Transcript Sept2_Lecture3

Lecture 8
Vertebrate immunity
Lymphocyte receptor diversity
•
Humans have about 30,000 genes, so there’s clearly not one
gene for each of the tens of millions of different receptors on
our T-cells
•
Instead we have a combination of three things:
1. Receptors (at least B-cell ones) are composed of two protein
chains, each different
Lymphocyte receptor diversity
Each chain is built of multiple segments that are combined by
specially controlled recombination (somatic recombination)
•
Heavy chains have three regions that affect recognition
(receptor binding), variable (V), diversity (D), and joining
(J)
•
Light chains have only V and J regions
•
In humans there are about 100 different V genes, 12 D
genes, and 4 J genes
Lymphocyte receptor diversity
•
Each progenitor of a B-cell clone undergoes somatic
recombination that brings together a V-D-J combination for
the heavy chain
•
There are 100X12X4 = 4,800 V-D-J combinations
•
Similar recombination events lead to the light chain
How many possible light chain combinations are there?
And heavy plus light chain combinations?
Lymphocyte receptor diversity
•
4,800 V-D-J combinations for the heavy chain
•
400 V-J combination for the light chain
= 1,920,000 different B-cell receptors (aka immunoglobulins,
aka antibodies)
Plus there are random DNA bases added between segments, so
the possible diversity is pretty much infinite
There are lymphocytes with around 100 million specificities
floating around inside each of us…
Lymphocyte receptor diversity
Finally, the six areas of the genes that code for the parts of the
receptor that do the recognizing can undergo further small
changes due to mutations within individual lymphocytes
•
The V-D-J shuffle will be different for each lymphocyte, and
is then locked in for that lymphocyte
•
The glass slipper doesn’t change…much. But it changes a
bit through somatic hypermutation (Haldane’s idea)
•
Somatic recombination gives a combinatorial pool of
diversity which is then fine tuned
Lymphocyte receptor diversity
•
Upon infection, one of the clones generated by VDJ
recombination might fit a pathogen epitope like Cinderella’s
slipper
•
This stimulates amplification of that clone
•
The new generation of clones increase their mutation rate at
recognition site
•
This creates slight variation in the clone population, and
variants with tighter binding are stimulated to divide more
rapidly = affinity maturation
Remind you of anything?
Lymphocyte receptor diversity
Clonal selection
•
The process that underlies lymphocyte specificity and
differentiation is akin to natural selection
•
only those lymphocytes that encounter an antigen to which
their receptor binds will be activated to proliferate and
differentiate into effector cells
•
This selective mechanisms was first proposed in the 1950s
by the Australian biologist Frank MacFarlane Burnet…
•
…at a time when nothing was known about lymphocyte
receptors, or even that lymphocytes were important
Clonal selection
•
It wasn’t until the 1960s that James Gowans removed
lymphocytes from rats and noticed that their adaptive
immunity disappeared
•
Peter Medawar removed the last conceptual problem in the
1950s by showing how the problem of immune responses to
“self” is solved
How?
(Burnet and Peter Medawar were co-recipients of the 1960 Nobel Prize in
Physiology or Medicine for demonstrating acquired immune tolerance.
This research provided the experimental basis for inducing immune
tolerance, the platform for developing methods of transplanting solid
organs.)
Clonal selection
•
Exposure to foreign tissues during embryonic development
of mice caused them to become tolerent of those tissues
later (I.e. no immune response)
•
Led to the idea that developing lymphocytes that are
potentially self-reactive are removed before they can mature
= clonal deletion
•
these sorts of experiments are why we call MHC MHC
(major histocompatibility complex)
Figure 1-15
Figure 1-14 part 1 of 2
Figure 1-14 part 2 of 2
Clonal selection
The proliferation of lymphocytes after clonal
selection leads to immunological memory
•
•
•
After a lymphocyte is activated, it takes 4-5 days
of proliferation before clonal expansion is
complete
That’s why adaptive responses occur only after a
delay of several days
After this primary response, some antigen-specific
cells persist and lead to a more rapid and effective
secondary response, and lasting immunity =
immunological memory
Clonal selection, adaptive immunity, and
diversity generation
The proliferation of lymphocytes after clonal
selection leads to immunological memory (and
vaccines)
Types of lymphocytes
•
B-cells produce immunoglobulins, molecules produced by
adaptive immunity to dispose of particular threats
•
Antibody = immunoglobulin = free-floating B-cell receptor.
