Lymphatic System - Network of lymphatic vessels, lymph nodes, and organs that transport cells of the immune system - Lymph is the clear fluid of the network - About 3L of fluid is lost everyday due to capillary filtration - Subclavian veins picks up interstitial fluid and returns it to the circulatory system Secondary lymphoid organs - Sites of adaptive immune cell activation - Lymph nodes - screens lymphatic fluids - Spleen - screens blood - MALTS - screens mucosal surfaces - Tonsils and adenoids Lymph Nodes - Bean shaped tissues - Inline lymphatic filters - Provide immune surveillance for foreign antigens Roles of lymph nodes - Concentration site of B and T cells - Increases efficiency of screening for antigens - Optimizes B and T cell activation Functional regions of the lymph nodes - Cortex - Lymphoid nodules (follicles) (LN) - Paracortex - Medulla - Medullary sinuses (MS) - Medullary cords (MC) Lymph nodes seeding - antigens/antigen presenting cells from infected tissues drain from the afferent lymphatics into the node - Naive Lymphocytes (from bone marrow and thymus) enter the cortex via the high endothelial venules of the vascular supply - Naive lymphocytes congregate and segregate in the: - B cells: concentrate in follicles of the cortex - T cells: concentrate in paracortex Lymph node structural organization - Cortex (follicle) - B cells - Follicular dendritic cells - Paracortex - T cells - Dendritic cells - Fibroblast reticular cells - Medulla - Plasma cells Cortex follicular dendritic cells - Distinct from hematopoietic-derived cells - Promotes the retention of B cells within the lymph node cortex follicle - Promotes the productive movement and interactions of B cells within the lymph node cortex follicle Lymph node paracortex - fibroblast reticular cell conduit - Paracortex fibroblast reticular cells provide a matrix to promote T cell interactions with antigen presenting cells; this occurs by channeling these cell types along extended cellular processes Exodus out the medulla (efferent lymphatics) - what is leaving the medulla? - Conditioned T and B lymphocytes - Antigen presenting cells - Antibodies Spleen - Removal of wasted RBCs - Immune surveillance of blood Microanatomy of the spleen - Capsule: fibrous sheath that surrounds the spleen - Trabeculae: fibrous projections that extend down the capsule (provides structural support) - White pulp includes - Follicles containing B cells - Periarteriolar lymphatic sheath (PALS) - T cells - Marginal zone: border that surrounds white pulp, contains marginal zone, B cells, and dendritic cells - Red pulp: site for recycling wasted RBCs & macrophages Blood flow throughout the spleen - Antigens in bloodstream enter the spleen - B cells in follicles/marginal zone trap antigen - CD4 T cells encounter antigen presented by dendritic APCs within the PALS (T cell zone) - B cells migrate to PALS, activated CD4 T cells then provide help to B cells - Activated B cells and CD4 T cells migrate back to follicles to form germinal centers - Germinal B cells proliferate and turn into plasma cells; these release antibodies into the circulation or become memory B cells Mucosa - Moist inner lining of the surfaces of some organs and body cavities - Glands within mucosa make mucus, which function to lubricate and protect Mucosa associated lymphoid tissue (MALT) - mucosa surfaces are the primary sites of pathogen entry - MALTS are positioned within tissues of mucosa - MALTS protect against exposure to external environments MALT abundance - Contain approx. 70% of immune cells in all secondary lymphoid tissues - Contain more Ab-producing plasma cells than the spleen, lymph nodes, and bone marrow combined MALT subdivisions - BALT - bronchus associated lymphoid tissue - NALT - nasal associated lymphoid tissue - GALT - gut associated lymphoid tissue MALT organs - Tonsils - back of throat - Adenoids - posterior nasal cavity - Appendix - large intestine - Peyers patch - small intestine - Only organ that doesn’t help filter bacteria to prevent infection Innate Immune System Nonspecific internal innate defenses inhibit invaders - Antimicrobial proteins complement - Fever - Inflammation - White blood cells / macrophages / dendritic cells / NK cells Barriers to Infection - Physical/chemical barriers - Cells of the innate response Physical barriers - Epithelial layers - Skin - Mucosa - Glands - Additional barriers - Glycocalyx - Mucus Chemical barriers - Chemicals - Acidic pH - Antimicrobial agents Physical barriers to infection: skin - Stratified epithelium (keratinocytes) - Epidermis - Stratified epithelium - Keratinized