Innate Immune Protection

• Innate immunity is a non-specific defence mechanism that a host uses immediately or within several hours (the exact time range being 0-96 hours) after exposure to a foreign antigen.

  A response in:

  • 0-4 hours is from innate immunity
  • 4-96 hours is an early, induced response from innate immunity
  • longer than 96 hours is a response from adaptive immunity.
• • The innate immune system is a very fast immune system one is born with. It is present in all organisms.
  • The innate immune system responds the same way each time an infection occurs (doesn't gain a memory).
  • The innate immune system uses a handful of molecules to recognise that an infection is present, since the innate immune system needs to distinguish between self and non-self material.
  • The innate immune system induces and directs the adaptive immune response.
• The innate immune system is composed of:

  • Physical/Anatomical barriers
  • Secreted compounds
  • Cellular components
• The physical/anatomical barriers of the innate immune system are the skin, GI tract, respiratory tract, and mucosal epithelia. These barriers are present at all times, and prevent the pathogen from entering the body.
• The secreted compounds of the innate immune system are antimicrobial compounds, complement, natural antibodies, and cytokines. These compounds are secreted in the presence of an infection.
• The cellular components of the innate immune system are phagocytes and natural killer (NK) cells. These cellular components of innate immunity eliminate/destroy the pathogen.
• The skin is an effective physical barrier against infections as it has a dry and thick protective outer layer of keratin.

  Under normal circumstances, the skin is a hard physical barrier for pathogens to penetrate. As a result, pathogens can only typically get past the skin through cuts or breaks in the skin.

  The skin also produces antimicrobial compounds, and these compounds are always present on the skin. An example of such a compound is Psoriasin, which protects the body from the bacteria E.coli.
• The respiratory tract is an effective physical barrier in the innate immune system because in the respiratory tract:

  • Tight junctions make it difficult for pathogens to enter the body via in between the cells.
  • Mucus present in the airways traps pathogens and prevent then from getting into the lungs.
  • Cilia are continuously moving, and they waft the mucus containing the pathogens up to the throat where they can be removed either via coughing, or swallowing into the stomach.
• The GI tract is an effective physical barrier of the innate immune system because of its peristaltic contractions, which keep the bacteria moving. Within the stomach, hydrochloric acid is produced, and this creates a very acid, low-pH environment that pathogens typically can't survive in.
• Tears, sweat, an saliva are secreted compounds that contain lysozymes, and these enzymes can destroy bacterial cell walls.

  Blinking also prevents pathogens from entering the body through the eyes.
• Within the gut and skin, there are already bacteria, and these bacteria will pose competition to any foreign bacteria over space, water, and nutrients.
• To recognise foreign pathogens, the innate immune system recognises a few highly conserved molecular structures present in many different microorganisms.

  These conserved molecular structures are called Pathogen Associated Molecular Patterns (PAMP).
• For a molecule to be considered a PAMP, it must be present in the microorganism, but not the host, and the molecule has to be essential for the survival of the pathogen. This is because some pathogens, such as the influenza virus, can undergo genetic mutations when under the pressure of the innate immune system to increase their chance of survival. However, PAMP molecules, being necessary for survival, can't be mutated if the pathogen is to continue to survive. Hence, they can be used in immunorecognition.
• PAMPs are recognised by specific receptors known as Pattern Recognition Receptors (PRR). When a PRR binds to a PAMP, a signal is sent to the nucleus of the cell to increase the rate of production of molecules necessary for immunity.
• There are 3 types of pattern recognition receptors (based on their location).

  • Collectins (soluble, and present in the tissue fluid).
  • Toll Like Receptors (membrane-bound to the cell surface membrane, and on membranes of organelles).
  • Nod Like Receptors (free in the cytoplasm
• Collectins are a family of PRRs that are present in solution outside the cells. They have a collagen-like region that interacts with the effector parts of the immune system, and a lectin region, which binds to sugar molecules on the surface of the pathogen.

