IMM250F 2020 Lecture 3
The acute inflammatory response
Cellular and humoral mediators of innate defense
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Cellular and humoral mediators of innate defense
Cells of the innate system (phagocytes, degranulating cells, NK cells)
• phagocytosis
Secreted mediators of the innate system (cytokines, complement system)
All immune cells are formed from the hematopoietic stem cell
Multipotent, can self-renew
After entering tissues, pathogens are recognized by PRRs expressed on phagocytic sensor cells: macrophages and dendritic cells
Macrophage
• Macrophages and dendritic cells express signaling receptors and receptors that induce phagocytosis.
• Some receptors directly recognize microbial component, some recognize host molecules (called opsonins) deposited on pathogens
Dendritic cell
present at the front-lines of the host prior to infection
Image: top, Icons from Biorender.com, bottom: L facilitate phagocytosis
• Opsonization prepares a particle for phagocytosis by coating it with host molecules. Phagocytes have receptors that recognize these host molecules and can mediate internalization.
• The main opsonins are antibodies and complement fragments (e.g. C3b) that bind to the microbial surface. Antibody- and C3b-coated pathogens are recognized by complement receptors and Fc receptors, respectively, expressed on phagocytic cells.
• Other molecules that act as opsonins are produced during the acute phase response (e.g. C-reactive protein, MBL)
Lclemenza. Created with Biorender. com
Macrophages
Phagocytes
Dendritic cells
Neutrophils
Macrophages are found in many tissues. Some resident macrophages have different names
Blood monocyte
Monocyte/Macrophage
• Derive from “monocytes” in the blood (except for self-renewing types, previous slide)
• When monocytes reach the tissue they differentiate into “macrophages”, flat with lots of protrusions to seek out pathogens
• Ingest and kill pathogens, remove aging, dying and dead cells (recognition of DAMPs)
• Produce inflammatory cytokines
• APC for activated T cells
• Longer life than neutrophils, sustain inflammatory response for several days
Resident in mucosal and cutaneous tissues àmonitor major routes of pathogen entry
Huge extensions or “dendrites
Major roles:
• PRR-mediated uptake of pathogen
• Degrade pathogens
• Travel via lymphatics to local lymph nodes for
antigen presentation to naïve T cells
DCs are main ‘antigen presenting’ cell type for naïve T cells
àkey to ‘priming’ adaptive immunity
Innate and adaptive immunity work together to clear infections
Neutrophils
The first leukocytes recruited by inflammation
• Highnumberinblood(50-70%of circulating leukocytes)
• Alsocalledpolymorphonuclear leukocytes (‘PMNs’)
• Mainrole:Gettositeofinfection rapidly, enter tissues, use phagocytic receptors to ingest and kill extracellular microorganisms.
• Cytoplasmisfullofgranulesthat contain toxic products to kill pathogens
• Aftertakingupmicroorganisms and releasing their granules, the neutrophil will die
Neutrophil ingesting S. aureus
NIAID / CC BY (https://creativecommons.org/licenses/by/2.0)
Neutrophil ingesting
Neutrophil migration to site of injury
Courtesy of Dr. , University of Calgary
Phagocytosis and intracellular killing of pathogens
Neutrophils can kill microbes through a number of different mechanisms
Neutrophils phagocytose bacteria and kill them within the phagosome:
– Antimicrobial peptides (lecture 2)
– Lysozyme (an enzyme that
degrades bacterial cell walls,
lecture 2)
– ROS (through the respiratory burst,
next slide)
Netosis. Formation of NETS that serve to trap bacteria, facilitating their killing (slide 17).
Formation of Reactive Oxygen Species (ROS) in phagocytes is mediated by the enzyme complex NADPH oxidase (Respiratory burst)
ROS are powerful oxidizing factors that kill pathogens
Defects in killing bacteria: Chronic granulomatous disease (CGD)
• Mutations affecting NADPH oxidase (the multi-subunit enzyme that generates ROS). Most common mutation is X-linked thus disease mainly affects males.
