The branch that breaks Is called rotten, but Wasn’t there snow on it? Bartolt Brecht Haiti after a...
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Transcript of The branch that breaks Is called rotten, but Wasn’t there snow on it? Bartolt Brecht Haiti after a...
The branch that breaksIs called rotten, butWasn’t there snow on it?
Bartolt Brecht
Haiti after a hurricane
Your body has evolved complexmechanisms of recognizing “non-
self”and fighting against it
The Immune System is the Third Line of Defense Against
Infection
Antibodies are Produced by B Lymphocytes(B cells totheir friends)
T Lymphocytes(T cells)provide“cell based”immunity
Overview of Human Immunity:Adaptive and Innate
Lymphocyte Origins
16-22
Let’s start with the role of B cells and antibodies in the immune response
Some definitions are in order
Antigen
A substance produced by a pathogen (e.g., protein, complex sugar) capable of producing an immune response
Some definitions are in order
Antibodies
Protein molecules (immunoglobulins) produced by B lymphocytes to help eliminate an antigen
Molecular Biology of the Cell Alberts et al
B cells Make
AntibodiesIn response to
antigens
Molecular Biology of the Cell Alberts et al
These antibodiescan bind to
and “neutralize”Viruses
or can directimmune attack
of virus-infectedcells
Molecular Biology of the Cell Alberts et al
Antibodies can also direct phagocytosis of pathogens
Cytotoxic (Killer) T Cells Recognize, Attack and Kill
Virus-Infected Cells
CELLS alive!
Let’s focus first on antibodies
Molecular Biology of the Cell Alberts et al
Molecular Biology of the Cell Alberts et al
Antibodies are proteins that have evolved to recognize molecules from pathogens
These molecules from pathogens are called Antigens
Molecular Biology of the Cell Alberts et al
The variable and constant regions ofantibodies are related = Ig domains
Constant Region
Hypervariable Region
Light Chain
Heavy Chain
Antigen Binding Region
Let’s use as an example an antibody that recognizesa protein on the surface of flu (influenza) virus
courses.washington.edu/medch401/pdf_text/401_07_lect2.ppt
Hemagglutinin
Here is the antibodyBound to the “antigen” = influenza hemagglutinin
Human antibody
Rotate ~90
Add all atoms
The antibody recognizes the antigen by a lock-and-key fit
Antigen residues at the interface = epitope
Epitopes are typically ~5 residues long
This interaction is VERY specific
hemagglutinin
antibody
Space-filling mode
Grey now = mainchainof hemagglutinin
Epitopes reside in turns and loops
This interaction is VERY specific
You can generate antibodies against HIV
like you do against other viruses
Molecular Biology of the Cell Alberts et al
Given thousands of pathogens each of which is constantly evolving
how do we generate antibodies against each?
Molecular Biology of the Cell Alberts et al
We cannot dedicate all 25,000 genes in the genome
just to make antibodies.
What’s the solution?
Molecular Biology of the Cell Alberts et al
We cannot dedicate all 25,000 genes in the genome
just to make antibodies.
What’s the solution?
Put antibodies together by a mix-and match approach!
Molecular Biology of the Cell Alberts et al
requires rearranging the DNA
Molecular Biology of the Cell Alberts et al
requires rearranging the DNA
Molecular Biology of the Cell Alberts et al
The result:an antibody light chain
Since there are multiple types of each gene segment, there are
thousands of possible V-D-J combinations
Each B cell gets a unique combination
Since there are multiple types of each gene segment, there are
thousands of possible V-D-J combinations
Each B cell gets a unique combination
Other mechanisms
further increaseantibodydiversity
Molecular Biology of the Cell Alberts et al
When a pathogen enters the bodyit stimulates proliferation of the
specific B Cells that recognize its Antigens
Once you are exposed to an antigenyour B cells “remember” this
CELLS alive!
OK, that explains antibodies and B
cellsbut what about us?
Molecular Biology of the Cell Alberts et al
T cells carry antibody-related proteinson their plasma membranes
called T cell receptors
Molecular Biology of the Cell Alberts et al
T cell receptors are also assembledby gene rearrangement, creating great diversity
However, T cell receptors (unlike antibodies) cannot
recognizeantigens from pathogens all by
themselves!!
