Intro to Immunology- Exam 2

Two classes of molecule used by the adaptive immune system to specifically recognize and respond to antigens:

Antibodies and T cell receptors

Two forms of antibodies

1. Membrane-bound antibodies on the surface of B lymphocytes that function as antigen receptors

2. Secreted antibodies that protect against microbes

What are immunoglobins?

• antibodies are a type of immunoglobin (Ig)

• primary mediator of humoral immunity

• produced exclusively by B cells

• incredibly diverse and specific in their ability to recognize foreign molecular structures

Where are antibodies found?

secreted forms reside in the circulation, tissues, and mucosal sites

Antigen

a molecule that binds to an antibody or a T cell receptor

Structure of an antibody?

• most of the antibody variability is contained within three short regions called the hypervariable regions

• different isotypes and subtypes perform different effector functions

Where does the antigen bind to an antibody?

The V (variable) region

What is the Fc region of an antibody?

The effector portion; bind to specific receptors on innate cells to activate phagocytosis or other immune functions

Antibody isotypes

• IgA —> mucosal immunity

• IgD —> native B cell antigen receptor

• IgE —> defense against helminthic parasites, immediate hypersensitivity

• IgG —> opsonization, complement activation, antibody-dependent cell-mediated cytotoxicity, neonatal immunity, feedback inhibition of B cells

• IgM —> naive B cell antigen receptor, complement activation

How are antibody classes determined?

Antibodies can be divided into classes and subclasses based upon their heavy chain C regions

Which antibody isotypes are made first?

During an immune response, IgM is always produced first

How do antibodies change during the immune response?

1. Affinity maturation (somatic mutations in variable region)

2. Change from membrane form to secreted form

3. Switching of isotypes

Polymorphism

a trait that described

Co-dominance

when two alleles of the same gene are expressed separately to yield different traits in an individual

Syngeneic

when individuals express the same alleles

Allogeneic

when individuals express at least one differing allele

Allele

MHC restriction

A given T cell can only recognize its specific antigen in the context of a specific MHC molecule

MHC haplotype

the set of alleles for a given individual

Class I molecule structure

• heterotrimer consisting of an alpha chain, 2 beta microglobulin, and a bound peptide

• all three must be present for expression on the cell surface

Class II molecule structure

consists of two alpha chains and two beta microglobulin

Characteristics of peptide-MHC interactions

• polymorphic residues are located both on the helices and on the floor of the peptide-binding groove

  • these residues add greatly to the diversity of peptides that can bind to a given MHC molecule

Expression patterns of class I MHC molecules

this class of MHC molecule is constitutively expressed on all nucleated cells

Expression patterns of class II MHC molecules

this class of MHC molecule is expressed only on professional antigen presenting cells

Properties of antigens recognized by T cells

• most T cells recognize antigens and no other molecules, since only peptides bind to MHC molecules

• T cells recognize cell-associated and non-soluble antigens

• CD4+ and CD8+ T cells preferentially recognize antigens sampled from the vesicular and cytosolic pools, respectively

Role of an antigen-presenting cell in T cell activation

• Dendritic Cells: naive T cell activation —> clonal expansion and differentiation into effect T cells

• Macrophages: effector T cell activation —> cell-mediated immunity through activation of macrophages

• B cells: effector T cell activation —> B cell activation and antibody production (humoral immunity)

What is the function of an adjuvant?

to enhance and modulate the immune response to make it stronger and more efficient

What cells are “professional APCs”?

1. Dendritic cells

2. Macrophages

3. B cells

What is the general process of antigen capture and presentation?

1. Antigen uptake

2. Antigen processing

3. MHC biosynthesis

4. Peptide-MHC association

Class I pathway of antigen processing

1. Production of proteins in the cytosol

2. Proteolytic degradation of proteins

3. Transport of peptides from cytosol to ER

4. Assembly of peptide-class I complexes in ER

5. Surface expression of peptide-class I complexes —> bind to CD8+ T cell

Class II pathway of antigen processing

1. Uptake of extracellular proteins into vesicular compartments of APC

2. Processing of internalized proteins in endosomal/lysosomal vesicles

3. Biosynthesis and transport of class II MHC molecules to endosomes

4. Association of processed peptides with class I MHC molecules in vesicles

5. Expression of peptide-MHC complexes on cell surface —> bind to CD4+ T cells

Endosome

organelles inside eukaryotic cells that are important for sorting and transporting materials within the cell, including proteins and lipids

What is CLIP?

