Download Lymphocytes

Human Organs and Systems 2216
14 September 2009
Prof John Campbell
LYMPHOCYTES
Most kinds of cells belong to discrete "cell types" in which all
cells are equivalent. In contrast, the small lymphocyte is a
resting stage for cells while they circulate in blood or wandering
in connective tissue. A variety of quite different cell types
assume the form of "small lymphocytes" when they are inactive.
Lymphocytes are extraordinarily heterogeneous. Various types of
cells that interact with lymphocytes also are diverse, including
reticular cells, dendritic cells, and endothelial cells.
Four major classes of lymphocytes are B, T, NK and stem cells.
Most of these have subclasses and sub-subclasses.
Classes and subclasses are hard to distinguish morphologically.
Their most diagnostic features are the proteins on the cells'
surfaces, called cell differentiation (CD) proteins.
Classes are developmentally distinct: e.g. consider B and T cells,
the most abundant categories of lymphocytes.
In birds, pre-T and pre-B cells originate in red bone marrow and
migrate to separate organs to mature:
Pre-T cells go to the thymus,
Pre-B cells to the bursa of Fabricius.
Removing either organ at birth abolishes that class of cells.
Mammals have a thymus for T cells but no bursa.
Pre-B cells probably mature in the bone marrow.
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Lymphocytes do vary in appearance but mainly due to differences in
their states of commitment to divide.
Small lymphocytes cannot divide as such.
They first must dedifferentiate into lymphoblasts.
Lymphoblasts have large pale nuclei and substantial
amounts of faintly basophilic cytoplasm. They
can actively divide.
Prolymphocytes are intermediates.
lymphoblasts --> prolymphocytes --> small lymphocytes
<---------------------WARNING: The "large" lymphocytes of blood are not lymphoblasts.
Lymphoblasts and prolymphocytes do not circulate in blood.
Cells can cycle repeatedly between blast and small lymphocyte forms
as in this possible killer T cell lineage.
Stage
Morphological form
Blast
Resting stem cell
Location
Yolk sac
SL
Bone marrow
Dividing stem cell
Blast
Pre T cell
SL
Thymic lymphocyte
SL
Killer type T cell
SL
Stimulated killer
Memory killer
2nd response killer
Blood
Blast
Thymus
Blast
SL
Peripheral C.T.
Blast
(SL=small lymphocyte, Blast=lymphoblast)
B CELLS
B cells can be induced to mature into plasma cells. These then
secrete massive quantities of antibodies.
Antibodies are proteins with binding sites for antigen molecules.
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A person can make any of 10,000,000 or so types of antibody
molecules, each with a different amino acid sequence.
--> Problem #1: How do genes encode this repertoire of information?
Crucial fact: Each B cell can make only a single type of antibody.
Antibody diversity comes from diversity among B cells.
A variety of genetic mechanisms generate the enormous diversity
among antibodies.
a. Antibody proteins are encoded by multigene families.
b. The two polypeptide chains of antibodies, L and H, can pair in
different combinations (“combinatorial association”).
c. Antibody genes are split into several segments and each
segment is tandemly repeated. During development one randomly
chosen member of each segment is spliced together to form a
complete L gene and an H gene.
H gene formation
mRNA transcript
Remove introns
d. The exact position of gene splicing is variable.
|G.G.A|C.A.T. …………… T.C.G|T.C.C|G.G.G
variable splicing
|G.G.A|C.A.G ----------- |T.C.C|G.G.G
|G.G.A|C.A. ----------- G|T.C.C|G.G.G
|G.G.A|C. ----------- C.G|T.C.C|G.G.G
|G.G.A|C.A.T|------ T.C.G|T.C.C|G.G.G
|G.G.A|------------------ T.C.C|G.G.G
e. Extra nucleotides are inserted into the DNA helix
at junction in a non-templated manner.
|G.G.A|C.A.T. ………………………… T.C.G|T.C.C|G.G.G
|G.G.A|C.A.T|N.N.N|N.N.N|T.C.G|T.C.C|G.G.G
g. Other mechanisms:
Pre-messenger RNA's can be differentially spliced.
