Lecture slides on Cells of the immune system

1. Cells of the Immune
System

2. Cells of the Immune System
• During embryonic development blood cell precursors
mainly originate in fetal liver and yolk sac
• In postnatal life, all derive from the bone marrow
• Two main lineages derive from the bone marrow
hematopoietic stem cells:
1. Lymphoid lineage
T cells, B cells, Natural Killer (NK) cells
2. Myeloid lineage
• Monocytes, Macrophages, Dendritic cells,
Megakaryocytes, Granulocytes (eosinophils, neutrophils,
basophils)
• In mammals, T cells develop in the thymus while B
cells develop in the fetal liver and bone marrow

3. Hematopoiesis
Myeloid
progenitor
Lymphoid
progenitor
Tc
cell
Th
cell
Hematopoietic
Stem cell
NK
B cell
Granulocyte
Platelets Megakaryocyte
Macrophage Monocyte
Dendritic cell
AFC
Plasma
cell
Mast cell

4. Thymic Processing of T cells Hematopoietic
Stem cell
CD4-
CD8-
TCR-
CD4+
CD8+
TCR+
CD4+
CD8-
TCR+
CD4-
CD8+
TCR+
• T-cell precursors differentiate into
immunocompetent T-cells within thymus
• Prior to thymus all lack antigen receptors &
CD3, CD4 & CD8-Double negative
• In thymus first differentiate to express both
CD4 & CD8 - Double positive
Double negative
Double positive
• A double-positive will become CD4+ if it
contacts a cell bearing MHC II proteins
• A double-positive will become CD8+ if it
contacts a cell bearing MHC I proteins
• These are now Single positive
MHC I
MHC II
No MHC
engagement
Single positives
Apoptosis

5. Thymic Processing of T cells
Hematopoietic
Stem cell
CD4-
CD8-
TCR-
CD4+
CD8+
TCR+
CD4+
CD8-
TCR+
CD4-
CD8+
TCR+
Double negative
Double positive
• Double-positive & double negative
cells are located in thymus
• Single-positive are located in medulla and
migrate to blood and extrathymic tissues
MHC I
MHC II
• Each double-positive T cell synthesizes a
highly specific Ag receptor (TCR)
• The rearrangement of variables, diversity
accounts for the remarkable ability of T
cells to recognize millions of different Ags No MHC
engagement
Single positives
Apoptosis

6. • CD4+ & CD8+ cells bearing Ag receptors for “self”
proteins are killed (clonal selection) by a process of
programmed cell death called apoptosis
• The removal of these self reactive cells called negative
selection results in self tolerance and prevents
autoimmune reactions
• CD4+ & CD8+ cells bearing Ag receptors that do not react
with self MHC proteins are also killed: Positive selection
for T cells
• Thymic Processing produces T cells that are selected for
their ability to react with foreign Ags and with self MHC
Thymic Processing of T cells:
Thymic Education

7. CD3+ Cells
• All T cells have CD3 proteins on their surface in
association with TCR
• Is complex of 5 transmembrane proteins that
transmit information inside the cell that TCR is
occupied
CD4+ & CD8+ cells
• T cells are divided into two major categories
whether they have either CD4 or CD8 proteins
on their surface but not both
T cells Types

8. • 65-80% of circulating pool of small
lmphocytes
• Within LN, are located in the inner
subcortical region
• Life span is months to years
Features of T cells

9. TCR: Specificity of Immune
Responses Resides in Receptors
• T cell receptor (TCR) recognizes pathogen
(antigen)-associated with MHC and is univalent.
• TCR consists of two polypeptides α & β
associated with CD3: are similar to
immunoglobulin heavy chain
• Each T cell has a unique TCR on its surface;
millions of T cells exist in each person
• Clones of T cells are produced specific for an Ag
T
cell

10. What Does the TCR Recognize?
 Only fragments of proteins (peptides) associated with
MHC molecules on surface of cells
 Helper T cells (Th) recognize peptide associated with
MHC class II molecules
 Cytotoxic T cells (Tc) recognize peptide associated with
MHC class I molecules
 Generally, Class I MHC proteins present endogenously
synthesized Ags e.g. viral proteins and class II MHC
presents Ags of extracellular microbes which are
phagocytised like bacterial proteins

