Exam 1 Key Terms/Ideas
Levels of receptor and ligand expression can vary during an immune response
-Aggregation due to ligand binding enhance binding Kd - Cell-cell interactions rely on a binding affinity to maintain contact over long periods of time - Extended contact facilitates signal transduction and exchange of cytokine signals
Tertiary lymphoid tissues also organize and maintain an immune response
-Tertiary lymphoid tissue is a site of active infection and immune activity. - Can generate new microenvironments that organize lymphocyte responses - Common at sites of chronic infection (worn in over time)
What points in complement activation are regulated? Why is regulation of complement activation important?
1) initiation 2) amplification - by DAFS + C3 convertases 3) Effector function - protectin/S protein (MAC), Carboxypeptidases (anaphylotoxin), FI (opsonization) To protect self cells from opsonization, lysis, unnecessary inflammation
What are the two lines of defense that comprise the innate immune system?
1) physical/ chemical 2) cellular
What cells in your body express pattern recognition receptors (PRRs)? Where in/on/around the cell would you expect to find PRRs located?
1. Innate immune response, epithelial barriers 2. Located on plasmid membrane (recognize extracellular), cytosol, nucleus, endosomes/lysosomes
1. In general, which cell lineage is involved in the innate immune response? 2. In general, which cell lineage is involved in the adaptive immune response?
1. Myeloid 2. lymphoid
Four main types of cells develop from common myeloid progenitors
1. Red blood cells (erythrocytes) 2. monocytes - migrate into tissues and differentiate into macrophages - function to repair/remodel, destroy pathogens, present antigens - can also differentiate into dendritic cells - HIGH degree of function as "ingesters" of antigens, followed by presentation to naive T lymphocytes for initial activation 3. granulocytes - Neutrophils - direct harm to pathogens - Basophils (tissues)/mast cells (blood) - inflammation/allergies (herapin, histamine) - Eosinophils - antiviral activity, antiparasite activity 4. megakaryocytes
Neutrophil extracellular traps (NETs) and NETosis
4 steps in lecture slides
Receptors and Cell Signaling
A cellular signal is any event that instructs a cell to change its metabolic or proliferative state Signals are usually generated by the binding of a ligand to a complementary cell-bound receptor A cell can become more or less susceptible to actions of a ligand by increasing or decreasing expression of the receptor for that ligand Cell signaling often induces a change in the transcriptional program of the target cell Sometimes multiple signals through multiple receptors are required to effect particular outcomes Integration of all signals received by a cell occurs at the molecular level inside the recipient cell
Interactions between innate and adaptive immune systems
A constant interplay between the two systems exists - Several innate systems have been co-opted by adaptive immunity to contribute to antibody- mediated pathogen elimination - Opsonization, complement activation Some lymphocytes express TLRs, but use them as costimulatory receptors Dendritic cells are a key bridge - They bring antigens from the site of infection and present them to T cells in lymph nodes - This activates the T cells, allowing them to differentiate into particular pathogen-specific subsets for the best antigen clearance - TH cell subsets - Tc cells PRR > Dendritic cell > cytokine or PRR > Dendritic cell > break down > TH cell > differentiate
B-cell receptor structure
A quaternary protein with two identical heavy chains and two identical light chains Antigen specificity is by the interaction between light/heavy chain variable regions Antibody effector activity, e.g., phagocytosis and complement fixation, is a function of the interaction of the constant regions of the heavy chain Three hypervariable regions of amino acids are found in variable heavy (VH) and variable light (VL) regions - These variable regions come together to form the antibody combining site - These regions are known as complementary-determining regions known as CDR1, CDR2, and CDR3 in VH and VL - Interspersed near each CDR is an invariant amino acid that forms the framework region and is responsible for the folding of CDRs to form the antibody combining site
ALRs bind cytosolic DNA
AIM2-like receptors (ALRs) are cytosolic receptors that bind bacterial and viral double-stranded DNA Binding of multiple ALRs to dsDNA via pyrin domains yields long filaments that together with other cellular proteins form inflammasomes Inflammasome generates IL-1β and IL-18
Epithelial layers produce protective substances
Acidic pH Enzymes and binding proteins Antimicrobial peptides Antimicrobial proteins and peptides kill what would be invaders Pathogens can evade anatomical barriers to infection
Innate and adaptive immunity work cooperatively
Activation of innate immune responses produces signal molecules (often cytokines) These signal molecules stimulate and direct adaptive immune responses
When an antigen binds to its receptor, what cell signaling steps may happen next? What key features do we see in this cell signaling diagram?
Adapter protein: provides scaffold Tyrosine kinase: phosphorylation Changes in gene expression from NF-KB Integration of pathways NF-KB bound to IKB(P) (diagram in CH 3 in class activity)
Where in the cell do you expect to find receptors of the adaptive and innate immune response? What cells express these receptors?