Birds use gene conversion with pseudogenes.
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B cells differ in their genetic (DNA) sequence from each other and
from every other cell type.
--> Problem #2: How do antigens induce only appropriate antibodies?
(ones which will bind to foreign antigens)
The clonal selection theory proposes a 2-step process.
B stem cell
Proliferation
Pre B cells
Diversification
B cells
o
o
o
o
Antigen
stimulation
o
o
o
o
o
o
o
o o oo
o
B cell  Lymphoblast o o
o o o
Proliferation
Clone
Differentiation
Plasma cells
Antibodies
B memory
cells
1. During its differentiation into a B cell each pre-B cell generates
a unique L and H gene, thus irreversibly committing the cell to a
particular antibody specificity before encountering any antigen.
This happens in the bursa of birds and bursa equivalent in mammals.
The B cell synthesizes a small number of antibody molecules and
attaches them to its surface as a receptor to detect that antigen.
A trillion lymphocytes are coursing through your blood, lymph
and connective tissues looking for antigens. This collection
includes cells committed to millions of possible antigens which
you have not yet encountered, and in most cases never will.
2. When antigen binds to its receptors, a B cell reverts to a
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blast form and divides into a clone of identical cells.
B cell --> immunoblast --> immunoblasts
plasma cells
memory B cells
Some progeny cells differentiate into plasma cells (veritable
factories for secreting antibody). Others differentiate into
memory cells and revert back to the morphology of small
lymphocytes. These are easier to reactivate to immunoblasts
than before, and allow a person to respond more rapidly and
vigorously when re-exposed to an antigen (i.e. "booster"
response or "amnestic response").
3. We now know that there is a third component of clonal selection.
A mutation mechanism generates further variation.when activated
lymphoblastes are stimulated to divide a mutator mechanism
introduces point mutations into the antibody gene. The mutant
B-cells compete for available antigen. Those with the most
avid antibody out reproduce the others. Thus, the ongoing
immune response "evolves" towards better and better antibodies.
This mutation-selection process occurs in special tissue
structures called lymphatic nodule. The antigens are held on
the surface of dendritic cells. Mutant B cells that can hang
onto the antigen elicit signals from the dendritic cell to
proliferate.
Another major factor is that chemical signals from infection
and inflammation have large effects on the reactivity of
lymphoid tissue. Distinguishing between diseased and normal may
be almost as important as between self and non-self for
generating appropriate responses.
T CELLS
T cells resemble B cells but are more diverse and complex in function.
1. T cells make a "T cell receptor protein” (TCR) instead of ntibody.
The TCR: 1. is homologous to the antibody molecule.
2. forms through gene rearrangement.
3. varies in binding specificity from one T cell to another.
But it: 4. is always bound to the cell membrane.
5. binds only to antigens which has been digested into
small peptides and are held on the surface of other
cells by a "major histocompatibility (MHC) protein.
2. T cells can be activated only by contact with antigen presenting
cells (APC's).
Antigen presenting cells phagocytize extracellular protein
antigens, digest them, place peptide fragments (about 10 AA long)
on Class II MHC molecules and "present" them on their cell surface
to T cells. They activate those T cells whose receptors recognize
those peptides, through a combination of direct cell-cell contact
and secretion of a chemical messenger called IL-1.
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Antigen presenting cells include a variety of cell types
including; macrophages, lymphocytes and dendritic cells.
T cells also can turn still other cells into auxiliary APC's by
secreting gamma interferon (e.g. endothelial cells).
3. Subtypes of T cells carry out individual functions.
Subtypes of T cells used to be named for their particular
functions (“helper cells”, “suppressor cells” etc.) Now they
are recognized and usually referred to by the presence of
particular proteins on their surface (CD4+, CD8+ . . .)
Cytotoxic T cells (CD4- CD8+) provide "cellular immunity", distinct
from the "humoral immunity" from B cells and antibody.
