Immunology Exam 2

Antibody responses in vivo: Isotype switching depends on T cells

-CD40 binding to CD40 ligand -IL-4 induces switching to IgG1 and IgE -IL-5-induces switching to IgA

Antibody responses in vivo: Immunological Memory

-the capacity to mount a secondary response is based on immunological memory -during the primary response, expansion of an antigen specific lymphocyte population is generated, so that there is an increased frequency of resting B and T cells capable of responding to that antigen in the future -Memory B cells differ from unprimed B cells in that they make IgG earlier and usually have higher affinity antigen receptors due to selection in the primary response

TH1 cells release...

IL-2 and IFN-gamma and tend to activate macrophages

signal transduction

the intracellular CD3 chains contain the ITAM phosphorylation sequence, while the intracellular CD4 chain has attached the enzyme p56^lck (a lymphocyte specific tyrosine kinase); together they are involved in signal transduction

Protein-polysaccharide conjugate vaccines

(pneumococcal conjugate vaccine PCV13) use polysaccharides to bind to carrier proteins, allowing the protein to present antigen on B cells and CD40/CD40L interaction, while T cells allow antibody class switching from IgM to IgG producing memory cells and longer response

Antibody responses in vivo: Affinity maturation depends on cell selection

-Antibodies produced in a secondary response have a higher average affinity than those produced in the primary response due to hypermutation acting on the recombined antibody genes -concentration of antigen drives the selective expansion of high affinity clones-in the presence of low antigen concentration, only B cells with high affinity receptors will bind sufficient antigen and are triggered to divide and differentiate

Differentiation into plasma cells and memory cells (Bm)

-B cells do this by receiving signals from the T cells, mainly IL-6 and IL-10 -plasma cells seem to have committed so much of protein-synthesizing machinery to making antibody that they are incapable of further growth and division. -Many die after several days-some survive months or years (up to 40 years) in bone marrow and continue to secrete antibodies into the blood

CD80 and CD86

-CD80 and CD86 are upregulated on interdigitating cells, monocytes, B cells, and other APCs -CD80 and CD86 bind to the CD28 present on T cells; CD28 stimulation augments production of IL-2 (causes T cell proliferation) and other cytokines

types of cell mediated cytotoxicity that do NOT require antibody: Cytotoxic T cells (CTLs)

-CTLs are generated by activation of Tc cells-binding of TCR to peptide associated with MHC I, and IL-2R expression; IL-2 released by TH cells results in the proliferation and differentiation of Tc cell into an effector cytotoxic T cell -CD8+ CTLs recognize antigen in association with class I MHC molecules -most important role of cytotoxic T cells is to eliminate virus-infected cells

Cytokines produced by APCs

-IL-1 and IL-6, induce the expression for IL-2 and IL-2 receptors on TH cells -IL-12 activates IFN-gamma production by TH cells -TNF-alpha (tumor necrosis factor) induces inflammation and further increases the release of IL-1 and IL-6

Interaction between B and T cells also involves multiple surface molecules

-Interaction b/w B and T cells is a two way process; B cells present antigen to T cells, and B cells receive signals from T cells for division and differentiation -MHC II-antigen complex and TCR; augmented by the interactions of LFA-3, CD2, ICAM-1, and LFA-1; the CD80 and CD86 surface antigens on the B cells interact with CD28 on the T cell causing IL-2 production **central, highly specific interation -CD40 on the B cell binds to CD40L (CD40 ligand) that is transiently expressed on T cells and delivers the most potent activating signal to B cells; Resulting in B cell undergoing division, antibodies switching isotypes, and plasma cells differentiate -IL-1 and IL-6 released by B cells enhance expression of IL-2 and IL-2 receptors on B cells

B cells as antigen presenting cells

-Major type of APC for secondary immune response where memory cells are high -they bind to a specific antigen, internalize it and then degrade it into peptides which becomes associated with MHC II -if antigen concentrations are low, the specific, high affinity receptors (Ig) on B cells makes them the most effective APCs because other APCs cannot capture enough of the antigen

"second" signal

-Molecules such as ICAM-1, LFA3, CD80 and CD86 that induce positive activation of T cells are termed the "second" signal -T cells cannot respond without this second signal -if T cells recognize the antigen in a non-stimulating manner (this is without this signal) they become inactivated and produce a state of anergy resulting in immunological tolerance

Oxygen reactive intermediates (ROIs)

-Peroxidase-independent: NADPH oxidase in the phagosome membrane reduces oxygen to superoxide anion (02); this can give rise to hydroxyl radicals; singlet oxygen and hydrogen peroxide which are all potentially toxic -Peroxidase -dependent: if lysosome fusion occurs, myeloperoxidases, and catalases act on peroxides in the presence of halides to form hypohalites (bleach-like compounds)

