Immuno Exam 3

Primary (central) lymphoid tissue

Bone marrow, thymus

Secondary (peripheral) lymphoid tissue

Lymph nodes, spleen, MALT

For primary lymphoid tissue, are they smaller or larger in young?

larger

Positive selection = ?

Selection to live

Does T-cell receptor self bind MHC?

It should, but only moderately. Not so tight where it activates a response to self-MHC

Negative selection = ?

Selection to die

Negative selection is important for which cell(s)?

B and T cells

How is negative selection important for T-cell receptors?

•those that bind self MHC tightly enough to cause signaling and activation must be weeded out

How is negative selection important for b-cell receptors?

those that are activated by self Ag’s must be weeded out

Stromal cell produces what?

various proteins, most importantly IL-7

Development steps from stem cell to naive B cell

Stem, Pro-B, Pre-B, Immature, Naive

Pre-B cell receptor light chain difference

•Surrogate light chain stands in for normal light chain

Gene rearrangement in Pro-B cells

Rearrangement of heavy chain

Gene rearrangement in Pre-B cells

complete heavy chain + surrogate light chain, goes to surface

–Ig on surface signals to stop gene rearrangement for heavy chain

Gene rearrangement on immature B cells

makes complete IgM heavy + light chains

Allelic exclusion

•Only one allele for each chain (heavy and light) makes functional protein.

(B cells in a/b heterozygote make only a or b surface Ig—not “ab” receptor)

Which non-productive gene rearrangement is common?

Frameshift

Can frameshift mutations be “rescused?”

Yes, but only in light (B cell) and alpha (T cell) chains only

How are frameshift mutations rescued?

•If joining one V to one J region does not work, then try a different V and J from the same chromosome (homologue)

Cells that live are “tolerant” of ___ antigens

Self

Central tolerance

–Due to weeding out in central (primary) lymphoid tissue

Peripheral tolerance

–Due to weeding out in peripheral (secondary) lymphoid tissue

If a B-cell recognizes multivalent “self” cell-surface molecules, what happens?

Apoptosis or further recombination

If a B-cell recognizes soluble “self” cell-surface molecules, what happens?

They become anergic (inactive)

What is the second chance that B cells are given?

Receptor editing

What is receptor editing

•Further light chain rearrangements could produce non-self antibodies

•If this doesn’t work, then apoptosis (similar to “rescue”)

Do any self-reactive B cells still slip through weeding out?

Yes, but requirement for T cell help provides an extra layer of protection

Athymic (nude) mice

•Do not reject tissue transplants, including cancerous and virus-infected tissue

Bone marrow, but no thymus

SCID mouse

Thymus, but no T cells

What happens if the SCID mouse and Nude mouse are combined?

•Bone marrow from nude mouse will “rescue” SCID mouse

•Thymus from SCID mouse will “rescue” nude mouse

When is the thymus most active? When does it atrohpy?

•Most active and largest in fetus/child – atrophy begins at puberty

A developing T cell is a ____

thymocytes

What do thymocytes bind to and where do they move?

Epithelial cells, move from cortex to medulla through maturation

What CD protein are young thymocytes associated with in the start?

Neither, they start off as a “double negative”

Are there any TCR that remain double negative?

γ:δ TCRs

Are there any double positive?

α:β TCR first become “double positive” – express both CD4 and CD8.  These cells then must choose to turn one of these off and keep the other

If a TCR gene rearranges alpha, what happens with the delta gene?

It is eliminated since the delta gene is an intron of the alpha gene

In α:β T cells, is α or β expressed first?

•There is a surrogate α chain, pTα, (similar to B cells) that allows β chain to be expressed on surface before α chain is finished

How many T cells in the thymus die? Why?

98%

•Some die due to self-reactivity or no reactivity

•Some die because gene rearrangement does not make functional TCRs

If a TCR does not bind with either MHC I or MHC II, what happens?

the α chain of the TCR may rearrange again (rescue), but if that fails, the cell is not selected to live

If TCR binds MHC I, what occurs to CD4 protein?

The CD4 gene is turned off, creates a CD8 only producing cell

If TCR binds MHC II, what occurs to CD8 protein?

The CD8 gene is turned off, creates a CD4 only producing cell

AIRE

transcription factor that is expressed in the thymus.  It turns on expression of many “self” proteins for the purpose of weeding out T cells that would attack them later

If TCR binding to MHC presenting self peptide in thymus would cause activation of T cell, what happens?

T cell is selected to die

How does most negative selection occur?

pairing of macrophages and dendritic cells (which express both MHC I and II) with T cells in thymus

Where do the B cells migrate to in lymph node?

Primary lymphoid follicle

Where do T cells migrate to in the lymph node?

Paracortical area

What are T-cells activated by ___ in secondary lymphoid cells?

Dendritic cells

What do dendritic cells do?

