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Welcome to the lecture series
INTRODUCTION TO IMMUNOLOGY
Valerio Izzi, Ph.D.
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Key literature
• Abbas et al. Cellular and molecular immunology. 7th Edition (2012) (Elsevier)
• Abbas et al. Basic Immunology. Functions and disorders of the immune system.
3rd Edition (2011) (Saunders, Elsevier)
• Kindt et al. Kuby Immunology. 6th Edition (2007) (Freeman)
• Original research and review articles
Questions?
RAISE YOUR HANDS!
(don’t be shy)
Get in touch:
[email protected]
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Lesson 1: General view of the immune system, its components and the immune responses
Immunology is a matter of SELF VS NON-SELF DISCRIMINATION
Self: A unique set of molecules
normally expressed by our cells
(MHC) identifies every cell of our
body as “normal, non harmful”.
The immune system WILL NOT
ATTACK CELLS IDENTIFIED AS SELF!
Non-self: A wide set of molecules not
normally expressed by our cells
(ANTIGENS) identifies the cells which
harbor them as “non-normal,
potentially harmful”.
The immune system WILL ATTACK
CELLS IDENTIFIED AS NON-SELF!
The outcome of self recognition is
TOLERANCE.
Errors in self recognition lead to
AUTOIMMUNE DISEASES.
The outcome of non-self recognition
is SURVEILLANCE.
Errors in non-self recognition lead to
INFECTIOUS DISEASES and CANCER.
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Introduction to immunology. Lesson 1
Tasks of the immune system
As multicellular organisms “immersed” in a
living environment, we are currently exposed
to many possible threat to our integrity,
including:
• Bacteria
• Viruses
• Parasites
• Fungi
• Toxins
• Environmental poisons
• Transformed/neoplastic cells
Immunity is defined as resistance to disease, specifically infectious disease. The collection of
cells, tissues and molecules that mediate resistance to infections is called the immune
system, and the coordinated reaction of these cells and molecules to infectious microbes is
the immune response. The physiologic function of the immune system is to prevent
infections and to eradicate established infections.
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Introduction to immunology. Lesson 1
Cells and tissues of the immune system
Leukocytes
Cells
(Each has a form and a specific function)
phagocytes lymphocytes
Tissues
(Each made of equal cells)
Organs
(Each made of different tissues)
Systems
Primary  where leukocytes develop
Secondary  where immune reactions take place
(Each made of similar organs)
Organism
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Introduction to immunology. Lesson 1
Cells of the immune system – The phagocytes
This group includes different types of cells, all of them originating from a common precursor
(the MYELOID lineage precursor) and endowed with phagocytic (“eating”) activity.
Those cells are:
•
The first cellular line of defense of the body
•
Aspecific and inborn – do not discriminate what kind of pathogen they’re dealing with
•
Needed for the proper activation of other, more specific cells (the lymphocytes)
•
Attack pathogens with oxidant molecules and enzymes, and “eat” them
Monocyte
Macrophage
Neutrophil
Basophil
Eosinophil
Mast cell
Granulocytes
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Introduction to immunology. Lesson 1
Cells of the immune system – Other myeloid cells
This group includes cells with none or very poor phagocytic activity, or tissue-specific forms of
the phagocytes found in specific locations of the body
Megakaryocyte
Platelets
Erythroid cells
Tissue-specific phagocytes:
• Kupffer cells (Liver)
• Alveolar macrophages (Lungs)
• Langerhans cells (Skin)
• Histiocytes (Blood vessels)
• Dendritic cells (Lymph nodes)
Red blood cells
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Introduction to immunology. Lesson 1
Cells of the immune system – The lymphocytes
This group includes different types of cells, all of them originating from a common precursor
(the LYMPHOID lineage precursor) and endowed with specific anti-microbial recognition
activity.