Multiple cell surface molecules interact during antigen presentation to T cells

-TCR complex (TCR and CD3) recognize specific peptide lodged in the peptide binding groove of the MHC II molecule -TCR complex can transmit messages to the interior of the cell

types of cell mediated cytotoxicity that do NOT require antibody: NK cells

-do not express antigen-specific receptors and are not MHC-restricted -they have an activating receptor on their surface that binds to oligosaccharides present on surface of tumor and virus-infected cells that have abnormal patterns of glycosylation; this triggers the NK cell to kill -have an inhibitory receptor that binds to class I MHC molecules on the target cell and overrides the activation signal telling the NK cell not to kill the cell -since tumor and virus-infected cells have reduced expression of MHC I, the inhibitory signal is allowed to induce killing -NK cells provide the first line of defense against virus infection, they control viral replication during the time when Tc cells are becoming activated and are proliferating; the NK population peaks at 3 days; Tc population peaks at day 7 -IFN-alpha and IFN-beta are released by virally infected cells which stimulate NK cells early on in the infection; once the CTLs are activated the virus titer rapidly decreases

T-independent antigens

-do not require T cell help to stimulateactivate B cells -large polymeric antigens with repeating antigenic determinants and are particularly resistant to degradation -primary and secondary immune response is similar, that is, weak Ab response with and IgM component (instead of IgG the typical Ab in the secondary response) -products of bacteria

cell-mediated and humoral immune responses serve different functions and involve different effector mechanisms for generating immunity

-effectors of the humoral branch are secreted antibodies which neutralize soluble antigens, mainly extracellular bacteria and their products -effectors of the cell mediated branch are various effector cells which include antigen-specific cells (CD4+ THI and TH2; CD8+ TC) and non-specific cells (macrophages, neutrophils, eosinophiils and natural killer cells) which serve mainly to eliminate intracellular pathogens, virus-infected cells, tumor cells, and foreign grafts -both specialize and non-specialize components of CMI depend on highly localized concentrations of various cytokines -antibody plays only a secondary role in CMI

Interdigitating Dendritic Cells (IDCs)

-found in the T cell dependent areas of lymph nodes and spleen, and are the most effective for initial activation of resting CD4+ T cells -they express high levels of MHC II -Major APC of primary immune response because they induce T cell proliferation more effectively than any other APC; they multiply the numbers of antigen-specific T cells

cell mediated immune reactions (CMI)

-important in intracellular bacterial infections (tuberculosis) and viral infections, in defense against fungi, parasites, and cancers. -main mechanism in rejection of organ transplants

The differentiation into TH subsets is an important step in selecting effector mechanisms

-pathogen triggers release of cytokines by macrophages and NK cells (determines which TH subset will develop, either TH1 or TH2) -if organism triggers release of IL-12 and IFN-gamma from macrophages and NK cells, TH subset will be TH1 -if IL-4 and IL-10 is released by TH cells, TH2 subset will develop

cytokines

-regulate important biological processes -proteins of low molecular mass (8-25 KDa) that regulate cell growth, cell activation, inflammation immunity, tissue repair and morphogenesis -some cytokines are chemotactic for specific cell types are termed chemokines

Space-occupying lesions

-size of the granuloma may compromise the individual (M. leprae causes nerve damage) -sometimes the cell-mediated response fails to eliminate an infecting organism or the antigenic material cannot be cleared -in this case T cells continue to accumulate and release ctyokines recruiting macrophages; this gives rise to a granuloma where the macrophage adhere closely to one another and sometimes even fuse to give rise to multinucleated giant cell -these giant cells release large amounts of lytic enzymes causing tissue damage and blood vessel damage -T cells involved are CD4+ TH1 type producing IL-2 and IFN-gamma and macrophages producing TNF-alpha

Interaction with antigen-presenting cells is essential for T-cell activation

-this is the first interaction to occur after antigen challenge -if a sufficient number of CD4+ TH cells are triggered then the activation of B cells will follow

What is an effective antigenic signal?

