The document discusses the mechanisms of T-cell and B-cell development and tolerance in the immune system, focusing on the processes that enable high specificity and sensitivity without self-attack. It details T-cell maturation in the thymus, including positive and negative selection, the roles of different T-cell lineages, and the influence of the autoimmune regulator (AIRE) protein. Additionally, it covers B-cell development, the importance of immunoglobulin expression, and how receptor editing can prevent self-reactivity.

1. Presented by
Somen Kumar Mistri
Department of Microbiology
University of Dhaka
Dhaka, Bangladesh

2.  How does the immune system provide a high degree of
sensitivity and specificity to the broad array of pathogens
without attacking self?
 How does a T-cell know whether to be CD4+ T-cell or
CD8+ T-cell?
 How does T-cells come in contact with all the self
antigens in thymus?
 What is the role of IgD in the development of B-cell
tolerance?

3. Tolerance
Central Peripheral
(Bone marrow , (Spleen , Lymph
Thymus) node etc)

4. Central Tolerance
Thymic
B-cell tolerance
tolerance to
to self antigen
self antigen (In Bone Marrow)
(In Thymus)

7. Thymus - Site of T-cell
Maturation
Cortex
Medulla

9.  In a young mouse , the thymus contains around
108 to 2×108 T-cells.
 Each day about 5×107 new T-cells are generated.
 Only 106 to 2×106 (Roughly 2 - 4%) of these will
leave the thymus each day as mature T-cells
 So approximately 98% of T-cells that develop in
thymus also die within the thymus.
 Death is usually by apoptosis.

10. Positive Selection
Recognition of Self -MHC
Negative Selection
Avoid strong binding to self antigen

14. 1 2 3 4

15.  Particular combinations of cell surface proteins can be
used as markers for T-cells at different stages of
differentiation.
 Two distinct lineages of T-cell: α:β and γ:δ.
 When progenitor cells first enter the thymus from bone
marrow, they lack most of the surface molecules
characteristic of mature T-cells. Their TCR genes are not
rearranged at that time.
 In fact, proteins such as RAG1 and RAG2
(Recombination Activating Gene ) are not expressed that
are required for rearrangement.
 At first T-cells do not contain :
CD3 : TCR complex
CD4 or CD8

16.  Once DN1 cells encounter the thymic
environment , they begin to proliferate and
express CD25.
 During DN2 stage , rearrangement of the genes
for TCR γ , δ , β chains begins. RAG expression
gets turned on at this stage.
 Cells destined to become γδ T-cells (˂ 5% of
mature lymphocytes) diverge at the transition
between DN2 and DN3.
 After DN3 stage DN4 stage comes. RAG
expression stops here and proliferation of DN4
cells occurs.

17.  Formation of
pre-TCR activates a signal
transduction pathway that has several
consequences :
 Indicates that a cell has made a productive TCR β-chain
rearrangement and signals its further proliferation and
maturation.
 Suppresses further rearrangement of TCR β-chain genes,
resulting in allelic exclusion.
 Renders the cell permissive for rearrangement of the
TCR α-chain.
 Induces the developmental progression to the CD4+ 8+
double positive (DP) state.

18.  Newly formed DP cells reactivate RAG genes allowing
rearrangement of the α-chain.
 Pairing of α and β-chains allow T-cells to recognize a wide
range of antigens (Both self and foreign).
 At this point positive selection occurs.
 Cells that recognize MHC with moderate affinity on
cortical epithelial cells survive.
 Cells whose TCR interacts with MHC class I becomes
CD4+ T-cell (SP).
 Cells whose TCR interacts with MHC class II becomes
CD8+ T-cells(SP).
 Cells whose TCR fails to engage either a class I or class II
MHC molecule undergo programmed cell death.

19. Proliferatio
n
Positive
selectio
n
Negative
Selection
•Positive selection involves recognition of self MHC at cortical epithelial cells.
Mechanism not really known.
•Negative selection predominantly in medulla (Medullary epithelial cells)

20. Positive selection for thymocytes bearing receptors capable of
binding self-MHC molecules, which results in MHC
restriction. Cells that fail positive selection are eliminated
within the thymus by apoptosis.
Negative selection that eliminates T-cells bearing high affinity
receptors for self-MHC molecules alone or self antigen
presented by self-MHC, which results in self-tolerance.

