Host response to extracellular infections Flashcards
Explain how a host responds to extracellular bacterial infection?
How is the immune system altered to extracellular bacterial infection?
First, the local macrophages become activated by ligation of patter recognition receptors (TLR4). Then, they activated macrophages secrete inflammatory cytokines (TNF-alpha and IL-1beta) (and chemokines; CXCL8), so that these cytokines can activate the endothelium in local blood vessels. Finally, the activated endothelium recruits phagocytes (monocytes & neutrophils) to the infected tissue.
Which cell types and receptors are involved in the innate respond to the extracellular bacterial infection?
Cells: tissue-resident macrophages, dendritic cells, and mast cells
Receptors: extracellular pattern recognition receptors: TLRs (TLR4)
Describe mechanisms of detection and of altering for different cell types in the innate response to an extracellular bacterial infection?
Detection: the bacteria is recognised by specific, surface molecules on the pathogen. Ex: TLR4 recognises LPS (G- bacteria)
The mechanisms of altering different cell types are the secretion of cytokines and chemokines. The mast cells granules contains high levels of cytokines which lead to a faster response to the infection. Whereas, the macrophages and DCs must transcribe new cytokines as a response to the infection.
What is inflammation contribute to eliminate infection with extracellular bacteria?
Inflammation contributes to the enhancement of phagocytosis by opsonization and acute-phase proteins (CRPs). In addition to activation of the NETosis by the recruited neutrophils.
Opsonization: the Fc receptors binds to bacteria and serves as "labels" that enhances phagocytosis. Phagocytes bind to the Fc portion of the bound antibodies with a receptor for IgG antibodies. The, complement protein contributes by activation of the complement system which results in deposition of complement factor C3b on the bacterial surface. Phagocytes express complement receptors that bind to C3b. Thereby, opsonization of the bacteria with C3b leads to an enhanced phagocytosis.
Acute-phase proteins: C-reactive proteins (CRPs) binds to bacteria for enhanced phagocytosis, opsonise and activate the complement system. CRPs are produced in the liver (by hepatocytes) in response to inflammatory cytokines (IL-6) that are secreted by macrophages and enter the blood.
How does opsonization contribute to elimination of extracellular bacteria?
Opsonization contributes to elimination of extracellular bacteria by the enhancement of phagocytosis through FcR/antibodies and/or complement proteins.
What is the relationship between tissue resident macrophages and monocytes? How early in life are tissue resident macrophages established in peripheral tissues?
Tissue-resident macrophages: represents 20-25% of leukocytes in humans. They are self-renew within the tissue. These are derived from embryonic-origin cells, which are selected to sites of the body before birth. Ex: Kupffer cells (liver) and Langerhans skin cells. Traditionally, macrophages are developed only from circulating monocytes.
Monocytes: represents ca. 10% of leukocytes in humans. They are developed from haematopoietic cells in the bone marrow and circulate in the blood for 1-2 days, after which they die unless recruited to tissues for differentiation. They, are the precursor cells for some macrophages and dendritic cells.
Describe phagocytosis of bacteria. What type of receptors are involved?
It starts with recognition and attachment: phagocytes (macrophages and neutrophils) recognise bacteria through pattern recognition receptors that identify pathogen-associated molecular patterns (PAMPs) on the surface of bacteria. Opsonins, which are proteins like antibodies and complement factors, can coat bacteria, enhancing their recognition and binding to phagocytes via specific receptors (Fc receptors for antibodies; Scavenger receptors for charged molecules (polysaccharides, lipids, DNA (CpG motifs), etc.).
Once the phagocyte binds to the bacteria, it extends pseudopodia (membrane extensions) around the bacterium, eventually engulf it. The bacterium is internalized into a membrane-bound vesicle called phagosome. Then, the phagosome undergoes a series of maturation steps, where it fuses with lysosomes (organelle containing digestive enzymes and antimicrobial substances). This fusion forms a phagolysosome, where the internal environment is acidic and enzymatically active (including; reactive oxygen species (ROS), nitric oxide, and various hydrolytic enzymes). These agents kill and break down the bacteria into smaller components. After the bacteria is degraded, the residual debris is expelled from the phagocyte through exocytosis, clearing the waste from the cell.
How are neutrophils recruited to infected tissues?
When they are “outside” of the infected tissue, they use their expressed adhesion molecules (integrins and selectins) to bind to the epithelium. It is the chemokines on the surface of the endothelial cells that signals changes in integrin formation that allow high-affinity interactions with its counter-ligand. Thus, facilitating cellular arrest under conditions of blood flow.
When inside the tissue: they use the G protein-coupled receptors which detect endogenously (CXCL8, cytokines, DAMPs) or exogenously (MAMPs) derived motifs to migrate towards the site of injury. Chemokines can be tethered to the extracellular matrix via hairpin groups and from a trail along which cell can migrate (haptotaxis).
What is NETosis, and how does it contribute to elimination of extracellular bacteria?
NETosis is an alternative cell death orchestrated by the neutrophils, a district cellular program from apoptosis or necrosis.
It involves several distinct and sequential morphological changes in the neutrophils, including: the characteristic lobulated architecture of the nucleus is lost, the nuclear and granular membranes become permeable, histone inactivation leads to chromatin expansion into the cytosol, chromatin mixes with granule content, the loss of internal membranes leads to the disappearance of cytosolic organelles. And the plasma membrane becomes permeable and the NETs are released into the extracellular space. The bacteria are killed by the NETs due to toxic components that destroy the DNA.
