31.2 Acute Inflammation Flashcards
What is acute inflammation?
Short-term, beneficial response in which a heightened immune state is locally induced in response to stimulation of the innate (rarely adaptive) immune system by a pathogen.
What is the time course for acute inflammation?
minutes to days
How does acute inflammation fight pyogenic bacterial infection?
Neutrophil recruitment –> to facilitate phagocytosis
What are the cardinal features of acute inflammation?
*Heat (vasodilation)
*Redness (vasodilation)
*Swelling (oedema)
*Pain (chemical mediators)
What are some of the cells that can begin acute inflammation at the site of infection/damage?
Resident macrophages, mast cells and dendritic cells.
What are the cells that are released into the site of infection as part of the acute inflammatory exudate?
Rapidly recruited:
- Neutrophils
- Platelets
Recruited soon after:
- Monocytes (that become macrophages)
- T-lymphocytes
If the site is of allergic inflammation:
- Eosinophils/Basophils
What are the histological differences between healthy and inflamed tissue?
*Vasodilation - increased vessel size + increased presence of blood cells
*Oedema - cells more spread out/ less concentrated on slide.
*Cellular infiltration by leukocytes
What are the different proteins that are released into the site of infection as part of the acute inflammatory exudate?
- Albumin
- Antibodies
- Complement proteins
- Coagulation factors
What is responsible for much of the swelling at sites of acute inflammation?
Albumin released as part of the exudate leads to osmotic pressure that pulls water in.
What are the main stages of leukocyte recruitment in inflammation?
- Margination
- Leukocyte rolling
- Leukocyte activation
- Leukocyte tight adhesion
- Diapedesis (Leukocyte extravasation)
- Chemotaxis
Describe the concept of how neutrophils and macrophages know where to go during acute inflammation.
- The cells at the site of inflammation send out cytokines and other molecules to partially activate neutrophils and macrophages passing through an adjacent venule
- This triggers them to cross the blood vessel barrier
- Once crossed, the neutrophils and macrophages follow chemokines, pathogen molecules and other molecules to the site of infection
- Here they are fully activated
What is pus?
An accumulation of fluid, living and dead white blood cells, dead tissue, and bacteria or other foreign invaders or materials.
What is an abcess?
- A localized collection of pus in any part of the body, often caused by an infection by pyogenic bacteria.
What are the different possible fates of an abscess, in order of desirability?
- Resolution with no scarring
- Resolution with scarring
- Rupture external
- Rupture internal -> Leading to a septic embolus
- Cyst formation (neutral outcome)
What are the best treatment options for abscesses?
Surgical drainage and antibiotics
What makes abscesses difficult to treat?
Their interiors have little or no vascularization, making it difficult to deliver therapeutic agents effectively.
How does pus form?
During immune response to a pathogen, neutrophils and surrounding tissue die accumulating as pus. This collects in a cavity created by the breakdown of tissue (Abscess).
Describe the appearance of neutrophils.
- Multi-lobed nucleus joined by filaments (sausage-like appearance)
- About 10µm
- Has some granules
What percentage of leukocytes are neutrophils?
40-70%
Where does proliferation and differentiation of neutrophils take place?
In the bone marrow.
Where is the reserve pool of neutrophils?
Bone marrow (50% of the nucleated leukocytes in bone marrow are PMNs)
When are neutrophils mobilised from the reserve pool in bone marrow? What is this called?
- In response to infection
- This is called granulocytosis/neutrophilia
Are neutrophils terminally differentiated?
Yes, so they do not synthesis DNA, although they do synthesis some limited mRNA and proteins.
What could an increased neutrophil count indicate?
Bacterial infections and stress.
What are the actions of neutrophils? (5)
- Increase their numbers during acute bacterial infection
- Adhere to vascular endothelium –> migrate into tissues in acute inflammation
- Release granules - contain antibacterial molecules
- Generate toxic mediators derived from H2O2
- Phagocytosis - ingest, kill and digest bacteria (produce pus)
What is diapedesis?
Passage of leukocytes through intact capillary walls
How does leukocyte (neutrophil and monocyte/macrophage) activation occur in acute inflammation?
- It occurs via GPCRs on the leukocytes
- The molecules that stimulate these mostly include cytokines, C5a, PAF and others
Note that partial activation occurs during recruitment of the leukocytes, while complete activation occurs at the site of inflammation.