Those cells are:
•
The second cellular line of defense of the body
•
Specific and adaptive – they discriminate (at the finest molecular level) what kind of
pathogen they’re dealing with
•
Needed for pathogen eradication and proper orchestration of the immune response
•
Attack pathogens with antibodies and lytic enzymes
T-lymphocyte
B-lymphocyte
Natural Killer (NK) cell
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Introduction to immunology. Lesson 1
Cells of the immune system – Other lymphoid cells
This group includes cells which, though being similar to their common counterpart, have more
specific or restricted activities and which are generally present in specific tissues only.
gd T-lymphocytes
•
•
B1-lymphocytes
Recognize phosphoantigens
Found predominantly in the
intestinal tract and the skin
•
•
NKT cells
• Faster antibody production
• No immunological memory
• Found predominantly in the
pleural and peritoneal cavities
• Recognize glycolipidic antigens
Fundamental for T-cells development
Found predominantly in the mucosae
and the thymus
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Introduction to immunology. Lesson 1
Immune system organs – Primary and secondary
Primary lymphoid organs are those where T- and B-lymphocytes develop.
Those are:
• The bone marrow, where T- and B-cells, as well as all myeloid cells, are born. All immune
cells, apart from T-cells, also mature here.
• The thymus, where immature T-cells (coming from the bone marrow) mature.
Secondary lymphoid organs are those where lymphocytes get activated, and they are
arranged as a series of filtering stations along the lymphatic transit.
Those are:
• Lymph nodes
• Tonsils
• Spleen
• Peyer’s patches
• Mucosa-associated lymphoid tissue (MALT)
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Introduction to immunology. Lesson 1
Hematopoiesis – the production of all blood cells (white and red) in the bone marrow
Hematopoietic
stem cells
Myeloid lineage
Lymphoid lineage
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Introduction to immunology. Lesson 1
Hematopoiesis – a stressful but necessary event for our survival
The hematopoietic process is probably one of the most-demanding tasks for our body. It needs
to:
1) be able to produce a steady number of red and white blood cells every day to supply
oxygen and nutrients to every tissue, and to efficiently patrol the body;
2) be able to boost the production of red blood cells in case of low oxygen tensions and of
leukocytes in case of infection;
3) be able to perform those actions for the whole life of an organism, by saving and keeping
constant a certain amount of hematopoietic stem cells.
It has been calculated that:
• Approx. 1011-1012 cells are produced daily by the hematopoietic process
• Hematopoietic stem cells replicate once each 48-50 weeks, meaning that they replicate
100 times in 80 years.
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Introduction to immunology. Lesson 1
Immunity – innate/adaptive or humoral/cell-mediated?
We already said that immunity is defined as resistance to infectious disease. There are two
different ways to “approach” the study of immunological mechanisms, either by considering
their specificity/ontogenetic derivation or their effector mechanisms.
Specificity and ontogeny:
innate and adaptive
Effector mechanisms:
humoral and cell-mediated
Innate immunity is:
• Aspecific
• Faster
• Parentally acquired
• Equal for all human beings
Humoral immunity is:
• Mediated
by
macromolecules
(antibodies, complement system, antimicrobial peptides)
• Occurs in blood and mucosae
• Effective against bacteria and toxins
Adaptive immunity is:
• Very specific
• Slower
• Somatically variable
• Particular for each human being
Cell-mediated immunity is:
• Mediated by leukocytes
• Occurs in almost every body district
• Effective against bacteria, worms and
viruses
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Introduction to immunology. Lesson 1
Innate and adaptive immunity
Abbas et al.
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Introduction to immunology. Lesson 1
Innate immunity
The innate immunity is our first line of defense, and relies on cellular and humoral mechanisms
to actively match the invading pathogens as fast as possible. Its main targets are microbes and
cells infected by microbes. The outcome of innate immunity is INFLAMMATION and
activation of the ANTIVIRAL STATE.
Recognition of microbes and infected cells occurs via:
SIGNALS
Pathogen-Associated
Molecular Patterns
(PAMP)
Damage-Associated
Molecular Patterns
(DAMP)
RECEPTORS
Microbes
TLRs, RIGs, NOD,
Mannose receptor,
Dectins, Scavenger
receptors
Phagocytes
Damaged/
Infected
cells
Antibodies,
complement system,
antimicrobial
peptides
Mucosae and
circulating
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Introduction to immunology. Lesson 1
Innate immunity – PAMPs, DAMPs and PRRs
Pathogen- Associated Molecular Patterns (PAMPs) are approx. 103 different molecules which
are normal components of viruses and bacteria (while our cells do not normally express them).
