Immunology Exam 3 Lectures 8 -10

Primary Immunity Organs

Thymus

Bone marrow

DP, SP and DN definition

Double positive

single positive

double negative

Steps of Thymocyte Maturation

1. 1. Early development in bone marrow

2. 2. Migrate through the blood to thymus

3. 3. Mature in the Thymus

Selection steps of T cells in thymus

Positive/negative selection

Removal of autoreactive cells

Then release into the peripheral bloodstream

Double Negative cell has no

CD4 or CD8

Double positive cell has both

CD4+ and CD8+

Positive/negative selection

stages for a cell to become single positive CD4+ or CD8+

αβ T cell Majority or Minority?

Majority

γδ T cell Majority or Minority?

Minority

Autoreactive cells

reactive to yourself (can cause autoimmune disease)

When cells arrive at the thymus, what assigned cell types can they become?

T cells

NK cells

dendritic cells

B cells

Myeloid cells

Notch

Receptor of cell that can assign a cell to the T lineage

Thymocytes can express either _______ or _______ receptors

TCRαβ or TCRγδ

Relationships between TCRαβ and TCRγδ outcomes

– TCRβ rearrangements are the first to take place.

– TCRαβ outcomes are more likely than TCRγδ

Thymocytes

cells in the thymus

_____ Thymocytes undergo β-selection, resulting in ________/__________

DN

Proliferation/ differation

Process of β-selection

1. 1. Successfully produced β chain is paired with the pre-Tα chain

2. 2. After selection occurs, thymocytes are at DP stage of development

3. 3. Pos./neg. selection occurs, yielding mature SP T cells

Proliferation

multiplication of same cell

Differentiation

changes of same cell to another function

DP thymocytes make up _____% of thymic cells

80

undergo thymic selection

Positive selection

Selects thymocytes bearing receptors capable of binding self-MHC molecules, resulting in MHC restriction

Negative Selection

Selects against thymocytes bearing high-affinity receptors for self-MHC/self-peptide complexes, resulting in self tolerance

_____% cells fail positive selection and fail to receive needed survival signals

95

Self MHC restriction

self MHC binding w/ T cell receptor only

Affinity

Binding strength

Positive Selection ensures ______ ______ and how?

MHC restriction

• - most cells die by neglect before reaching negative and so doesn’t bind to MHC structures

High affinity interactions (2-5% of cells) result in what outcome of T cell development?

Negative selection (cell does not become a T cell)

Low/intermediate affinity interactions (2-5% of cells) result in what outcome of T cell development

Positive selection, cell does become a T cell

No affinity interaction (90-96% of cells) outcome in T cell development

Death by neglect, cell does not become a T cell

Negative Selection ensures ________

and how?

self-tolerance

by clonal deletion

Clonal Deletion

Induction of apoptosis in cells with too strong anti-self signaling/binding

Other mechanisms of self-tolerance

Clonal Arrest

Clonal anergy

Clonal editing

Clonal Arrest

autoreactive T cells are prevented from maturing further

Clonal anergy

autoreactive T cells are inactiveated

Clonal editing

second or third chances at rearranging a non-self-reactive TCRα gene

Models that explain lineage commitment

Instructive model

Stochastic model

Kinetic signaling model

Instructive model

TCR/CD4 and TCR/CD8 co engagement generate unique signals

the signals instruct the T cells which lineage to commit to

• - Class I MHC leads to CD8 + TCR engagement resulting in CD4-8+ T cell (low CD4 expression, high CD8 expression)

• -Class II MHC leads to CD4 + TCR engagement resulting in CD4+8- T cell (low CD8 expression, high CD4 expression)

Thymocytes learn MHC restriction in the ______

thymus

Stochastic model

Positive selected thymocyte randomly down-regulates either CD4 or CD8

Only cells with correct coreceptor receives signals to continue development

• - CD4+8+ cell randomly gets down regulated CD4 or CD8, the down regulated CD cell then gets tested if it can bind to the correct class of MHC (Class I for CD8 high, Class II for CD4 high)

• - if CD cell cannot bind to its necessary MHC class, it results in apoptosis, if it does bind then it continues its development

Kinetic Signaling model

cells commit to the CD4 lineage if they receive continuous signal

cells commit to the CD8 lineage if the stimulation signal is interrupted

DP thymocytes may commit to these other lymphocytes:

NKT cells

Intraepithelial lymphocytes (IELs)

Regulatory T cells (TREG)

NKT cells

• Express a TCR with an invariant TCRα chain

• Interact with CD1 molecules presenting lipid antigens

Intraepithelial lymphocytes (IELs)

• Usually CD8+, but also have features of innate immune cells

Regulatory T cells (T_REG)

• CD4+ subset that helps to quench adaptive immunity

Apoptosis

programmed cell death

• Controlled process of dismantling cells

• Doesn’t trigger inflammation

Necrosis

results from injury

• Release of cell contents = inflammation

Antibody diversity results from _____ and _____ combinations

VJ and VDJ

There are multiple B cell types because

the cell only has 1 type of receptor

μ_m mRNA features

5’ cap and poly A tail

Contains μ1, μ2, μ3, and μ4

δ_m mRNA features

5’ cap and poly A tail

Contains δ1, δ2, δ3, and δ4

mRNA splicing

control whether the cell produces membrane-bound or secreted IgM

B-cell development begins in the__________ and is completed in _________.

