Week 14 Flashcards
Role of PARP inhibitors
PARP inhibitors kill cancer cells with defective Brca gene
Function of brca
Recombination mediated double strand break repair.
Role of PARP protein
PARP [Poly (ADP ribose) polymerase] proteins are involved in repairing single strand breaks.
In absence of PARP, what happens?
In absence of PARP, BRCA (BReast CAncer) can compensate. The WT cell will detect that they don’t have a single strand repair pathway to use, so they will introduce another lesion so that the BRCA pathway can be induced and the break fixed.
Why does DNA damage to cancer cells result in permanent damage?
- Cancer cells have mutated away their BRCA genes.
- If a cancer cell were to have a single strand break and were exposed to PARP inhibitors, now the PARP pathway cannot be used to fix the break.
- It also does not have the BRCA pathway, so it has no way to repair that break.
- This results in permanent damage that it cannot overcome.
Cell targets of drugs that have been developed to target PARP.
Both cancer and WT cells.
Immune therapy for cancer
Because cancer cells have a lot of mutations, they make proteins with novel variants. The immune system recognizes these antigens as foreign and targets them.
What type of cell targets cancer cells in immune therapy?
A T-cell detects that the cell is expressing a foreign variant of a protein and infer that the cell is infected and will target it for destruction.
Why does antibody therapy for cancer sometimes have serious side effects?
Antibody therapy can neutralize immunosuppressive signaling pathways, unleashing T cell attack on cancer cells. Can have serious side effects.
Role of PDL1 system
Check and balance built into the immune system to ensure proper targeting.
How does the PDL1 system work?
The immune cell has a receptor for PDL. When PD1 is activated, it inhibits the T-cell response.
Cancer cells and PDL1
Normal cells will not express PDL1 while cancer cells overexpress PDL1.
Treatments that affect PDL1
There are antibodies that have been developed against PDL1.
Can block the function of PDL1 and PD1.
CAR-T
Chimeric antigen receptor T-cell therapy
How does CAR-T work?
The mutant variant (antigen) makes its way to the cell surface, and then an immune cell tries to recognize the mutant variant. However, sometimes there are not enough immune cells or they do not have the correct receptor.
Can engineer immune cells in the lab–this is the basis of the therapy.
Characteristics of the innate immune response
- Most cell types capable of mounting an innate immune response
- Innate immunity against pathogens is hardwired
- Fast–minutes to hours to days
- Provides short-lasting protection
- Ancient
Characteristics of the adaptive immune response
- Response mediated by specialized cells called lymphocytes, a subgroup of white blood cells (leukocytes)
- Adaptive immunity against pathogens is acquired.
- Slow–days to weeks
- Provides long-lasting protection
- More recent invention
What type of receptors are Toll-like receptors (TLR)?
A pathogen recognition receptor (PRR).
Structure of TLRs
Toll-like receptors form a dimer whose substrate is the ligand double-stranded RNA.
What happens when TLRs detect double-stranded RNA?
They will elicit an immune response. Why? Because our bodies do not have double-stranded RNA–it is a foreign molecule (sometimes found in viruses).
Pathogen-associated molecular patterns (PAMPs)
Pattern recognition receptors (PRRs) in host cells recognize conserved features of pathogens called pathogen-associated molecular patterns (PAMPs) to activate innate immune response
Effect of PRRs
PRRs induce a transcriptional program that fights the infection in the infected cell. PRRs also induce the infected cell to secrete cytokines that attract immune cells.
What type of cell secretes antibodies?
B cells. This is a part of adaptive immunity
What triggers the generation of antibodies?
An antibody is generated in response to an antigen.
How do antibodies work?
Antibodies bind to antigens on the pathogen surface to neutralize them and to mark them for recognition by other immune cells.
Origin of B cells and T cells
Lymphoid progenitor cells develop into B cells in the bone marrow and into T cells in the thymus. Both are activated in peripheral lymphoid organs.
Clonal expansion
Once a B cell encounters an antigen that is a match for its receptor, that B cell will selectively proliferate and differentiate.
Difference between B cell receptors and antibodies
Once the B cells are made, it will create a secreted form of its receptor—these are the antibodies. The only difference is that antibodies do not have a transmembrane domain. Otherwise, the B cell receptors are the same as antibodies.