They are mostly structural components of bacterial walls, or viral genomic elements. Their
recognition by innate immunity ensures non-self discrimination and major damage of
microbes. When cells get infected by microbes, they will eventually die by necrosis, releasing
intracellular proteins. Those self nuclear/cytoplasmic proteins recognized by innate immunity
are called Damage-Associated Molecular Patterns (DAMPs). Receptors for PAMPs and DAMPs
are called Pattern Recognition Receptors (PRRs).
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Introduction to immunology. Lesson 1
Innate immunity – PAMPs and DAMPs
PAMPs
Microbes
Nucleic Acids
Proteins
Wall lipids
Carbohydrates
ssRNA
Viruses
dsRNA
Viruses
Non-meth. CpG
Viruses, Bacteria
Pilin
Bacteria
Flagellin
Bacteria
LPS
Gram- bacteria
LTA
Gram+ bacteria
Mannans
Fungi
Glucans
Fungi, Bacteria
All of these molecules are
necessary for the normal
functions and survival of
microbes. Their recognition
ensures wide coverage and a
strong
interference
on
microbial life cycle.
DAMPs
Stress proteins
Crystals
Nuclear proteins
HSP
Monosodium urate
HMGB1
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Introduction to immunology. Lesson 1
Innate immunity – PRRs
Toll-like receptors (TLRs) are a family of
receptors evolved from Drosophila’s Toll
receptors. They exist as membrane-bound and
cytosolic forms, and show the widest ability to
recognize PAMPs. They also recognize DAMPs.
In humans 9 TLRs are known (TLR 1-9).
Their activation by intracellular or extracellular
ligands determines transcriptional activation
of:
• antiviral genes (Type-I interferons)
• cytokines genes
• chemokines genes
• adhesion molecules genes
• co-stimulatory molecules
• oxidative burst genes
Abbas et al.
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Introduction to immunology. Lesson 1
Innate immunity – PRRs
NOD-like receptors (NLRs) are cytosolic receptors for PAMPs and DAMPs. More than 20 NLRs
are known, but the best characterized are NOD1 and NOD2. NOD1 recognizes preferentially
bacterial peptidoglycans, while NOD2 recognizes muramyl dipeptide from Gram+ and Grambacteria. NLRs activation leads to gene activation in a similar way to TLRs. NOD1 and NOD2 are
especially important against Helicobacter pylori and defects in NOD2 might be involved in
Chron’s disease.
RIG-like receptors (RLRs) are cytosolic receptors for viral RNA (both ssRNA and dsRNA). Best
characterized RLRs are RIG-I and MDA5, which recognize different viral nucleic acids. Once
activated, they will lead to transcription of Type-I interferons and antiviral defenses activation.
These include suppression of transcription factors needed for viral genes (like EIF2a), the
synthesis of RNAse which degrade viral RNA and suppression of viral proteins assembly
complexes.
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Introduction to immunology. Lesson 1
Innate immunity – other PRRs
Microbial carbohydrates are recognized by calcium-dependent Lectins (C-type Lectins). These
include:
• Mannose receptor (CD206), which recognizes D-mannose, L-fucose and N-acetyl-Dglucosamine.
• Dectins, which recognize fungal b-glucan (dectin-1) and mannose-rich oligosaccharides
(dectin-2).
• Langherin (CD207), DC-SIGN, and others
Other microbial PAMPs are recognized by:
• Scavenger receptors (like SR-A and CD36). Initially identified as receptors for oxidized LDL
(involved in atherosclerosis), they bind to bacterial polyanions (LPS, LTA, nucleic acids etc.).
• N-FMLP receptors (like FPR and FPRL1). Those receptors bind to n-formylmethionine
leucyl-phenylalanine, a sequence which characterizes all bacterial proteins but not human
ones (apart from mitochondrial proteins).
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Introduction to immunology. Lesson 1
The innate immunity in action – a 3 step process
STEP 1
Epithelia:
- Physical barrier
- Defensive molecules
Abbas et al.