Adaptive; plasma membrane or circulating. Cells expressing: B cells (BCR) or T cells (TCR) Innate; (PRR) plasma membrane, cytosol, nucleus, endosomes (taking things in) Cells expressing: neutrophils, dendritic cells, macrophages, most nucleated cells
Protein partners of the B-cell receptor
Antibody molecules form a B-cell receptor (BCR) complex with molecules involved in signal transduction - IgA, IgB: transduce signals via ITAMs (Immunoreceptor typrosine-based activation motifs) - CD19, CD81, CD21: transmit and relay signals to cell interior (CD21 also serves as a coreceptor)
Immune Responses:the outcomes of immune system recognition
Antigen signaling includes: Bringing dendritic cells into the required locations Macrophages and neutrophils upregulate phagolysosome activity and cytokine production Dendritic cells exhibit antigen peptides on MHC class I and MHC class II Cytoplasmic proteasomes process antigen to peptides Dendritic cells induced to secrete cytokine
Ligand binding can induce dimerization or multimerization of receptors
Antigen-mediated receptor clustering initiates signaling in B and T cells - Receptor dimerization is often a result - Multimerization can also occur - Clustered receptors are localized in lipid rafts
PRR signaling pathways activate expression of a large variety of genes
Antimicrobial peptides Type I interferons (potent antiviral activity) Cytokines (inflammatory IL-1, TNF-a, and IL-6) Chemokines Enzymes: iNOS (reactive nitrogen) and COX2 (both generate inflammatory molecules)
Isotypes of B-cell receptor
Antiserum to the constant region of the heavy chain identifies five distinct classes of antibody called isotypes - IgA, IgD, IgE, IgG, IgM Light chain isotypes are: - Kappa and lambda
Secondary lymphoid organs: where the immune system response is initiated
Areas where lymphocytes encounter antigens, become activated, undergo clonal expansion, and differentiate into effector cells Secondary lymphoid organ areas include: lymph nodes, spleen, mucosa-associated lymphoid tissue (MALT), other diffuse and loosely organized areas These are connected to each other via the blood and lymphatic circulatory systems
Complement aids in the contraction phase of the immune response
As lymphocytes are no longer required, complement aids in disposal of apoptotic cells and bodies Complement also aids in removal/disposal of immune complexes formed during responses These responses avoid damaging inflammation induction in the absence of antigens following clearance of an infection extra immune cells transported to liver/spleen for phagocytosis (too much can be damaging. Ex. lupis)
CH 1 REVIEW QUESTIONS: Application Question: Vaccination is one approach to immunity. Recently we have started seeing that vaccinated individuals with symptomatic COVID-19 show symptoms of COVID-19 earlier in the course of their infection than unvaccinated individuals. Why might we expect this result?
B cell (humoral) T cell (cell-mediated) Antibodies (humoral) Fast (innate) Slow (adaptive) Memory (adaptive) nonspecific (innate) specific (adaptive) phagocytic (innate) humoral & cell mediated responses (adaptive) AQ: Vaxx have prior exposure (recognition) which leads to more rapid responses. Symptoms due to immune response (working!).
Lymphoid cell lineage
B cells, T cells (include NKT cells), NK cells Lymphocytes appear very similar, but different sets carry different clusters of differentiation (CD) molecules on their surface. B cells express the B-cell receptor (BCR) - soluble antigen binding to B cell T cells express the T-cell receptor (TCR). - APC presentation to a T cell (CD, MHC, peptides)
Immune antigen receptor systems
B-cell receptor (BCR) contains an antibody of defined specificity T-cell receptor (TCR) specificity is for peptides derived from APC-degraded antigen presented on MHC molecules CD4 and CD8 are T-cell coreceptors that define different subsets of T-cell function
The C1 inhibitor, C1INH, promotes dissociation of C1 components
Binds in the active site of serine proteases Causes C1r2s2 to dissociate from C1q No further cleavage of C4 or C2 is possible Inhibits initiation of classical and lectin complement pathways
CD21 (CR2) acts as a coreceptor on B cells
Binds to C3b breakdown products (C3d, etc) on opsonized bacteria/antigens Helps provide secondary signals to B cells through BCR complex for more efficient activation (avidity)
C3aR/C5aR on granulocytes
Binds to either C3a or C5a Stimulates release of proinflammatory cytokines and granule components from basophils, eosinophils, neutrophils (amplification step)
Where would you expect to find hematopoietic stem cells in an adult human?
Bone marrow
Path of T cell from development to immune response
Bone marrow --> Thymus --> lymph node, spleen, MALT
Path of a B cell from development to immune response
Bone marrow --> lymph node (follicle)
The three complement pathways converge at the formation of the C5 convertase
C5b initiates the generation of the Membrane Attack Complex (MAC) Membrane attack complex is the result of deposition of C5b, C6, C7, C8, and C9 in target cell membranes This pore structure disrupts osmotic integrity, resulting in cell death
Tyrosine phosphorylation is an early step in many signaling pathways
CD3 (T cells) and Igα/β (B cells) are phosphorylated on ITAMs Immunoreceptor tyrosine activation motifs (ITAM) Phosphorylated tyrosines serve as docking points for adapter molecules
You are a physician scientist working with a family that is prone to fungal infections. Genome sequencing and further experiments revealed that members of this family have an inactivating mutation in a gene of interest. What do you predict that this gene encodes? There are probably more than one good answer.