What are the components of neutrophil extracellular traps (NETs)? What does each component do?
Components of neutrophil extracellular traps (NETs): Histones (H2A, H2B, H3, H4) are important in the condensation of the chromatins, Granule proteases (alpha-defensin, lysozyme C) which remain enzymatically active even when the NET is exposed to endogenous protease inhibitors, Myeloperoxidase (MPO) generates multiple oxidants to kill pathogens and binds to extracellular matrix proteins, plasma (lipo)proteins, etc., lactotransferrin function as an intrinsic inhibitor of NETs release into the circulation, cytoplasmic and cytoskeleton proteins (calcium-binding proteins, actin, myosin, and cytokeratin) forms the network of the web that traps the bacteria.
What factors determine whether a neutrophil will use phagocytosis or NETosis as an effector mechanism?
The microbe size-sensing mechanism and location: small microbes are readily taken up by phagocytosis are poor NET stimulants because the phagosome fuses with neutrophil azurophil granules, and neutrophil granule protease are thus unavailable for initiating NETosis. Whereas, some microbes are too large to be engulfed by phagocytosis (ex: C. albicans that forms large filamentous hyphae or aggregates of M. bovis). If a neutrophil encounters these large microbes, the azurophil granule protein NE is released into the cytoplasm to initiate NET formation. Leading to neutrophil deficient in dectin-1 (a receptor that mediates phagocytosis of fungi) are hindered in their ability to determine which antimicrobial defence mechanism to apply. Thus, resulting in uncontrolled NET release.
Noncanonical inflammasome machinery: gram-negative bacteria that escape the phagosome to translocate to the cytosol induce NETs. Thereby, evading phagocytosis.
Metabolic state of the host - for ex: lysosomal efficiency.
Describe mechanisms used by macrophages and neutrophils to kill bacteria.
Macrophages: uses phagocytosis; produces ROS (superoxide that can be converted into variety of different ROS, all of which are toxic to some degree. It is considered the key killing mechanism for both macrophages and neutrophils, and it is important for clearance of S. aureus) and RNS (production of NO radicals is catalysed by inducible nitric oxide synthase (iNOS). Upon reaction of NO with superoxide, peroxynitrite is formed which is toxic to phagocytosed microbes’ proteins and DNA); phagosomal acidification enhanced by enzymes (acidification is generated by an influx of protons in to the phagosome by vacuole-type proton transporting ATPase, which is present in phagosome membranes. The activation of ATPase reduces the pH of endosomes and lysosomes. Fusions of endosomes and lysosomes, which are enriched with ATPases is an important part of phagosome maturation, the continued delivery of ATPase causes increased acidification throughout sequential stages of phagosome maturation); NADPH oxidase (NOX2) is located in the phagosome membrane and when its assembly is induced by signals from the FcR-gamma and macrophage-presented antigen, it catalyses super-peroxidase production and ROS resulting in oxidation burst; iNOS is only expressed in response to inflammatory stimuli, with IFN-gamma being the key cytokine required fro iNOS induction in macrophages; Hydrolytic enzymes (proteases (cathepsins), lipases, phosphatases and glycosylases) has their optimal efficacy in acidic conditions. Ex: Cathepsins is located in the lysosomal compartment and kill bacteria (S. aureus) by direct proteolytic damage; Antimicrobial enzymes - ex: phospholipases.
Neutrophils: phagocytosis - the physical uptake of a microbe by neutrophils forming a pinched off intracellular cytoplasmic membrane-bound compartment called phagosome. These are a physically walled off vesicle in which the neutrophil can focus antimicrobial activity while limiting collateral cell damage. Granules fuse with the phagosomal membrane, delivering soluble or membrane-embedded effector proteins onto or in close proximity to the compartmentalized microbe. The NADPH oxidase (higher in neutrophils than macrophages) complex is embedded within the phagosome membrane-granule bilayer and produces intra-phagosomal superoxide, which fuels the heme-bound enzyme myeloperoxidase to generate injurious chemicals like hypochlorite (bleach); NETosis; degranulation and exosomes; antimicrobial peptides (AMPs) which are positively charged and thus damage the membrane of pathogens.
Describe the role of dendritic cells in the initiation of adaptive immune responses to extracellular bacterial infection.
The role of DCs in the initiation of adaptive immune response to extracellular bacterial infections is to take up bacterial antigen (engulf the bacteria) and migrate to secondary lymphoid tissue (lymph nodes) and maturate/differentiate due to recognition signals, cytokines and/or a combination of the signaling. The mature DCs express high levels of MHC-II molecules and are effective at activating CD4+ T cells (in vesicles). They can also activate the cross-presentation involving the MHC-I molecules activating the CD8+ T cells (in cytosol).
How do antibodies contribute to eliminate extracellular bacteria?
Antibodies that bind to bacteria serves as “labels” that enhances phagocytosis. This is known as opsonization. Phagocytes bind to the Fc-portion of the bound antibodies with a receptor for IgG antibodies. Opsonization enhances phagocytosis.
They can also activate the complement system, induce neutralization by neutralizing toxins from the pathogens, perform antibody-dependent cellular cytotoxicity (ADCC) where NK cells recognize virus-antibody on an enveloped virus, and aiding mast cells where the bacteria-bounded IgE has an high affinity to FcR leading to the absorption of the IgE into the cell membrane and onto the cell surface, resulting in annulation of the pathogen.
