T1 L5 Adaptive immune system 2 Flashcards
What is ITAM?
Immunoreceptor tyrosine-based activation motif
Enables the receptor once engaged with the antigen to signal inside the cell to generate activation signals
Describe the steps from a resting B cell with membrane bound IgG to secretion of immunoglobulins
Resting B-cell with membrane bound IgG (IgM, IgD) encounters antigen
Stimulated B cell gives rise to antibody-secreting plasma cells
Secretes immunoglobulins
What are the 7 steps of genetic alteration to immunoglobulin genes?
1) V-region assembly from gene fragments
2) Generation of junctional diversity
3) Assembly of transcriptional controlling elements
4) Transcription activated with coexpression of surface IgM and IgD
5) Synthesis changes from membrane Ig to secreted antibody
6) Somatic hypermutation
7) Isotype switch
What is the mechanism of V-region assembly from gene fragments and what is the nature of the change to B cells’ genome?
Somatic recombination of genomic RNA
Irreversible change
What is the mechanism of generation of junctional diversity and what is the nature of the change to B cells’ genome?
Imprecision in joining rearranged DNA segments adds non-germline nucleotides (P and N) and depletes gremlin nucleotides
Irreversible change
What is the mechanism of assembly of transcriptional controlling elements and what is the nature of the change to B cells’ genome?
Promoter and enhancer are brought closer together by V-region assembly
Irreversible change
What is the mechanism of transcription activated with co-expression of surface IgM and IgG and what is the nature of the change to B cells’ genome?
2 patterns of splicing and processing RNA are used
Reversible and regulated
What is the mechanism of synthesis changes from membrane Ig to secreted antibody and what is the nature of the change to B cells’ genome?
2 patterns of splicing and processing RNA are used
Reversible and regulated
What is the mechanism of somatic hypermutation and what is the nature of the change to B cells’ genome?
Point mutation of genomic DNA
Irreversible
What is the mechanism of isotype switch and what is the nature of the change to B cells’ genome?
Somatic recombination of genomic DNA
Irreversible
What is an irreversible change?
Requires changes in base composition within DNA of different loci within cell
What is a reversible change?
Requires change in splicing of RNA, doesn’t require change in genetic code
Why is it important that the constant region is changed?
Different immunoglobulins perform different functions in the cell
What are the 2 types of light chain in a B cell?
Lambda chain on chromosome 22
Kappa chain on chromosome 2
Only one of these is expressed as the 2 light chains are identical
What are the 2 types of heavy chains?
Alpha chain on chromosome 14
Beta chain on chromosome 7
What is V(D)J recombination?
Genetic recombination of a variable region with diversity region and a joining region
Produces new gene sequences to generate immune cell diversity
Occurs in B cells and T cells
What are the key enzymes for V(D)J recombination?
Recombination activating genes RAG1 and RAG2
How many variable regions, diversity regions and joining regions in a B cell?
44 variable regions
27 diversity regions
6 joining regions
How many possible combinations are there for B cells?
3 x 10^11
How many possible combinations are there for T cells?
10^18
Where does rearrangement occur?
Between specific sites on DNA called recombination signal sequences
What additional proteins get incorporated into large complexes with RAG proteins?
DNA dependent protein kinase
Ku
Artemis
Dimer of DNA ligase / XRCC4
What does TdT do?
adds additional nucleotides to ends of DNA strands
Briefly state the steps of V(D)J recombination
1) Generation of junctional diversity
2) RAG complex cleaves heptameter from D and J gene segments to yield DNA hairpins
3) RAG complex opens hairpin by nicking one strand of DNA generating palindromic P-nucleotides
4) N-nucleotide additions by TdT
5) Pairing of strands
6) Unpaired nucleotides are removed by an exonuclease
7) Gaps are filled by DNA synthesis and ligation to form coding joint
Describe affinity maturation
Only the highest affinity centrocytes remain from variable population after competitive selection
Continue to mutate and divide with range of affinity providing basis for selection by competitive binding
Unsuccessful cells undergo apoptosis which increases the affinity with each generation
What is somatic hypermutation?
Further refinement of B-cell receptor recognition of antigen
Targets rearranged gene segments encoding variable region
What is the critical enzyme involved in somatic hypermutation?
Activation induced cytidine deaminase
What is class-switch recombination?
Last step in B-cells response to antigen
Replaces the constant region of the antibody so it can interact with different effector molecules
Why aren’t IgM and IgD produced by class switch recombination?
The naive B-cell hasn’t experienced antigen
How are IgM and IgD produced instead of class switch recombination?
Alternative mRNA splicing due to close proximity of u and delta constant regions in immunoglobulin gene structure
Which is the dominant immunoglobulin?
IgG
What immunoglobulins sensitisation by killing NK cells?
IgG1, IgG3
What immunoglobulins sensitise by mast cells?
IgE
What immunoglobulins sensitise by basophils?
IgD
What immunoglobulins activate the complement system?
IgM
IgG3
What immunoglobulins transport across epithelium
IgA
IgM
What immunoglobulins diffuse into extravascular sites?
IgG
IgA
