Antigen Receptors and MHC Flashcards
What is the structure of B Cell Receptors (BCRs)?
= surface membrane-bound form of IgM
= have variable (antigen binding) part and a non-viable (constant) parts
= BCR on surface of B cell and (eventually secreted) antibody from progeny have same antigen binding sites
= each B cell has : one heavy + one light chain gene
How is BCR diversity generated?
= vast number of antibody molecules that can be made per person
= diversity achieved by bringing together different combinations of multiple gene segments to form genes that encode antibodies
= RECOMBINATION
What are the possibilities of recombination of gene segments?
Each Ig light chain has 3 parts:
= variable (V)
= joining (J)
= constant (C)
Each Ig heavy chain has 4 parts:
= variable (V)
= joining (J)
= constant (C)
= + diversity (D)
= around 2 million possible different combinations of light and heavy chain molecules possible
What is the mechanism for gene segment recombination?
V(D)J recombination
= catalysed by lymphoid-specific enzymes: RAG1 and RAG2
= recognise conserved RSSs (recombination signal sequences) next to coding regions
= recombination mechanism involves cleavage of hairpin structures in DNA and joining of DNA ends
= cleavage can be asymmetric = addition of extra P nucleotides (to fill gaps)
= addition / removal of nucleotides increases diversity
= in heavy chain genes = extra N nucleotides can be added during D-J joining
= exonuclease nibbling can also modify (reduce) number of N and P nucleotides in the final genes
How are B-cell receptors expressed? In theory vs reality?
B cells have two copies of each chromosome
= therefore could in theory express more than one light or heavy chain
BUT
= B cell would have multiple specificities
= cause issues in eliminating self-reactive B cells (cause autoimmunity)
= and would dilute the effectiveness of clonal selection
Prevented by allelic exclusion
= if productive gene rearrangement occurs in one copy of chromosome (allele)
= expression from the other is silences
Rearrangements can be unproductive
= if P/N nucleotide addition / exonuclease trimming causes a frame shift
What is the expression of IgM and IgD in B cells?
Membrane bound IgM with μ constant region
= first Ig expressed by immature B cells in bone marrow
BUT as they mature
= also express membrane bound IgD on their surface
= has same antigen binding specificity but has a heavy chain with a constant δ region
= caused by alternative splicing
(joining the same variable region to a different constant region)
= mature B cells express both IgM and IgD on their surface
How does BCR signalling work?
= BCR does NOT signal directly
= requires association with Igα / Igβ
How are different BCR and antibody sub-classes produced?
= same variable regions can be connected to different constant regions
How does Antibody function and diversity occur?
Functional diversity of antibodies
= achieved by class switching
(from IgM to another class / subclass)
Additional diversity generated by:
= somatic hypermutation
= affinity maturation
General Info about T-Cell Receptors (TCRs)?
= most T cells cannot bind antigen alone
(must be presented by MHC on a APC - e.g. macrophage , dendritic cell)
= TCR expression controlled by allelic exclusion
(like B cells)
= the β gene rearranges first, followed by α gene
= there is only one kind of constant region in TCRs
(unlike antibodies with different classes with different constant regions)
Similarities between diversity expression of TCRs and BCRs?
TCRs / BRCs have many similarities
= both members of Ig superfamily of proteins
= both have variable antigen-binding and constant regions
(repetoire potentially very high)
= one domain of TCR α or β chain is variable / one is constant
(similar to an antibody light chain)
Which co-receptors are involved in TCRs?
TCR complexes with coreceptors involved in antigen recognition:
CD3
= contains ITAMs that transmit signal to cell
CD4, CD8
= increase avidity of peptide binding by TCR
CD28
= engages CD80 or CD86 on APC to fully activate a naive T cell
How does TCR signalling work?
- TCR complex and coreceptors are clustered within membrane lipids rafts by antigen recognition
- Lck phosphorylates tyrosine in ITAMs
- ZAP-70 binds to phosphotyrosines and phosphorylates adaptor protein , including LAT
- Assembly of adaptor protein and enzyme scaffolds, multiple signalling pathways are activated
What are MHC molecules?
MHC
= major histocompatibility complex
= proteins found to have role in acceptance or rejection of organ transplants
= also play key role in T cell biology
= encodes class I and II molecules
(function in antigen presentation to T cells)
= class III have other functions
(EXTRA READING)
What is the peptide binding groove?
= located in between α1 and α2 domain of MHC
= have a floor composed of:
antiparallel β sheets
= have sides of:
α helices
Class I molecule with a peptide in binding groove
= each individual nucleated human cell can express several unique class I molecules
= each has slightly different peptide binding specificity
Class II molecule with a peptide in binding groove
= MHC class II expression restricted to antigen-presenting cells
What are the two processing and presentation pathways for MHC?
MHC class I (endogenous pathway)
= endogenous antigens degraded into peptides within cytosol by proteasomes
= assemble with class I molecules in the RER
= presented on membrane to CD8+ CTL cells
MHC class II (exogenous pathway)
= exogenous antigens are internalised and degraded within the acidic endocytic compartments
= combine with class II molecules
= presented to CD4+ TH cells
What is licensing and cross presentation?
Exogenous antigens in some cells (mainly DCs)
= can gain access to class I presentation pathways
= CROSS-PRESENTATION
= allows APCs to stimulate CD8+ T cells against intracellular microbes
= enables APCs to capture antigen from dying cells or extracellular environment and activate CTLs
What does the presentation of non-peptide antigens do?
Non-protein antigens can also be recognised by T cells
= presented by CD1 family of non-classical class 1 molecules
= glycolipids or lipoproteins are bound
(with the lipid part fitting deep into binding groove)
(hydrophilic part exposed for TCR engagement)