11. T Cell Functions
Regulatory: mainly mediated by CD4+ cells (Th
cells)-Th 1 & Th 2. Play a central role in humoral
and cell mediated immune responses
Th1 cells:
• Help CD8 T cells by IL-2 to become activated Tc cells
• Activate macrophages by Gamma-INF in delayed
hypersensitivity
Th2 cells:
• Help B cells by IL-4 & IL-5 to develop into Ab-
producing plasma cells
* IL-12 produced by macrophages is important regulator
of balance between Th1 & Th2

12. Helper (Th1) T Cells
Macrophage Macrophage
Th1
cell
Lysosome
Mycobacteria Antigen
Infected macrophage
Activated infected
macrophage
Mycobacteria resist intraphagocytic killing by inhibiting fusion with lysosomes.
Th1 cell recognizes an infected macrophage and releases macrophage-activating
molecules that induce lysosomal fusion and the activation of macrophage
bactericidal activities

13. • Four types of T cells : Th-1, Th-2, Th-17 (CD4+ cells) &
CD8+ cells
• Th-1 cells: mediate delayed hypersensitivity reactions
against intracellular organisms including certain fungi
and bacteria. Th-1 cells secrete gamma INF and activate
macrophages to kill the organism
• Th-2 cells along with eosinophils mediate
protection against worms by producing IL-4 which
activates eosinophils to secrete enzymes to kill worms
• Th-17 cells : protect against the spread of bacterial
infections by recruiting neutrophils by IL-17 at mucosal
surfaces
T Cell: Effector Functions

14. • CD8+ cells (T cytotoxic cells) :
Protect by killing:
 Virus infected cells
 Tumor cells
 Allografts
T Cell: Effector Functions

15. Cytotoxic (Tc) T Cells Effector Function
Virus infects cell
A
B
C
Cell expresses
viral antigens
Cytotoxic
T cell
Infected cell is killed by cytotoxic T cell
by activation of perforins and granzymes
that cleave host and viral DNA
Th 2 cells recognize viral Ags
bound to MHC II on APC and
secrete IL-2
IL-2 activates Tc cell to kill virus
infected cell

16. Fas-Fas ligand (FasL) Interaction
• Fas is a protein on surface of many cells
• FasL is in Tc cell
• When Tc TCR recognizes an epitope on surface
of target cell, FasL is induced in Tc cell
• When Fas & FasL interact apoptosis of target
cell occurs
ADCC
• Ab bound to the surface of infected cell is
recognized by IgG receptors on surface of
phagocytic cell and the infected cell is killed
Cytotoxic (Tc) T Cells Effector Function

17. B Cells Processing
• B Cell precursors differentiate into
immunocompetent B cells in BM
• B cells also undergo clonal deletion of cells
bearing Ag receptors for self proteins and
reduce occurrence of autoimmune
diseases.
• The site of clonal deletion is uncertain

18. B Cells Processing
• Maturation of B cells has two phases:
• Ag-independent phase consists of stem
cells, pre-B cells, B cells
• Ag-dependent phase consists of activated
B cells and plasma cells
• Pre-B cells lack surface receptors (Ig) and
light chains. Are found in bone marrow

19. • 30% of circulating pool of small lmphocytes
• Within LN, are located in the germinal center,
also in spleen and Peyer’s patches
• Life span is days to weeks
• 109 B cells are produced daily
Features of B cells
B cell receptor (surface immunoglobulin-
IgM) recognizes antigen and is bivalent.
This IgM is monomer
B
cell

20. B Cell Activation
• An Ag interacts with IgM receptor on B cell (APC)
• The Ag is processed and epitopes appear on the
surface in conjunction with class II MHC
• This complex is recognized by Th2 cell with a
receptor for the Ag on its surface
• Th cell gets activated and produces IL-2, IL-4, IL-5
that activate B cells

21. Cellular Events in 1o Response to
T-dependent Ags
• Lag Phase
• Clonal selection
• Log Phase
• Initially IgM
• Class switching:
some B cells switch
from making IgM to
IgG, IgA or IgE.
• Stationary Phase
• Decline
• Memory Cell Pool
IgM
Memory Cells
IgG
1o Ag
IL2,
BCGF (IL4),
BCDF (IL5)
Activated
Th2 cell
Activated B cells transform to plasma cells to
produce thousands of specific Ig molecules
per sec for few days and then die.