layer - Dermis Physical barrier to infection: mucosal epithelia - glycocalyx/mucus layer - Tight junctions Chemical barriers to infection - pH - making the surroundings less hospitable to microbes Directional flow of secreted fluids can carry pathogens out the body - These contain antibacterial/antiviral agents - mucus/urine/milk/sweat Chemical barriers to infection Epithelial secretions: controlling surface microbes populations *now showing class epithelial secretions and example* - Proteolytic enzymes : lysozome Metal ion chelators : psoriasin (Ca2+) Antimicrobial peptides : a-defensin / b-defensin - Defensin functions: - Defend from pathogens - Shape microbiota - Protect stem cells - A-defensin in small intestinal paneth cells (peyers patch) - Collectins : surfactant protein A (SPA) / surfactant protein D (SPD) - Microbe binding proteins - SP-A and SP-D bind different patterns of carbohydrates, lipids and proteins - Neutralization: coating blocks pathogen infection - Opsonization: promotes pathogen clearance - Proteolytic enzyme : lysozyme - Secreted into tears, milk, saliva, respiratory tract - Cleaves peptidoglycan molecules in bacterial cell walls - Metal ion chelators : - Lactoferrin - Calprotectin - Psoriasin Sites and cells of origin - epithelia: constitutive secretion - Neutrophils: secretory granules - Infected tissues: induced expression PAMPs: pathogen associated membrane patterns that are part of a microbe - Composed of: - Proteins - DNA/RNA - Sugars - Lipids - Features: - Distinct to the group of pathogens PRRs: pattern recognition receptors are located on immune cells that recognize and react to PAMPs - Innate immunity: 1st step - Detect and identify PAMPs on microbes - Extracellular surface (on pathogens) : PAMPs - Plasma membrane bound (on immune cells) : PRR PRR location - Located on the cell surface or within the cytoplasm of white blood cells including: - Neutrophils / eosinophils / basophils - macrophages/monocytes - Mast / dendritic / NK cells - Subsets of T and B cells Pathogens distribute intracellularly - PRRs in or on immune cells may recognize PAMPs located within the infected cell - PRRs on immune cells may also recognize damage-associated molecular patterns (DAMPs) on damaged or dying cells due to tissue damage, trauma, or an infection by a pathogen 5 Main PRR families - Toll like receptors (TLR) - C type lectin receptors (CLR) - Retinoic acid-inducible I-like receptors (RLR) - Stimulator of interferon receptors (STING) - Nod-like receptors receptors (NLR) End result of PRR recognition of microbial PAMPs is inflammation Toll-like receptors - PRRs located within cell membranes of immune cells that recognize microbial PAMPs - Detect PAMPs from bacteria, viruses, fungi and parasite - Detect DAMPS from damaged cells and tissues - Several different types of TLRs that recognize different PAMPs - TLRs located on cell membrane/membrane of endosomes Toll like receptor structure - Extracellular ligand-binding domain - Leucine-rich repeats Single transmembrane domain TIR domain (toll intracellular domain) - Helps dimerize with TLRs - Dictates intracellular binding partners - Various TLRs form dimers that re located on the surface of immune cells/surface of cell endosomes TLR locations reflect PAMP distributions - Different TLR forms recognize specific pathogens - Some cell surface TLRs can redistribute to the membranes of cell endosomes and can bind PAMPs in both places Signaling pathways of TLRs - TLR binding to PAMPs = induction of transcription factors that stimulate transcription and expression of anti-microbial proteins and inflammation. - Ex: NF - kB and AP-1 Pattern Recognition receptors on immune cells - C-Type Lectin Receptors (CLR) - Located in the plasma membrane - Downstream signaling pathways - Includes NF-kB and AP-1 transcription factors - Stimulate transcription and expression of anti-microbial and inflammation - Effects: - Induces phagocytosis - Pro-inflammatory signal transduction - Reinoic acid inducible I-like Receptors (RLR) - Detects viral dsRNA; virus specific sequence motifs - Involves mitochondrial antiviral signaling (MAVS); these are proteins that recruit downstream signaling components - Signals thru IRF3 and NF-kB transcription factors - These induce the synthesis and secretion of: - Interferons - Antimicrobial proteins - Chemokines - Inflammatory cytokines Inflammation What causes inflammation? - PAMP recognition by PRRs on innate WBC induces phagocytosis and inflammation Inflammation - Increased diameter of small blood vessels - Vasodilation, warmth, redness - Vascular permeability-leakage of