STEP 2
STEP 3
Circulating defenses:
- Natural antibodies
- The Complement system
Cellular defenses:
- Phagocytes
- NK cells
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Introduction to immunology. Lesson 1
The innate immunity in action – anatomical barriers
Epithelia play a big role in the very first match
with invading pathogens. As most of the
microbes enter the body via the skin, the
respiratory and the gastrointestinal tract, these
epithelia evolved specific strategies to
counteract pathogens, like:
• Expression of sialomucins on the external
side of the plasma membrane, which
electrostatically repel bacterial walls
• Production of antimicrobial peptides (like
defensins and cathelicidins), which both
directly attack the microbes and help their
recognition by innate immunity cells
• Hosting of intraepithelial lymphocytes.
While some of them are “classical” ab Tcells, many are gd T-cells, which are
particularly good at recognizing bacterial
phospholipids. They can directly attack
microbes
and
stimulate
phagocyte
responses.
Abbas et al.
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Introduction to immunology. Lesson 1
The innate immunity in action – circulating defenses
Once microbes enter the blood flow or the extracellular fluids, they get in contact with the
humoral effector mechanisms of the innate immunity: natural antibodies, the complement
system and the pentraxins/collectins/ficolins family members.
Natural antibodies belong to the pentameric IgM class. They
are mainly produced by B1 lymphocytes in a spontaneous
way which does not require any presence of the pathogen.
These natural antibodies recognize PAMPs and DAMPs,
especially carbohydrates, lipids and lipoproteins. The AB0
blood group system depends on the presence of natural
antibodies.
Once bound to microbes, they activate the complement and
stimulate phagocyte response to eradicate the threat.
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Introduction to immunology. Lesson 1
The innate immunity in action – circulating defenses
The complement system is a network of many (> 20) soluble and membrane-bound proteins
which are normally present in the blood flow in the form of inactive precursors (pro-enzymes).
The term complement (C) was used to refer to a heat-instable serum
component which was able to lyse bacteria when incubated at 37oC, but lost
when incubated at 56oC for 30 minutes (antibodies are thermo stable!).
•
•
Jules Bordet
•
•
•
The complement system is activated by microbial macromolecules or by
antibodies bound to them. There are 3 complement activation pathways.
Complement activation proceeds through sequential cleavage and
activation of circulating inactive forms of C proteins. Once cleaved, those
precursors become active and acquire proteolytic activity.
The final stages of complement activation lead to formation of MAC
(Membrane Attack Complex), which is a canal (similar to acquaporins)
structure allowing water to enter into the microbes and kill them by
osmotic lysis.
Microbes bound to complement fragments can be more-easily recognized
and killed by phagocytes
Self cells, but not microbes, express proteins which inhibit complement
activation.
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Introduction to immunology. Lesson 1
The innate immunity in action – complement activation
C3 has a spontaneous
cleavage turnover in
body fluids, so it can
directly
recognize
microbes.
The
classical
and
lectins
pathway
provide specificity to
complement activation
Abbas et al.
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Introduction to immunology. Lesson 1
The innate immunity in action – complement activation
Abbas et al.
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Introduction to immunology. Lesson 1
The innate immunity in action – other functions of the complement
The complement system provides a fast and reliable way to directly attack microbes, but it is
also connected to the cells of the innate immunity thanks to COMPLEMENT RECEPTORS (CR):
• CR1 (or CD35) promotes the phagocytosis of microbes bound to C3b and C4b. It is
expressed by almost all immune cells, in which determines activation, but also by red blood
cells. This provides a mean for the elimination of microbes by hemocatheresis.
• CR2 (or CD21) promotes the activation of B-cells and antibody production. It’s also
expressed by specific epithelial cells and is involved in Epstein-Barr virus pathogenesis.
• CR3 (or CD11b/Mac-1) binds a fragment deriving from C3b (iC3b) and is a strong promoter
of phagocytes’ activation.
• CR4 (or CD11c) belongs to the same family of CR3 and has similar functions, even though is
more restricted to specific cell subtypes (like dendritic cells)
• CR5 (or CRIg) binds to C3b and iC3b and is expressed only by liver’s Kupffer cells. It is
mostly involved in the direct elimination of circulating microbes by hemocatheresis.
