3. Innate immune defences & inflammation 2 Flashcards
Innate immune cells
neutrophils, macrophages, dendritic cells, natural killer cells
Neutrophils
phagocytosis
reactive oxygen and nitrogen species
antimicrobial peptides
NETs (neutrophil extracellular traps)
Macrophages
phagocytosis inflammatory mediators antigen presentation reactive oxygen and nitrogen species cytokines complement proteins
Dendritic cells
antigen presentation costimulatory signals reactive oxygen species interferon cytokines
phagocyte recruitment steps
Rolling and extravasation
- rolling
- activation
- arrest/adhesion
- transendothelial migration
How does phagocyte recruitment happen?
- cytokines e.g. TNF-alpha dilate local blood vessels
- chemokines attract monocytes and neutrophils to the infection
- cell adhesion molecules (ICAM-1 and ICAM-2) are upregulated on the endothelium which bind to integrins (family of adhesion molecules) on the leukocytes
What is phagocytosis?
Phagocytosis is the capture and digestion of foreign particles
Performed by neutrophils and macrophages
Put out processes to feel for foreign objects, but not always recognise them sometimes recognises opsonins on opsonised things
What initiates phagocytosis
Active process initiated by binding to pathogen
How does phagocytosis occur?
Macrophage receptors recognise components of microbial surfaces
Microorganisms are bound by phagocytic receptors on the macrophage surface
Microorganisms are internalised by receptor-mediated endocytosis
Fusion of the endosome with a lysosome forms a phagolysosome in which microorganisms are degraded
Antimicrobial mechanisms of phagocytes
Acidification, toxic oxygen-derived profucts, toxic nitrogen oxides, antimicrobial peptides, enzymes, competitors (e.g. lactoferrin)
What are NETs?
When activated some neutrophils undergo a special form of cell death termed ‘NETosis’
During NETosis nuclear chromatin is released from cells trapping microorganisms thus aiding phagocytosis
What are pattern recognition receptors (PRRs)
Receptors able to recognise conserved structures
They recognise patterns termed:
pathogen-associated molecular patterns (PAMPs)
What are some examples of PRRs?
Toll-like receptors (TLRs)
NOD-like receptors (NLRs)
Rig-I like receptors (RLRs)
Cytosolic DNA sensors (CDS)
Pathogen-associated molecular patterns (PAMPs)
PAMPs - Microbes evolve rapidly, so innate immunity must focus on highly conserved and essential components of microbes (cell wall structures; nucleic acids)
DAMPs
DAMPs – Damage associated molecular patterns, molecules released from necrotic cells
Drosophila Toll receptor
Mutagenesis work on Drosophila revealed two members of the Toll family, dToll and 18-wheeler
Important for development
Important for immunity to the fungal and bacterial infections
Mammalian equivalent are the Toll-like receptors
Toll-like receptor structure
Extracellular:
LRR domain – site of pathogen binding
Cytosolic side:
TIR-domain - conserved stretch of ~200 amino acids
TLRs form functional hetero/homodimers
The convex surfaces of TLR-1 and TLR-2 have binding sites for lipid side chains of triacyl lipopeptides
Binding of each TLR to the same lipopeptide induces dimerization, bringing their cytoplasmic TIR domains into close proximity
Different TLRs and their ligands
Extracellular: TLR1 and 2- triacyl lipopeptides TLR2 and 6- diacyl lipopeptides TLR5 - flagellin TLR4 - LPS
Intracellular:
TLR3, 7, 8 and 9 - DNA and RNA
TLR signalling
TLR signalling induces genes that function in host defense
Pro-inflammatory cytokines
Chemokines
MHC & co-stimulatory molecules
antimicrobial peptides & complement components
What does TLR activate?
IRF3/7, AP1 (thru MAPKs), NFK-beta
TLR adaptor proteins
Myd88 (used in all pathways except TLR3)
Mal used by TLR1/2 and TLR4
TRAM just TLR4
TRIF TLR3 and TLR4
Myd88 gain of function mutation
Waldenström macroglobulinemia is a rare type ofnon-Hodgkin lymphoma.
B cells make large amounts of IgM that can cause excess bleeding, vision problems and headaches.
Lymphoma cells proliferating in the bone marrow can cause anaemia (low levels of red blood cells), neutropenia (low levels of neutrophils) and thrombocytopenia (low levels of platelets).
Myd88 loss of function mutation
Nine MyD88 deficient children suffered from life-threatening, often recurrent pyogenic bacterial infections, but were otherwise healthy, with normal resistance to other microbes.
Their clinical status improved with age, possibly due to a compensatory effect of adaptive immunity or other innate immune mechanisms.