CLR --> Pattern recognition receptor for fungi or Downstream of CLR could be mutated
Regulated cell death contributes to pathogen elimation
Cell death induced by receptor-activated signal pathways Apoptosis induced by TNF binding (innate cell signaling) in TNFR, NK cells, and T cytotoxic cells Neutrophil extracellular traps (NETs) activation requires NADPH oxidase and generation of ROS Pyroptosis induced by inflammasome activation eliminates infected macrophages allowing release of IL-1β and IL-18
Chemokines direct movement of leukocytes
Chemokine structure: small (7.5-12.5 kDa) proteins with highly conserved disulfide bonds that dictate both structure and category Chemokine receptors are G-protein-coupled receptors -Seven-pass transmembrane receptors - Tranduce signals via interactions with a polymeric GTP/GDP-binding G protein - Many receptors can bind to more than one chemokine; and several chemokines are able to bind to more than one receptor Chemokines direct leukocyte migration - Signaling through chemokine receptors helps cells move to different body areas
Class 1 Cytokine Family
Class I cytokines are diverse in action and cell target Structurally class I cytokines are a single protein family (similar structure) Class I cytokine receptors are made up of multiple subunits
Draw a chart or concept map of the 3 major pathways of complement activation including: factors that initiate each pathway, the types of enzymes involved, and outcomes. What's different and what is the same?
Classical: antibody/antigen complexes Alternative: spontaneous hydrolysis Lectin: PAMP recognition - All use C3+C5 convertases - Serine proteases used by lectin and classical paths
Cell membrane proteins expressed by cells of the immune system are referred to by ___.
Cluster of differentiation (CD)
Immune Homeostasis
Commensal organisms that live in and on us that cause no harm. Function in metabolic and immune balance (homeostasis). Imbalance or dysbiosis leads to immune overstimulation resulting in inflammation. (dietary changes, environment, stress, etc).
The regulation of complement activity
Complement activity is passively regulated by protein stability and cell-surface composition (general) Short half-life of C3 convertase unless stabilized by properdin (5 minutes) Self-cells possess different carbohydrate structures that are more effectively bound by fluid-phase proteases - These more readily inactivate C3b through hydrolysis, protecting self-cells
What is a cytokine? What is a chemokine? How are they related?
Cytokine; signaling protein Chemokine; type of cytokine that causes cell migration (chemotaxis)
General properties of cytokines and chemokines
Cytokines are proteins that mediate the effector functions of the immune system They can act in several different ways: Endocrine action - released into the bloodstream to affect distant cells Paracrine action - released to effect nearby cells Autocrine action - released, but then bind to receptors on the cell that produced them
Cytokines and their receptors
Cytokines: proteins used by cells to communicate Cytokine signals are usually generated by the binding of a ligand to a complementary cell-bound receptor Cytokine-receptor binding is noncovalent, although it may be of generally high affinity Cytokine-signaling end results often induce a change in the transcriptional program of the target cell A cytokine signal is any event that instructs a cell to change its metabolic or proliferative state Chemokines are cytokines that attract cells (direct immune cell movement)
Phagocytosis
Defined as engulfment and internalization of materials such as microbes for their clearance and destruction Microbes are recognized by receptors on phagocytes - May recognize PAMPs directly (5 Steps in lecture slides)
Innate lymphoid cells (ILCs)
Derived from common lymphoid progenitor cells Include NK cell and six other ILC populations - NK cells found in lymphoid tissue and recirculate, other ILC mainly found in epithelium - Each group of ILC based on cytokines produced - ILCs lack PRRs and thus and not activated directly by pathogens
TLR binding of PAMPs activates signaling pathways
Different TLRs recruit different adapter proteins to the Toll/IL- 1R domain Different adapter proteins lead to different events (after activation) - MyD88 - TRIF Pathways include: NF-κB transcription factor activation Interferon regulating factor (IRF) pathways MAP kinase pathway downstream transcription factors such as AP-1
Generation of memory T and B cells in the lymph node
Differentiation into effector cells takes place in follicles of secondary lymphoid organs B cells further mature in germinal centers - Antigen affinity is increased - Class switching can take place Both B and T lymphocytes will develop into long-lived memory cells in these areas
Primary lymphoid organs: where immune cells develop
During embryogenesis and the fetal period, blood cell formation shifts from site to site. - Hematopoiesis begins in the yolk sac - Fetal HSCs arise near the kidney - Mature HSCs capable of populating the hematopoietic system can be isolated from yolk sac, placenta, and fetal liver - HSCs seed the bone marrow late in fetal development - HSCs ultimately populate the bone marrow postnatally Order: Yolk sac > kidney > placenta/fetal liver >> bone marrow *post fetal* Bone marrow is the main site of hematopoiesis in the adult. - B lymphocytes develop in contact with stromal cells of the bone marrow - Stromal cells facilitate HSC proliferation, direct migration, and stimulate differentiation The thymus is the primary lymphoid organ where T cells mature - T cells develop initially in the bone marrow but then migrate to the thymus to achieve full maturity - The microenvironment of the thymic cortex and medulla directs stepwise changes in thymocytes - TCR affinity of binding with MHC-peptides drives positive and negative selection
Complement acts at the interface between innate and adaptive immunities
Enhances antigen uptake of antigen bound to MBL, C1q, C3b, and C4b that binds receptors on APCs (increasing avidity) Enhances B-cell responses by an increasing avidity of B-cell binding to complement-bound antigen Lyse immature T cells with low sialic acid content, a carbohydrate that increases in concentration as a protective coating on maturing T cells (weed out underdeveloped T-cells) Binding of C3a, C5a, and C3b to their receptors on mature T cells facilitates their growth, differentiation, and survival
