Exam 2 Review - Lecture notes Immunology Exam 2 Study Guide PDF

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Immunology Exam 2 Study Guide...

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Immunology Exam 2 Review!

! Adaptive Immune System! !-

Antigen Receptors: encoding & diversity ! - Germline: receptors encoded by several separate gene segments! - Somatic recombination (different gene combinations) during lymphocyte maturation produces diverse antigen receptors ! - Does NOT happen in cells of other lineages ! - Ig heavy or light chain and TCR alpha and Beta chain each contain multiple V region genes ! - Also contains one or a few C region genes! - Between V and C genes: small stretches of nucleotides, called joining (J) and diversity (D) gene segments ! - All antigen receptor gene loci have V, J, and C genes! - ONLY Ig heavy and TCR Beta loci have D genes ! - VDJ recombinase !

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(Ques: What do mature lymphocytes not have? RAG-1)

- These genes expressed only in developing lymphocytes when they are assembling their antigen receptors !

- Recognize specific DNA sequences that flank all Ag receptor V, D, and J gene segments and makes the cut !

- HEAVY chains: D and J combined first then combine V with them. Now the transcription can happen. LIGHT chains: There’s no D, so only one combining event: J with V and then transcription takes places ! - Ligases: repair DNA breaks after recombination but are not unique to lymphocytes only ! - Combinatorial diversity: varying combinations of V, D, and J gene segments in (These processes are different lymphocyte clones! NOT for affinity - Junctional diversity: even greater diversity! ! and avidity. For those, - Addition and subtraction of nucleotides at joints between different gene segments ! somatic hypermutation. - Exonucleases may remove nucleotides! These processes are - Lymphocyte-specific enzymes, terminal deoxyribo-nucleotidyl transferase (TdT), only for diverse antigen catalyses addition: “N regions”! receptors) - Overhanging sequences filled in by “P nucleotides” ! (VDJ segments of heavy chain of primary response antibodies - B cell maturation and selection! produced “before exposure to antigen” in bone marrow) - Maturation mainly in bone marrow ! - Specific checkpoints occur throughout, so that B cell expresses only ONE receptor specificity (Different B cells have different receptors and hence recognize so many antigens but one B cell will always have the same kind of receptor) ! - Bone marrow stromal cells (connective tissue):! - Produce external signals switching on key developmental genes! - Form specific adhesive contacts with developing lymphocytes via cell-adhesion molecules! - Provide soluble and membrane-bound cytokines and chemokines, controlling differentiation and proliferation, hence B cell development happens only in the bone marrow!

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- B lymphocyte development: stages! - Stem cell -> Pro-B cell -> Pre-B cell -> Immature B cell -> Mature (but naive) B cell! - Stages marked by cell surface markers, receptors, and transcription factors! - Stages also marked by rearrangement of immunoglobulin genes (V, D, J segments)! - Stem cell to pro-B cell: committed to B cell lineage; lots of proliferation! - Change from pro-B cell to large pre-B cell involves lots of cell division! - Expands population of cells with successful V(D)J rearrangement of one heavy chain locus !

(First one expressed during B cell development)

- Some μ protein expressed on surface in association with two other invariant surrogate light chain proteins: IMPORTANT CHECKPOINT!!

- Pre-B-cell to Immature B cell! - μ protein and pre-BCR complex signal two other processes ! -

(Ques: Through which mechanism do T cells produce different heavy chains -> allelic exclusion) )

- Recombination of Ig light chains, first k and then n: if functional, it -

associates with μ chain to form complete membrane-associated IgM receptor SECOND CHECKPOINT! ! Immature B cells! - Successful surface IgM (sIgM) expression = immature B cell! - Negative selection here: if binds self Ag with high affinity, further maturation is stopped! - Final maturation involves co-expression of IgM and IgD! - IgM+IgD+ cell = MATURE B cell! (Which Ig is only membrane bound and never secreted?

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

IgD)

- T cell development! - Develop from lymphoid progenitor in bone marrow ! - Some travel to thymus for further development ! - Cells pass through series of distinct phases marked by changes in: ! - Status of TCR genes! - Expression of TCR at cell surface ! - Expression of other cell-surface proteins (CD3)! - Expression of co-receptors, CD4 and CD8! - No expression upon arriving in thymus = double-negative thymocytes ! - Both expressed = double-positive thymocytes ! - Only one expressed = single-positive thymocytes [Correct!]! - Stages:! - Stem cell -> double-negative T cell (Pro-T cell) -> Pre-T cell -> double-positive T-

(Ques: What’s the sequence to get to CTLs? Double negative -> double positive -> CD8)

cell (immature T cell) -> single-positive T cell!

