Clonal selection theorie (MacFarlane Burnet, 1957)
each lymphocate carries an AG receptor with a unique specificity
AG recognition by a specific lymphocyte triggers its activation and proliferation
daughter cells carry identical AG receptor to the parental cell (=clone)
one specific lymphocytoe trapped/slected - gets active an proliferates (1/1000000) all the daughter cells who arise arry aG receptor of parental cell.
Clonal selection - selection of one cell
clonal proliferation - proliferation leads to gen. identical clone of daughter cells
Clonal selection : proliferation and differntiation
One t-cell can be specific for peptides for different pathogens
how fast is the reproductive rate of T cells - not clear (4-7d)
per cell division -> few hours
leads to elimination of infection
exponential increase after threshold for protection is reached
Importatn developmental stages of T cells - double negative -> bo CD4 and D8
double pos - have CD4 and CD8 -> bc important for TCR signal, must interact with MHC/self peptides
ensures posibility of positive selection.
single pos - either CD4/CD8
single neg - no CD5 or CD8
negative selection latest (small resting single-pos. thymocytes)
rearrangement of the TCR a and ß chain
somatic cells of the body - when does recombination takes place - 1. crossing over = increasing diversity -> germ line; 2. cnacer cells -> somatic line! just wants to differentiate and proliferate
Problem: how could you combine the sections ? later on mRNA lever -> but still large genome => recombination in genome -> somatic recombination on DNA level
high number of V/J segemnts - high variety of AGs. -> LC - only 1 recombination event here - one J with one V
3 kind of gene segemnts for HC -> 2 recombination events here - 1 D with 1 J - 1 V with 1 DJ.
sequencial process - does not take place on the same time.
afterwars transcritoption, splicing, translation and protein synthesis - normal
ß comparable to VDJ
a comparable to VJ
stages of the gene rearrangement in “normal” (aß) T cells
Process always in beginning: Germline gene configuration in maturing CD4-8- thymocyte ->Dß-Jß rearrangement (y and sigma chain rearrangement may also occur) in CD25+ CD44^lowthymocyte rearranging ß-chain genes -> Vß -DJß rearrangement in frame. ß chain protein produced - in CD25+ CD44^low thymocyte cytoplasmic ß+ ->surface expression of ß chain with durrogate a chain. ß rearrangement stopf cell proliferates CD4/CD8 induction a transcription starts - in CD4- 8- ->CD4+8+ -> Va-Ja rearrangement - surface expression of a:ß:CD3, selective events begin -> in CD4+8+
stages of gene rearrangement in “normal” Tcells
positive selection does take place in double neg. stage - immediated by taq.
how do we get to the signel positive stage? positive selction can develope in CD4 or CD8. the other one will be downregulated
positive selection by cortical thymic epithelia cells
normal MHC 2 expression -> both CD8 and CD4 T cells mature (by epithelia cells)
MHC2 negative mutatnt (knock out of MHC2 processing -> only CD8 Tcells mature
mutant with MHC 2 transgene expressed in thymic epithelium -> both CD8 and CD4 T cells mature (rescue mutant)
Mutant with MHC2 transgene expressed that cant interact with CD4 -> only CD8 T cells mature
MHC2 = B cells, macrophages, DCs and epithelia cells -> absolutley needed for CD4 development
B cell maturation: an overview / Where do the B cells mature?
first always heamapoetic stem cells -> pluripotent.
B cell precursor rearranges its immunoglobulin genes -> generation of B-cell receptors in the bone marrow
immature B-cell bound to self cell-surface AG is removed from the repertoire -> negative selection in the bone marrow -> autoreactive B cells recognize
matture B cell bound to foreign AG is activated -> B cells migrate to the peripheral lymphoid organs
Activated B-cells give rise to plasma cells and memory cells -> AB secretion in bone marrow and lymphoid tissue and memory cells in lymphoid tissue
=> B cells need to be negative selected.
rearrangement of gene segments encoding for the variable Ig regions _ Important
somatic cells of the body.
when do recombination takes place?
crossing over = increased diversity -> germ line!
cancer cells -> somatic line -> just wants to differentiate /proliferate
problem: How could you combine the sections? -> later on mRNA level -> but still large genome
=> recombination in genome
somatic recombination on DNA level!
