Chapter 8- BOOK- Development of B and T lymphocytes

Central/primary lymphoid tissues

Bone marrow for B cells and thymus for T cells

Secondary lymphoid tissues/peripheral

lymph nodes, spleen, mucosal tissue

Positive selection

Cells with R that bind weakly to self antigens survive

Negative selection

lymphocytes which bind strongly to self reactive receptors must be eliminated to prevent autoimmune diseases→ self tolerant

Development of B lymphocytes: stromal cells

a network of specialized nonlymphoid connective tissue which produce signals for develpment

Have adhesive contact with lymphocytes + provide cytokines and chemokines that contribute to differentiation + proliferation

First step of development of B cell

Hematopoeitic stem cells differentiate into lymphoid myeloid primer multipotent progenitors (LMPPs)

LMPPS

produce 3 major subsets of progenitors:

EILP→ produces ILCs

CLP→ mostly B cells (pro-B cells)

ETP→ T cell progenitors

Interleukin-7 (IL-7)

Cytokine secreted by bone marrow stromal cells

ESSENTIAL for growth and survival of innate lymphoid cells

Composed of 2 polypeptides

B cell development

CLP → pro B cell→ receive proliferation ans survival signals from IL-7→ immature B cell→ Mature B cell

Pro B cell

Rearrangement is initiated→ induced expression of RAG1 and RAG2

1. Early: Joining of D and J segment

2. Late: Joining of V to DJ segment

   Has: IL-7 R, Kit (CD117)

Successful rearrangement→ production of 2 u heavy chains→ if not → elimination (NON productive)

Pre B cell

Pre B receptor is formed

Heavy chain is formed + surrogate light chains

Progression to light chain rearrangement

Large→ proliferates→ small→ cease expression of the surrogate light chains

Express u heavy chain alone

Immature B cell

Express Ig M at cell surface

Stop light chain gene rearranement

Mature B cells

Produce delta heavy chain + u heavy chain

Ig M + Ig D

TdT

Expressed in pro B cell during heavy chain rearrangement

Adds non template nucleotides (N-nucleotides) between gene segments

Helps with the diversity of the BCR

Found mostly in heavy chains

Pre B cell R rests production of heavy chain

Is the heavy chain produced functioning?

How? Incorporate a functional heavy chain into a receptor that can signal its successful production.

Absence of light chains-loci not rearranged→use 2 surrogate light chains

Surrogate light chains

Pro cells make 2 surrogate proteins that can form a similar surrogate light chain and can pair with u chain to form pre B cell R

The assembly of pre BCR signals to B cell → productive rearrangement→ The cell is a PRE B cell now

Ig Beta and Ig alfa chains

Pro and pre B cells

They are signaling components of the BCR complex on mature B cells→ they transduce signals by interaction with tyrosine kinase through cytoplasmic tails

Crossliniking

Pre B cell receptor and signaling

Checkpoint between pro and pre B cell

Allelic exclusion

To prevent the formation of 2 successful receptors of different antigen specificity

Signaling by the pre B cell receptor→ allelic exclusion

Only one of the 2 allels is expressed in a diploid cell

Occur in both heavy and light chain loci

Signaling from pre B cell R (allelic exclusion)

3 ways to promote heavy chain allelic exclusion:

1. Reduces the V(D)J recombinase activity by reducing RAG1, RAG2

2. Reduces RAG-2 by indirectly causing the protein to be targeted for degradation

3. Reduces access of heavy chain locus recombinase machinery

→ pre B cell receptor signaling→ profund reduction in numbers of pre B cells and mature B cells that develop

Pre- B cells function

Large pre B cells become small

RAG are produced again in small

Rearrangement of light chain occur

Rearrangement only one allele at a time

If VJ rearrangement fails from one allele→ use the other allele→ can happen multiple times

Isotypic exclusion

Light chain only express one type of chain kappa or lambda

Kappa rearranges first→ most common

When is Ig M expressed?

Once a rearranged light chain has paired with a u chain it can be expressed on the cell surface

The pre B cell becomes an immature B cell

Immature B cell

Antigen R is if first tested for self antigens or autoreactivity

Autoreactivity cells are eliminated or replace receptor by receptor editing

Enter the spleen

Central tolerance

Arises in a central lymphoid organ- bone marrow

Elimination of autoreactive B cells

Transitional B cells

B cells emerge from bone marrow into the periphery , not fully matured

Final maturation occur in periphery

Express mostly Ig M but little Ig D

(the opposite expression in a mature B cell)

Follicular dendritic cells

FDCs

Non hematopoetic cells in B cell follicles, captures antigens for recognition by B cell antigen receptors

Follicles

Part of the lymph node where transitional B cells enter

Provides signals for B cell survival

Complete their maturation here

Transitional stages for transitional B cells

Called T1 and T2

Then enter B cell pool

Defined by the abscence or presence of CD21

Follicular B cells

Reside in spleen and other peripheral organs

Marginal zone B cells

Minor population of B cells in spleen

Tyrosine kinase Syk

Involved in signaling from B cell receptor

Without SYK→ fail to develop mature B cells

Survival or mature B cells

B1 B cells

Subset of B cells involved in the innate defense

Source of natural antibodies→ consistently produced antibodies secreted prior infection