fluids into the infected tissue (edema) Local tissue macrophages are activated when PRRs react with PAMPs of foreign microbes - Activated macrophages release inflammatory mediators: - Chemokines - Leukotrienes - Prostaglandins - Cytokines - Induction of the complement system Chemokines - Chemokines (or chemo-attractants) - activated macrophages in infected tissues release molecules (chemokines) - Chemokines recruit leukocytes to the site of injury by concentration gradient - Attracted cells then gain access to infected tissues (extravasion) Extravasation - Selectins, a cell adhesion molecule, are expressed on vascular endothelial cells - only during inflammatory conditions - Selectins on endothelial cells bind to OSGL-1 adhesion molecules on neutrophils Extravasation - activation & arrest - Chemokines (CXCL-1) bind to receptors on neutrophils; this induces a change in the LFA-1 integrin adhesion molecule - Neutrophil LFA-1 binds to ICAM-1 adhesion molecule on the endothelial cell surface - This causes arrest Extravasation - Diapedesis - Endothelial cell junctions expand and loosen - Neutrophils then change shape and cross the endothelial cell border to enter the infected tissue Cytokine response in inflammation - TNFα, IL-1, and IL-6 are key cytokines generating the inflammatory response. - These cytokines are produced by activated macrophages and other leukocytes at the site of infection - These bind to receptors on leukocytes to stimulate NFkB & other transcription factors; this results in activation of gene expression of other inflammatory products - Other functions: - Enhances synthesis of cytokines from leukocytes Induces fever and vasodilation Activates efficient leukocyte phagocytosis Type 1 interferon - Produced by the following cells: - Virus infected cells - Macrophages - dendritic cells - (produced during the inflammatory response to viral infection) - Interferon is induced by double stranded RNA or proteins in virus infected cells - Attaches to receptor on neighboring cells, which induces an antiviral state Interferon response to virus infection - Interferon induced antiviral proteins inhibit viral replication Phagocytosis - In tissues: - Macrophages - Dendritic cells - Neutrophils - In vasculature: - Monocytes 5 steps in phagocytosis - Phagocytosis begins when PRRs on leukocytes recognize PAMPs on pathogens 1. Bacterium becomes attached to pseudopodia, a membrane evaginations 2. Bacterium is ingested, which forms phagosome 3. Phagosome fuses with lysosome 4. Bacterium is killed & digested by lysosome enzymes 5. Digestion products are released from cell Lysosomal degradation in phagocytosis - Antimicrobial peptides - Defensins & cathelicidins - Reactive oxygen species - Low pH - Acid-activated enzymes What can cause death in phagocytosed microbes? - Reactive death and nitrogen species produced in activated macrophages - During phagocytosis, leukocytes have a respiratory burst - This increases oxygen consumption and generating toxic reactive oxygen species Enzymes produced during inflammation - Inducible nitric oxide synthase - Generated by activated macrophages and other leukocytes - Produces nitric oxide, which kills microbes - Cyclooxygenase -2 (COX-2) - Generated by activated macrophages and other leukocytes - Converts lipid arachidonic acid to prostaglandins (proinflammatory mediators) Prostaglandins play a major role in inflammation Chronic Inflammation - A slow, long term inflammation that lasts for prolonged periods of time - The extent and effects of chronic inflammation vary on the cause of injury and the body’s ability to repair and overcome the damage AntiInflammatory medications Corticosteroid anti-inflammatory drugs - steroid hormones normally produced by the adrenal cortex - These drugs induce prednisone and dexamethasone - Used to treat a variety of inflammatory diseases, such as: - Asthma - Eczema - Arthritis - Mechanism of action - Inhibits NFkB transcription factor - Blocks transcription and gene expression of inflammatory proteins - chemokines, cytokines, interferons Nonsteroidal anti-inflammatory drugs - Reduces inflammation, pain, and fever - Aspirin, advil, aleve Production of prostaglandins - Prostaglandins are lipids made at sites of tissue damage or infection that mediate inflammation - NSAIDs block inhibit cyclooxygenase (COX 1 and 2) to block prostaglandin synthesis - Since all NSAIDs work the same, they shouldn’t be combined Acetaminophen (tylenol) - A pain reliever but not anti-inflammatory - Inhibits prostaglandin synthesis, but not in the same way as NSAIDs - This means it could be combined with NSAIDs