  An example of a collectin is mannose-binding lectin (MBL). The lectin region binds to mannose molecules on the pathogen surface. While mannose is also present on host cells, MBL will only bind to mannose when they are arranged with the correct spacing for the binding to occur. This arrangement is only present on pathogens.
• Toll Like Receptors (TLR) are receptors bound to membranes, and they have at least 10 mammalian homologues that recognise different PAMPs.

  Examples:

  • TLR 1 and TLR 2 recognise Gram-positive bacteria
  • TLR 4 recognises Gram-negative bacteria
  • TLR 5 recognises flagellin - the protein used in flagella.
  • TLR 9 recognises unmethylated CpG DNA (pathogenic DNA is unmethylated, whereas host DNA is methylated).
• NOD (Nucleotide Oligomerisation Domain) Like Receptors (NLR) are PRRs located in the cytoplasm, and are particularly adept at recognising Gram-positive and Gram-negative bacteria.
• The entire cell is covered inside and out by PRRs to recognise PAMPs and trigger an immune response. Once PAMPs are recognised, signals are sent to the nucleus of the cell to upregulate the production of molecules important to the innate immune response. Such molecules include cytokines, MHC molecules, and costimulatory molecules.
• Once a pathogen has been recognised, that innate immune system has 4 effector mechanisms to deal with the pathogen:

  • complement
  • phagocytosis
  • cytokines
  • activation of adaptive immunity.
• • Complement is a series of proteins that are synthesised in the liver in response to inflammation (innate immune action) and are released into the bloodstream to circulate in the blood and tissue fluid.
  • Complement operates as an enzyme cascade.
  • There are 3 complement routes: the Classical, the MBL, and the Alternative. However, all of these 3 routes will result in the production of C3 convertase.
• In the complement effective mechanism, C3 convertase will cleave the protein C3 into two parts: C3a (smaller part) and C3b (larger part).

  C3a diffuses away from the site of infection and binds to a C3a receptor on cells that are needed in the innate immune response - macrophages and neutrophils. C3a recruits these cells to the site of infection.

  C3b molecules make pathogens more of a target to macrophages and neutrophils by coating the surface of the pathogen in a process called opsonisation.
• Terminal components C5b, C6, 7, 8, and 9 form a membrane attack complex (MAC) that inserts into the wall of the bacteria. This forms pores in the bacterial wall that will disrupt osmotic regulation of the bacterial cell, eventually causing it to lyse.
• Phagocytosis is another effective mechanism that involves mainly macrophages and neutrophils.

  The pathogen first detects the pathogen by chemotaxis, causing membrane-bound pseudopodia to be produced. These will engulf the pathogen into membrane-bound vesicles known as phagosomes. A lysosome will then fuse with the phagosome, turning the structure into a phagolysosome. The lysozymes within the vesicle digest the pathogen, and the debris is either released from the phagocyte via exocytosis or is presented on the phagocyte.
• Macrophages are found in the interstitial fluid and mature from monocytes found in the blood. Macrophages are found in large numbers, particularly in the GI tract, lungs, liver, and spleen, and are relatively long-lived.
• Neutrophils are typically only found in the blood unless they receive a signal indicating the presence of a pathogen. This signal prompts neutrophils to leave the blood and go to the site of infection in the tissues. Neutrophils are short-lived compared to macrophages.

  (Pus that leaks out from wounds is made up of dead neutrophils and their dead bacterial debris).
• Not all pathogens are killed by phagocytosis, so macrophages and neutrophils have other mechanisms they employ to kill pathogens. These mechanisms involve oxygen radicals and nitrogen radicals.
• Reactive oxygen intermediates/oxygen radicals are mainly employed by neutrophils.

  Following phagocytosis, there is a significant increase in oxygen uptake by the cell, and this is called a respiratory burst. In the respiratory burst, oxygen is reduced by an enzyme called NADPH oxidase found on the cell surface membrane of the phagocyte. The reduction of oxygen forms the oxygen radicals.
• NADPH oxidase doesn't exist as a full complex when it's active. When in the inactive state, 2 subunits are embedded in the membrane, and the rest reside just underneath in the cytoplasm. The increase in oxygen uptake due to phagocytosis is the signal that causes the subunits to come together to form the full complex, and for the enzyme to be activated.
• Oxygen radicals damage DNA and cause alterations in bacterial membranes. However, oxygen radicals aren't specific to bacterial cells; they can act on host cells as well, so oxygen radicals are rightly regulated by:

  • ensuring that these oxygen radicals are rapidly converted back into harmless products.
  • producing oxygen radicals close to the bacteria.
• Reactive nitrogen intermediates/nitrogen radicals are mainly employed by macrophages.