Anatomical barriers to infection
Epithelial barriers prevent pathogen entry into the body's interior - skin - mucosal membranes
Name one (or more) physical barrier that is part of the innate immune system
Epithelial, lungs, mouth, mucus, tears, urine
Cellular innate response receptors and signaling
Families of PRRs recognize a wide variety of PAMP ligands Toll-like receptors(TLRs) C-type lectin receptor(CLR) RIG-I-like receptor(RLR) NOD-like receptors(NLRs) AIM2-like receptors(ALRs) Signaling pathways are activated, contributing to innate/inflammatory responses
Innate immune responses
First line of defense (Fast, but nonspecific) Uses germ-line-encoded recognition molecules Also uses phagocytic cells
C-type lectin receptors (CLRs) activate innate and inflammatory responses
Generally recognize carbohydrate components of fungi, viruses, mycobacteria, parasites, allergens Trigger signaling pathways leading to transcription Transcription factors induce expression of proinflammatory cytokines, IL-1β, TNF, and IL-23 (having 15 receptors allows for more recognition)
The evolutionary origins of the complement system
Genes for complement components belong to five families - Alternative pathway genes appear first in evolution -Terminal complement components appear last Five families of genes for complement components - Complement served to assist phagocytosis prior to evolution of adaptive immunity
Complement deficiencies
Genetic deficiencies have been described for each of the complement components, but outcomes vary - Patients with any C1q, C1r, C1s, C4 or C2 deficiency often present with immune complex disorders due to inadequate clearance - Some with MBL deficiency may exhibit greater frequency of infections by encapsulated bacteria due to inefficient opsonization and phagocytosis Animal models exist for most complement deficiencies, allowing for further study
The complement system
Group of serum proteins circulating in an inactive form Once activated, multiple possible outcomes: Target cell membrane lysis Chemotaxis Opsonization to enhance phagocytosis Inflammation 50 proteins found in blood plasma, surface of cells
Toll-like receptors (TLRs) recognize many types of extracellular pathogen molecules.
Homologous to fruit fly Toll (cool) receptor Dimers with extracellular leucine-rich (LRR) domains that bind PAMPs and DAMPs Toll-like receptors recognize many types of pathogen molecules (found throughout the cell) - Location helps determine what each TLR will bind
How are foreign substances recognized by the immune system?
Humoral and cell-mediated immunity relies on surface receptors (B and T cell receptors). These are randomly generated by gene segment rearrangements. B cells that encounter antigen that binds to their immunoglobulin receptor survive and produce and refine their antibody. T-cel receptors bind specific peptides presented by MHC molecules.
Adaptive immune responses
Humoral and cell-mediated responses (Using B and T lymphocytes, respectively) (Slower to develop) (5-6 days (or more)) Use randomly generated antigen receptors Highly specific to individual antigen molecules
Cytokines of the IL-1 family promote inflammation
IL-1 stimulated by viral, parasitic, or bacterial antigens Secreted very early in immune responses by macrophages and dendritic cells Acts locally on capillary permeability and to pull leukocytes to infected tissues Acts systemically to signal the liver to produce acute phase proteins Can help to activate adaptive immune responses
IL-17 Family Cytokines
IL-17 Family cytokines are proinflammatory molecules expressed on a variety of cells - Receptors found on neutrophils, keratinocytes, and other nonlymphoid cells - Tend to work at the interface of innate and adaptive immunity
Many immune receptors include immunoglobulin domains
Immune receptors bear immunoglobulin domains Immune receptors can be transmembrane, cytosolic, or secreted Immunoglobulin lacking the carboxyl terminus transmembrane segment is secreted
Cells of the immune system derive from HSCs
Immune responses result from coordinated activities of many cells, organs and microenvironments in the body. Hematopoietic stem cells (HSC) have the ability to differentiate into many types of blood cells - all red and white blood cells develop from a pluripotent HSC during hematopoiesis. - Hematopoiesis is a highly regulated process. In adult vertebrates, hematopoiesis occurs in the bone marrow. Within the bone marrow, HSCs are constantly renewed and directed to differentiate into two major types of progenitor cells. - Common myeloid - Common lymphoid
Immunis meaning "exempt"
Immunology
What are the 3 main outcomes of complement activation as part of immune response?
Inflammation <-- C3A + C5A Lysis (MAC) Opsonization (phagocytosis) <-- C3B
The alternative protease-activated pathway
Initiation of clotting cascades has also been shown to stimulate cleavage of complement proteins - Thrombin cleaves C3 and C5 in vitro - Platelet activation releases ATP, calcium ions, and serine/threonine kinases that could stabilize C3b in fluid phase This indicates that strong inflammatory reactions could potentially activate complement systems
Ubiquity of innate immunity
Innate immunity is evolutionarily "older" than adaptive - TLRs are unique to animals - PRRs with leucine-rich repeats (LRRs) are found in virtually ALL higher plants and animals
Innate immunity vs adaptive
Innate: minutes to hours, limited and fixed (specific), response is same each time, major components; barriers, phagocytes, PRRs Adaptive: days, highly diverse (adapts to improve during the course of immune response, nonspecific), more rapid response and effectiveness with each subsequent exposure. Major components; T and B lymphocytes, antigen-specific receptors, antibodies
Match the class of cytokine with one of its key features:
Interleukin-1 family: secreted early in immune response to promote inflammation Class 1: Grouped based on a common structure Class 2: Interferon family; includes antivirals Tumor necrosis (TNF) family: secreted or membrane-bound; developmental roles Interleukin 17 family: Most recently discovered; promote inflammation Chemokines: serve as chemoattractants
What is immunity?