1. For a T cell, a single TCR is not sufficient; few tens to few hundreds of TCR molecules need to be activated 2. Mitogens: a class of molecule that can activate lymphocytes in a non-antigen-specific manner (ex. PHA-phytohaemagglutinin) - they bind to T cell surface molecules involved in activation (ex. CD2) 3. Superantigens: toxins produced by bacteria can activate T cells non-specifically-by binding to TCRs but not the antigen binding site; they bind on the V-beta chain of the TCR and crosslink with macrophage on the alpha chain of MHC II; no peptide in bound in the cleft; this is sufficient to activate T cells non-specifically; they release high amounts of IFN-gamma which in turn stimulates macrophage to produce IL1, IL6, and TNF-alpha; since T cells are activated without specificity this results in polyclonal activation of T cells and induces tolerance to antigen--the high levels of TNF-alpha can induce systemic inflammation and lead to multiple organ failure, shock, comma, and death

Cytokines produced by TH cells

1. IFN-gamma induces expression of MHCH, Fc receptors and more adhesion molecules, including CD80 and CD86 on the APCs 2. GM-CSF (granulocyte-macrophage colony stimulating factor) stimulates the proliferation of granulocytes and monocytes 2. IL-4 stimulates proliferation of activated B cells and T cells 3. TNF-beta (tumor necrosis factor beta) upregulates the production of adhesion molecules

during the T-B cell interaction, T cells can secret a number of cytokines that have a powerful effect on B cells

1. IL-2 a proliferation inducer for B cells (as well as for T cells) 2. IL-4 acts early on B cell activation and proliferation and induces differentiation of B cells into plasma cells, induces class switch 3. IL-5 class switch 4. IL-6 strong signal for differentiation 5. TNF-alpha inflammation

molecules involved in signal transduction with co-stimulatory function

1. Intercellular adhesion molecule-1 (ICAM-1) on the APC is known to interact with lymphocyte functional antigen-1 (LFA-1) present on all immune cells 2. the most potent co-stimulatory molecules are CD80 (B7-1) and CD86 (B7-2) that bind to CD28 present on the T cells 3. the lymphocyte functional antigen (LFA-3) is widely distributed on cells and on APCs binds the CD2 molecule present on T cells; involved in activation of T cells

Types of antigen presenting cells

1. Interdigitating dendritic cells (IDCs) (Major type of APC in primary immune response) 2. B cells (major type of APC during secondary response where the # of antigen specific aka memory cells is high) 3. Macrophages (important but they must upregulate MHC II expression)

2 types of T-independent Antigens

1. TI-type 1: the BCR binds to the antigen, then the B cell receives a second activation signal by TLR (toll-like receptors) binding to antigen (ex. LPS) 2. TI-type 2: the antigen has repeating subunits which can simultaneously crosslink IgM to fully activate the B cell (ex. capsular polysaccharides)

The effector cells of cell mediated cytotoxicity are...?

1. cytotoxic T cells bind to target cell by recognizing antigen 2. NK cells recognize absence of MHC class I molecules (decreased expression of MHC I occurs in virally infected cells)

Induction of apoptosis via perforins and granzymes

1. cytotoxic cell binds to the target cell 2. cytotoxic cell degranulates releasing perforin near the target cell membrane 3. presence of Ca2+ there is enzymatic polymerization of perforin to form polyperforin channels on the target cell 4. degradative enzymes (granzymes include trypsin and chmotrypsin) released by the cytotoxic cell can now enter the target cell through these channels 5. granzymes are proteases that cleave caspases; caspases activate nucleases that cleave DNA and induce apoptosis; DNA fragmentation and disintegration of the cell into small membrane bound fragments

Humoral (antibody-mediated) immunity is directed primarily against

1. exotoxin-mediated disease (ex. tetanus and diphtheria) 2. infections in which virulence is related to polysaccharide capsules (ex. pneumococci, meningococci, Haemophilus influenza) 3. certain viral infections

TH select and activate effector mechanism

1. help B cells to make antibody 2. activate or suppress functions of effector cells such as TC, NK, macrophages, granulocytes (neutrophils, eosinophils, mast) and K cells

Antibody responses in vivo: secondary antigenic response differs from the primary

1. it has a shorter lag phase (3-5 days) and an extended plateau and decline 2. the plateau levels of the antibody titer are much greater than the primary response 3. IgM was the main component of the primary response' the secondary response consists mostly of IgG 4. the affinity of antibodies in the secondary response are much higher (affinity maturation)

The central role of macrophages in the immune response...

1. they are involved in all stages of the immune response 2. they act as a rapid protective mechanism that can respond before T cell amplification 3. they process and present antigen to T cells 4. important role in the effector phase of cell mediated response, that is, in inducing inflammation, tumoricidal activity and microbicidal activity

Induction of apoptosis via Fas ligand

1. this perforin-independent mechanism of cytotoxicity involved the binding of Fas in the target cell membrane by the Fas ligand, which is present in the membranes of activated cytotoxic T cells 2. Ligation of Fas leads to activation of capsases, which induce apoptosis in the target cell

Cytotoxicity

CTLs may kill virus-infected cells that are essential to the host (ex. nerve cells)

Antibody-dependent cell-mediated cytotoxicity depends on binding the cytotoxic cell to the target via antibody

Cells that have cytotoxic potential and also posses Fc receptors for IgG may bind to and lyse cells coated with IgG -this is referred to as killer (K) cell activity -several cell types have cytotoxic ability plus Fc receptors: T cells, NK cells, macrophages, monocytes, neutrophils, and eosinophils

TH2 cells release...