22. Cellular Events in 2o Response to
T-dependent Ags
• Lag phase
• Virgin cells
• Memory cells
• Log phase
• Pool size
• IgG, IgA or IgE
• Stationary
• Decline
• Sustained
production
IgM
Memory
Cells
IgG
IgG
Memory
Cells
Memory
Pool
Virgin B cell

23. Comparison of T cells and B cells
Features T cells B cells
• Ag receptor on surface Yes Yes
• Ag receptor recognizes only processed Yes No
peptides in association with MHC
• Ag receptor recognizes unprocessed No Yes
proteins without requirement of MHC
• IgM on surface No Yes
• CD3 proteins on surface Yes No
• Ig synthesis No Yes
• Regulator of Ab synthesis Yes No
• Effector of CMI Yes No
• Maturation In thymus Bursa or
its equivalent

24. Lymphocyte Recirculation
Naïve lymphocytes
enter lymph nodes
from the blood circulation
Lymphocytes return
to blood
via thoracic duct
Antigens from infected area
go to lymph nodes
via the lymphatic system
Lymphocytes recirculate
and encounter antigen in
peripheral lymphoid tissues

25. Leukocyte Migration and Localization
T
cell
T
cell
T
cell
B
cell
B
cell
B
cell
APC
T
cell
B
cell
Naive
lymphocytes
Bone marrow
Thymus
Spleen and lymph nodes Tissues
Primed lymphocytes
Dendritic
cell
Macrophage

26. Classes of Major Histocompatibility
Complex (MHC) Molecules
MHC class I and MHC class II

27. Class I MHC Molecules
• Are about 20 glycoproteins expressed on
surface of all nucleated cells
• Polymorphism of MHC is important in
recognition of self & non-self
• Are recognized by TCR of Tc (CD8+) cells
• CD8 binds to class I MHC-peptide complex

28. Class II MHC Molecules
• Are glycoproteins expressed on
surface of some nucleated cells,
mainly on APC, B cells, dendritic
cells and Langerhans cells
• Are recognized by TCR of Th
(CD4+) cells
• CD4 binds to class II MHC-peptide
complex

29. Cells Expressing Class I and Class
II MHC
Class I MHC
Class II
MHC
All nucleated cells
express class I MHC
Cells expressing
class II MHC also
express class I MHC

30. Biological importance of MHC
The ability of T cells to recognize Ag is
dependent on association of the Ag
with either class I or II MHC

31. Self MHC Restriction in the Thymus
4 -
8 low
4 low
8 low
4 + 8 +
TCR
4 + 8 +
TCR
macrophage
Productive TCR
rearrangement
Non-productive TCR
rearrangement
Recognise self MHC
Not recognise self MHC
TCR does not recognise self
Antigens Negative selection
TCR recognises
self antigens
APOPTOSIS
4 + 8 +
TCR
4 - 8 -
Sub-capsular region
Cortex
Cortico-medullary region
4 + 8 -
TCR 4 - 8 +
TCRvessel
Medulla

32. Class I MHC Pathway
Viral protein is made
on cytoplasmic
ribosomes
Plasma membrane
Proteasome
degrades
protein to
peptides
Peptide transporter
protein moves
peptide into ER
MHC class I alpha
and beta proteins
are made on the rER
Peptide associates
with MHC-I complex
Peptide with MHC
goes to Golgi body
Peptide passes
with MHC from Golgi
body to surface
Peptide is presented
by MHC-I to CD8
cytotoxic T cell
Golgi body
rER
Globular viral
protein - intact

33. Class II MHC Pathway
Globular
protein
Endocytosis
Protein is processed to
peptides in endosome
or lysosome
Endosome
Lysosome
Fusion of endosome
and exocytic vesicle
Endoplasmic reticulum
Class II MHC
Synthesis
3 chains: α,β and Ii
Golgi
body
Exocytic vesicle fuses
with endosome
releasing Ii from αβ dimer
α
β Ii
Immunodominant
peptide binds
to class II MHC
Endosome fuses with
plasma membrane
Peptide MHC-II
complex is presented
to CD4 helper T cell
CD4 helper T cell

34. Leukocyte Migration and Localization
• Bone marrow and thymus (primary lymphoid tissues)
produce B cells and T cells, respectively
• B cells and T cells recirculate through spleen and
lymph nodes (secondary lymphoid tissues)
• Recirculation of lymphocytes through lymphoid
tissues optimizes productive encounters with Ag to
initiate response
• Antigen presenting cells (APC) pick up antigen and
migrate to secondary lymphoid tissues (main sites
where lymphocytes encounter Ag) and interact with
T cells and B cells