•
B-cells’ main job is to produce humoral immunity, to
neutralize pathogens floating anywhere outside of cells
(extracellular)
•T-cell receptors (TCRs) have a
Figure 3-11
very similar structure to
BCRs/immunoglobulin molecules
•Formed by the same sort of
somatic recombination, but no
affinity maturation
•Host cells continually break up
intracellular proteins into little
peptides around 10 aa long
Figure 1-27
•The host MHC molecules bind
these little peptides within the cell
•The cell then transports the
MHC/peptide combination to the
cell surface for presentation to
roving T-cells
•The two main classes of MHC
molecules present antigen from
cytosol (MHC class I) and vesicles
(MHC class II)
•TCRs recognize (bind) the
Figure 8-10
MHC/antigen complex presented
by infected cells
• TCRs also recognize antigen
presented by cells (e.g. B-cells)
that have ingested antigen
MHC class I molecule presenting an
epitope
Figure 3-23
•Peptides eluted from two
Figure 3-24
different MHC molecules
•One MHC can bind multiple
epitopes, often with similar
anchoring residues
Figure 1-28
•MHC class I molecules present antigen derived from proteins in the cytosol
•Basically, MHC I is all about viruses, obligate intracellular parasites
•As the virus synthesized proteins in the cytosol, they are transported to the cell
surface
•Contrast this with early (wrong) theories that postulated that T-cells had some
kind of “finger” with which they could probe the interior of a cell (supported
with electron microscopy)
•CD8-bearing T-cells (aka cytotoxic T lymphocytes, CTLs, killer T-cells) then
kill the infected cell
Figure 1-28 part 1 of 2
Figure 1-28 part 2 of 2
Figure 1-30
Figure 1-28
•MHC class II molecules present antigen derived from proteins originating in
intracellular vesicles
•Some bacteria (like M. tuberculosis) infect cells and reside in intracellular
vesicles
•Peptides derived from them they are transported to the cell surface by MHC II
molecules
•CD4-bearing T-cells, specifically TH1 cells and TH2 cells (TH2 are more
commonly called helper T-cells ) then facilitate the disposal of the pathogen
Figure 1-29 part 1 of 2
•TH1 cells tell an infected macrophage
to go ahead and fuse lysozomes with
the vesicles
Figure 1-31 part 1 of 2
•This leads to the breakdown of the
pathogens
Figure 1-29 part 2 of 2
Figure 1-31 part 2 of 2
Figure 3-19
•Every nucleated cell in the body
expresses MHC (class I always
present: why?)
•Pattern of MHC class, expression
level varies among cell types
•Erythrocytes do not express MHC.
This may be one reason for success
of Plasmodium spp.
•Why is the lack of MHC class I on
the surface of RBCs not a problem
with respect to viral infection?
The architecture of
immunity
The white blood cells of the immune system derive
from precursors in the bone marrow.
•
The main cell types are macrophages, Tlymphocytes of various subclasses, and Blymphocytes.
•
Other cells types are involved in innate immunity
and allergic responses (eosinophils, neutrophils,
basophils
The architecture of
immunity
Distribution of lymphoid tissues in the body:
Central lymphoid tissues.
•
Bone marrow: houses stem cells
•
Thymus gland: where T-lymphocytes differentiate
from stem cells.
The peripheral lymphoid organs are
specialized to trap antigen and allow the
initiation of adaptive
Figureimmune
1-7 responses
The architecture of
immunity
Peripheral lymphoid tissues.
1. Lymph glands: T-cells and B-cells migrate and
occupy lymph glands. Macrophages and dendritic
cells are present to trap antigens entering the glands
(inducer cells)
2. Spleen: another filter of the blood and lymph
3. GALTs: gut associated lymphoid tissues are
aggregates of cells found in various organs. Especially
associated with mucosal membranes of respiratory and
gastrointestinal tract (e.g. tonsils)
4. Blood and lymph: where B- and T-lymphocytes
circulate, passing continuously in and out of the system
and through lymphoid organs via lymphatic
circulation
Lymph nodes are highly organized structures that
are the sites of convergence of an extensive system
of vessels
that collect
extracellular
Figure
1-8
part 1 fluid
of 2(lymph)
from tissues and return it to the blood
The architecture of
immunity
Lymphatic circulation.