Immunoglobulins (Igs)
B cell receptors (BCRs) and antibodies are called immunoglobulins (Igs), because they have Ig domains
How does the structure of BCRs and antibodies contribute to the diversity of their targets?
Both heavy and light chains have variable and constant regions.
Differences in the variable regions allows different Igs to bind to different antigens.
Within the variable region are hypervariable loops, which are extremely divergent between different Igs.
What connects the heavy and light chains on an antibody?
Disulfide bridges.
How many antigens can one molecule of antibody bind to?
Two antigens.
Number of hypervariable loops within the variable domain.
Within the variable domain, there are three hypervariable loops.
How is diversity in the variable regions of the heavy and light chains generated?
V(D)J recombination via recombination activating genes (RAGs) generates diversity in the variable regions of the heavy and light chains
How is the constant part of the light chain generated?
- Transcription starts in the V region closest to the J region
- Splicing of the mRNA fragment eliminates J regions
- The result is a mature mRNA with one V region, one J region, and one C region
Junctional diversification
V(D)J recombinase does a ‘sloppy’ job of recombination, inserting or deleting a few nucleotides during recombination.
This increases variability (referred to as junctional diversification).
Junctional diversification and TdT
Junctional diversification is also a result of terminal deoxynucleotidyl transferase (TdT) that adds random nucleotides at junctions.
Why are two B cells with the same V, D, and J segments not the same?
Combined, two B cells, each with the same V, D, and J segments will not be the same because the junctions between each segment will be different.
How are Igs with greater affinity for their antigens generated?
Affinity maturation via somatic hypermutation
How does affinity maturation via somatic hypermutation occur?
Igs generated via V(D)J recombination can bind to antigens weakly
(with Kd in the range of 10-5-10-7 M).
- When a B cell binds to an antigen and is activated, it starts proliferating.
It is in these proliferating B cells that somatic hypermutation occurs via AID.
- This mutational process combined with selection for B cells with greatest affinity for antigens results in production of Igs with greater affinity.
- After multiple rounds of mutagenesis and selection, Igs are produced that can bind antigen with Kd in range of ~10-11 M.
Role of activation-induced cytidine deaminase (AID)
Creates C to U changes in the DNA that results in U:G mismatches. DNA repair pathways try to fix this and depending on the repair pathway used, different mutations are introduced.
Hypermutation is confined to which regions?
The V(D)J regions
A given heavy chain can have how many different constant regions?
A given heavy chain can have one of five different constant regions.
Constant regions determine the type of activity the Igs elicit
Naive B cells
Those that have not yet encountered an antigen that matches their receptor.
What version of BCR do naive B cells express?
IgM and IgD versions of BCR.
Switching of constant region in B cells derived through clonal expansion
B Cells derived through clonal expansion can switch the constant region of their Igs to either IgG, IgA, or IgE
Primary Ig repertoire
IgM and IgD
Secondary Ig repertoire
IgG, IgE, IgA
Four steps of Ig generation and diversification
- Combinatorial joining of gene segments
- Junctional diversification during gene-segment joining
- Combinatorial joining of L and H chains
- Somatic hypermutation and class switch recombination
Three main classes of T cells
- Cytotoxic T cells
- Helper T cells
- Regulatory T cells
Role of cytotoxic T cells
kill cells infected with intracellular pathogens and viruses
Role of helper T cells
stimulate responses of other immune cells (e.g., macrophages, B cells, and cytotoxic T cells)
Role of regulatory T cells
suppress activity of other immune cells
T cell receptors vs Ig molecules in B cells
- T cells receptors, unlike Ig molecules in B cells, are always membrane bound and consist of alpha and beta chains with variable and constant regions
- V(D)J recombination and junctional diversification but no somatic hypermutation
- No class switching of the constant region
- Ig-like domains
How do vaccines work?
Vaccines introduce antigens into the body, ‘tricking’ the immune system to generate B cells and T cells against antigens
Primary vs secondary response of exposure to antigen
The secondary response induced by a second exposure to antigen A is faster and greater than the primary response and is specific for A, indicating that the adaptive immune system has specifically remembered its previous encounter with antigen A.
AIRE (autoimmune regulator)
Promiscuous transcription factor expressed in the thymus that turns on expression of most genes in the genome.
