Step 3 of the Inflammatory response
Phagocytosis
- means cellular ingestion of the offending agent
- 3 criteria for phagocytosis:
1. the surface has to be rough
2. shouldn’t have a protective protein coat
3. an antibody needs to adhere to the bacterial membrane
- the process by which a pathogen is selected for phagocytosis is called opsonization
- the initial step is contact between the surface of the phagocyte and pathogen
- the neutrophil projects pseudopodia in all directions around the particle. The pseudopodia meet one another on the opposite ends and fuse
- trigger for phagocytosis during this contact is the interaction of phagocyte surface receptors with certain carbohydrates or lipids in the pathogen or microbial cell wall
- opsonins help to bind phagocytes tightly to pathogens
- once the pathogen is enclosed in a chamber, the chamber breaks away from the outer cell membrane of the phagocyte to form a free-floating phagocytic vesicle or phagosome within the cytosol
- the phagosome membrane makes contact with one of the phagocyte’s lysosomes, which is filled with a variety of hydrolytic enzymes
- the membranes of the lysosome and the phagosome fuse and is now called a phagolysosome
- the lysosomal enzymes break down the microbe’s macromolecules
- other enzymes produce nitric oxide and hydrogen peroxide, which are also highly destructive
- lysosomes of macrophages also contains lipase, which digests the thick lipid membranes possessed by some bacteria like tuberculosis bacillus
- neutrophils and macrophages also contain bactericidal agents that kill most bacteria even when the lysosomal enzymes fail to digest them
Mediators of the inflammatory response
- key mediators are cells that function as phagocytes (neutrophils, macrophages and dendritic cells)
- 2 general categories:
1. polypeptides (for example kinins) generated in the infected area by enzymatic actions on precursor proteins that circulate in the plasma
2. substances secreted into the extracellular fluid from cells that either already exist in the affected area (injured cells or mast cells) or enter it during inflammation (neutrophils)
Step 1 of the Inflammatory response
Vasodilation and increased permeability to proteins
- mediators cause mircrocirculation vessels in the infected area to dilate
- mediators increase the capillaries’ permeability to proteins (contract endothelial cells, opening spaces between them)
- the increased blood flow to the inflamed area increases the delivery of proteins and leukocytes
- increased permeability to protein ensures that the plasma proteins participate in inflammation
- causes net filtration of plasma into the interstitial fluid, causing an edema
- this accounts for swelling in an inflamed area, which is simply the consequence of the changes in the microcirculation and has no adaptive value of its own
Step 2 of the inflammation response
Chemotaxis
- involves a variety of protein and carbohydrate adhesion molecules on both the endothelial cell and the neutrophil
- regulated by messenger molecules released by cells in the injured area, including mast cells and endothelial cells (chemoattractants)
- cause increased expression of adhesion molecules, like selectins and intracellular adhesion molecule-1 (ICAM-1) on the surface of endothelial cells in the capillaries and venules
- adhesion molecules react with integrin on the neutrophils, causing the neutrophils to stick to the capillary and venule walls in the inflamed area. This is called margination
- since the endothelial cells have contracted, loosening the intracellular attachments, the openings are large enough for the neutrophils to enter the tissue through diapedesis
- a narrow projection of the neutrophil is inserted into the space, whereafter the entire neutrophil squeezes through the endothelial walls and into the interstitial fluid
- Once in the interstitial fluid, neutrophils follow a chemotactic gradient and migrate toward the site of tissue damage. Chemotaxis
- this occurs because pathogen-stimulated innate immune cells release chemoattractants
- chemokines, a type of cytokine often acts as these chemoattractants
Step 4 in the Inflammatory response
Tissue repair
- fibroblasts (connective-tissue cells) that reside in the area divide rapidly and begin to secrete large quantities of collagen, and blood vessel cells proliferate in a process called angiogenesis
- brought about by chemical mediators, namely a group of locally produced growth factors
- remodeling occurs as the healing process winds down
Alternative to Phagocytosis: Complement
- certain complement proteins are always circulating in the blood in an inactive state
- upon activation, a cascade occurs through which multiple active complement proteins are generated in the extracellular fluid
- the central protein in the complement cascade is C3
- Activation of C3 initiates the deposition of C3b on the microbial surface
- C3b can act as an opsonin. One portion binds nonspecifically to carbs on the surface of the bacterium, whereas another portion binds to specific receptor sites for C3b on the plasma membrane of the phagocyte
- C3b is also part of a proteolytic enzyme that amplifies the complement cascade and leads to the development of a multiunit protein called the membrane attack complex (MAC)
- the MAC embeds itself in the bacterial plasma membrane (or the virus protein coat) and forms porelike channels in the membrane, making it leaky
- water, ions and small molecules enter the pathogen, disrupting its intracellular environment and killing it
- some of the activated complement proteins can directly or indirectly cause vasodilation, increased microvessel permeability to protein and chemotaxis
- antibodies are needed to activate C1 in the full sequence of the classical complement pathway
- with nonspesific inflammation the alternative complement pathway occurs that bypasses C1
- or the interactions between carbohydrates on the surface of the microbe and inactive complement molecules beyond C1 can lead to the formation of active C3b
- in addition to complement C3b, other plasma proteins can also facilitate opsonization. These C-reactive proteins are produced by the liver and are always found at some concentration in the plasma. Their production are greatly increased during inflammation
Interferons in the Innate immune response