- Stages marked by cell surface markers, receptors, and transcription factors! - Stages also marked by rearrangement of TCR genes! (What do you need to express at Pre-T cell receptor stage with receptor? Beta chain with surrogate alpha chain (TCR) expressed with CD3)

- Expression of CD3:pre-T-cell receptor complex causes:! - Cell proliferation and enlargement ! - Beta chain rearrangement stopped ! - Expression of both CD4 and CD8 (cells become double-positive) ! - Once proliferation ceases, cells are small double-positive thymocytes; alpha chain rearrangement occurs !

- Small double-positive cells express low levels of TCR (positive and negative -

selection occurs)! - Only the ones that recognize MHC and self-antigen appropriately pass ! Selection of T cells! - Positive Selection: Permits survival of only those T cells capable of self-MHC antigen recognition ! - 95% die: apoptosis in thymus! - Ensures MHC restriction! - Cells ultimately express either CD4 or CD8 !

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(What is the most accurate description of negative selection? binding with self antigens)

Apoptosis induced by strong

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Animation: T cell development

(Ques: What do B and T cells have in common?

They cause phosphorylation of proteins inside cells)

(Ques: B and T cell activation is similar except they have what?

Different transcription factors)

- Adaptive immunity initiated in peripheral lymphoid organs! - Naive T cells specific for invader’s antigen must be induced to respond to infection by APC!

- Dendritic cells must present Antigen in peripheral lymphoid organs: spleen, lymph

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nodes, MALT! - Can pick up Ag at sites of infection and travel or:! - Free Ag can travel (lymph or blood) to lymphoid organs for uptake and presentation there! Naive T cells: on the prowl! - Naive T cells continuously circulate through blood and lymph, passing through lymphoid tissues along the way! - Meet dendritic cells in lymphoid tissues! - Sample the wares: T cells sample peptide-MHC complexes on surface of dendritic cells! - If encounter their specific Ag-MHC complex on mature dendritic cell, migration stops: proliferation (clonal expansion) and differentiation proceed! - Clonal expansion and differentiation of T cells: APC signals! - Activation: engagement of TCR with specific peptide:MHC complex ! - Not enough for expansion and differentiation ! - Survival: promote or inhibit survival and expansion of T cell! - B7: member of Ig superfamily; found exclusively on surfaces of cells that stimulate native T cell proliferation (meaning APCs)! - Differentiation: promote differentiation to effector cells ! Recognition of MHC Complex-Associated Peptides! - TCR for Ag and CD4 or CD8 co-receptor: recognize complex of peptide Ags and MHC on APCs! - Initiates T cell activation! - CD8 T cells: Recognize peptides presented by MHC Class I! - Differentiated into cytotoxic effector T cells (CTLs)! - CD4 T cells: Recognize peptides presented by MHC Class II! - Differentiate into various effector cells, but all helper (TH1, TH2, TH17; activate target cells) or regulatory subsets (inhibitory)! Biochemical signals! - Signals needed for T cell activation triggered by proteins linked to TCR, forming TCR complex, and by CD4 or CD8! - Diverse antigen receptors, so can recognize many microbes; CAN’T transmit (What signals are involved in “Two signal” activation? Signal signals ! AND from response to antigen (innate response)) - Signaling Complex ! - TCR non-covalently associated with 3-protein complex, making up CD3, and w/ homodimer of signaling protein, zeta chain! - TCR complex = TCR, CD3, zeta chain! - TCR alpha and Beta chains: Variable, recognize Ag!

from antigen

(Which two molecules do NOT bind to provide activation signals? CD3 and zeta -> they both transduce signals but do NOT bind each other)

- Adhesion molecules’ role! - Naive T cells bind transiently to APCs, sampling MHC molecules + specific peptide! - If encounter correct specific complex, TCR signaling induces conformation change in LFA-1 (on T-cell)!

- Conformational change increases

affinity for its ligand, ICAM-1 (on APC):

tight binding ! (What adhesion molecule is found on lymphocytes in peripheral lymph nodes?

LFA-1)

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !-

- Proliferation of naive T cells occurs! - Progeny bind APC, too; differentiate into effector cells! - APCs give signals for clonal expansion and differentiation !

Co-stimulators needed for T cell activation !

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! ! ! ! ! ! Remains a naive T-cell !

