Adaptive immunity 2+3 Flashcards
Purpose of VDJ
Generates massive diversity go B and T cell receptors
Number of splicing combinations in B cells
3x 10 (11)
- 44 V
- 27 D
- 6 J
Recombination signal sequence
Sequence of non-coding DNA that RAG 1/2 recognise in immature B and T cells.
RAG enzyme binds and flanks region of DNA at this point.
RAG 1/2
Recombination activating genese
Enzymes in T and B cells that recognise RSS in DNA.
Splices introns between the RSS and V/D/J segment
Mechanism of VDJ
- RAG1/2 binds to RSS in DNA, flanking a V/D/J gene segment
- RAG breaks DNA between RSS and V/D/J segment- leaving double stranded break at the end of coding segment (DNA hairpin)
- Loop of DNA spliced out to form signalling loop.
- Terminal, deoxynucleotridyltransferase (TdT) adds nucleotides to cleaved ends in a non-template fashion.
- DNA ligase ligates gene segments
Terminal deoxynucleotidyl transferase (TdT)
DNA polymerase that adds nucleotides to cleaved hairpin end of DNA in random fashion
Somatice hypermutation
Only occurs in B cells
- Occurs after B cell has encountered antigen
- A component of affinity maturation
Function
- Diversifies B cell receptors used to recognise foreign
Cytidine deaminase
Enzyme involved in somatic hypermutation.
- Causes random mutation in V gene segment
Somatic hypermutation process
Occurs in B cells, Variable (V) region.
Cytidine deaminase causes nucleotide change to uracil from cytosine, in V region of DNA.
- Since uracil is not stable, this is fixed by base-exicison repair enzymes
- Uracil is removed and error prone DNA-polymerases fill in the gaps with = new mutations
When B cells undergoes devision with new mutation, the cells with highest affinity are positively selected.
Class switch recombination
- Description
Occurs in B cells after antigen recognition
- Immunoglobulins switch from IgD/M isotope to another (IgA, IgG)
- Constant region of immunoglobulin changes, whilst keeping the variable region the same
Purpose
- Allows antibody to retain affinity to antigen, whilst changing its interaction with effector molecules.
Class switch recombination
- Process
- The first regions on the heavy chain exon are:
- Cu (IgM)
- C-delta (IgD)
Stimulation of B cell by T cells (CD40L) and cytokines triggers class switching.
- Activated cytidine deaminase is involved in breaking DNA at IgM region.
- Second cut is make in the isotype region that will be switched to (e.g IgG).
MHC 1 pathway
Processes endogenous antigens
- Intracellular antigens are present and are processed in proteasome.
- Into peptides - Peptides are transported into the endoplasmic reticulum via Tap proteins
- Peptides are bound to MHC 1 in ER
- Peptides are presented on cell surface via MHC
Mechanism of peptides binding to MHC class 1 binding
- MHC in ER originally has clanexin bound to it
- Released when beta-microglobulin binds to it - TAP proteins load peptides into ER- then MHC
- MHC class 1 dissociates with protein and presents peptides
HSV and MHC 1
HSV can bind to Tap proteins
- Inhibits transport of peptides into ER
MHC Class 2 pathway
Processes exogenous antigens.
- Whole antigen is taken into cell via endocytosis, into a vesicle
- Vesicle fuses with lysosome and undergoes acidification
- Activates proteases to break down antigen into peptides. - Vesicle fuses with another vesicle containing MHC 2
- MHC have invariant chain blocks that initially block them, but this Is cleaved by CLIP fragment - HLA-DM allows MHC to bind to peptide by releasing CLIP
- MHC 2 presents peptide on plasma membrane