21. POSITIVE NEGATIVE SELECTION
SELECTION
APC APC APC
MHC MHC MHC
Self
antige
TCR n
TCR TCR
T-cell T-cell T-cell
Binding of TCR and MHC Weak binding of TCR Strong binding of
(weak), results in positive and MHC-Peptide TCR and MHC-
selection of T-cells complex :T-cell is peptide complex : T-
positively selected cell is negatively
selected

23.  Engagement of TCR by the MHC-peptide complex on some
type of antigen presenting cell underlies both positive and
negative selection. Positive and negative selection may occur at
low and high degrees of TCR ligation, respectively.
 Experiments show that the same peptide will induce positive
selection at low concentration and negative selection at high
concentration.
 This has led to the avidity model, which postulates that a
functionally low avidity interaction between T-cell and peptide-
MHC involving a relatively low number of TCRs will
positively select Double positive (DP) CD4+8+ thymocytes,
while a high avidity interaction will lead to clonal deletion.
 The overall avidity of the T-cell interaction is a function of :
Ligand Density TCR Density Affinity

24. It is postulated that a low avidity interaction between the T-cell and
APC will give positive selection and that high avidity will give
deletion.
* Refers to affinity of peptide for the MHC or of the MHC-peptide
complex for the TCR

25.  The autoimmune regulator (AIRE) is a protein that in humans
is encoded by the AIRE gene. AIRE is a transcription factor
expressed in the medulla of the thymus and controls the
mechanism that prevents the immune system from attacking
the body itself.
 AIRE allows endothelial cells in thymus as well as dendritic
cells to express other proteins that they normally do not
express.
 Due to the presence of AIRE medullary epithelial cells as well
as dendritic cells present self peptides in low amount to T-cells.
This serves the negative selection purpose.
 Mutation in AIRE results in Autoimmune Polyendocrinopathy–
Candidiasis–Ectodermal Dystrophy (APECED).

30.  B-cell development shows similar features to T-cell
development, but takes place largely in the bone marrow.
 Checkpoints in B-cell development include :
 Successful expression of Igα and Igβ in late pro-B
cells.
 Successful rearrangement at the heavy chain locus in
pre B-cells.
 Successful rearrangement at the light chain locus and
receptor editing.
 Mature B-cells express IgD at higher levels than IgM.
 Self tolerance begins when IgM first appears at the surface
of the developing cell.

31. / Immature B

32.  Expression of IgD is an important checkpoint in terms
of eliminating self reactive B-cells.
 Normally in a mature B-cell, when Antigen binds to
mIg of that B-cell, it turns the B-cell on.
 However, before IgD is expressed, if antigen (Self)
binds to mIg then the B-cell gets turned off. Further
development of that B-cell does not occur, but light
chain rearrangement can continue.

33. If in bone marrow,
IgM binds to a non-
cross linking self
molecule it can not
give strong signal to
the B-cell to die,
instead it generates a
B-cell that may not be
as responsive or
clonally ignorant.

36.  Allowing light chain rearrangement to continue among
immature cells permits the B-cell to edit its receptor and
rescue potentially autoreactive cells from inevitable death.
 Thus by changing the light chain, a self reactive B-cell can
be converted to a non-self reactive one.
 After making a new light chain, the mIgM is again tested
to see if this is also self reactive.
 If the heavy chains association with antigen is very strong,
then light chain rearrangement alone can not alter self
reactivity. Ultimately that B-cell would not be allowed to
leave the bone marrow.

37.  If cross linking occurs, but least
sufficiently to produce strong
signal, the B-cell does not die by
apoptosis.
 It migrates to the periphery and
actually become anergic B-cells.
 Even though they are self
reactive they won’t make
antibodies. Therefore they won’t
cause any kind of autoimmune
diseases.