Unresponsiveness = Anergy

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- Termination of immune response! - Proteins homologous to CD28 limit or terminate immune response ! - Different members of CD28 family participate in T cell activation and inhibition ! - Inhibitory receptors include CTLA-4 and PD-1!

(Ques: An antagonist of CTLA-4 binds ____ and leads to _____ inhibition => Answer? binds B7 and leads to decreased)

- PD-1 recognizes many related ligands on multiple cell types! - Both induced on activated T cells and inhibit them !

- Application: Orencia (abatacept)! - Immunotherapeutic agent used to treat rheumatoid arthritis ! - Fusion protein: CTLA-4 extracellular domain fused with Fc region of IgG1! - T cell activation is inhibited ! - Stimuli for CD8+ T cell activation! - Dendritic cell is presenting to the helper T-cell at the same time it is presenting to naive T cell. CD8+ gets the cytokines from the helper T cell that might be enough to be activated and proliferate clonally. !

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T cells recognize Ag and MHC and co-stimulators: results in protein expression!

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- Proteins involved in proliferation, differentiation, and effector functions ! - Gene transcription and protein synthesis occurs within mins of Ag recognition !

- Immunologic synapse ! - Region of contact between T cell and APC: Confluence of TCR complex (includes

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zeta and CD3), CD4/CD8 co-receptors, and CD28 at center of contact; integrins in ring around this! - Believed to induce interactions required for optimal signaling within T cell for activation ! - Inhibitory signaling molecules are also recruited here! Signaling!

- Cytokines ! - Large group of proteins -> mediators in immunity and inflammation !

- Innate immunity -> macrophages! - Adaptive immunity -> secreted by T cells! - General properties:! - TCR signal and costimulation (after exposure to antigen) induce cytosine gene transcription!

- Usually acts autocrine or paracrine -> T cell activation induces expression of both cytokines and high-affinity receptors for cytokines!

- Pletiotropism: each cytokine has multiple biological actions -> different cell types express receptors for a particular cytokine!

- Redundancy: multiple cytokines may share same biological activities -> many cytokines use same conserved signaling pathways.!

- Application:! - Multiple therapeutic agents target IL-2! - Some are monoclonal antibodies that bind IL-2 receptor: Monoclonal antibodies

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block the IL-2 Receptor and hence reduce proliferation of T cells -> usually used during organ transplants for immunosuppression ! - Some are simple infusions of IL-2: More IL-2 available to bind to the naive (resting) cells and lead to more T cell activation, proliferation, and differentiation ! - One is a fusion protein: Used for T cells to attack cancer cells -> Fusion protein – Diphtheria toxin -> fused to IL-2 (binds IL-2R α chain) -> Toxin now bound to proliferating T cells ! will kill T cells -> Used for T cell lymphomas ! T cells: clonal expansion ! - Within 1-2 days of activation, get huge expansion of antigen-specific clones only (not nonspecific cells)! - Expansion of CD8+ cells > CD4+ cells! T cell differentiation: effector cells! - Differentiation occurs 3-4 days after rapid growth ! - Process occurs via changes in gene expression ! - Effector cell has distinguishing characteristics:! - Co-stimulation (B7, CD28) NOT needed to be activated anymore - encounter with specific Ag is enough to activate ! - Changes in cell-adhesion molecule and receptor expression: allows movement to infection site! !

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- Effector cells make molecules required for specific functions ! - CD8 bearing cells: ALL Differentiate into CTL -> kill target cells! - Cytotoxicity is SPECIFIC: Need specific Ag receptor (TCR) and infected cell must present intracellular pathogen Ags at cell surface with MHC I!

- Activated CTLs induce apoptosis in target cells ! - Effector molecules found in intracellular granules! - Granzymes: cleave, thus activating, caspases in cytoplasm of target cells -

(What happens in target cells when CTLs kill them? 1) Granzyme Perforin *3) Caspases 4) 1 and 2 5) None ANS: 3

-> caspases induce apoptosis! 2)

(Produced by CTLs, NOT helper cells)

- Fas ligand expressed: Membrane protein! - Binds Fas receptor (CD95, death-inducing receptor) on target cells! - Binding activates caspases => apoptosis !

- CTLs can also increase cytokines for effects (TNF-a, IFN-y, LT-a (TNF-B))! - IFN-y activates macrophages to destroy phagocytoses microbes and enhances leukocyte recruitment !

