Epithelial Defense Measures
Lysozymes (tears, saliva, milk, mucus, etc.)
- degrades peptidoglycans
Antimicrobial Peptides (insert into and disrupt bacterial membranes)
- alpha defensins = produced by PMN (granulocytes, especially neutrophils) and Paneth cells in the gut
- Beta defensins - produced by many epithelial cells including the skin, resp. and genitourinary tract
- Cathelicidins - produced by epithelial cells, macrophages, and neutrophils
Physical and Chemical Barriers
Epidermis
- stratum layers
- lamellar bodies produce watertight lipid layer
Bronchial Epithelium
- Cilia
- Goblet cells
- Mucous glands
Gut Epithelium
- Goblet cell
- Paneth Cells (Defensins)
- Macrophages
PAMPs and PRRs
PAMP (Pathogen-Associated Molecular Patterns)
- Ex: LPS (Gram Negative), Lipoteichoic acid (Gram Positive)
- expressed by microorganisms; recognized by immune system and used to target pathogens
PRR (Pattern Recognition Receptors)
- Receptors designed to target PAMPs
- soluble versions exist; called complement components
- Types: Soluble, Membrane Bound, Intracellular
Pathogen Recognition by TLRs (Toll-Like Receptors)
TLR2:TLR6 - Gram positive bacteria, Fungi
TLR4 - Gram Negative bacteria
TLR7, TLR8, TLR3 - RNA Viruses (Intracellular Receptors; attached to endosomes)
TLR9 - Bacteria, DNA viruses (Intracellular Receptors; attached to endosomes)
Note: TLRs activate transcription factors (like NFkB) to induce inflammatory cytokine expression
NOD-like Receptors
Intracellular sensors of bacterial infection (not attached to endosome)
Note: they activate NFkB just like Toll Receptors do
Type I IFN and the “Antiviral State”
Type I IFN = IFN alpha/IFN Beta
Infection of a neighboring cell can cause Type I IFN to bind to a receptor in primary cell, causing upregulation of expression of three enzymes:
- 2’, 5’ Oligoadenylate
- RNAse L = mRNA degradation
- PKR (Protein Kinase R) = inhibition of protein synthesis
Cytokine Nomenclature
3 Groups:
- Interleukins (IL-1, IL-2, etc.)
- Chemokines (CC and CXC)
- chemotactic activity - Interferons (Type I (IFN a/B); Type II (IFN-Gamma)
- antiviral activity
Note: Important Interleukins:
- IL-2 = T Cell Promoting Factor
- IL-4 = B Cell Promoting Factor
Target Activity
Autocrine (IL-2)
Paracrine (IFN Gamma)
Endocrine (IL-6 from Macrophage)
Interaction Properties of Cytokines
Pleiotropy
- 1 cytokine = multiple effects on different cells
Redundancy
- Two or more cytokines mediate similar/identical functions on same target cell
Synergy
- effect of 2 or more cytokines are greater than the sum of their parts.
Antagonism
- cytokine effects fight each other (decrease/inhibition)
Common Gamma Gene
IL-2R Gamma
- it is a subunit present in several interleukin receptors (IL-21R superfamily)
- A defect in this gene causes X-linked SCID; no signalling possible through IL receptors, and no T-cell development from IL-7
Complement Pathway: Alternative
Mechanism
- Spontaneous hydrolysis of C3 into C3b and C3a; C3b binds to pathogen surface
- Factor B binds to C3b
- Factor B undergoes conformational change and is cleaved by Factor D into Bb and Ba (which is released)
- Resultant C3bBb acts as surface C3 convertase, creating an amplification loop.
Additional Elements:
- Factor P (Properdin) = binds and stabilizes C3 Convertase
- Factor H = assists in inactivation of C3 Convertase
- Factor I = assists in inactivation of C3 Convertase
Function:
- Opsonization via C3b (binds to CR1 on phagocytic cells)
- Generation of C5 convertase by binding C3b to C3 convertase
- FIRST TO ACT
Complement Pathway: Lectin Pathway
Mechanism
- Soluble Receptors (Ficolin or MBL) bind to pathogen
- Binding to pathogen activates MASP 1, which in turn activates MASP 2.
- MASP 2 cleaves C4, causing C4b to bind to pathogen surface, and cleaves C2, causing C2a to bind to C4b, creating C3 Convertase
- C3 is cleaved by C4b2a, causing C4b opsonin to bind to pathogen
Function:
- Recognition of specific pathogens due to specificity of binding sites
- Cleave C4, C2, and C3 (via MASP 2 and C3 Convertase aka C4b2a)
- SECOND TO ACT
Complement Pathway: Classical Pathway
Mechanism
- Identical to Lectin Pathway, except for recognition event: Activation of C1 complex, which binds to antibodies that have bound to antigen.