Bone marrow

periphery

Steps in B cell maturation

Early Pro B → Late Pro B → Pre-B → Immature B → Mature B

Early Pro B

DJ H chain recombination

Start of V DJ H chain recombination

Heavy chain is formed first

Late Pro B

V DJ H chain recombination

Pre-B

μ H chain expressed as pre-BCR

Several rounds of cell division

VJ L chain recombination

Immature B

mIgM expression

negative selection (deletion and receptor editing)

Goes into central sinus and then blood vessel

Mature B

Undergoes Transitional -1 and Transitional 2 in Spleen

Transitional 2 adds IgD onto B cell

Bone marrow exists right at fertilization

True or false

False

Blood cell development location before Bone marrow formation

Changes locations, major location in yolk sac

Stromal cells in the BM

provide support and growth factors to developing cells

The later steps of B-cell development result in

commitment to the B-cell phenotype

Pre-B cell info

receptor is expressed in early pre-B stage– Heavy-chain rearrangement is finalized– Ig heavy chain complexed with VpreB and λ5 (SLC)

• CD25 (IL-2R α chain) expressed

• Late pre-B stage marked by initiation of light-chain gene rearrangement, then completion

• IgM receptor is expressed on the cell surface, transitioning the cell into immature B-cell stag

Immature B cells in the bone marrow are exquisitely sensitive to _____ _______.

Tolerance induction

– Bear membrane IgM, B220, CD25, IL-7R, and CD19

– Surface receptors are tested against self-antigens

Testing surface receptors against self antigens results in three possible outcomes:

Clonal deletion

Receptor editing

Anergy

Clonal deletion

takes out strongly autoreactive cells

– Through apoptosis

– Termed central tolerance as it occurs in bone marrow

Receptor editing

reactivation of recombination machinery

Anergy

induction of nonresponsiveness to further stimuli (even self-antigen stimuli)

B cell deletion processes

Many, but not all , self-reactive B cells are deleted within the bone marrow

Some are released to the periphery and subject to further round of selection

Is bone marrow the only testing ground for B cells? Yes or no? Why or why not?

No

Bone marrow does not have all the testing needed for B cells so it must go elsewhere for further testing

B cells exported from the bone marrow information

are still functionally immature

– Two subsets:

Transitional B cells 1 (T1) and T2 (these are NOT T cells!)

How are Transitional B cells 1 (T1) and T2 differ?

Differ in gene expression as they progress through the spleen for further maturation

T1 and T2 both have high mIgM but T1 has absent/low mIgD while T2 has intermediate mIgD levels

Mature, primary B-2 B cells migrate to the ________ ________ and info.

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 approximately 4.5 months in periphery

Two other important subsets of B cells also exist

B-1 B cells

Marginal zone B cells

B-1 B cells

derived from a separate developmental lineage

– Develop from fetal liver to protect fetus from common microbial antigens

– 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 antigens

• Bind with relatively low affinity

• Much more similar to PRRs of innate immunity

– Undergo apoptosis unless they interact with self-antigens

Marginal-zone (MZ) 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 receptor

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

Similarities of B- and T- pathways

– Rearrangement of gene segments

– Screening processes to avoid self-reactivity

– Production of small subsets with discrete functions

– Production of larger “general purpose” subset

B- and T-cell developmental pathways differences

– Location of maturation and screening

– Screening processes used

• Positive and negative in T cells

• Negative in B cells

– Specific arrangements and identities of gene segments recombined together

– Eventual outcomes of antigen receptor stimulation

• T cells require presentation and differentiate into helper or killer subsets

• B cells require T-cell help and secrete antibodies

T cells require _______ presentation as a first signal

antigen

Other molecular interactions can provide the second required activation signal

Once activated, T cells differentiate into their effector forms

• CD8+ T cells go on to become killer T cells

• CD4+ T cells differentiate into several different subsets

Successful T cell–APC interactions organize signaling molecules into an ________ ______

immunological synapse

–TCR/MHC-peptide complexes

–Adhesion molecules/bound ligands

Costimulatory signal:

Additional signal that is required to induce proliferation of antigen-primed T cells and is generated by interaction of CD28 on T cells with CD80/86 on antigen-presenting cells.

Costimulatory signals are required for optimal T- cell _________ and ________.

activation and proliferation

signal 1-3

Signal 1

antigen-specific TCR engagement

Signal 2

contact with costimulatory ligands

Signal 3

cytokines directing T-cell differentiation into distinct effector cell types

Positive costimulatory receptors

—facilitate activation

• CD28 and ICOS

Negative costimulatory receptors

—help turn activation off

• CTLA-4, PD-1, and BTLA

Clonal anergy

A physiological state in which cells are unable to be activated by antigen