• Class II-associated invariant chain peptide

• binds to the peptide-binding groove of MHC class II and remains there until the MHC receptor is fully assembled

• plays a critical role in regulating MHC class II folding, transport, and peptide occupancy

HLA-DM

• human leukocyte antigen DM

• non-polymorphic MHC class II molecule that plays a key role in presenting peptides from outside the cell and interacting with CD4+ T helper cells on immune cells such as B cells and APCs

What is ubiquitin and what does it do?

it is a protein modifier that attaches to and “tags” proteins to mark them for degradation via proteasomes

Proteosome

a protein complex that breaks down unwanted proteins in a cell

What is TAP and what does it do?

• transporter associated with antigen processing

• a protein that is essential for peptide delivery from the cytosol into the lumen of the ER, where these peptides are loaded on MHC class I molecules

What is the general model of ligand-receptor signaling?

“signaling from the cell surface causes something to happen”

ITAMs

• “immunoreceptor tyrosine-based activation motif”

• sends an activating signal into the cell

ITIMs

• “immunoreceptor tyrosine-based inhibition motif”

• sends an inhibitory signal into the cell

ITSMs

• “immunoreceptor" tyrosine-based switch motif”

• can send either an activating or inhibitory signal into the cell depending on other signals present in the cellular environment

Somatic recombination (DNA rearrangement)

• the method by which functional lymphocyte receptor genes are created

• involves the rearrangement of gene segments that code for specific portions of lymphocyte receptors into a functional gene

Lymphocyte repertoire

• all of the unique TCR and BCR genetic rearrangements within the adaptive immune system

• generated by lymphocytes through the recombination of genes

What are the sites of lymphocyte rearrangement and maturation?

• rearrangement —> primary lymphoid organ (bone marrow or thymus)

• maturation —> secondary lymphoid organ or tissue

Checkpoints for lymphocyte development

1. After first proliferation, expression of one chain of Pre-B/T antigen receptor —> failure to express results in cell death

2. After second proliferation, complete expression of immature B/T cell receptor —> strong/absent recognition results in cell death, weak recognition results in maturation

What does single or double positive cells man?

• “single positive” cells —> express only one type of surface marker

• “double positive” cells —> express both markers simultaneously, indicating an earlier stage of development in the thymus before full maturation

Death by neglect

failure of TCR beta-chain rearrangement during checkpoint 1 —> the cell does not receive a survival signal and eventually dies by apoptosis

Failure of positive selection

failure to interact successfully with self-MHC during checkpoint 2 —> leads to death by apoptosis

Negative selection

too strong of an interaction with any autoreactive cells leads to active initiation of death by apoptosis

Central tolerance

cells that are positively selected, and not deleted by negative selection —> leads to the release of mature single-positive T cells into the circulation

Junctional diversity

• the process of DNA sequence variation that occurs when gene segments are joined incorrectly during recombination

• this process results in the insertion of additional nucleotides, which can change the amino acid sequence —> contributes to the variability of the CDR3

Combinatorial diversity

• contributes to the diversity of B cell receptors

• random recombination of separate V, D, and J gene segments to form a complete V-region exon

B-1 B cells

• derived from a separate developmental lineage

• constitute 30-50% of B cells in pleural and peritoneal cavities of mice

• have a relatively limited receptor repertoire

• receptors tend to bind microbial carbohydrate anions from microbes generally found in the gut

  • bind with relatively low affinity

  • much more similar to PRRs of innate immunity

• undergo apoptosis UNLESS they interact with self-antigens

Marginal zone (MZ) B cells

• found in white pulp outer regions of the spleen

• appear to be specialized for blood-borne Ag recognition

• recognize protein and carbohydrate antigens, similar to B-1 cells

  • some may be able to do so without T cell help

• characterized by low levels of IgD and Fc receptors

• seem to be derived from T2 cells with strong self-Ag signaling through BCR and binding of Notch ligands

B-2 B cells

• mature, primary cells migrate to lymphoid follicles

• express high levels of IgM/IgD on their surfaces

• recirculate between blood and lymphoid organs

• help to respond to antigens with T-cell help by producing antibodies

• half-life of approx. 4.5 months in periphery

PD-1

• Expression: activated T cells

• Function: inhibition of T cell activation

CTLA-4

• Expression: regulatory T cells, activated T cells

• Function: inhibition of T cell activation

ICOS

• Expression: activated T cells, T follicular helper (Tfh) cells

• Function: generation of Tfh cells

Central memory T cells (Tcm)

