Immunology Chapter 3

• 1. Specificity is inherited in the genome
• 2. Expressed by all cells of a particular type (e.g. macrophages)
• 3. Triggers immediate response
• 4. Recognizes broad classes of pathogens
• 5. Interacts with a range of molecular structures of a given type

• 1. Encoded in multiple gene segments
• 2. Requires gene rearrangement
• 3. Clonal distribution
• 4. Able to discriminate between even closely related molecular structures

• 1. Free receptors in serum
• 2. Membrane-bound phagocytic receptors
• 3. Membrane-bound signaling receptors
• 4. Cytoplasmic signaling receptors

• 1. Mfs (Macrophages) - mostly in tissue
• 2. Granulocytes - neutrophils are the most active
• 3. DC's (Dedritic cells)
• a) Conventional DC's - process ingested microbes, present peptides to and activate T cells and produce cytokines
• b) Plasmacytoid DC's - produce interferons

• 1. Macrophages have phagocytic receptors (mannose, Dectin-1 (Beta-glucan receptor), C, scavenger, lipid) that bind microbes and their components
• 2. Bound material is internalized in phagosomes and broken down in phagolysosomes

G-protein-coupled receptors - receptors on phagocytes (Mfs, granulocytes, DC's, neutrophils) that signal the phagocyte to kill the ingested pathogen

• 1. Before ligand (chemokine or fMet-Lec-Phe) binding, GPCR is not associated with the inactive G protein which is bound to GDP
• 2. Ligand binding causes a conformational change in the receptor which enables it to associate with the G protein which releases GDP and binds GTP
• 3. G protein dissociates into alpha and beta-gamma subunits, both of which activate antimicrobial compounds

• 1. Neutrophils engulf and kill the microbes to which they bind
• 2. Bacterial fMet-Lec-Phe peptides activate Rac 2 and bacteria are taken up into phagosomes
• 3. Phagosomes fuse with primary and secondary granules
• 4. Rac 2 induces assembly of a functional NADPH oxidase in the phagolysosome membrane, leading to generation of antimicrobial superoxide and hydrogen peroxide

• 1. Macrophage detects invading pathogen and releases cytokines
• 2. Cytokines cause dilation of local small blood vessels
• 3. Leukocytes, plasma proteins, and fluid move from blood to tissue causing inflammation
• 4. Clotting in infected area prevents spread of pathogen

• 1. Deliver effector molecules and cells from blood
• 2. Induce local blood clotting to prevent spreading of pathogen
• 3. Promote tissue repair

• 1. Monocytes bind adhesion molecules on vascular endothelium near site of infection and receives chemokine signal
• 2. Moncytes migrate into the surrounding tissue
• 3. Once in tissue, monocytes differentiate into macrophages and migrate to the site of infection

Toll-like receptors - represent an ancient host defence signaling system in vertebrates, plants, and invertibrates and is important for defense against G+ bacteria and fungi

• 1. TLR:TLR2 and TLR2:TLR6 heterodimers - Lipoproteins
• 2. TLR3 - dsRNA
• 3. TLR4 - LPS
• 4. TLR5 - Flagellin
• 5. TLR7 and TLR8 - ssRNA
• 6. TLR9 - DNA
• 7. TLR11 - Profilin

• 1. Extracellular recognition - The cell surface of DCs, Mfs and B cells
• 2. Intracellular recognition - In the walls of endosomes

• 1. They have binding sites for triacyl lipopeptides
• 2. Binding of each TLR to the same lipopeptide induces dimerization and brings their TIR (Toll-IL-I receptor) domains close together
• 3. This initiates signaling

• 1. Lipopolysaccharide
• 2. Cell wall component of Gram- bacteria
• 3. In mass amounts can cause septic shock due to mass secretion of cytokines
• 4. Recognized by TLR-4

• 1. MD-2 binds TLR-4 to aid recognition
• 2. CD14 helps TLR-4 recognize free LPS (LPS detached from the pathogen)

• 1. Dimerized TLRs recruit IRAK1 and IRAK4, activating the E3 ubiquitin ligase TRAF-6
• 2. TRAF-6 and NEMO (NFkB essential modifier) are polyubiquitinated, creating a scaffold for activation of TAK 1
• 3. TAK1 associates with IKK and phosphorylates IKKb, which phosphorylates IkB
• 4. IkB is degraded, releasing NFkB into the nucleus to induce expression of cytokine genes

• 1. TLR-3 in endosome binds dsRNA and signals via TRIF to induce IFN gene expression
• 2. TLR-7 in endosome binds ssRNA and signals via MyD88 to induce IFN gene expression