- Work in cooperation with CD4+ cells to eradicate intracellular microbes. -

Phagocytosed and sequestered vesicles eliminated by CD4+ and microbes that escape vesicles or inhabit cytoplasm are eliminated by CD8+! CD4 bearing cells: Differentiate into multiple effector subsets:! -> Helper T cells -> activate macrophages for microbial killing by CD40 ligand-CD40 interactions ! - Cytokines produced: , IL-2, TNF. ! - Macrophages with microbes in phagolysosomes process microbes, display microbial peptides on surface via MHC II molecules. TH1 cells specific for these antigens respond by expressing CD40L, which binds CD40 on macrophage and induces IFN-y secretion from TH1 cells ! - IFN-y activates macrophages: Macrophage responds by increased production of lysosomal proteases, production of ROS and NO, Production of IL-2 (stimulations differentiation of CD4+ T cells to TH1 subset, More IFN-y, more IL-12, more IFN-y and so on). Production of TNF, IL-1, chemokines for inflammation! ! - IFN-y also stimulates antibody isotypes promoting phagocytosis of microbes (antibodies and activated complement coat microbes, bind Fc and complement receptors), amplifies T cell responses: expression of MHC class II, B7 costimulators on macrophages and dendritic cells! - TH1 cells help CD8+ cells differentiate into CTLs! - TH1 cells help B cells differentiate into Antibody-producing cells! - Ability to activate macrophages depends on Antigen recognition: specific response!! - Same reaction as delayed-type hypersensitivity: Inject microbial protein into skin, if individual immunized against microbe, response seen on skin 24-48 hours later, response takes time b/c circulating T cells that recognize Ag must home to injection site, respond, induce detectable rxn ! -> Helper T cells -> get rid of parasitic infections (also involved in allergic reaction). ! - Cytokines produced: , IL-13. !

- Stimulate phagocyte-independent, eosinophil-rich inflammation:

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Helminth parasites! - IL-4 stimulates IgE production and IL-5 activates eosinophils: activated eosinophils release granule contents, IgE mediates mast cell degranulation, result is release of mediators that kill parasites or promote expulsion ! - TH2 cells also activate Macrophages but in a different way -> enhances synthesis of extracellular matrix proteins involved with tissue repair. ! - Also produces IL-4, IL-10, and IL-13 -> inhibit macrophage microbicidal activity = limit injurious consequences of macrophage activation ! - IL-4 and IL-13: cause macrophage to express mannose receptors! - IL-13: acts on fibroblasts, increases collagen synthesis and fibrosis -> wound healing ! - Efficacy of cell-mediated immune responses determined by balance between TH1 and TH2 cells ! TH17 -> Produce mediators involved in inflammation and other immune responses. Induces chemokine production to recruit macrophages and neutrophils. Expresses IL-17 (host defense against some bacteria and fungi) and IL-22. Appear to be involved in inflammatory disorders !

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CD4+ Helper T cells: activation ! - Response to antigen: production of surface molecules and cytokines that activate phagocytes and B cells! - CD40 ligand (CD40L): surface molecule ! CD4 subset development! - Stimuli received by naive T cells during microbial encounters drives TH1, TH2, and TH17 subset development.!

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- From macrophages, dendritic and NK cells meeting microbes! - Transcription factors in T cells activated promote differentiation and production of IFN-y!

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- IFN-y activates macrophages with ingested microbes AND more TH1 development! - Thus, innate immunity drives adaptive immunity ! - TH2 cells and lack of IL-12 (which induces TH1 development) may drive activated T cells to produce IL-4 by default !

- Helminths may activate mast cells to secrete IL-4!

- TH17: Driven by inflammatory cytokines, IL-6, IL-1, IL-23 from macrophages and -

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dendritic cells! - TH17 cells promote PMN and macrophage recruitment ! CD8+ T lymphocytes activated by Ag and co-stimulator differentiate into cytotoxic T lymphocytes (CTLs)! - CTLs kill infected cells ! - Kill infected cells via secretion of proteins that: ! - Create pores in membranes of infected cells! - Induce DNA fragmentation! - Induce apoptosis !-> granzymes (caspases) and perforins Development of memory T cells ! - Portion of Ag-activated T cells differentiates into long-lived memory cells! - Survive even after infection eradicated ! - Found in lymphoid organs, mucosal tissues, and circulation !

(IL-7 also needed for proliferation of Immature lymphocytes) - 2 types: central and effector memory cells!

- Decline of immune response ! - Return to steady state occurs when survival and maintenance signals for T cells

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are removed:! - Antigen, co-stimulatory signals from CD28, cytokines (IL-2) ! - Cells die via apoptosis ! Microbes meet T cells!

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T cells: they can get there from here! - Effector T cell migration to sites of infection depends upon:! - Expression of adhesion molecules and ch...