- Type I interferons include several proteins that nonspecifically inhibit viral replication inside host cells
- in response to infection by a virus, many immune cells produce and release these interferons into the extracellular fluid
- Type I interferons then bind to plasma membrane receptors on the secreting cell as well as other cells, whether they are infected or not
- binding triggers the synthesis of dozens of different antiviral proteins by the cells
- these proteins interfere with the ability of the viruses to replicate
- also functions in the killing of tumor cells and in generating fever during infection
- severe acute respiratory syndrome coronavirus 2 (SARS-CoV 2) have evolved mechanisms that prevent or limit cells from producing interferons, making it harder for the body to rid itself of the infection
- Type II interferons- interferon-gamma potentiates some of the actions of Type I interferons, enhances the bacteria-killing activity of macrophages and acts as a chemokine in the inflammatory process
Monocyte-Macrophage cell system (Reticuloendothelial system)
- after monocytes enter tissue and become macrophages, another large portion becomes attached to the tissues and remains attached for months or years until they are called on to perform specific local protective functions
- have the same abilities than mobile macrophages to phagocytise
- when appropriately stimulated, they can break away from their attachments and once again become mobile macrophages that respond to chemotaxis
- the total combination of monocytes, mobile macrophages, fixed tissue macrophages and a few specialised endothelial cells in the bone marrow, spleen, and lymph nodes are called the reticuloendothelial system
Macrophages throughout the body
Skin and subcutaneous tissue
- histiocytes
Lymph nodes
- large numbers of macrophages line the lymph sinuses, and if any particle enters the sinuses by way of the lymph, macrophages phagocytise them and prevent general dissemination throughout the body
Lungs
- present in the alveolar walls
- can phagocytise particles that become entrapped in the alveoli
- if the particles are digestible, macrophages digest them and release the waste into the lymph
- if undigestible, macrophages form giant cell capsules around the particle until it can be slowly dissolved, if ever
- frequently formed around tuberculosis bacilli, silica dust particles and carbon particles
Liver
- bacteria from ingested food constantly pass through the gastrointestinal mucosa into the portal blood
- before entering the blood it passes through the liver sinusoids, which are lined with tissue macrophages called Kupffer cells
- so effective that almost none of the bacteria from the gastrointestinal tract pass from the portal blood into the general circulation
Spleen and bone marrow
- the trabeculae of the red pulp and the venous sinuses are lined with vast numbers of macrophages
- phagocytises unwanted debris in the blood, especially old and abnormal RBC’s
The role of neutrophils and macrophages in inflammation
Neutrophils
- within a few hours after the onset of acute inflammation, the number of neutrophils increases fourfold to fivefold
- this is called neutrophilia and is caused by products of inflammation that enter the blood stream, are transported to the bine marrow, and there act on stored neutrophils of the marrow to mobilize these into the circulating blood
Macrophages
- along with the invasion of neutrophils, monocytes from the blood enter the inflamed tissue and enlarge to become macrophages
- blood concentration and bone marrow storage is lower than neutrophils
- even after invading the inflamed tissue, monocytes require 8 hours to mature
- play an important role in the initiation of the development of antibodies
Inflammation lines of defense
- Tissue macrophages
- neutrophil invasion of the inflamed area
- second macrophage invasion into the inflamed tissue
- Increased production of granulocytes and monocytes by the bone marrow:
- results from the stimulation of granulocytic and monocytic progenitor cells of the marrow
- takes 3-4 days for the new granulocytes and monocytes to reach the stage of leaving the bone marrow
- if the stimulus persists, the bone marrow can continue to produces these cells in large quantities for months and even years, at a rate 20-50 times normal
Feedback control of the Macrophage and Neutrophil response
5 dominant factors:
1. Tumor necrosis factor (TNF)
2. Interleukin-1 (IL-1)
3. Granulocyte-monocyte colony stimulating factor (GM-CSF)
4. Granulocyte colony stimulating factor (G-CSF)
5. Monocyte colony stimulating factor (M-CSF)
- formed by activated macrophage cells in the inflamed tissues
Eosinophils in the Innate Immune response
- produced in large numbers when parasitic infection is present
- cannot phagocytise
- attach themselves to the parasites by way of special surface molecules and release substances that kill many of the parasites by way of the following:
1. releasing hydrolytic enzymes from their granules which are modified by lysosomes
2. releasing highly reactive forms of oxygen that are lethal to parasites
3. releasing a highly larvacidal polypeptide called major basic protein - collect in the tissues in which allergic reactions occur, such as the peribronchial tissues of the lungs in people with asthma
- the mast cells and basophils release an eosinophil chemotactic factor that causes eosinophils to migrate toward the inflamed allergic tissue
- detoxify some of the inflammation inducing substances released by the mast cells and basophils
- phagocytise allergen-antibody complexes to prevent the spread of the local inflammation process
Basophils in the Innate Immune response
- liberate heparin into the blood which prevents blood coagulation (clotting)
- also release histamine, and smaller quantities of bradykinin and serotonin
- plays an important role in some types of allergic reactions
- the type of antibody that causes allergies, immunoglobulin E (IgE), has a special propensity to attach to mast cells and basophils
- when the antigen for this antibody attaches to it, it causes the basophil to rupture and cause a large release of histamine, bradykinin, serotonin, heparin, slow-reacting substance of anaphylaxis and a number of lysosomal enzymes
- these substances cause many of the allergic manifestations