–senses inflammation

–phagocytizes pathogen, presents it  on MHC II (or MHC I if pathogen is cytosolic)

–migrates to lymph node to find matching T cell

Dendritic cells location + function

T cell areas, most important APC, initiates activation of T cell

Macrophage cell location + function

•concentrated at outer edge

–Eat pathogens that enter node

–Promotes continued activation of T cells if infection persists

B cell location and function

•in follicles / germinal centers

–Not strong phagocytes, but can endocytose small soluble molecules

The plasmacytoid dendritic cells look like normal DCs but have a different function, what is it?

•pDCs make large amounts of Type I interferon (IFN a/b) upon sensing viral infection

Which APC has a lower diversity of antigen?

B cells

Which APC has specific endocytosis of particle that binds to surface ig

B cells

There are 3 signals sent from APCs

1) TCR recognizes MHC/antigen complex.  CD4 or CD8 stabilizes interaction

2) CD28 on T cell binds B7 on APC

3) Various cytokines push T cell differentiation and vary with circumstances

• (ex. IL-12 promotes T helper 1 cells)

Activated T cells are more sensitive to _____

IL-2

IL-2 does what?

stimulates proliferation and differentiation

CTLA-4 is a competitive inhibitor of ____

CD28

CTLA-4 sends signals to _______ to downregulate activation. Why?

Downregulate IL-2, •This prevents dangerous over-activation of T cells

Sometimes APCs present antigen, but do not express B7. When does it happen?

in the absence of pathology, infection, or inflammation—PRRs are not sufficiently stimulated

CD8 T-cell activation usually needs help from ______

CD4 T cell

How does CD4 help CD8 T cell?

–APC activates CD4 T cell to produce cytokines, which in turn cause APC to upregulate B7 (signal 2)

–Higher levels of B7 on APC and IL-2 from CD4 T cell increase likelihood of CD8 T cell activation

•How does APC present a cytosolic antigen?

–Cross- or straight presentation of viral Ag’s on MHC I

–Straight presentation on MHC II after phagocytosis or destruction of infected or cancer cell

TH₁₇

Pro-inflammatory (help neutrophils)

T_reg

Anti-inflammatory

T_FH

Help B cells in germinal centers (isotype switching)

Development of one T_H pathway _____ the other 2

inhibits

T_H1 response to bacterium (tuberculoid)

macrophages suppress bacteria

T_H2 response to bacterium (lepromatous)

•unactivated macs don’t kill bacteria, but protect them from antibodies (bacteria live inside macs)

Cytotoxic (CD8+) T cells kill ____

Up close by reorientation of cytoskeleton + lytic granules

How do T_c cells kill?

One at a time to trigger apoptosis

T_h1 cells activate _____

Macrophages

What signals does macrophage activation invole?

–CD40L

–IFN-γ

How is CD40L produced?

•produced by activated (not naïve) T-cells

–CD40L binds to CD40 on the cell being “helped” (here, a macrophage)

–We saw CD40L in Ch7 as the co-stimulatory signal for B cells; here it stimulates macs

How do activated macrophages keep themselves activated?

–TNF-α

How do macrophages make their APC stronger?

•Upregulates B7 and MHC

Failure to kill bacteria in lysosomes can lead to ____

Granulomas

Where are granulomas seen?

tuberculosis and tuberculoid leprosy

Dendritic cell migrates ____ to present antigens to ___ cells

secondary lymphoid tissue, T_H

What are the two signals B cells require for activation?

1) Binding of antigen to B cell receptor

2) T_H cell w/ CD40L and matching TCR

What interaction is required for class/isotype switching in B cells?

CD40 – CD40L interaction

Mutation in CD40L results in what syndrome?

hyper-IgM syndrome

If a T cell matches a different epitope in the same large antigen it was taken it, can it still be activated and activate the B cell?

Yes

Can Igs recognize things other than peptides?

Yes, other molecules like carbs too. Can only present peptides in MHC though

What are specialized macrophages

recognize and “pass” complement-coated antigens  on to FDC

Whats FDC

•FDC holds antigens so B cells can “see” them (not the same process as presentation to T cells though!)

How are plasma cells different from B cells

•Plasma cells can no longer act as antigen presenting cells or modify their immunoglobulin genes. 

T cell induces B cell to mutate ____ region

Variable

Activated B cell that cannot interact with T cell will ___

Die

TI-1 antigens at high concentrations induce ____

•general B cell proliferation (polyclonal activation).  Also called “B cell mitogens.”

TI-1 antigens at low concentrations activate_____

a small subset of B cells that have BCRs that recognize specific antigen

TI-2 antigens are repetitive and therefore ______

cluster on B cell surface

B-2 B cells

Main B cells

B-1 B cells

•Recognize carbs, lipids, & nucleic acids, not protein

•Don’t need T cell help; antigens are T-independent

Transcytosis

IgA is transported across epithelial cells

Antibodies can block ____ of pathogens

Endocytosis

Antibodies help clear soluble _______ from bloodstream

Antigen, by scrubbing RBCs

Timeline for adaptive response to start

4-7 days to start

T_H17 cells cytokine

TGF-B, IL-6