  Nitrogen radicals are produced from the reaction of oxygen and L-arginine. This produces NO radicals and citrulline. This reaction is catalysed by the enzyme inducible nitric oxide synthase (iNOS). iNOS is activated/induced by cytokines and bacterial components; the two main activators of iNOS are interferon-gamma and tumour-necrosis-factor. These two activators bind to their respective receptors and in doing so, send a signal to the nucleus to upregulate iNOS expression.
• NO radicals also cause DNA damage and alterations in bacterial membranes.

  Both macrophages and neutrophils can generate oxygen and NO radicals, and generally, both radicals are generated in the innate immune system at the same time, so they operate simultaneously.
• Cytokines are another effector mechanism used by the innate immune system.

  Cytokines are soluble proteins that act as intercellular messengers that direct cellular function and communication. Cytokines bind to specific receptors on cells to instruct the appropriate cells. Cytokines can be activating or deactivating, but in the innate immune response, they are mainly activating.

  Interleukin 1 (IL-1), interleukin 6 (IL-6), an tumour necrosis factor alpha (TNF-alpha) are 3 cytokines involved in triggering inflammation.
• Chemokines are a class of cytokines. They have a particular arrangement of cysteine residues within their structure, and the arrangement of these residues classifies chemokines into 4 families: cxc, cc, cx3, and xc.

  Chemokines are pleiotropic because one chemokine can bind to more than one receptor, and one receptor can bind to many chemokines.

  Chemokines promote inflammation by enabling cells to adhere to the surface of blood vessels and have them migrate to the infected tissue.
• Interferons are another group of cytokines, and there are 2 types: Type 1, and Type 2.

  Type 1 interferons are known as IFN-alpha and IFN-beta. They are produced in response to virally infected cells.

  Type 1 interferons active Natural Killer (NK) cells. NK cells kill virally infected cells and tumour cells, produce IFN-gamma, and are responsive to TNF-alpha.

  Type 2 interferons are the IFN-gamma molecules, which are important in activating macrophages before the adaptive immune response is activated.
• Cytokines and chemokines are important for directing macrophages and neutrophils to where the bacteria and infections are located, and then in instructing these cells what to do. Without cytokines and chemokines, there would be no innate immune response.
• Activating the adaptive immune response is another effector mechanism of the innate immune system. This is done by a naïve T-cell being activated into an effector T-cell. This activation requires 2 signals:

  • recognition of small, immunogenic molecules present on the surface of antigen-presenting cells (such as on a phagocyte after phagocytosis).
  • A costimulatory molecule generated following the recognition of PAMPs by PRRs.

  There are many types of effector T-cells. The cytokine used will determine what type of effector T-cell a naïve T-cell will turn in to.
• Antigen present cells (APC) include macrophages, B-cells, and Dendritic cells.
• Dendritic cells (DCs) are the most important APC in the innate immune system, as they are the best at activating naïve T-cells.

  DCs mostly exist as immature cells. Immature DCs are mostly round and have a lot of molecules important in presenting antigens, but these molecules aren't displayed on the cell surface. However. immature DCs do display a lot of PRRs.

  When a PAMP of a pathogen binds to a PRR, that will trigger the DC to mature. A mature DC has many dendrites and expresses a large number of molecules for T-cell activation, such as MAC molecules and costimulatory molecules.
• The function of the innate immune response is:

  • recognise the pathogen (with PRR and complement receptors).
  • ingest the pathogen (via phagocytosis and opsonisation by complement).
  • recruit cells using cytokines and chemokines.
  • Induce a specific immune response with the production of IL-12, IFN-gamma, and APCs.