Test 2 Vet Immunology

Where do T-cell progenitors come from, and where do they develop?

T-cell progenitors come from the bone marrow, are already committed to the T-cell lineage, and migrate to the thymus for development.

What are “nude” or double-negative T cells?

Immature T cells that have no TCR, CD4, or CD8 expression. They enter the thymus and undergo TCR development.

What is the basic structure of a T-cell receptor (TCR)?

A TCR consists of 2 protein chains: either αβ or γδ. Each chain contains a variable and constant region; the constant region is attached to the cell through a transmembrane domain and cytoplasmic tail.

What proteins are associated with the TCR and what is their function?

CD3 and ζ (zeta) chains, which function as signal-inducing/transducing proteins associated with the TCR.

What are the T-cell co-receptors, and what MHC does each recognize?

• CD4 → T-helper cells → recognizes MHC II

• CD8 → cytotoxic T lymphocytes (CTLs) → recognizes MHC I

How are TCR genes generated?

Genes encoding α, β, γ, and δ chains are assembled by mixing and matching gene segments, producing enormous TCR diversity.

What is V(D)J recombination?

A stepwise rearrangement of V (variable), D (diversity), and J (joining) gene segments that allows a limited number of genes to generate millions of unique TCRs.

Which TCR gene segments contain D segments?

The β, γ, and δ chains use V, D, and J segments; the α chain uses V and J segments

What is the general sequence of TCR development?

β-chain rearrangement → pre-α chain expression → pre-TCR + CD3 → proliferation → α-chain rearrangement → expression of both CD4 and CD8 → double-positive thymocyte → selection

Why can two T cells have different TCRs even if one chain is identical?

Because TCRs contain two chains, and each chain is independently generated through gene rearrangement. Different chain combinations create different TCRs.

What is the major TCR type in most T cells?

αβ TCRs are found on most T cells.

What are γδ T cells, and where are they especially important?

γδ T cells are a minority of T cells (except they are prominent in ruminants), are found mainly in mucosal tissues, recognize common pathogen motifs, have less diversity, and may recognize non-protein peptides presented by CD1.

What are the three major developmental stages of thymocytes based on CD4/CD8 expression?

• Double negative: CD4− CD8−

• Double positive: CD4+ CD8+

• Single positive: CD4+ OR CD8+

What happens during positive selection?

Double-positive thymocytes are tested for whether their TCR can interact with host MHC. Cells that cannot recognize MHC die.

How does positive selection determine whether a T cell becomes CD4 or CD8?

• TCR recognizes MHC I → survives → becomes CD8 single-positive

• TCR recognizes MHC II → survives → becomes CD4 single-positive

• TCR recognizes neither → dies

What happens during negative selection?

T cells that strongly recognize self-peptide + MHC are eliminated to prevent autoimmunity.

What is the outcome of weak vs. strong recognition of self-peptide/MHC?

• Low-affinity interaction → survives and becomes a mature naïve T cell

• Strong interaction → dies
  This is part of central tolerance.

What is AIRE and why is it important?

AutoImmune REgulator (AIRE) promotes transcription of a wide range of self proteins by medullary epithelial cells and dendritic cells, allowing autoreactive T cells to be identified during negative selection.

How long does T-cell selection take, and what percentage graduate?

Selection takes about 3 weeks, and only about 2% of double-positive thymocytes successfully graduate.

What are naïve T cells?

Mature, single-positive CD4 or CD8 T cells that have successfully completed thymic selection and leave the thymus to circulate.

How do naïve T cells recirculate, and what provides them a survival signal?

They circulate from blood → lymph nodes → blood. Interaction with MHC provides a survival signal. Their function is to recognize dendritic cells displaying foreign antigen on MHC I or II.

Where are B cells produced?

B cells are produced in the bone marrow.

What determines BCR/antibody diversity?

1. Many V, D, and J genes

2. Random V(D)J recombination

3. Insertion/deletion of nucleotides at V-D-J junctions

4. Random association of heavy and light chains

5. Somatic hypermutation during B-cell activation in secondary lymphoid tissue

What are antibody/BCR isotypes?

Heavy-chain constant-region variants determine whether antibody is IgM, IgG, IgE, or IgA. IgD is another isotype that is always membrane-bound in the context of this lecture

Which immunoglobulin do developing B cells initially express?

They initially use the Cμ segment, producing membrane-bound IgM.

What antibody does a mature activated B cell secrete first?

IgM, which is secreted as a pentamer.

What is B-cell central tolerance?