• 1. Cytoplasmic NOD proteins reside in the cytoplasm in an inactive form
• 2. Binding of bacterial ligands to NOD proteins induces recruitment of RIPK2, which activates TAK1, leading to NFkB activation

• 1. Efflux of K⁺ ions from damages cells induces dissociaton of cytoplasmic proteins from NALP3
• 2. NALP3 dimerization recruits PYCARD, causing aggregation and proteolytic activation of caspase 1
• 3. Caspase 1 releases mature inflammatory cytokines such as IL-1 and IL-18 from their proproteins

RIG-I-like helicases - they detect cytoplasmic viral RNA and stimulate interferon production

• 1. Cytoplasmic replication of virus produces uncapped RNA with a 5'-triphosphate
• 2. Viral RNA binding to RIG-I induces RIG-I to associate with the adaptor MAVS, causing dimerization
• 3. MAVS induce production of inflammatory cytokines and interferons via activation of NFkB and IRF's

• 1. Immature DC's ingest pathogens
• 2. They migrate to lymph nodes
• 3. Increased expression for cell surface molecules B7.1 (CD 80) and B7.2 (CD 86) that stimulate T-cells

• 1. Extracellular recognition receptors activate a protease cascade leading to cleavage of Spatzle
• 2. Ceaved Spatzle homodimer binds to Toll, causing its dimerization
• 3. The TIR domains of Toll recruit the adaptor dMyD88, which activates a signaling pathway similar to the NFkB pathway

• 1. Autocrine manner - affect the behavior of the cell that releases the cytokine
• 2. Paracrine manner - affect the behavior of adjacent cells
• 3. Endocrine manner - affect the behavior of distant cells

• 1. TNF-alpha - induces local inflammation and fever
• 2. CXCL8 - induces local inflammation by attracting neutrophils
• 3. IL-1Beta, IL-6 - induces fever and acute-phase response in liver
• 4. IL-12 - activates NK cells and induces differentiation of CD4 T cells into T_H1 cells

• 1. CC chemokines - have 2 adjacent cysteine residues near the amino terminus
• 2. CXC chemokines - have 2 cycteine residues near the amino terminus that are separated by a single amino acid

• 1. Selectins - Bind carbs; initiate leukocyte-endothelial interaction; located in endothelium
• 2. Integrins - Bind to cell-adhesion molecules and extracellular matrix; located in monocytes, T cells, Mfs, neutrophils, DCs, and NK cells
• 3. ICAMs - ligands for integrins; located in activated leukocytes and endothelial cell-cell junctions

• 1. Rolling adhesion
• 2. Tight binding
• 3. Diapedesis
• 4. Migration

• Locally
• 1. Mfs activated to secrete TNF-alpha in tissue
• 2. Increased release of plama proteins into tissue; increased phagocyte and lymphocyte migration into tissue; increased platlet adhesion to blood vessel wall
• 3. Phagocytosis of bacteria; local vessel occlusion; plasma and cells drain to local lymph node
• 4. Removal of infection - adaptive immunity

• Systemic (Sepsis)
• 1. Mfs activated in the liver and spleen secrete TNF-alpha into the bloodstream
• 2. Systemic edema causing decreased blood volume, hypoproteinemia, and neutropenia, followed by neutrophilia; decreased blood volume causes collapse of vessels
• 3. Disseminated intravascular coagulation leading to wasting and multiple organ failure
• 4. Death

• 1. Bacteria induce Mfs to produce IL-6, which acts on hepatocytes to induce synthesis of acute-phase proteins
• 2. C-reactive protein binds phosphocholine on bacterial surfaces, acting as an opsonin, and also activating C
• 3. Mannose-binding lectin binds mannose residues on bacterial surfaces, acting as an opsonin, and also activating C

• 1. They activate gene (Mx proteins, oligoadenylate sysnthetase, and PKR) that cause the destruction of mRNA and inhibit the translation of viral proteins
• 2. They induce MHC I expression in infected cells which makes them more susceptible to being killed by CD8 cytotoxic T cells
• 3. They activate NK cells which selectively kill virus-infected cells

Activated NK cells serve to contain virus infection while the adaptive immune response is generating Ag-specific cytotoxic T cells and neutralizing Abs that can clear the infection.

• 1. MHC I on normal cells is recognized by inhibitory receptors that inhibit signals from activating receptors - NK cell does not kill the normal cell
• 2. Absent MHC I can't stimulate a negative signal; the NK cell is triggered by signals from activating receptors - Activated NK cell releases granule contents, inducing apoptosis in the target cell

• 1. B-1 Cells - Make Ab
• 2. Epithelial y:g cells - Produce cytokines rapidly
• 3. iNKT cells - Produce cytokines rapidly