•
Allows access of lymphocytes to body tissues and
organs to respond to any invading foreign antigen
•
Allow recruitment of lymphocytes to an
inflammatory site and thus enables the initiation of
the immune response
•
Allows replenishment of lymphoid organs
damaged by trauma or infection
Figure 1-11
Figure 1-3 part 1 of 4
Figure 1-3 part 2 of 4
Figure 1-3 part 3 of 4
Figure 1-3 part 4 of 4
Evolution of the immune
system
•
In its present state, the immune system of
mammals is a coherent system of interacting cells
and molecules, but this state has been pieced
together by tinkering
•
Its components have been co-opted from different
physiological functions and shaped by
evolutionary pressures to fit relatively seamlessly
together
•
The distribution of the components of the immune
system in different species can shed light on its
overall evolutionary history
Evolution of the immune
system
•
•
•
The most ancient
immune defenses lie
within the innate
immune system
Drosophila spp. Have
well developed innate
immune system
The first defense
molecules in
evolutionary terms were
probably antimicrobial
peptides, produced by
plants and animals
Evolution of the immune
system
•
For a long time the evolution of
adaptive immunity was obscure,
because it seemed to emerge
suddenly as a complete biological
system at roughly the same time
as the vertebrates
•
The picture is now becoming
clearer….
Evolution of the immune
system
•
Adaptive immunity appeared
abruptly in the cartilagenous
fishes
•
It’s been known for 50 years that
all jawed fish can mount an
adaptive immune response
•
Jawed fish have B-cells, T-cells,
MHC, Lymphoid organs,
immunological memory
Evolution of the immune
system
•
hagfish and lampreys lack most
signs of an adaptive immune
system; no organized lymphoid
tissue, no immunological memory,
no T-cells, etc.
Evolution of the immune
system
•
Both prokaryotic and eukaryotic genomes contain
mobile DNA elements known generally as
transposable elements
•
Transposable elements can move themselves, or
copies of themselves to different positions on
chromosomes: “selfish DNA”
•
TEs contain two essential elements: a sequence
encoding transposase, a DNA recombinase that is
able to cut DNA and excise the element; and terminal
repeat sequences that are recognized by the
transposase and are required for excision and
insertion
Evolution of the immune
system
•
The recombination-activating genes that catalyze
somatic recombination are called RAG-1 and RAG-2,
and they make RAG-1 and RAG-2 proteins
•
They are essential for receptor gene rearrangements
•
Mice lacking either gene cannot form lymphocyte
receptors and hence don’t have adaptive immunity
•
RAG-1 and RAG-2 are unusual genes in vertebrates
in that they don’t have introns
•
They look an awful lot like transposons
Evolution of the immune
system
•
The evolution of adaptive immunity
seems to have been made possible by
the invasion of a putative
immunoglobulin-like gene by a
transposable genetic element
•
This conferred on the ancestral
immunoglobulin gene the ability to
undergo somatic gene rearrangement,
and thus to create genetic diversity
Evolution of the immune
system
•
When a mobile DNA element excises
itself from a piece of DNA, alterations in
the original sequence are introduced into
the “host” DNA when cut ends are
rejoined
•
This is the origin of antigen receptors
in the adaptive immune system!
Evolution of the immune
system
•
By luck, a normally “selfish” DNA element, a sort of genomic
pathogen, gave vertebrates the machinery to cut and join
independent loci
•
This paved the way for the full-blown somatic gene
rearrangement seen in the immunoglobulin and T-cell receptor
genes today
Landmark papers:
Is V(D)J recombination “irreducibly
complex”?
“We can look high or we can look low, in books or in
journals, but the result is the same. The scientific
literature has no answers to the question of the
origin of the immune system.”
-Michael Behe
Evolution of the immune
system
•
Why did adaptive immunity arise in vertebrates?
•
Brainstorming contest
Recapitulation
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The immune system defends the host against
infection
•
Innate immunity serves as a first line of defense
but lacks the ability to provide the specific
protective immunity that prevents re-infection
•
Adaptive immunity is based on the clonal selection
of lymphocytes bearing highly diverse antigenspecific receptors
•
Adaptive immunity is an evolutionary novelty
relative to innate immunity, and likely arose after a
transposon invaded a proto-immunoglobulin-like
gene
Recapitulation
•
In the adaptive immune response, antigen-specific
lymphocytes proliferate and differentiate into
effector cells that eliminate pathogens
•
Host defense requires different recognition
systems and a wide variety of effector
mechanisms to seek and destroy the wide variety
of pathogens in their various habitats within and
on the body
Recapitulation
•
Not only can the adaptive immune response
eliminate a pathogen, but can deliver
immunological memory
•
This depends on a pool of differentiated memory
lymphocytes generated through clonal selection
•
Memory cells engender a more rapid and effective
response upon re-infection