• Neutrophils and macrophages from CGD patients can’t undergo the respiratory burst to generate ROS
• Microbes cannot be killed and patients have recurrent infections
• ‘Granulomas’ are formed (in lungs, lymph nodes, skin, bowel)
Ø Cyst-like formations of immune cells surrounding un-digested microbes
Ø Microbial clearance has failed so the body ‘walls off’ the microbe to prevent spread
“Dying for a cause”: Neutrophils die and trap pathogens in NETs
• NETs = neutrophil extracellular traps, chromatin structures loaded with antimicrobial proteins
• As neutrophil dies it extrudes its DNA
• DNA is very sticky and traps bacteria, killed by the antimicrobial proteins. Macrophages clean up the area
Dying neutrophil
Phagocytes – Summary Phagocytes include:
Macrophages
Main functions: phagocytosis and killing of pathogens, cytokine production, antigen presentation
Dendritic cells
Main functions: phagocytosis and cytokine production, best antigen presenting cellèbridge innate and adaptive immunity
Neutrophils
Also classified as Granulocytes
Main functions: phagocytosis and killing of pathogens, NET formation
Three types of granulocytes
Neutrophil Eosinophil Basophil
Cytoplasmic granules stain differently in the three different cell types with the hematoxylin-eosin stain commonly used in blood smears
Some pathogens cannot be handled by phagocytosis as a defense
Copyright © 2006 Elsevier Inc.
Photo credit: E.M.Unit, Royal Free Hospital School of Medicine/Wellcome Trust Photo Library
What about a pathogen that is extracellular, but very large; much larger than a phagocytic cell?
(e.g. parasitic worms)
Phagocytosis is suited for microbes that are small and extracellular
Other Innate Cells – “degranulating cells”
Mast cells
• Contain granules but historically not classified
as granulocytes because not found in the
• Reside in tissues
• Involved in parasitic infections and allergy.
Release histamine and other mediators
• Least common of the innate immune leukocytes
• Are granulocytes like neutrophils- lots of toxic
substances contained in granules. Produce
• Involved in defense against parasites
• Recruited from blood during allergic responses
Eosinophils
• Also granulocytes
• Involved in parasitic infections and allergy
• Granules contain toxic cationic proteins
Degranulation products are responsible for immediate allergy symptoms
Pixaby, PublicDomainPictures Pixaby, , PublicDomainPictures
• Designed to eject parasites from the body, but in response to harmless entities cause
symptoms of allergy
• Immediate effects due to release of histamine and other preformed (esp. by mast cells)
• Cause vasodilation, mucus production, itching, gut motility
FceRI-mediated degranulation of mast cells
Degranulation of mast cells takes place within seconds of IgE-mediated antigen recognition because mast cells are pre-loaded with IgE attached to their Fc receptors
Suggested Prompt for Graded Discussion 3
(Remember: lecture prompts are optional. You can choose your favorite lecture 3-related topic for your post on Graded Discussions)
“Why might neutrophils play a less important role in host defense against intracellular bacteria than extracellular bacteria?”
Soluble mediators of defense
• Cytokines
• Complement
• Antiviral response: Interferons
Infection (or trauma) triggers an acute inflammatory response
Controls the production of soluble mediators of innate immunity (among other things!) – one class of soluble mediators are CYTOKINES which trigger an acute inflammatory response
• Release of PAMPs/DAMPs
• Detected by PRR on sensor cells
• Activates signal transduction
• Activates NFkB
General scheme of an acute inflammatory response
(sentinel cell)
Bacterial infections or wounds activate epithelial cells and resident effector cells via PRRs to secrete cytokines
(modified)
Redness, heat, swelling and pain: cardinal signs of inflammation. Recruited inflammatory cells in turn release other mediators that amplify the response
Inflammatory cytokines induce vascular changes in acute inflammation
Cytokines and complement factors act on vascular endothelial cells:
– dilation of vessels
– Increase permeability
– Increase expression of adhesion molecules that trap white blood cells
These changes allow escape of white blood cells so they can migrate to injured or infected site
Inflammation is triggered by cytokines and chemokines released during an immune response
Cytokines:
• A diverse collection of soluble proteins that, like hormones and neurotransmitters, are used extensively in cellular communication, particularly between leukocytes
• Modulate the behavior of cells at extremely low concentrations
• Act in general at low range but some can act systemically
• Pleiotropic – an individual cytokine can have many biological
activities
• Includes interleukins, interferons, chemokines, growth factors
Chemokines:
• A family of small chemotactic cytokines that act to attract other cells to the site where they are produced
Pro-inflammatory cytokines include:
• IL-1β, IL-6, IL-12, IL-18, tumor necrosis factor (TNF), others.
• And IL-8, also known as CXCL8 (a chemokine)
General properties of cytokines and chemokines
Most cytokines are soluble proteins
– Releasedbyonecell
– Bindtoreceptorsonanothercell(oron the same cell – autocrine effect)
– direct leukocyte migration
Inflammatory cytokines mediate leukocyte extravasation into tissues
Leukocyte extravasation (exit from blood vessels) during inflammation is mediated by increased interaction between cell adhesion molecules on leukocytes and blood vessel endothelial cells.