Before immature B cells enter the blood, they undergo central tolerance that removes B cells that recognize self antigens. Their surface IgM BCR interacts with soluble self-antigens presented by stromal cells.

What are stromal cells?

Non-lymphoid cells in central and peripheral lymphoid tissues that provide soluble and cell-bound signals required for lymphocyte development, survival, and migration.

Where does B-cell maturation occur, and what is receptor editing?

B-cell maturation occurs in the bone marrow and extramedullary locations, including ileal Peyer’s patches; in birds, the bursa of Fabricius is an important site. Receptor editing is part of the process used to address self-reactive B cells.

What characterizes mature naïve B cells, and what happens after antigen recognition?

Mature naïve B cells:

• Survive central tolerance

• Enter the circulation and populate secondary lymphoid tissues

• Express surface IgM and IgD

• Remain naïve until encountering their cognate antigen
  After antigen recognition, they can become germinal-center B cells, plasma cells, or memory cells. Genetic defects in V(D)J recombination can cause SCID in animals.

Where are T cells made and where do they mature?

Answer: Made in the bone marrow; educated/matured in the thymus.

What happens to T cells during thymic education?

They develop their receptor repertoire, undergo positive/negative selection, and become either CD4 or CD8.

What is the status of T cells released from the thymus?

Mature but naïve T cells.

Where are B cells made and negatively selected?

In the bone marrow.

What is the status of mature B cells entering the blood?

Mature but naïve.

What is the main peripheral role of dendritic cells?

Collect antigen, then switch to antigen presentation

Where do dendritic cells go after collecting antigen?

They migrate via lymphatics to secondary lymphoid organs.

What is unique about follicular dendritic cells?

They are stromal cells, non-migratory, lack MHC II, and collect antigen for transfer to B cells.

Where do T cells, B cells, and dendritic cells meet?

In secondary lymphoid organs.

What initiates the immunologic synapse?

A naïve TCR recognizes its specific peptide-MHC complex on a dendritic cell.

What does LFA-1 bind during T-cell/DC interaction?

ICAMs on dendritic cells.

What is the function of LFA-1–ICAM binding?

It stabilizes T-cell/DC binding and prevents the cells from being separated by lymph flow.

What are the 3 steps of T-cell activation?

1. 1) TCR-MHC interaction

2. Co-receptor/co-stimulatory interactions

3. Cytokine stimulation

Which MHC does CD4 recognize?

MHC II.

Which MHC does CD8 recognize?

MHC I.

What provides the co-stimulatory signal to naïve T cells?

B7-1/B7-2 on activated dendritic cells, macrophages, and B cells.

What receptor on naïve T cells binds B7?

CD28.

What does CD28-B7 binding induce?

Entry into G1 phase, IL-2 synthesis, and synthesis of high-affinity IL-2 receptors

What is IL-2 autocrine stimulation?

T cells produce IL-2 and IL-2 receptors; IL-2 binds their own receptors and promotes T-cell proliferation.

Why do CD8 T cells need CD4 T-cell help?

CD8 cells require higher co-stimulation. CD4 cells provide additional co-stimulatory molecules and IL-2.

What is the result of the 3rd T-cell activation signal?

Cytokines drive T-helper cell differentiation into Th1, Th2, Th17, Treg, etc.

What happens if T-cell co-stimulation is missing?

T cells become anergic and are unable to activate or respond.

How do cytotoxic T cells (CTLs) kill target cells?

Perforin forms pores in the target membrane; granzymes enter and induce apoptosis. FasL can also trigger apoptosis.

What is usually the first step in B-cell activation?

Cross-linking of BCRs on mature B cells by antigen.

How can antigen epitopes be brought close together to cross-link BCRs?

Antigen clumping can bring epitopes into close proximity; opsonization enhances this.

What receptor helps opsonized antigen cross-link BCRs?

B cells have C3b receptors; C3b binding helps pull BCRs together.

What happens to BCRs during activation?

BCRs in lipid rafts cluster together after antigen binding and cross-linking.

What happens after BCRs cluster on the B-cell surface?

The BCR/antigen complex is internalized.

What happens to a resting B cell after initial antigen recognition?

It is put on alert and remains on alert until co-stimulated by a Th cell.

What happens without B-cell co-stimulation?

The B cell remains anergic and may eventually die by apoptosis.

What 2 criteria must a Th cell meet to activate a B cell?

It must be activated by a DC and recognize peptide from the same antigen recognized by the BCR.

What must the Th cell recognize on the B cell?

The same MHC II/peptide complex it recognized on the dendritic cell.