- Requires either 1 IgM antibody or at least 2 IgG antibodies to activate. Can also initiate after binding to C-Reactive Protein (CRP) bound to pathogen surface
Note: C1s behaves like MASP 2 from Lectin Pathway, and C3 Convertase remains identical.
Membrane Attack Complex (MAC ATTACKKKKKKKKKK)
Mechanism
- C5b binds C6 and C7
- Binding of C7 allows complex to bind to membrane
- C8 binds to the complex, inserting it into the cell membrane
- C9 binds to the complex and polymerizes, and forms a pore in the membrane with about 10-16 molecules.
Complement Receptors
CR1 = C3b, C4bi (promotes C3b/C4b decay, phagocytosis with help of C5a, and Erythrocyte transport of immune complexes
CR3 = iC3b (Phagocytosis)
CR4 = iC3b (Phagocytosis)
C5a receptor = C5a
C3a receptor = C3a
Immunological Steps of Inflammation
- Initiation (Trauma, tissue necrosis, infection, or immune reaction
- Acute Vascular Phase
- Vasodilation and Endothelial Cell contraction - Note that process may stop here if due to nonimmune stimulus
- Acute Cellular Phase
- Transmigration/Diapedesis of Neutrophils
- Resolution may occur if stimulus is adequately removed at this stage. - Chronic Cellular Phase
- Lymphocytes, Macrophages, and Eosinophils may get involved at this point (parasitic infection or allergic response) - Resolution
- IL10 and TGF-Beta (produced by macrophages) involved in process
5 Signs of Inflammation
Rubor - Redness Tumor - Swelling Calor - Heat Dolor- Pain Functio Laesa - Loss of Function
Gross Manifestation Timeline
- Immediate: White line of vasoconstriction
- 3-50 seconds: redness from vasodilated capillaries
- 30-60 seconds: increased redness from vasodilation of arterioles
- 1-5 mintues: wheel formation due to increased vascular permeability
Substances released by Mast Cells
Histamine
- Stored in granules
- Causes dilation of arterioles and TRANSIENT increase in vascular permeability
TNF-Alpha and IL1
- Induce vascular endothelial cells to contract to allow leukocytes and fluid (contains Bradykinin) to pass through
- Induce adhesion molecules on endothelial cells to ‘catch’ leukocytes in bloodstream
- SUSTAINED EFFECT
IL-8 (CXCL8)
- Chemokine that attracts monocytes and neutrophils
Prostaglandin
- Lipid mediator that induces vasodilation
Acute Phase Response
Induced by cytokines released in response to injury or inflammation (IL1, IL6, and TNF-Alpha)
Acute Phase Proteins produced by Hypothalamus, Liver, and Bone Marrow
- Reinforce innate defenses against infection
- Limit Damage
- May increase inflammation in unresolved cases
- CRP-C reactive protein is an important component.
Acute Phase Proteins and Functions
Pathogen Recognition
- CRP (C-reactive Protein)
- MBL (Mannose-Binding Lectin)
- LPS Binding Protein
Pathogen Elimination
- Complement Components C3, C4, C9, Factor B
Chemokine Classification
CXC
- Contains Cys-X-Cys Sequence
CC
- Contains Cys-Cys sequence
(X)C
- Contains single Cys
CX3C
- Contains Cyx-X-X-X-Cys sequence
Note: All Chemokine receptors activate G proteins and have 7 alpha-helical transmembrane regions
Cell Adhesion Molecules (CAMs)
Regulate infiltration of immune cells into sites of infection.
- Vascular Adressin (CD34)
- Selectin (L-Selectin)
- Integrin (LFA-1)
- Immunoglobulin-like Molecule (ICAM-1)
Extravasation (4 Steps)
- Rolling
- Sialyl-Lewis (Adressin?) on leukocyte and E-selectin on endothelium.
- Note: insufficient to halt leukocyte on its own - Tight Binding
- LFA-1 on leukocyte and ICAM-1 on endothelium
- helps bring the leukocyte’s rolling motion to a halt - Diapedesis
- CXCL8 (chemokine) binds to CXCL8 receptor, switching leukocyte to a high-affinity state.
- Leukocyte squeezes between cells - Migration
- Follow the chemokine gradient in the tissue towards site of infection