• express CCR7 and home to lymph nodes

• limited effector function but expand rapidly and gain effector function

Effector memory T cells (Tem)

• home to peripheral tissues, especially mucosal tissues

• upon stimulation, rapidly produce cytokines like IFN-gamma or become cytotoxic but do not proliferate much

Tissue-resident memory T cells (Trm)

• present in non-lymphoid tissues and provide rapid defense against microbes

• not in circulation

Peripheral memory T cells (Tpm)

similar to Trm but can enter blood

What is the role of IL-2 in T cell activation?

promotes T cell proliferation, especially for cytotoxic CD8+ T cells

What is the role of the high affinity receptor in T cell activation?

can minimize the number of peptide-ligand interactions required for T cell activation and optimize the signal activation dwell time

Two signals needed to activate naive T cells:

1. Antigen-specific signal that occurs when the TCR binds to a peptide-MHC complex on an APC

2. A costimulatory signal that occurs when the co-receptor protein CD28 on the T cell recognizes the B7 proteins (CD80 and CD86) on the APC

What happens if a naive T cell receives signal 1 without signal 2?

it becomes unresponsive or inactivated

What is the role of CD40/CD40L interaction on T cell activation?

• leads to dendritic cell expression of B7 —> secretion of cytokines

• enhanced T cell proliferation and differentiation

Properties of memory T cells:

• express increased levels of anti-apoptotic proteins which may allow them to survive for prolonged periods

• respond to antigen more rapidly than naive cells

• number of memory cells for a specific antigen are greater than the number of naive cells for that antigen

• can migrate into peripheral tissues

• undergo a slow proliferation and a long life

• survival is dependent on cytokines (IL-7) but does not require antigen

Different types of memory T cells:

1. Central memory (Tcm)

2. Effector memory (Tem)

3. Tissue-resident memory (Trm)

4. Peripheral memory (Tpm)

How does the immune system respond to microbes that live in the phagosomes of macrophages?

• Activates CD4+ effector T cells (T helper cells)

• Helps the phagocyte kill the ingested microbe

How does the immune system respond to microbes which do not live in phagosomes?

• Activates CD8+ T cells (killer cells)

• directly kill the antigens/microbes

What are the CD4+ T cell subsets?

• Th1

• Th2

• Th17

What cytokines are secreted by the CD4+ T cell subsets?

• Th1 —> secretes IFN-gamma

• Th2 —> secretes IL-4, IL-5, IL-13

• Th17 —> secretes IL-17, IL-22

How do leukocytes find sites of infection?

migration of leukocytes to sites of infection is stimulated by cytokines, which induce the expression of adhesion molecules on the surface of endothelial cells and the chemotaxis of leukocytes

What happens to T cells that locate their antigen at a site of infection?

they are activated and retained at the site

What happens to T cells that do not find their cognate antigen at the site of infection?

they return to the circulation, largely through lymphatic vessels

What role does CD40/CD40L and IFN-gamma play in macrophage activation?

• IFN-gamma is the major inducer of CD40 expression in macrophages

• CD40 is a type of tumor necrosis factor —> expression is crucial for T cell development

• CD40L is a costimulatory protein displayed by Th1 effector cells that binds to CD40 on macrophages

How do macrophages facilitate tissue repair?

Granuloma

• nodules of inflammatory tissue surrounding particulate sources of antigen

• produced when activated macrophages are unable to eradicate an infection

How does the immune system respond to helminth infections?

Using the Th2 differentiation pathway —> chronic T cell stimulation without a significant innate immune response of classical macrophage activation —> activation of eosinophils in response to helminth infection

Two main mechanisms by which CD8+ T cells kill target cells:

1. Perforin and Granzymes

2. Fas ligand (FasL)

CD8+ T cell exhaustion

• strong and persistent immune responses to chronic antigen exposure can result in damage to host

• may have evolved as mechanism to limit immunopathology

• repeated antigen stimulation decreases T cell proliferative capacity, and effector function (IFN-gamma production/cytotoxicity)

• results in increased expression of inhibitory receptors by T cells

Functions of peripheral gamma-delta T cells

• recruited/expand in response to infection

• major innate producer of IL-17 —> recruit inflammatory cells (neutrophils) to site of infection

• can activate macrophages via production of IFN-gamma or IFN-alpha

• possess cytolytic activities (Granzyme B)

Functions of epithelial/mucosal gamma-delta T cells

• expand in response to tissue injury

• help repair intestinal (colitis) and cutaneous tissue damage

• produce tissue growth factor in response to tissue injury

• cell-produced growth factors preserve integrity of epithelium under stress

What types of antigens do gamma-delta T cells respond to?