Rejection of a graft of foreign tissue is an example. T cells
migrate into the graft, divide, become activated as killer T
cells and kill the cells of the graft.
1. by punching holes in the cell membrane.
They secrete proteins (perforins) which go into the
membrane of target cells and assemble into pores.
2. by inducing the target cell to commit suicide.
They trigger the pathway for apoptosis.
The main function of cytotoxic T cells is to kill cells that
are infected by viruses.
Helper T cells (CD4+ Cd8-) help to activate B cells.
In general, B cells cannot be activated by antigen unless they
are aided by activated helper T cells.
When antigen binds to the antibody receptors on a B cell, the
cell internalizes some of the antibody-antigen complexes. It
chops the antigen into peptides, places the peptides onto MHC
molecules and displays them on its cell membrane. If an
appropriate T cell contacts the MHC-peptide with its TRC it
signals the B cell to proliferate.
T cells also regulate the activation of other T cells.
T cells may regulate formation of other blood cell types:
eosinophils, mast cells and maybe red blood cells.
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Suppresser T cells (CD4- CD8+) inhibit the activation of B cells.
They are important for preventing persons from making
antibodies to their own proteins. A variety of complex
mechanisms participate in this self-tolerance.
1. the immune system tolerates proteins that are present in
the animal or person at birth.
2. A complex selection process in the thymus removes newly
formed T cells that would react to self proteins.
3. Suppressor cells inhibit B cells that are commited to a
self-antigen.
4. A bunch of other complex, confusing and controversial
mechanisms that immunologists quarrel about. For
example, all cells of the body (well, all but
erythrocytes which are only quasi-cells) hydrolyze a
small sample of the proteins that they make and place
the peptide fragments on class I MHC proteins. In this
way the immune system displays the repertoire of selfproteins, not to be responded to.
5. T cells regulate other cell types by secreting chemical messengers
called interleukins as another part of their functions.
Evolutionary considerations (if you are interested):
1. B cells probably evolved from a subfamily of T cells.
2. MHC proteins are homologous to antibody molecules.
3. A dozen or two other proteins with functions in immunity are
homologous to antibodies. This immunoglobulin superfamily
probably evolved by repeated gene duplication and divergence as
the immune system grew more elaborate.
4. The enzymes for rearrangement of the antibody and T cell
receptor genes is thought to have come from the transposase of
a transposon.
NATURAL KILLER CELLS
Natural killer cells (NK) survey the body for tumor cells.
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They
correspond to those "large lymphocytes" with scattered granules that
you saw last semester in blood.
The distinctive feature of NK cells is that they do not requiring
prior induction before killing their target cells.
(Yes, there appear to be more than one sort of NK cells.)
STEM CELLS
The stem cells of bone marrow take the form of a (medium to large
sized) lymphocyte when inactive. This is most of the time.
Stem cells of bone marrow differentiate into various end cell types
through a series of progenitor stages. These progenitor cells also
take the resting form of a lymphocyte most of the time. Presumably
there is a blast form for each progenitor type.
OTHER POSSIBLE CLASSES OF LYMPHOCYTES
B and T cells make up about 95 percent of the lymphocytes of the
body. The others are called Null lymphocytes. Most of these are NK
and a few more are circulating stem cells. However, other minor
classes of lymphocytes probably await discovery.
For example, mast cells and follicular dendritic cells originate
in bone marrow and migrate through the blood to their final
location They probably circulate in the form of small
lymphocytes.
(Again, there are at least two subtypes of mast cells and
dendritic cells are considerably more heterogeneous and
confusing).
* * * *
Objectives: after reading these notes you should be able to:
1. answer the questions:
In what way is a small lymphocyte not a “cell type”? What is it?
How does the T cell receptor differ from the receptor on B cells?
How does cellular immunity differ from humoral immunity?
How does a small lymphoblast differ from a lymphoblast?
What are natural killer (NK) cells. What and how do they kill?
What are antigen presenting cells and how do they present antigens?
2. describe:
The clonal selection theory.
The mechanisms that increase the variety of B cells.
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