L4 Lymphocyte Development

Where is the pluripotent hematopoeitic stem cell

bone marrow

what transcription factor do the precursor cells produce to become B cells which stay in the bone marrow

Pax5

how many B cells are produced per day (continuous process)

~3×10^10

what does B cell development involve from Ig genes

re-arrangement and expression

give examples of B cell specific markers with the expression of the lymphocyte

• CD45

• CD19

what does the removal of self-reactive cells attempt to avoid

autoimmunity (negative selection)

B-cell precursor rearranges its immunoglobulin genes causing…

generation of B-cell receptors in the bone marrow

immature B cell bound to self cell-surface antigen is removed from the repertoire causing…

negative selection in the bone marrow

if the B cell does not recognise anything in the bone marrow it means the cell receptor is not self-reactive so they…

migrate through the circulatory system to lymphoid organs and B-cell activation

activated B cells give rise to plasma cells and memory cells which causes

antibody secretion and memory cells to form in bone marrow and lymphoid tissue

describe H chain genes rearranging first (mu chain)

moves to the cell surface with Igalpha and Igbeta and expressed with surrogate light chain

what is the surrogate light chain a product of

V preB and lambda 5 genes

why is the pre-B cell receptor there

to make sure that the random B cell receptor heavy chain is capable of subsequently binding a light chain

what happens if the B cell receptor heavy chain is capable of binding a light chain

the light chains rearrange

describe light chain rearrangement

displacement of V preB and lambda 5 chains (associating with the heavy chain)

• IgM BCR

what are B cells called when they have rearranged heavy and light chains

immature B cells

Describe the signal from pre-BCR

• turns off RAG-1, RAG-2 genes

• 5-6 rounds of cell division

• surrogate light chain expression stops

• RAG-1 and RAG-2 turned on again

• L chain rearrangement starts

what are RAG genes needed for

gene rearrangement

what does the signal from pre-BCR show

H chain looks functional

• no Ag required yet

what happens as kappa light chain genes rearrange before lambda genes

more B cells will express kappa than lambda light chains

how many goes can the B cell have on each chromosome

five

describe the error prone way of immunoglobulin gene re-arrangements

if cell fails to productively re-arrange both H and L genes it dies

what happens if there is successful H chain re-arrangement but rearranged genes may be out of frame

• pre-B cells that initially fail to generate non-productive re-arrangements of light chain kappa genes can be ‘rescued’ by up to 10 further rearrangements at the same locus

• 5 Jk genes on each chromosome

• if after all these attempts and still out of frame the lambda locus will begin t rearrange

what do immature B cells only express

membrane IgM

what do immature cells that bind multivalent self-antigen undergo

• clonal deletion

• receptor editing

what is clonal deletion

cells die by apoptosis

what is receptor editing

further light chain gene rearrangements of variable regions (get another chance)

what happens to immature B cells that bind soluble self-antigens

the cell becomes unresponsive (anergic)

T cells do not express Pax5 so they instantly go to the

thymus (from the bone marrow)

how are self-reactive T cells eliminated

negative selection

what are the alternative lineages of T cell rearrangement

TCRalpha and TCRbeta genes are rearranged (CD4+ or CD8+) or TCRgamma dn TCR delta genes

T cells expressing TCRalpha and TCR beta must bind with…

self MHC expressed in the thymus

• positive selection (but fine line → need to avoid excessive response)

precursors commit to the T cell lineage, following Notch signalling and initiate…

T-cell receptor gene rearragements

Immature T cells which recognise self MHC recieve signals for survival. Those that interact strongly with self antigen are…

removed from the repertoire

T-cell progenitors develop in the bone marrow and migrate to the thymus where the cells complete their development by…

rearranging their antigen - receptor genes and undergoing repertoire selection

Mature T cells encounter foreign antigens in the…

peripheral lymphoid and are activated and migrate to peripheral lymphoid organs

once in the thymus, T cells (alpha/beta) develop into thymocytes which…

• re-arrange TCR genes (beta first) and express TCR

• acquire other markers e.g. CD3, CD4,CD8 (can have multiple)