It is the state of protection against foreign pathogens or substances (antigens). Observations go back 2000 years (Thucydides wrote of plague in 430 BC)
Application: An individual is being treated for cancer via chemotherapy. This chemotherapeutic drug affects all cell division. What outcomes would you expect regarding this individual's immune system? Why?
It would be weakened; less clonal expansion. Less hematopoietic stem cell division happening.
Inflammatory responses
Later stages of inflammation are the acute phase responses (APRs) Induced by proinflammatory cytokines (IL-1β, TNF-α, IL-6) APR involves: Increased synthesis/secretion of antimicrobial proteins by the liver MBL, CRP, Complement components
Cell surface receptors cluster when binding multivalent antigens
Ligand-receptor binding induces molecular change in the receptor - conformational, dimerization/clustering, location in the membrane, covalent modification Receptor alterations induce cascades of intracellular events - Activation of enzymes, changes in intracellular locations of molecules (down stream cascade)
CR1(CD35) on leukocytes and erythrocytes
Ligands include: C3B, C4b, C1q, MBL (binds) On erythrocytes, helps to bring immune complexes to the liver for clearance by phagocytes On phagocytes, helps bind to complement-coated bacteria to enhance ingestion and destruction On B cells, helps bind to complement-coated antigens, enhancing ingestion for processing and presentation to helper T cells
Lymph Nodes (secondary lymphoid organ)
Lymph nodes: B-cell and T-cell activity is separated into distinct microenvironments, cortex and paracortex, respectively Macrophages and dendritic cells are found in the innermost lymph node medulla Antigen enters via afferent vessel Naive lymphocytes enter by the High Endothelial Venule (HEV), lymphocytes exit via an efferent vessel. Lymphoid cells actively migrate toward each other during activation events for their required interactions Lymphoid fibroblastic reticular cell conduit (FRCC) guides T cells and APCs toward activation interactions Differentiation into effector cells takes place in follicles of secondary lymphoid organs: - B cells undergo clonal expansion in germinal centers - CD4+ T cells differentiate into helper T cells that assist in B-cell differentiation - CD8+ T cells differentiate into killer (or cytotoxic) T cells that destroy virally infected cells
What are the two "branches" of the hematopoietic family tree? In other words, what two types of cells can an HSC make in addition to making another HSC?
Lymphoid and myeloid
Complement enhances host defense against infection
MAC-induced cell death Promotion of inflammation Promotion of opsonization - Opsonized microbes easier to ingest/destroy - Opsonized immune complexes easier to clear
Phagocytosis and antigen-presenting cells
Macrophages and neutrophils are specialized for phagocytosis . Macrophages can also present antigens to T cells via MHC molecules. Immature dendritic cells capture antigen, then mature and migrate out of that location to another to present antigen to T cells. - Dendritic cells are the most potent antigen-presenting cells for activating naive T cells
Structures of opsonins
Mannose-binding lectin (MBL found in blood and resp fluids) C1 bound to LPS
If you were a pathogen and could direct your own evolution, what complement evasion strategy would you want to have?
Mimic C3 convertase protease - Knock out all 3 pathways
Pathogen-associated molecular patterns (PAMPs)
Molecules associated with groups of pathogens that are recognized by cells of the innate immune system (first recognition system)
Barrier organs also have secondary lymphoid tissue
Mucosa-Associated Lymphoid Tissue (MALT) - important layer of defense against infection at mucosal and epithelial layers - Organizes responses to antigens that enter mucosal tissues - Includes a network of follicles and lymphoid microenvironments associated with the intestines (gut-associated lymphoid tissue, or GALT)
Interactions between receptors and ligands can be multivalent
Multivalency increases avidity of the interactions - Individual interactions have an affinity, the strength of an individual bond - Avidity is the combined strength of binding of multiple interactions - An interaction may have weak affinity but high overall avidity
What cells work as part of the innate immune system? What characteristics and/or processes do you remember about these cells?
Myeloid branch --> innate lymphoid cells Phagocytosis, regulate cell death, antigen presentation, degranulation, NETosis
Natural killer (NK) cells are ILCs with cytotoxic activity
NK cells are lymphocytes with innate immune functions Express a limited set of receptors for self-proteins induced by: - Infections • Malignant transformations - Other stresses NK cells also express inhibitory receptors that recognize normal healthy cells Activated NK cells perform one of two functions: - Kill the altered self-cell by releasing perforin and granzymes inducing apoptosis - Produce cytokines that induce adaptive responses against the altered self-cell
NLRs bind PAMPs from Cytosolic Pathogens
NLR=NOD-like receptor OR nucleotide oligomerization domain/leucine-rich repeat- containing receptor NOD1 and NOD2 bind PAMPs of bacterial cell walls (diaminopimelic acid and muramyl peptides), protozoan parasites, and some viruses. Induce expression of genes encoding antimicrobial proteins and peptides Initiate autophagy by forming autophagosomes that fuse with lysosomes to then kill bacteria Can form inflammasomes and induce pyroptosis - highly inflammatory programmed cell death
Is this cell derived from the ___ or ___ cell lineage?