Germline DNA—somatic recomb. -> D-J joined rearranged DNA—>V-J or V-DJ joined rearranged DNA—> Primary transcript RNA—Splicing—>mRNA—Translation—> polypeptide chain
what is somatic recombination? (Exam) -> DNA recombination in lymphocytes -> even on genome level in somatic cells, not restricted to germ cells
gene setments are encoding in human genome.
uncommon - bc recombination event = something unexpected happens
stages during rearrangement of Ig gene segments
does this rearangement take place on both chromosomes ? first rearrangement - yes, on both
how often is the cutting of DNA successful ? -> endonucleases (restrictions enzymes) cut and generate blund ends - problem -> V needs to find D segement; changing of gen. code - funcitonal protein ?
1/3 is working - 3 bp encode a AS - cant derive
2/3 unproductive rearrangement & no AB production
first HC in parallel for both chromosomes
allelic exclusion - means of molecular biology - rearrangement only on 1 chromosome at a time
B cell maturation -> ATP dependend
Pre-B receptor: an intermediate stage in B cell maturation (VPre B/ lamda 5) -> surrogate LC
formed by fully recombined HC & surrogate LC - mainly left out
Repeated rearrangements at the L-Chain loci possible
to prevent cell-loss!
only for LC gene-segments.
increases numbers of probability for sucessfull recombination but only on LC
opportunities to prevent loss of Bcell = increase number of recombination event on one chromosome -> frame shift would be unproductive.
icrease probability of successful recombination event
Elimination of self-reactive B cell
immature B cell (bone marrow) - shoud be selfactive
multivalent self molecule - crosslinking with AB (igM)-> clonal deletion or receptor bindng -> apoptosis (normally) or receprot additing (rescue) -> normally or receptor additing (rescure)
Soluble slef molecule ( b cell does recognize molecule, but without crosslinking -> survive but anergic => cant be activated again!) —migrates to periphery -> Anergenic B cell
Low affinity noncross-linking self molecule (most complex -> maybe a bit of recognition, very weak binding, less as when the cell gets anergic) -> migrates to periphery -> mature/ naive B cell (very clonally ignorant) => mainly potential autoreactive = could cause autoimmunereaction! =>crossreactivity
No self reaction (doesnt recognize) -> migrates to periphery ->Mature/naive B cell -> mainly potentiall autoreactive = could cause autoimmune reaction -> cross reactivity - prinicpal the same as ‘3’ -
Naive B cell
not yet been stimulated
hasnt seen the AG yet
same as ‘mature’ but he doesnt like that term
receptor editing to rescue self-reactive B cells
(only for LC) -> strong ligation of IgM by self AG -> Arrest of B-cell development and continued light chain rearrengement: low cell-surface IgM (still active) -> a new receptor specifity is now expressed —2 ways—> 1. if the new receptor is still self-reactive, the B cell undergoes apoptosis. 2. if the new receptor is no longer self-reactive, the immature B cell migrates to the periphery and matures
only one time possible still during selection process
a way to safe resourcess -> could go into apoptosis
another LC recombination
T cell maturation: an overview
thymus= branch between lungs
basically the same progress
starts in the Thymus: T cell precursor rearranges its T-cell receptor genes in the Thymus -> T-cell progenitors develop in the bone marrow and migrate to the thymus.
Immature T-cells that recognize self peptides MHC molecues receive signals for survival. those that interact strongly with self AG are removed from the repertoire. -> positive (first )and negative (second) selection in the thymus
mature T cells encounter foreign AGs in the peripheral lymphoid organs and are activated -> mature T cells migrate to the peripheral lymphoid organs -> AG presentation
activated T cells proliferate and migrate into peripheral sites to eliminate infection -> activated T cells migrate to sites of inflammation -> T cell differntiation
what does negative selection mean?
the ones self recognizing killed
in the inner part of the medulla/thymus
What does positive selection mean?
recognize MHC molecules - T cell only continues development if it interacts with MHC molecules! also with loaded self peptide molecules
but must not interact with MHC PEPTIDE/Self peptide
takes place in the outer part of the medulla
Structure of the human thymus
Located above the heart
cortex (=outer area) contains cortical epithelia cells, immature thymocytes (developing B cells), and macrophages
medulla (=central area) contains medullary epithelia cells, mature thymocytes, and DCs
fully developed in cortex.
negative selection mainly presented by DCs
cortex outer part of thymus, medulla inner part of thymus -> only here negative selection