• undergo positive and negative selection

which TCR gene is rearranged first

beta

describe the structure of the thymus

• bi-lobed organ in anterior mediastinum

• each lobe divided into many lobules

• each lobule has an outer cortex and inner medulla

what cells are found in the thymus

• lymphoid cells

• epithelial cells

• macrophages

• dendritic cells

how do pro-thymocytes enter the cortex

via blood vessels from the bone marrow

describe what happens when thermocytes re-arrange TCR genes once inside the thymus

• firstly, they rearrange TCRbeta genes (similar to H chain in BCR)

• expressed along with pre-T cell receptor

• cells proliferate and then re-arrange TCRalpha genes

what do T cells express during maturation in the thymus

• TCR together with CD3

• both CD4 and CD8

what do peripheral T cells express

either CD4 or CD8

what complex does TCR expression require

CD3 complex

describe the structure of the CD3 complex

• delta, epsilon and gamma chains

• zeta chain dimer

what does CD3 transmit signal to

T cell nucleus following TCR recognition of p/MHC

what do gamma/delta T cells not express

CD4 or CD8 markers

how diverse is the gamma/delta TCR than the alpha/beta receptor

less

where is the gamma/delta TCR expressed

• 1-5% in circulation

• majority in epithelial tissues at mucosal surfaces

what does the lineage commitment to gamma/delta TCR or alpha/beta TCR depend on

which genes are first to rearrange successfully

T cells expressing a randomly rearranged alpha/beta TCR may:

• recognise self MHC plus peptide from ‘foreign Ag’

• recognise self MHC plus peptide from ‘self’ Ag

• not be able to recognise self-MHC

what is the term for recognising self MHC plus peptide from ‘foreign Ag’

immunity

what is the term for recognising self MHC plus peptide from ‘self’ Ag

autoimmunity

what happens when TCRs are not able to recognise self-MHC

as all cells will express self-MHC, T cells with this TCR will be useless

reality is a fine balance based on TCR/MHC affinity, describe this

• need to keep T cells with TCR 1

• eliminate those with TCR 2 and 3

  • (positive and negative selection)

describe positive selection

cells which recognise self MHC (+self peptide)

• occurs when double-positive (DP, CD4+ CD8+) T cells recognise MHC on thymic cortical epithelial cells

• apoptosis if not recognised

describe how rearrangement gives a random TCR repertoire

• only 1-2% capable of recognising own MHC

• cells expressing TCR that doesnt bind self-MHC die via apoptosis

where do positively selected cells move to

the medulla

describe negative selection of T cells

• recognise self MHC (+ self peptide) on thymic medullary dendritic cells/macrophages with high affinity (as these T cells may induce autoimmunity if not removed)

what does TCR binding to MHC/self-peptide with a high affinity cause

T cell to die via apoptosis (clonal deletion)

positive and negative selection occur…

sequentially, in different regions of the thymus

Describe the paradox of positive and negative selection

T cells are positively and negatively selected on self MHC and self peptide

• why aren’t all T cells that have been positively selected subsequentially eliminated by negative selection?

describe TCR affinity for p/MHC being the basis for selection

• all T cells recognising MHC/self peptide are positively selected

• those with highest affinity TCRs are negatively selected

• goal: a population of T cells with low affinity for self peptide and self MHC

why do we want cells with low affinity for self peptide and self MHC

safest/most useful

• highest probability they will have high affinity for self MHC when presenting peptides derived from pathogens

describe T cells that survive thymic selection

• express TCR capable of binding self MHC

• depleted of self-reactive cells

• exit the thymus as mature, single positive (SP) T cells

• CD8+ T cells → MHC class I

• CD4+ T cells → MHC class II