• recognize antigen in the absence of MHC

• can directly respond to lipid antigens and microbial metabolites during infection —> results in pro-inflammatory cytokine production/macrophage activation to clear infection

• can directly respond to molecules released by sterile tissue injury —> results in release of growth factors to repair tissue

T-independent antibody response

• in response to multivalent, non protein antigens on microbial surfaces

• primarily made by B-1 B cells and marginal zone B cells

• they produce low-affinity IgM antibodies and short-lived plasma cells

T-dependent antibody response

• in response to protein antigens

• primarily made by follicular B cells in response to helper T cells

• produce isotype-switched, high-affinity antibodies, memory B cells, and long-lived plasma cells

Why are T-independent responses primarily IgM only?

there is a lack of T-mediated isotype switching processes —> IgM is the first antibody produced in a humoral immune response and does not require isotype switching

Antigen capture and delivery to B cells

in all cases, the antigen captured is presented to B cells generally in its intact, native conformation, and is not processed by antigen-presenting cells

What second signals can fully activate a B cell?

signal provided by complement receptors (CR2), pattern recognition receptors (PRRs), or T cell help is usually required for full activation

How do T cells and B cells move toward each other in the lymph node?

1. Migration of activation T cells to edge of follicle —> upregulation of CCR7 and downregulation of CXCR5

2. B cells present antigen to activated helper T cells —> B cells interact with T cells that recognize the same antigen

3. Antigen uptake and processing —> B cell activation

4. migration of activated B cells to edge of follicle —> expression of EBI2 and upregulation of CCR7

What is the role of CD40-CD40L in B cell activation?

1. activated helper T cell expresses CD40L, secretes cytokines

2. B cells are activated by CD40 engagement, produce cytokines

3. B cell proliferation and differentiation

Six broad categories of antibody effector functions:

1. Neutralization —> protects against viral or bacterial infection or the effects of toxins

2. Agglutination —> enhances neutralization and more efficient clearance of pathogens from the body

3. Opsonization —> promotes and/or enhances the engulfment of antigens by phagocytes

4. Antibody-Dependent Cell-Mediated Cytotoxicity (ADCC) —> activates the killing activity of several types of cytotoxic cells

5. Antibody-Dependent Degranulation and Mediator Release —> triggers mediator release from granulocytes

What is cross-presentation?

• antigens presented on class I MHC molecules come from the cytosol and are processed through the endogenous pathway

• microbes that do not infect dendritic cells will be acquired from outside the DC; the same will occur for tumor antigens or cells

• in order to be presented on class I MHC molecules, classical DC1 subset cells are used to ingest infected cells, tumor cells, or proteins produced by these cells, transfer these proteins into the cytosol, and process them for class I presentation

Perforin

• exocytosed in CTL granules and polymerizes in the target cell plasma membrane, forming pores

• induces uptake of granzymes into target cell endosome and release into cytosol, activating caspases

Granzymes

exocytosed in CTL granules, enter target cells through the perforin pores, and induce target cell apoptosis

Fas ligand (FasL)

expressed on activated CTLs, engages Fas (“death” receptors) on the surface of target cells, and induces apoptosis

Primary humoral response

• occurs immediately after antigen exposure

• smaller in magnitude

• antibody isotype is usually IgM > IgG

• lower average antibody affinity; more variable

• induced by all antigens

Secondary humoral response

• occurs a few days after antigen exposure

• larger in magnitude

• antibody isotypes: relative increase in IgG, often IgA, and sometimes IgE

• higher average antibody affinity (affinity maturation)

• induced by protein antigens only

Effector Functions of Antibodies

1. Neutralization of microbes and toxins

2. Opsonization and phagocytosis of microbes

3. Antibody-dependent cellular cytotoxicity

4. Phagocytosis of microbes opsonized with complement fragments

5. Inflammation

6. Lysis of microbes

7. Complement activation