T Cells Only Recognize Antigen when it is presented by another cell
Antigen presentation is done by another family of proteins called
MHC proteins
Molecular Biology of the Cell Alberts et al
Viral or bacterial proteins are digested by Cellular proteases inside the cell andpieces of them bind the MHC proteins
Molecular Biology of the Cell Alberts et al
This allows T cells to recognize HIV infected cells,for example, and even internal proteins
like reverse transcriptase can serve as antigens
Molecular Biology of the Cell Alberts et al
Here is where ourold friend CD4
comes into the picture
Let’s come back to the immune response
to HIV
People initially mount a strong immune response
However, this response ultimately fails for five reasons
However, this response ultimately fails for five reasons
Antigenic escape
Inaccessible epitopes
Downregulating MHC
Destruction of CD4+ T cells
Integration and latency
We already discussedtwo of these
Antigenic escape
Inaccessible epitopes
Downregulating MHC
Destruction of CD4+ T cells
Integration and latency
First, the ability to integrate into the host genome allows HIV to lurk
undetected
Second, by killing CD4+ Helper T CellsHIV ultimately disables both
antibody production andKiller T cells
What about the other three meansHIV uses for immune evasion?
Antigenic escape
Inaccessible epitopes
Downregulating MHC
Destruction of CD4+ T cells
Integration and latency
One way HIV “hides” is by hidingits most “antigenic” regions
Antigenic escape
Inaccessible epitopes
Downregulating MHC
Destruction of CD4+ T cells
Integration and latency
Most antibodies against the virusdo not block viral entry
Why not?
Regions of gp120 and gp41 key for viral entryare hidden until after the shape change we
discussed
Natural selection also shapesthe sequence of viral proteins
Antigenic escape
Inaccessible epitopes
Downregulating MHC
Destruction of CD4+ T cells
Integration and latency
Remember that while reverse transcriptase is an amazing Enzyme, there was something it lacks—which was….
Remember that while reverse transcriptase is an amazing Enzyme, there was something it lacks—which was….
This has major consequences
RT makes 1 error /10,000 bp=1 error per replicated genome
And since the viral generation timeIs 2.5 days and one infected cell produces
~1010–1012 new VIRIONS each day…..
Do the numbers!
Do the numbers!
Given that billions of cells are infected per day There will be thousands of copies of
EVERY possible mutationPresent in the gene pool!!
Recombination adds to the amount of variation
Many cells are co-infected by two or more viral variants
and RT can switch between viral templates
when copying the genome
Remember these sequence based “trees” we usedto study the evolution of different HIV and SIV strains?
We can use the same approach to study the evolutionof a single virus after it infects a single person
Viral diversity in 9 AIDS patients
HIV rapidly evolves into different “strains”after the initial infection
How couldThat happen?
Can you sayNatural selection?
We start with the tremendous amountOf viral variation caused by RT errors
Now we add the selective pressureExerted by the immune response
+
In response to antibody selectionViruses with mutations
in gp120 and gp41 accumulate
T cell selection selects for changes inpeptide “epitopes” so they no longer
bind to MHC proteins
The result: despite high levels of anti-HIV antibodies
viral variants escape from the immune response
HIV also has another trick
Antigenic escape
Inaccessible epitopes
Downregulating MHC
Destruction of CD4+ T cells
Integration and latency
Remember our discussion of Long-term non-progressors:Some are infected with a mutant HIV virus lacking the accessory gene Nef
What does Nef do?
Nef prevents infected cells from putting MHC proteinson their cell surface!
Without MHC proteins infected cells become
Invisible to T cells
HIV has even one more trick
Antigenic escape
Inaccessible epitopes
Downregulating MHC
Destruction of CD4+ T cells
Integration and latency
Blocking Cytosine Deamination
Cytosine DeaminationCauses Mutations
Dr. Weiguo Cao website, Clemson U.
APOBEC3G is a Cytosine Deaminase Present in Resting T cells, which
HIV doesn’t infect well
APOBEC3G
APOBEC3G is incorporated into HIV virions and inhibits viral replication
by inducing hypermutation
Dr. Warner Greene's laboratory at the Gladstone Institute of Virology and Immunology
The viral accessory protein vif blocks APOBEC3G function
Vif blocks APOBEC3G incorporation into virions and
targets it for proteolytic destruction
Antigenic escape
Inaccessible epitopes
Downregulating MHC
Destruction of CD4+ T cells
Integration and latency
Blocking Cytosine Deamination
This formidable array of defense mechanismsAllows HIV to avoid being suppressed by our immune system