What provides the B cell's second/co-stimulatory signal?

CD40 on the B cell interacting with CD40L on the Th cell.

What is the third signal for full B-cell activation?

Cytokines secreted by the Th cell after CD40/CD40L ligation.

Which B cells are more likely to take up antigen and present it to Th cells?

B cells whose BCRs have high avidity for the antigen.

What 3 major outcomes result from B-cell activation?

Isotype switching, somatic hypermutation, and affinity maturation.

What is isotype switching?

A change in the antibody heavy chain/isotype.

What enzyme is involved in isotype switching

AID (activation-induced cytidine deaminase).

Which antibody classes can result from isotype switching?

IgM/IgD, IgG, IgA, or IgE.

What determines which Ig class a B cell switches to?

The stimuli and cytokines involved.

What is somatic hypermutation?

Single-nucleotide substitutions in antibody variable regions occurring only in activated B cells.

How can somatic hypermutation affect BCR affinity?

It can increase or decrease affinity for antigen.

What happens to B cells with improved BCR affinity?

They out-compete lower-affinity B cells for antigen and T-cell help.

What happens to B cells with decreased BCR affinity?

They may fail to bind scarce antigen and eventually die from neglect.

What is affinity maturation?

Fine-tuning of B cells so those with the highest-affinity BCRs are selected.

What makes affinity maturation possible?

Somatic hypermutation plus competition for antigen

How do antibodies change during an immune response?

Early: low-affinity IgM; later: high-affinity, class-switched antibodies.

What is one possible B-cell fate in the germinal center?

Remain in the follicle and undergo affinity maturation to produce better antibodies.

What is the plasma-cell fate of activated B cells?

Become antibody-secreting plasma cells that live in MALT, inflammation sites, or LN medullary cords.

What is the memory B-cell fate?

Become non-antibody-secreting cells that recirculate in blood until reactivated by vaccination or reinfection

What are TI-1 antigens and how do they activate B cells?

Examples include bacterial polysaccharides, LPS, and bacterial DNA; they bind BCR plus other receptors and cause polyclonal, nonspecific activation.

What are TI-2 antigens and what response do they produce?

Primarily highly repetitive bacterial capsular polysaccharides; they produce a specific antibody response without T-cell help.

What are the 2 selection processes during T-cell development?

Positive selection ensures T cells recognize MHC; negative selection eliminates T cells that recognize self-peptides

What happens during positive selection of T cells?

T cells able to recognize MHC are selected; those unable to recognize MHC are discarded.

What determines whether a T cell becomes CD4 or CD8?

Recognition of MHC II → CD4; recognition of MHC I → CD8. T cells recognize one, not both

What is the role of AIRE in negative selection?

AIRE promotes expression of many self-proteins by medullary epithelial cells and DCs, allowing autoreactive T cells to be eliminated.

What are naïve T cells?

Single-positive T cells that successfully graduate from the thymus and begin circulating.

How long does T-cell selection take, and what percentage of double-positive thymocytes graduate?

About 3 weeks; only ~2% graduate.

How do naïve T cells recirculate?

They travel from blood → lymph nodes → back to blood

What provides a survival signal to naïve T cells?

Interaction with MHC.

What is the main function of naïve T cells?

Recognize dendritic cells displaying foreign antigens on MHC I or MHC II.

Where do T cells, B cells, DCs, and follicular DCs meet?

In lymphoid organs, especially the spleen and lymph nodes.

What are the 3 steps of T-cell activation?

• TCR–MHC interaction

• Co-receptor interactions

• Cytokine stimulation

What is the relationship between antigen-specific lymphocytes and viral infection?

For most viruses, declining infection titers coincide with development of antigen-specific B and T lymphocytes; CTLs are especially important for viruses.

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

IL-2 primarily acts through autocrine stimulation to promote T-cell proliferation.

What is the role of APC-produced cytokines during T-cell activation?

They generally act through paracrine stimulation to polarize/differentiate T cells

What type of pathogens induce Th1 cells?

Microbes that are ingested and activate phagocytes, especially intracellular pathogens residing in phagocytic vesicles.

What are the major cytokines produced by Th1 cells?

IFN-γ (signature cytokine), TNF-α, and IL-2.

How do Th1 cells enhance macrophage activation?

T-cell CD40L interacts with macrophage CD40, increasing macrophage activation; IFN-γ is the most potent macrophage-activating cytokine.

What type of infection induces Th2 cells, and what do they promote?

Parasitic worm/helminth infections; they promote IgE-, mast cell-, and eosinophil-mediated destruction.