Inflammation and repair

Inflammation is a response by the body to injurious agents like pathologic microbes, physical and chemical agents etc or caused by immune reactions. Depending on the timeframe, it can be acute or chronic inflammation. Acute inflammation typically lasts for less than 2 weeks and is the early, transient response to injury. Chronic inflammation is of longer duration and is due to persistence of the inciting stimulus or due to characters of the inciting injury like Mycobacteria and fungi which typically cause chronic inflammation. 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 happens immediately as the first response of acute inflammation. This is followed in a few minutes by progressive vasodilation of mainly arterioles, venules and capillaries. This results in local edema, warmth and increased capillary hydrostatic pressure. Vasodilation is followed by slowing or stasis of microcirculation resulting in increased blood viscosity. Stasis is followed by transmigration of leukocytes from the capillary into the interstitium.

There is increased vascular permeability causing more fluid and cells to leak out of the 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. There is disruption to normal axial flow of blood. As a result the blood cells are marginalized to the periphery where they come in contact with the vessel wall.

2. Rolling and adhesion: Neutrophils slowly roll over the vessel wall in “rolling phase” while transient bonds are formed between the endothelial cells and neutrophils in “adhesion phase”. Both of these phases are accomplished by specific factors on neutrophils and endothelial cells and their dysfunction can lead to pathologic states.

3. Transmigration: After adhesion to the endothelium, neutrophils transmigrate into the tissues with the help of pseudopod formation and breaking the basement membrane with collagenases. Simultaneously, RBCs also escape out 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 case of inflammation, it is the chemokine mediated movement of leukocytes which helps them reach the focus of inflammation. Potent chemokines or chemotactic factors for leukocytes are leukotriene B4/LTB4, complements C5a and C3a, IL8 and bacterial formylated peptides. Other important chemokines are eotaxin for eosinophils, MCP 1 for monocytes and platelet factor 4 for neutrophils, monocytes and eosinophils.

5. Phagocytosis: It 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: Receptors such as mannose receptor and scavenger receptor are expressed on the surface of macrophages, which help to identify microorganisms. Opsonins are molecules that coat the microorganisms and can be identified by phagocytes through specific opsonin receptors. Opsonin coated microbes are phagocytosed. IgG, C3b and lectins act as opsonins.

2. Engulfment: Phagocytes form pseudopods around a particle, with the help of actin filaments. It is followed by the formation of a phagosome. The phagosome 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 is dependent on free radicals like superoxide, H2O2, OH-, HOCl etc. The enzyme NADPH oxidase is present in the phagosome and it converts oxygen to superoxide free radical. Superoxide is then converted to H2O2. Respiratory burst is the phase of increased oxygen consumption by the phagocyte which is producing free radicals, for which oxygen is needed.

      1. Oxidative MPO dependent mechanism: The enzyme MPO or 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. All 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 the free radicals, causing degradation of the ingested particle.
      4. Non-oxidative mechanisms: Few lysosomal enzymes do not need oxygen for their activity. They are lysosomal hydrolases, defensins, lipases, proteases, DNAses and permeability increasing factor. Nitric oxide (NO) is produced both by endothelial cells and activated macrophages. NO forms free radicals that are microbicidal.

   2. Extracellular mechanisms: Apart from intracellular microbicidal activity in the phago-lysosome, phagocytic cells release hydrolytic and proteolytic enzymes extracellularly as well. Immune cells with the help of antibodies and cytotoxic T cells kill microbes by cytolysis and ADCC (antibody dependent cell mediated cytotoxicity).