Some adhesion molecules expressed by endothelial cells are called Selectins and interact with sugars displayed on leukocyte molecules causing “leukocyte rolling”.
Some adhesion molecules expressed by leukocytes are called Integrins (e.g. LFA-1). Integrin binding to their ligand ICAM on endothelial cells allows firm adhesion of leukocytes to the endothelial cells.
Copyright © 2006 Elsevier Inc.
Endothelial cells up-regulate selectin expression in response to inflammatory mediators
e.g. CXCL8 (IL-8)
Leukocytes are recruited to sites of infection in a process called extravasation regulated by inflammatory cytokines, and mediated by cell adhesion molecules
Cytokines released by macrophages act systemically to coordinate the body response to infection
An elevation of acute phase proteins is an indicator of inflammation
Zero indicates the basal level
Acute phase proteins have important immune functions
Complement Movie
The complement system – an enzymatic cascade: an activated complement protease cleaves and activates the next component
The three pathways of activation of complement
The terminal pathway of complement activation and the assembly of the membrane attack complex
Soluble anaphylatoxin
C5 convertase
C5b C6 C7 C8
The generation of the enzyme C5 convertase results in the cleavage of C5 and formation of C5b. C5b triggers the assembly of the membrane attack complex that ultimately leads to the formation of pores in the microbial membrane.
C5a, a powerful inflammatory by- product, is also generated
Assembly of MAC
[You do not need to know the composition of the C5 convertase (it’s in the book if you are curious)]
Complement can enhance inflammation through the actions of the anaphylatoxins C3a and C5a
Complement can directly kill complement-sensitive bacteria and viruses
C3b and C4b act as important opsonins that facilitate the phagocytic process by cells expressing complement receptors
Effector functions of complement
Several Regulatory Proteins Control Complement Activation
(Not for Testing)
C1INH inhibits the activation of C1 complex. Genetic deficiency of C1INH causes Hereditary Angiodema
C1INH dissociates C1q from the C1r/C1s in C1 complex and blocks classical complement activation. Individuals with hereditary angioedema have diminished levels of C1-INH and undergo spontaneous complement activation that results in higher than normal levels of circulating degradation products of C4 and C2.
One C2 by-product is called C2-kinin and has vasoactive properties that cause excessive swelling of several tissues, including life- threatening swelling of the larynx. Recombinant C1INH is given to patients to prevent attacks
The interferon response to viral infections
Several PRRs recognize viral genetic material and start signaling pathways leading to the activation of the transcription factors of the IRF family that control the production of IFN-α and IFN-β. The type I interferons prevent the spread of viral infection by acting on uninfected cells
Antiviral function of type I interferons
Type I IFNs are produced by virus-infected cells. They bind to the IFN receptor expressed by most cell types and induce in them an antiviral state.
Another important function of type I IFNs is their ability to activate natural killer cells (NK cells)
NK cells are cytotoxic cells important during the initial phase of viral infections. Their killing potential is regulated by a balance between activating and inhibitory receptors
Missing-self recognition
What we learned in this lecture
• Cellular effectors of innate responses include tissue-resident phagocytic cells, dendritic cells and macrophages that act as sentinel cells, and neutrophils which are recruited from blood in response to inflammatory cues.
• Extracellular bacteria are killed by phagocytosis: uptake of microbes leads to formation of a phagosome, which fuses with lysosomes, leading to microbial killing due to the action of microbicidal products
• We heard about CGD – a disease impacting infection control because of defective killing of bacteria by phagocytes
• How inflammation develops to control infection. Vascular changes are induced by cytokines released by sensor cells
• The complement system can be activated through three different pathways and provides three main functions: facilitation of phagocytosis through formation of opsonins (C3b), amplification of inflammation (C3a, C5a), and lysis of bacterial cells (C5b-9)
• Early protection against viral infection is largely due to the action of type I IFNs and NK cells
Suggested Prompt for Graded Discussion 3
(Remember: lecture prompts are optional. You can choose your favorite lecture 3-related topic for your post on Graded Discussions)
“What are the hallmark characteristics of a localized inflammatory response? How are they induced by the early innate immune response at the site of infection, and how do these characteristics contribute to an effective innate immune response?”
Enjoy your lectures on adaptive immunity with Dr. Singh starting next week! I’ll see you for the pre-midterm tutorial: Tuesday October 13 at 10am-noon
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