These free radicals have potent microbicidal activity.
      2. Oxidative MPO independent mechanism: Macrophages lacking the enzyme MPO produce OH- and superoxide free radical from H2O2 by the Haber-Weiss and Fenton (in the presence of Fe++) reaction.
      3. Killing by lysosomal enzymes: Preformed lysosomal enzymes such as proteases, trypsinase, phospholipase, and alkaline phosphatase are discharged into the phago-lysosome. These enzymes act synergistically with free radicals, causing degradation of the ingested particle.
      4. Non-oxidative mechanisms: Some lysosomal enzymes do not require oxygen for their activity. These include lysosomal hydrolases, defensins, lipases, proteases, DNAses, and permeability increasing factor. Nitric oxide (NO) is produced by endothelial cells and activated macrophages. NO forms free radicals that are microbicidal.

   2. Extracellular mechanisms: In addition to intracellular microbicidal activity within the phago-lysosome, phagocytic cells can also release hydrolytic and proteolytic enzymes extracellularly. Immune cells, with the help of antibodies and cytotoxic T cells, kill microbes by cytolysis and ADCC (antibody dependent cell mediated cytotoxicity).