IL-4, IL-5, IL-6, and IL-10 and tend to increase the production of eosinophils and mast cells, and enhance the production of antibody; they respond well to antigens presented by B cells

T-cell Independent Mechanisms

Play a vital role in the initial stages of an infection 1. phagocytes recognize the pathogen via PAMPS or complement 2. the organism binds to the phagocyte's cell surface which induces uptake and triggers killing 3. this triggers the release of cytokines by macrophages and NK cells which enhance their microbicidal activities -macrophages secrete IL-12 and TNF-alpha; IL-12 causes NK cells to release IFN-gamma; this increases the microbial capacity of macrophages -TNF-alpha causes phagocytes to adhere to the blood vessel walls increasing their entry to the site of inflammation

polysaccharide vaccines

Purified polysaccharide vaccines and Protein-polysaccharide conjugate vaccines

T-Cell Dependent Cell-Mediated Responses

TH cells play a central role in cell-mediated immunity -determine specificity in CMI: determine which antigens and which epitopes are recognized by the TCR -determine mechanism in CMI: determine which effector cell will be activated

synergistic effect on macrophages

The combination of IFN-gamma and TNF-alpha has a synergistic effect on macrophages, allowing the to produce nitric oxide (NO) and become more microbicidal

Purified polysaccharide vaccines

Work by activation of B cells in a T-independent manner, producing predominantly IgM and little memory B cells. (Pneumococcal polysaccharide vaccine, PPSV23); these work best in the older population but not in infants

IL-4

acts on B cells to induce activation and differentiation; acts on T cells promotes differentiation of TH2 cells

requirements of full T cell activation

antigen specific activation of lymphocytes involves the specific receptors on T and B cells

Chronic Inflammation

cell mediated mechanisms can be directed against autoantigens resulting in tissue damaging inflammation (RA, MS)

K cells

considered ADDC (antibody dependent cytotoxic cells) T cells, NK cells, macrophages, monocytes, neutrophils, eosinophils that have Fc receptors

Which effector mechanism is appropriate for each antigen?

decision is important; if an inappropriate effector mechanism is chosen it can lead to susceptibility to the pathogen

T-dependent antigens

depends on both T and B cells for recognizing the antigen

Excessive Cytokine Release

excessive release of cytokines can cause toxic shock syndrome (ex. superantigens)

Interferons are antiviral cytokines

fall into 3 groups: IFN-alpha and IFN-beta are made by cells in response to viruses, while IFN-gamma has much less antiviral activity and it is made in response to immune stimuli. IFN-gamma is produced by T cells and natural killer cells; they activate APCs partly by inducing MHC II molecules and they in turn can activate T cells by secreting IL-1 and IL-6 (positive feedback)

Antibody responses in vivo: the enhanced secondary response

following primary antigenic challenge there is an initial 5-10 day lag phase when no antibody can be detected; followed by the log phase where antibody titer increases logarithmically, followed by the plateau phase, during which the antibody titer stabilizes; lastly, the decline phase where the antibody is naturally catabolized or is bound to antigen and is cleared from the circulation

T cell cytokine secretion and action

important for B cell activation

mechanisms of killing of T cells, NK cells, and lymphoid K cells (ADDC)

lymphoid cells use similar mechanisms for killing -2 mechanisms: both mechanisms induce apoptosis-the distinction between apoptosis and cell lysis is important: lysing a virus-infected cell could potentially release the infectious virus whereas apoptosis leads to destruction of the virus from the inside

What makes Interleukins?

made by leukocytes, endothelial cells, fibroblasts

IL-1

made primarily by macrophages (but also by B cells, endothelial cells); stimulates T and B cells and induces inflammatory responses; induces fever; virtually all cells in the body have IL-1 receptors

Myeloid cells use a range of mechanisms to kill their targets

myeloid cells release reactive oxygen intermediates (ROIs), reactive nitrogen intermediates (RNIs), and hydrolytic enzymes

IL-2

produced by T cells (previously known as T cell growth factor) and LGLs (NK cells); most powerful growth factor and activator; it acts on LGLs and B cells to induce growth and differentiation

IL-6

produced by T cells, macrophages, B cells, fibroblasts and endothelial cells; acts on B cells to differentiate into plasma cells

IL-3

stimulates growth of precursors of all hemopoietic lineages (red cells, granulocytes, macrophages, and lymphocytes)