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Introduction to immunology. Lesson 1
The innate immunity in action – cellular immunity
Cellular defenses belonging to the innate immunity involve all myeloid cells and some specific
lymphoid cell types.
Myeloid
Lymphoid
Phagocytes
Phagocytosis and
RESPIRATORY BURST
CYTOKINES and
CHEMOKINES
Mast cells
BIOGENIC AMINES
NK cells and
gd T-cells
CYTOLYTIC
MECHANISMS
B1 cells
NATURAL
ANTIBODIES
BIOGENIC AMINES
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Introduction to immunology. Lesson 1
The innate immunity in action – phagocytosis and the respiratory burst
Elsevier 2005
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Introduction to immunology. Lesson 1
The innate immunity in action – phagocytosis and the respiratory burst
Once the microbe is engulfed, the phagosome (phagocytic vesicle) fuses with the lysosome. Its
acidification activates the pro-enzymes that it contains and allows microbial degradation.
• Elastase and Cathepsin-G. Wide-spectrum proteases able to degrade many bacterial
proteins. Elastase is a serin-protease while Cathepsin-G is chymotripsin-like protease.
Those enzymes are activated by the low pH and by radicals.
In the phagolysosome, the respiratory burst occurs. This is the most powerful antimicrobial
mechanism of the phagocytes. It involves both the consumption of oxygen and specific
aminoacids to produce toxic compounds.
• Reactive oxygen species (ROS) include H2O2, O2.-, O. and are generated by the NADPH
oxidase. Those compounds are able to destroy/destabilize all major classes of
biomolecules, and thus destroy bacterial walls, membranes and genomes. Deficit of
NADPH oxidase is responsible for the chronic granulomatous disease (CGD), a very severe
deficiency of the innate immunity.
• The inducible nitric oxide synthase (iNOS) catalyzes the conversion of arginine to citrulline.
During this process, nitric oxide is formed. In the phagolysosome, NO interacts with H2O2
and O2.- to form peroxynitrite (ONOO-) which is extremely toxic as the ROS.
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Introduction to immunology. Lesson 1
The innate immunity in action – biogenic amines and prostanoids
Phagocytes and mast cells secrete immunomodulatory compounds upon their activation,
including amines and prostanoids.
Histamine is produced via decarboxylation of
histidine. Once released, it will determine:
• Vasoconstriction of the major arteries
• Vasodilatation of arterioles
• Contraction of endothelial cells with
increased permeability of the capillaries and
venulae. This facilitates the immune cells to
penetrate in the site of infection (leukocyte
extravasation). Histamine is responsible for
itching at inflamed sites (think of mosquitos
bites)
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Introduction to immunology. Lesson 1
The innate immunity in action – biogenic amines and prostanoids
Phagocytes and mast cells secrete immunomodulatory compounds upon their activation,
including amines and prostanoids.
Prostanoids are produced from
arachidonic acid by the cyclooxygenase
(COX) enzymes (target of NSAIDS).
Prostaglandins (PGs), leukotrienes (LTs)
and thromboxanes (TXs) have different
systemic and local effects, but they
generally either:
• Stimulate
inflammation
and
leukocyte extravasation (LTs, and
TXs and some PGs)
• Suppress local inflammation (PGs
like PGE2)
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Introduction to immunology. Lesson 1
The innate immunity in action – lymphoid cells
Microbes can be recognized and killed by lymphoid cells too. Involved in this process are
mainly NK cells, NKT cells and gd T-cells. Recognition can be either direct or mediated by
antibodies.
DIRECT
F1000 2011
MEDIATED
Nature 2003
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Introduction to immunology. Lesson 1
The innate immunity in action – ADCC
NK cells can also directly recognize microbes if they are covered by antibodies (opsonized).
Abbas et al.
When antibodies bound to a microbe (or a damaged cell) are recognized by the NK cell
through their antibody receptor (FcgRIII), activation will occur even if the cells express
suppressive stimuli.
Note that opsonization works the same way to aid phagocytes with microbes recognition
and engulfment.
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Introduction to immunology. Lesson 1
The innate immunity in action – Inflammation
The 5 symptoms of
inflammation:
• Tumor (swelling)
• Rubor (redness)
• Calor (heat)
• Dolor (pain)
• Functio laesa (functional
damage)
Consequences of immune
system activation!