Recognize polysaccharides

DONT need T cells → produce IgM within 48 hours (no T present)

Development of T cells

Mature in thymus

Thymic stroma → T cell precursors embedded in a network of epithelia→ unique environment for development

Progenitor T cell

Cells entering thymus lack cell surface molecules

Their receptor gene is not rearranged

Early thymic progenitors ETPs

In thymus, receive signal from thymic epithelial cells transduced through a receptor called Notch to switch on specific genes

Notch signaling

Required through the T cell development

Increase the expression of 2 transcription factors

Essential for commitment to T cell lineage

Also induces expression of IL-7 alfa chain important cytokine-

Double negative thymocytes/ double positive

Absence of both CD4 and CD8 or presence of both

Regulation of T cell rearrangement

Cells that fail the rearrangement of the Beta chain locus → death

Successful rearrangement→ express beta chain protein→ next stage is called DN4 stage→ proliferation→ loss off CD25 expression

Pre T cell receptor

Th expressed Beta chains pair with a surrogate pre-Tcell receptor alfa chain called pTalfa→ assembly of pre TCR (analogous to the pre-beta R)

Expressed with CD3→ provide signaling components of receptor

Double positive thymocytes

Alfa chain locus rearranges

Alfabeta TCR is produced

One of the 2 receptor molecules (CD4/CD8) is down regulated → single positive thymocytes

Thymus structure

2 main regions:

Peripheral cortex and the central medulla

T cell development occur in cortex

Only mature single positive thymocytes are present in the medulla

Development of T cell

• Progenitors entering to the outer cortex→ here double negative thymocytes proliferates

• Positive selection- MHC molecules+epithelial cells +receptors

• Developing T cells migrate from cortex to medulla

• Negative selection in medulla→ dendritic cells express co-stimulatory molecules

delta-gamma TCR

Found primarly in epithelial and mucosal sites

Lack expression of CD4 and CD8

Mostly component of the innate

Can differentiate into dentritic epidermal T cells DETCs

Successful synthesis of rearranged Beta chain (alfa/beta T cells)

Allows the production of a pre -T cell receptor that triggers cell proliferation and blocks further Beta chain gene rearrangement

T cell receptor alfa chain gene

Comparable to kappa and lambda in Ig→ they dont have any D gene segments and are rearranged after their partner receptor chain has been expressed

Can produce two types of alfa chains→ expression of the TCR is not sufficient to shut of rearrangement of gene

Big difference between T cell and B cell devlopment

Multiple alfa chain are produced and rearranged and tested with the same beta chain

Not occur in B cells

Positive and negative selection of T cells

Positive selection=the rescue of a double positive cell from programmed cell death and their maturation into single positive cells

Negative selection= if maturing thymocyte has strong reactivity to a self antigen found in the medulla→ cells with weaker levels of self reactivity→ differentiate

Peptide:self MHC complexes

Only thymocytes whose receptors interact with self peptide:MHC complexes can survive and mature into naive CD4 or CD8 cells

Positive selection functions

Coordinates the expression of CD4 or CD8 with the specificity of the TCR

CD4 cells have R recognizing peptides bound to MHC class2→ become cytokine secreting helper T cells

CD8 cells recognize MHC 1→ become cytotoxic T cells

Positive selection determines the phenotype

Co receptors in positive selection

MoreBind to MHC: CD4 and CD8

Recruit- Lck= tyrosine kinase→ initiates signaling

The signal is only initiated with cells that have TCR bound to MHC→ survival

Stromal cells in positive selction

Important! Close contact with double positive cells

Display the MHC molecules to the T developing cell

Negative selection function

T cell reacting strongly to self antigens are deleted

More receptor specificity must be positively selected tha nehatively selected→ otherwise no T cells produced

AIRE

Autoimmune regulator

Gene controlling the expression of tissue specific proteins in thymic medulla→ lengthens transcripts that would otherwise be terminated

Fasciliatetes negative selection by proimoting expression of tissue specific antigens in epithelial cells

Affinity hypothesis

The choice between pos and neg selection is thought to depend on the strength of the self peptide:MHC binding by the TCR

Low affinity→ positive selection

High affinity→ apoptosis

T reg cell subset

A population of CD4+ T cells that function to maintain self tolerance

TCR have moderate high affinity for self peptides

Invariant NKT cells (iNKT cells)

Express a receptor called NK1.1 receptor

Part of the early response in immune defense

Recognize CD1 molecule rather thanMHC molecules

Thymic emigration

After 1-2 weeks of maturation in thymic medulla→enter blood stream

Emigration requires recognition of the lipid molecule S1P by the G protein coupled receptor S1PR1→ expressed by thymocytes during final maturation

Why does not a mature naive lymphocyte + self antigen lead to cell death? But with a pathogen?

Infection→ inflammation

Induces expression of co-stimulatory molecules on the antigen-presenting dendritic cells→ cytokines promote activation of lymphocytes