Adaptive immunity 2 Flashcards
How do we ensure that we always have a B cell an/or T cell that can recognise and respond to every antigen?
B cells:
V(D)J recombination in immunoglobin heavy chain gene chromosome 14 + kappa or lambda light chain genes on chromosome 2 + 22 respectively. only heavy chain gene contains diversity segments so light chain only undergoes VJ recombination.
T cells:
V(D)J recombination in alpha chain gene chromosome 14 + beta chain gene on chromosome 7. beta chain doesn’t contain diversity segments so only undergoes VJ recombination -> produces more diversity than B cells as new B cells born every day + B cells can induce further diversity due to somatic hypermutation unlike T cells.
How are new gene sequences produced through V(D)J recombination?
During pro B + pro T cell phase of development, cells undergo DJ recombination. In B cells this occurs in IgH gene + in T cells this occurs in alpha chain gene.
These genes recombine their new DJ segments with new V segment. Finally, light chain genes undergo VJ recombination.
new pieces of genetic code are transcribed into mRNA + translated into proteins. Assembly of IgH chain + one of light chains (either kappa or lambda) results in formation of membrane bound immunoglobulin IgM + IgD expressed on surface of immature B cells.
How does rearrangement occur?
Between specific sites on DNA -> recombination signal sequences (RSSs) -> contain conserved segments of DNA composed of a heptamer, spacer + nonomer, found on 3’ side of V segments, 5’ side of J segments, 3’ + 5’ side of D segments.
Catalysed by 2 Recombination Activating Genes -> RAG-1 + RAG-2.
Outline the steps in V(D)J joining
Rearrangement begins as RAG-1:RAG-2 complexes binds to RSSs which flank coding sequences to be joined.
RAG complex is activated to cut 1 strand of dsDNA precisely at end of heptamer sequences. 5′ cut end of reacts with complementary uncut strand, breaking it to leave double-stranded break at end of heptamer sequence + forms hairpin by joining to cut end of its complementary strand on other side of break. Through action of additional proteins that join complex, DNA hairpin is cleaved at random site to yield ssDNA end. end is modified by TdT, creating diverse, imprecise ends.
2 unmodified heptamer sequences are ligated to form precisesignal joint, whilecoding jointis also ligated, both by DNA ligase IV.
What is somatic hypermutation and when does it take place?
Following B cell recognition of antigen.
Accumulation of mutations occurs atcentroblaststage of B cell differentiation ingerminal centresof secondarylymphoid organs.
Produces high-affinity antibodies. in absence of selection, SHM doesn’t distinguish between favourable + unfavourable mutations + can produce antibodies with higher affinity for antigen, lower affinity for antigen, no change in affinity for antigen, non-functional antibodies that cannot fold correctly.
a diversity-generating process
Adds further diversity to already rearranged segments via point mutations.
What is affinity maturation?
Selection process forantigen binding occurs in light zone ofgerminal centre,selects for B cells that produce highest-affinity antibodies
Which gene segments does somatic hypermutation target?
Rearranged gene segments encoding variable region
Which enzyme triggers somatic hypermutation?
Activation-Induced Cytidine Deaminase (AID) -> deaminates cytidine to uracil which is incorrectly repaired by several different mechanisms to generate mutations –> mismatch repair + base excision repair.
Outline the process of affinity maturation
B cells produce antibodies with increased affinity for antigen during immune response.
Follicular dendritic cells found in germinal centres present antigen to B cells that have undergone SHM. only those with high affinity for antigen are selected to survive. others are out-competed or die via apoptosis.
What is Class Switch Recombination (CSR)?
Resulting antibodies of SHM are stillIgMisotype with very short half-life + limited functional capacity. CSR allows for substitution of 1 isotype for another -> provides antibody with new functions + different distribution throughout body. Induction of CSR orchestrated by differentcytokines resulting fromcellular interactionsof B cell with different types of T cells -> directs CSR toward specific isotype. CSR occurs exclusively atheavy-chain antibodylocus, as it's the location of different constant region genes that encode each isotype. CSR doesn't change affinity of BCR for antigen but allows for tuning of B cell response + long term B cell memory to occur as class switched B cells can become long lived plasma cells or memory B cells. Same receptor, different constant region, allowing cell to perform range of different effector functions.
What does CSR require?
Specific sequences of DNA that lie upstream of each set of constant region genes -> switch regions (S-regions). Each S-region is associated with upstream promoter + exon -> allows for production of sterile (noncoding)germline transcripts through S-region. IgDconstant region lacks defined S-region. Rather than CSR, IgD is generated through alternative splicingmechanism with μ constant region.
What does alternative splicing result in?
IgM + IgD in naive B cells
How is CSR achieved and which enzyme is critical in this process?
First targeted switch region is always Sμ switch region.
other partner switch region determined by cytokines.
Activation-induced cytidine deaminase (AID).
How are constant regions spliced out during CSR?
Using switch regions located upstream of each constant region.
First cut is always just before Cμ region.
Second cut determined by cytokines secreted by follicular T helper cells.
Why does CSR matter?
It determines type of response that immunoglobulin can promote. E.g. non class switched IgM can activate complement but can't sensitise cells for NK killing. IgG1 class switched immunoglobulins are effective at antigen neutralisation + opsonisation by making invading pathogen “visible” to phagocytes.