What does (partial) activation of leukocytes during recruitment in acute inflammation do?
- Leads to altered conformations of cell surface integrins on the leukocytes
- The integrins can now form tight interactions with cell adhesion molecules on endothelial cells
- Thus, the leukocytes adhese to the vessel wall
Describe how tight adhesion of leukocytes to the endothelium occurs (during acute inflammation).
ICAM interactions:
- Integrins on the leukocyte surface undergo a conformational change upon activation of the leukocyte
- This allows the integrins to bind to cell-adhesion molecules on endothelial cells
- This ICAM interaction is necessary to halt leukocyte rolling and start diapedesis
(NOTE: The diagram also shows the selectin interactions involved in leukocyte rolling)
What are chemokines?
A family of small cytokines that can induce chemotaxis in nearby responsive immune cells.
What is chemotaxis?
Movement of a cell in a direction corresponding to a gradient of increasing or decreasing concentration of a particular substance.
What receptors on leukocytes detect chemoattractants?
GPCRs
What are some chemoattractants involved in attracting leukocytes to the site of inflammation? What are their receptors?
- Bacterial products, namely N-formylmethionine (fMLP - an amino acid used to initiate protein synthesis in bacterial cells) -> mFLP receptor
- Cytokines and chemokines (e.g. IL-8, CCL5, etc.) -> IL-8 receptor, CCR5, etc.
- C5a (a chemoattractant product of complement) -> C5a receptor
- Platelet-activating factor (PAF) -> PAF receptor
Describe how leukocytes move during chemotaxis and describe their appearance.
- It moves by cytoskeleton remodelling
- The protrusions at the front are called pseudopods, while the tail at the back is called the uropod
Describe the process of phagocytosis
*Opsonisation by Ab and complement
*Lysosomal fusion
*Killing and digestion of micro-organism
What are the different phagocyte killing mechanisms?
*Respiratory burst (NADPH oxidase)
*Non-oxygen-dependent killing mechanisms (antimicrobial peptides, e.g. LL-37)
*Neutrophil Extracellular Traps (NETs)
What is the respiratory burst and why is it required?
- Following uptake of pathogens for phagocytosis, the oxygen demand of phagocytes increases.
- The oxygen burst is used largely to generate NADPH from glucose via the pentose phosphate pathway. The NADPH in turn can be used to generate reactive species that kill pathogens.
What are antimicrobial peptides and what is their function in phagocyte defence mechanisms?
- Peptides (AMPs) of fewer than 50 amino acids that are an evolutionarily conserved part of the innate immune response
- Primarily target the cell membrane and create transmembrane channels, meaning that they have broad specificity within bacteria and other pathogens.
- They are not affected by antibiotic resistance of a bacterium.
- Experimentally, the broad specificity of AMPs can be demonstrated by an in vitro assay.
What are NETs?
- Neutrophil extracellular traps
- Neutrophils have a short lifespan (relative to macrophages), but before and during apoptosis they leave an extracellular net of DNA and histones, with proteins such as MPO bound to them.
- These bound proteins have antimicrobial properties, so the net functions to kill pathogens outside of cells.
Give examples of defects in phagocytic killing mechanisms.
Chronic granulomatous disease, complement deficiencies.
What are some functions of neutrophils?
- Phagocytosis
- Degranulation (of anti-microbial agents)
- Production of NETs
- Mediation of inflammation (via release of cytokines)
What are some functions of macrophages? How do these compare to neutrophils?
Defence:
- Phagocytosis and killing
- Control of inflammation -> Via cytokines and interferons
- Antigen presentation
Tissue maintenance:
- Tissue homeostasis via scavenger receptors
- Tissue remodelling
- Apoptotic cell clearance
- Tissue repair (e.g. in wound repair)
Are all macrophages derived from blood monocytes?
No, some tissue resident macrophages are derived from myeloid precursors in embryonic life and proliferate in situ throughout adult life.
Describe the two origins of macrophages.
- Embryonic yolk sac derived tissue macrophages (long-lived, self-renewing)
- Macrophages that are formed by bone marrow-derived monocytes that infiltrate the tissue
Describe the two functional types of macrophages and what the function of each is.
- Tissue resident (e.g. Kupffer cells) macrophages (mostly arising from embryonic yolk sac) -> Mediate homeostasis, repair and remodelling
- Infiltrating monocytes that become inflammatory macrophages -> Antimicrobial functions