TLR-deficiencies
Herpes simplex encephalitis (HSE): Inflammationofthebraindue toinfection with herpes simplex virus (HSV-1)
HSV-1 is a dsDNA virus, but during viral replication it produces dsRNA
Defects in other signalling molecules involved in the TLR3 signalling pathway have also been associated with HSE
TLRs in infection
HIV – TLR8
Sepsis – TLR2 and 4
Tuberculosis – TLR2 and 4
TLRs in inflammation
Systemic Lupus Erythamatosus –
TLR7, 8 and 9
Alzheimer’s Disease – TLR2 and 4
Atherosclerosis – TLR2 and 4
TLR agonists
Infection- genital warts (TLR7)
Cancer - Melanoma (TLR7 ligand)
Allergy – Ragweed pollen (TLR9)
Vaccine adjuvant
TLR antagonists
Autoimmunity (TLR7,8,9)
Sepsis (TLR4)
Cancer
Nod-like receptors
NLR = Nucleotide-binding Leucine Rich
Cytoplasmic pattern recognition molecules
Two major groups- NLRCs and NLRPs - ‘C’ stands for ‘caspase recruitment domain (CARD)’ and the ‘P’ stands for pyrin domain.
NLRCs
Two examples:
NLRC1 (NOD1)
NLRC2 (NOD2)
They have a leucine rich domain which can bind to peptidoglycan which is present on the cell membrane of most bacteria
NOD1 and NOD2
NOD1 and NOD2 detect similar yet distinct peptides of peptidoglycan
What does NOD1 bind
NOD1 binds γ-glutamyl diaminopimelic acid (iE-DAP) (Mainly Gm-ve Bacteria)
What does NOD2 bind
NOD2 binds muramyl dipeptide (both Gm+ve and Gm-ve bacteria
NOD2 gain of function
NOD2 gain of function mutation linked to early onset sarcoidosis where granulomas develop in the organs of the body.
NOD2 loss of function
NOD2 loss of function mutation is associated with susceptibility to Crohn’s disease, a chronic intestinal inflammatory disorder
NLRPs
The best characterised is NLRP3 (NALP3)
NLRP3 is activated by cellular stress; K+ efflux, ATP, reactive oxygen species and lysosomal damage
Inflammasome is essential for IL-1 and IL-18 secretion
What activates inflammasomes
Cellular infection or cell stress
Uric acid crystals (gout) Asbestos Silica Amyloid beta (Alzheimer's) Islet amyloid polypeptide(Type II diabetes) Hemozoin (Malaria)
What does inflammasome activation lead to?
Inflammasome activation leads to cleavage of pro-IL-1 and pro-IL-18 to allow secretion
Gain of function mutations in NLRP3
Cryopyrin-Associated Periodic Syndromes (CAPS)
Caused by rare mutations in exon 3 of NLRP3 gene causing over production of IL-1
What is Muckle Wells syndrome
A cryopyrin-Associated Periodic Syndrome caused by mutations in NLRP3 -> overproduction of IL1
(Prevalence unknown)
Can occur spontaneously or be triggered by cold, heat, fatigue, or other stresses.
Symptoms of fever, rash, arthralgia, conjunctivitis, uveitis, sensorineural deafness, and potentially life-threatening amyloidosis
What is famial cold autoinflammatory syndrome
A cryopyrin-Associated Periodic Syndrome caused by mutations in NLRP3 -> overproduction of IL1
1: 1000000
Triggered by exposure to cold
Symptoms of fever urticarial rash with headache, arthralgia, and sometimes conjunctivitis
How can CAPS be treated?
Both conditions can be treated with anakinra (IL-1RA)
What do RIG-I-like receptors (RLRs) sense?
RIG-I and MDA5 are sensors of cytoplasmic RNA, a replication intermediate for viruses. They signal to induce pro-inflammatory cytokines and IFN
RIG-I
Binds to single stranded RNA containing 5’-triphosphate
Recognition of :
Flaviviruses (hepatitis C (HCV)
Orthomyxoviruses (Influenza)
MDA5
Preferentially recognizes long double stranded RNA
Critical for picornavirus detection
Mutations (rare) are beginning to be associated with IFN related diseases
Cytosolic DNA sensors
E.g. STING
dsDNA from viruses activates cGAS to produce cGAMP from ATP and GTP
cGAMP or other bacterial-derived cyclic dinucleotides bind to the STING dimer present on the ER membrane and activate its signalling
STING activates the kinase TBK1 to phosphorylate IRF3, which enters the nucleus and induces expression of type I interferon genes
SAVI
Stimulator of interferon genes (STING)-associated vasculopathy with onset in infancy (SAVI) is an autoinflammatory disease caused by gain-of-function mutations inTMEM173, the gene that codes for STING.
Patients have abnormalinflammationthroughout the body, especially in the skin, blood vessels, and lungs.
Acute phase response
Induced by cytokines such as TNF, IL-6 and IL-1 during infection and inflammation
Acute phase proteins are mainly produced by the liver
Induces opsonisation/phagocytosis
Can activate the complement pathway
Raised erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP (a pentraxin)) are characteristic of an acute phase response and are used clinically to detect inflammation