Neutrophil; myeloid T helper cell; lymphoid Macrophage; myeloid Dendritic; both Eosinophil; myeloid Innate lymphoid cell (ILC); lymphoid Erythrocyte; myeloid Cytotoxic T cell; lymphoid
Cells in human blood differ in number and life span
Neutrophils: 6 hours to 2 days (shorter lived) Monocytes: days to months
Dysfunctions of immunity
Overly active (misdirected): allergies/asthma. Autoimmune diseases (MS, Crohn's) Immunodeficiency: primary (genetic) loss of function. Secondary (acquired) loss of function (HIV, Cancers). These are opportunistic infections (oral thrush) can occur in people with impaired immune responses.
What receptors are associated with the innate immune response? What receptors are associated with the adaptive immune response?
PRR; B cell receptor/ T cell receptor
Compare PRRs and BCR/TCR
PRRS: innate PAMPS Families Multiple PRRs per cell Multiple locations in cell Bind anywhere Similar within family (bio) BCR/TCR: Specific antigen binding Variable and constant regions (heavy/light) 1 BCR/TCR per cell transmembrane BCR --> secreted TCR --> MHC BCR --> Bind directly to antigen Antibodies from mom but will be individualized later on
The classical pathway is initiated by antibody binding to antigens
Part of adaptive immune response IgM and IgG binds to a multivalent antigen This allows the binding of C1q, beginning the process of complement deposition C1 binds Fc on adjacent IgG molecules or on IgM bound to antigen C1qrs initiates a cascade of reactions enabling the next reaction in the sequence C1 binding is followed by cleavage of C4, then C2 C4b2a bound to the cell surface is C3 convertase C3 convertase cleaves many C3 proteins Some combine with C3 convertase to form C5 convertase C5 convertase cleaves C5 protein
The major pathways of complement activation: Lectin pathway
Part of innate immune response (detects PAMPs) The lectin pathway is initiated when soluble proteins recognize microbial antigens - Lectins (e.g., mannose-binding lectin, or MBL) bind to microbial surfaces - Lectins can serve as docking sites for MBL-associated serine proteases (MASPs) - MASPs cleave C4 and C2 to form the C3 convertase - Subsequent steps are the same as the classical pathway (see on viruses, bacteria, fungi)
The major pathways of complement activation: alternative pathway
Part of innate immune system The alternative pathway is initiated in three ways The alternative tickover pathway - Small amounts of C3 are always being cleaved - Usually quickly inactivated if nothing is around for it to bind to Activated C3b binds to membrane of target cell - Factor B binds and is cleaved by Factor D - C3bBb at the membrane is the C3 convertase - Properdin stabilizes the C3 convertase - After properdin stabilizes the C3 convertase, it cleaves many more C3 proteins - Newly active C3b bins to C3bBb to form C3bBbC3b, or the C5 convertase - C5 convertase cleaves many C5 proteins
Pattern recognition receptors recognize PAMPS/MAMPS/DAMPS
Pathogen associated molecular patterns (PAMPs), Microbe associated microbe patterns (MAMPs), and damage-associated molecular patterns (DAMPs) PAMPS present motifs of recurring patterns on bacteria, yeast and parasites Receptors for PAMPs may uniformly recognize large numbers of bacteria that share the same PAMPs Receptors for PAMPs are not clonally distributed (all cell types have same PRRs), but are expressed equally on the same cells Receptors for PAMPs may be integral membrane proteins or intracellular proteins
Ingested materials are taken into phagosomes
Phagosomes are fused with lysosomes or granules - Destruction occurs through enzyme degradation, antimicrobial proteins, and toxic effects of reactive oxygen and reactive nitrogen species (ROS and RNS) (oxidative attack)
Biological effects of cytokines
Pleiotropic activity induces different biological effect dependent on target cell Redundant activity mediates similar effects on target cell Synergy effect combines two cytokine activities to be greater than additive effect Antagonistic effect inhibits one cytokine's effect by another's action Cascade effect of one cytokine on one target cell to produce additional cytokine(s)
Match the type of lymphoid tissue to its main function and give an example:
Primary lymphoid tissue of chronic immune response; location of immune cell development Secondary lymphoid tissue cell development; site where an immune response initiated Tertiary lymphoid tissue response is initiated; loosely organized site of chronic immune response
Memory is the hallmark of adaptive immunity
Primary response is initiated upon first exposure to an antigen. (memory lymphocytes are left behind after antigen is cleared). A secondary response is initiated upon second exposure to the same antigen that stimulates memory lymphocytes (stimulation yields faster, and more significant better responses) Memory is NOT present in innate immunity!