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Introduction to immunology. Lesson 1
The innate immunity in action – Inflammation
Inflammation is the normal, second line of defense of our body against pathogens. It is defined
as acute inflammation when it proceeds normally and resolves spontaneously. It will
eventually turn to chronic inflammation, a pathological process leading to tissue destruction,
if the stimulus (microbial invasion) cannot be resolved by the immune cells. The clinical signs
of inflammation depend on the release of soluble immune mediators, including amines,
prostanoids, cytokines and chemokines.
cytokines
chemokines
direct regulation of immune processes
coordinated regulation of surrounding, non-immune cells
chemotactic factors for immune cells
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Introduction to immunology. Lesson 1
The innate immunity in action – cytokines
Cytokines can:
• Stimulate the functions of
phagocytes and lymphocytes,
empowering their antimicrobial
activity.
• Stimulate the proliferation of
leukocytes, to empower body
defenses.
• Stimulate
lymphocytes
differentiation, ensuring specific
immunity to be effective.
• Stimulate
multiple
organs
functions, to cooperate with
leukocytes activity.
• Suppress immune functions, to
end the reaction and restore the
normal functions of the body.
Abbas et al.
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Introduction to immunology. Lesson 1
The innate immunity in action – cytokines
Abbas et al.
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Introduction to immunology. Lesson 1
The innate immunity in action – chemokines
Chemokines are generally divided
into:
• CC chemokines (b-chemokines), if
they have two adjacent cysteines
at the C-terminus
• CXC chemokines (a-chemokines),
if the two N-terminal cysteines
are separated by another residue
(X)
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Introduction to immunology. Lesson 1
The innate immunity in action – activation of anti-viral state
The anti-viral state is a coordinated set of events occurring mostly in a paracrine way to
prevent viruses dissemination and further infection. It is considered as an ancillary mechanism,
as the major defense against viruses requires lymphocytes (adaptive immunity). It is mediated
by Type-I interferons, which are produced when viral PAMPs activate PRRs.
Type-I interferons
IFN-a (9 different types)
IFN-b (1 type)
Segregation of lymphocytes
in the lymph nodes, where
they will “learn” what type
of virus is infecting
Potentiation of NK cells
Th1 differentiation of T-cells
Increased MHC-I expression
Adaptive immunity
ACTIVATION OF THE
ANTIVIRAL STATE
Innate (passive)
immunity
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Introduction to immunology. Lesson 1
The innate immunity in action – activation of anti-viral state
The so-called antiviral state is a passive
defense,
which
prevents viruses to
replicate in cells
which have been
targeted by Type-I
IFN. But, it does not
kill infected cells.
The need for
Adaptive immunity
Abbas et al.
41
Introduction to immunology. Lesson 1
Inflammation switch-off – feedback mechanisms
Involving powerful cells (phagocytes, NK cells, etc.) armed with aggressive enzymes and toxic
products (lytic enzymes, vasoactive molecules, ROS, NO, etc.), inflammation always damage the
tissues. It is therefore necessary that it ends as soon as the microbial invasion is resolved.
Interleukin-10 (IL-10) is the prototypical anti-inflammatory cytokine. Its production is triggered
by inflammatory stimuli, but starts after the synthesis of inflammatory cytokines like TNF, IL-1,
IL-12 etc. Once secreted, it switches off the transcription of inflammatory cytokines genes.
IL-1 functions are suppressed by a soluble antagonist which binds to the same receptor as IL-1
but which has no biological functions, called IL-1 receptor antagonist (IL-1ra), produced by
phagocytes. There is also a Type-II receptor for IL-1 which is a decoy receptor with no activity
(scavenger of IL-1).
Inflammatory stimuli induce the expression of autophagy genes (Atg). Autophagy is the
process of a cell which degrades (eat) its own organelles. By “eating” organelles where PRRs or
cytokines are stored, a cell diminishes its ability to activate during inflammation.
Several suppressor of cytokines transduction pathways (SOCS, Suppressors Of Cytokine
Signaling) are induced by inflammatory stimuli and shut down pro-inflammatory cytokine
signals.
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