Local inflammation is triggered by innate immune response
Proinflammatory cytokines and chemokines triggered by innate responses to infection, damage, or harmful substances Early components of inflammation include - Increased vascular permeability - Recruitment of neutrophils and other leukocytes from the blood to the site of damage/infection
The alternative properdin-activated pathway
Properdin can directly bind to a surface - This can then recruit C3b and Factor B - Factor D is recruited and cleaves Factor B into Bb - The resultant C3bBb is an active C3 convertase - Subsequent steps identical to alternative tickover pathway (differences between this and alternative pathway is the ORDER of binding. Here, properdin binds first/early on)
Prevention of the MAC Attack
Protectin (CD59) inhibits the MAC attack - Binds C5b678 complexes deposited on host cells - Prevents their insertion into the plasma membrane - Also blocks C9 recruitment, preventing MAC formation Similarly, soluble complement S protein (vitronectin) binds fluid phase C5b67 to prevent insertion into host cell plasma membranes
Protein kinase C signaling
Protein Kinase C (PKC) activates _ Nuclear factor kappa B (NF-κB) NFκB inactive in the cytoplasm when bound to inhibitor (IκB) PKC phosphorylates IκB releasing it from NFκB - Diacylglycerol activates PKC, activating IKK complexes - IκB phosphorylation and ubiquitination targets it for destruction - NF-κB translocates to the nucleus to enhance transcription
The Ras/MAP kinase cascade activates transcription through AP-1
RasisaGprotein - Activated when GTP exchanged for GDP Guanine-nucleotide Exchange Factors (GEFs) activate Ras by inducing this exchange - GTPase activating proteins (GAPs) inhibit Ras - by stimulating its ability to break down GTP into GDP Ras (once active) participates in downstream signaling events - The Ras/MAP kinase cascade activates transcription through AP-1 -AP-1 can facilitate transcription of the IL-2 gene
Common features of receptor-ligand interactions
Receptor-ligand binding occurs via multiple noncovalent bonds - Each individual bond may be weak but in total deliver a strong binding affinity - Many such bonds occur between receptors and ligands, providing great cumulative bond strength - Dissociation constant (Kd) is a measure of strength of ligand binding
What is the role of pattern recognition receptors (PRRs) in our immune response? Are they part of the innate or adaptive immune response?
Recognize PAMPs on pathogens (innate) --> signaling activates adaptive immune response
RIG-I-like receptors (RLRs)
Recognize viral double-stranded RNAs RNA bound by RLR helicase domain Function as cytosolic PRRs Trigger signaling pathways that activate: - IRFs to trigger antiviral interferon responses - NF-κB transcriptionfactor
The spleen is the first line of defense against bloodborne pathogens
Red Blood Cells (RBC) are compartmentalized in red pulp White blood cells are segregated in white pulp A specialized region of macrophages and B cells known as the marginal zone borders the white pulp
Regulation and evasion of innate and inflammatory responses
Regulation and control of these responses are important Defects in PRRs and signaling pathways increase susceptibility to infections Defects that allow the systems to remain abnormally "turned on" contribute to inflammatory disorders - These can be cases where more and more of a good thing ends up being unhealthy and damaging Regulation includes both positive and negative feedback mechanisms Pathogens have evolved strategies to block, evade, and escape these responses SARS-CoV-2 nucleocapsid protein represses signaling via RIG-I to prevent IFN-Beta release
Carboxypeptidases can inactivate the anaphylatoxins C3a and C5a
Remove arginine residues from the C termini of C3a and C5a - Creates des-Arg (without arginine) inactive forms - Helps to shut down unnecessary or dangerous chemotactic and inflammation induction
3 lines of defense against infection
Several barriers, both physical and chemical, exist to prevent pathogens from gaining access to deep tissues - Should those barriers be breached, innate immune system receptors recognize the threat - Conserved pathogen-associated molecular patterns (PAMPs) found on microbes Aging, dead, or damaged self-structures can also be recognized - Damage-associated molecular patterns(DAMPs) Pattern recognition receptors (PRRs) recognize these structures and target them for clearance Barriers are just ONE difference between innate and adaptive immune responses - innate is older - adaptive is in vertebrates
Decay accelerating factors promote decay of C3 convertases
Several different proteins with similar activities - DAF (CD55), CR1, C4BP (C4 binding protein) - Factor H binds negatively charged cell surface sialic acid and herapin, molecules unique to eukaryotic cell surfaces Work to accelerate the decay of C4b2a (C3 convertase) on the surface of host cells
Why are ITAMs important? What would you expect if an ITAM was altered in a way to prevent its activity?
Signal transduction; lack of immune response
Adapter proteins gather members of signaling pathways
Signal-inducing phosphatidyl bisphosphate (PIP2) breakdown by phospholipase C gamma (PLCγ) causes an increase in cytoplasmic calcium ion concentration Calcium ions can then bind several cellular proteins, changing their conformation and altering activity Calmodulin (CaM) is an important member of this group
Factor I degrades C3b and C4b
Soluble, constitutively active serine protease Cleaves membrane-associated C3b and C4b into inactive fragments - Requires MCP (BD46) and CR1 (found on membranes of host cells) to function (cofactors which pathogens lack)
Ways pathogens avoid complement interaction
Some interfere with the first step of Ig-mediated complement activation Microbial proteins may bind and inactivate complement proteins Microbial proteases destroy and complement proteins Some microbes mimic or bind complement regulatory proteins
Src-family kinases play important early roles in the activation of many immune cells
Src-family kinases phosphorylate tyrosines These kinases become activated themselves when phosphorylated
Clonal selection theory
States that the antigen selects which lymphocyte will undergo clonal expansion and produce more lymphocytes bearing the same type of receptor. (many copies of one specific antigen) B and T cells each have an individual specificity for an antigen due to each cell having many copies of receptor on their surface that only binds one antigen. Binding = activation. >> proliferation, producing a large number of clones.
Structure of T-cell receptor
Structurally similar to immunoglobulin domains - Two subunits, a and B each have a constant region and variable region - Variable regions have three CDRs forming peptide specific bonding site - Constant regions each contain transmembrane regions Two TCR types, aB and gammaD, have diverse antigen binding characteristics
What do you know about the T- and B- cell receptors? Consider: Structure: Location in the cell: Activity of different regions (variable vs. constant):
Structure: diagram Location: B-cell receptor: cell membrane, outside the cell T-cell receptor: transmembrane Activity of different regions: Variable: specificity Constant: effector function
T-cell receptor binding to antigen
TCR recognizes and binds both antigen-derived peptide and MHC (major histocompatibility complex) to which peptide is bound - stabilized by CD4 Peptide sources can be from endogenously or exogenously processed antigens (Note: B cells can directly bind to antigen, and T cells can recognize 'digested/broken' antigens)
Protein partners of T-cell receptors
The T-cell receptor (TCR) complexes with coreceptor involved in antigen recognition -CD3 contains ITAMs that transmit signal to cell - CD4, CD8 function in increasing avidity (combined strength) of peptide binding by TCR - CD28 engages in CD80 or CD86 on APC to fully activate a naive T cell
Generation of diversity
The generation of a diverse repertoire of antigen-binding receptors on B or T lymphocytes that occurs in the bone marrow or thymus, respectively. Creates a menu of responding recognition molecules.
The major pathways of complement activation
Three activation pathways exist: Classical, lectin, alternative All three pathways generate C3b, an important, multifunctional complement protein All form C3 & C5 convertase. All have some sort of initiation
Manipulating our immune system
Transplanted tissues: rare case where we want to AVOID an immune response (rejection). Cancer: a situation where the dangerous cells we want to target are our own self-cells. (generally tolerated and hard to generate immunity against).
Tumor Necrosis Factor (TNF) family cytokines
Tumor necrosis factor (TNF) regulates development, effector function, and homeostasis of cells of the skeletal, neuronal, and immune system TNF cytokines may be soluble or membrane bound TNF-a is proinflammatory and produced by activated macrophages and other cell types TNF-B (lymphotoxin-a) is produced by activated lymphocytes, delivering signals to leukocytes and endothelial cells Five membrane-bound TNF members - Lymphotoxin-B: lymphocyte differentiation - BAFF and APRIL: B-cell development and homeostasis - CD40 Ligand (CD40L): T-cell differentiation signal to B cell - Fas Ligand (FasL): induces cell death (apoptosis) upon ligand binding
Class 2 Cytokines are grouped into 3 families of interferons
Type I interferons - IFN-a and IFN-B are 18-20 kDa dimers with antiviral effects - Secreted by activated microphages and dendritic cells - Induce synthesis of ribonucleases and inhibit protein synthesis Type II interferons (IFN-gamma) - Dimer produced by activated T/NK cells - Potent modulator of adaptive immunity Type III interferon (IFN-delta) - Secreted by plasmacytoid dendritic cells - Upregulate genes controlling viral replication and host cell proliferation
Induction of antiviral activities by type I interferons
Type I interferons (potent antiviral effects) include IFN-⍺ and IFN-β Type I Interferons (IFN) bind as dimers to IFN-alpha receptor (IFNAR) Four genes turned on by IFN signaling - Protein kinase R (PKR) - 2',5'-Oligoadenylate synthetase - Mx group proteins - IFIT
cell-mediated immunity
Type of immunity produced by T cells that attack infected or abnormal body cells. These can eradicate pathogens, clear infected self-cells, or aid in other cells in inducing immunity. T-helper (TH) and T-cytotoxic. T lymphocyte subpopulations (multi-functional) are used as soluble messengers.
Phagocytes
White blood cells that attack invading pathogens
Cytotoxic T lymphocytes
bind to viral proteins present in cytosol and initiate an early warning system, alerting the cell to the presence of an invader
Tolerance
ensures that the immune system avoids destroying the host. Many random arrangements used to create B- and T-cell receptors could be anti-self. Tolerance helps to keep these anti-self recognition molecules/cells from circulating the bloodstream.
Active immunity
natural infection or administration of vaccine
Name one (or more) chemical barrier that is part of the innate immune system
pH, antimicrobials, enzymes/binding proteins
Vaccinations
prepares the immune system to eradicate an infectious agent before it causes disease. (Dr. Jenner unethical, and Louis Pasteur with cholera attenuation).
Pattern recognition receptors (PRR)
proteins encoded in the genomic DNA and are always expressed by many different immune cells. Germ-line encoded (PRR) that bind to PAMPs (generic molecules found on my pathogens)
Extended binding can result in cytoskeletal reorganization
reorganization enhances cross-talk between cells
Lymphocyte
responsible for both cellular and humoral immunity
Antibodies
soluble active molecules in immunoglobulin fraction of serum
Humoral immunity
specific immunity produced by B cells that produce antibodies that circulate in body fluids. Antibodies bind specifically to an antigen. Antibodies can be transferred between individuals to provide passive immunity. (Bursa in birds). Other terms: immunoglobulin and gamma globulin.
Anitgen
substance that elicits a specific response by B or T lymphocytes
Neutrophils
swam to sites of tissue damage or infection
4 categories of human pathogens
viruses, bacteria, fungi and parasites Immune responses are tailored to these types of organisms and depend on the structure of the pathogen (intra or extracellular). Pathogen has to breach physical barrier!
Complement receptors connect complement-tagged pathogens to effector cells
yields many responses. (CR1, CR2, C3aR, C5aR)
