Antigen Recognition in Adaptive Immune System Flashcards
What do antigen receptors do for lymphocytes?
They serve roles in maturation of lymphocytes and all adptive responses.
Naive lymphocytes recognise antigens to initiate responses
Effector T cells and antibodies recognise antigens to perform their functions
What receptors do B and T cells express for antigens?
B cells (membrane bound antibodies)
T cells (TCRs)
What do lymphocytes do that allow them to distinguish many, closely related, chemical structures?
Antigen receptors are clonally distributed which means they each are specific for a distinct antigen and have unique receptors for different antigens.
What is the immune reportoire?
The entire collection of distinct clones
Are biochemical signals and cytokines antigen specific?
No, receptors are the same for cytokines.
How are B and T cell receptors similar?
Structurally they are similar. (The edge of the T cell receptor looks like one of the arms of an immunoglobulin)
Both T and B cells both use additional membrane proteins for signal transduction
What are the differences in recognition properties between B and T cells?
B cell receptors recognise much larger antigens directly without need for MHC. T cell receptors can only recognize MHC bound peptides.
BCRs recognise native conformation of antigens and for this reason have a much more broad specificity than TCRs.
BCRs can bind 2 antigens at the same time whereas TCRs only 1 antigen
How is the vast diversity of receptors structures in lymphocyte repertoire generated?
Many clones of lymphocytes with distinct specificity exist. (>10^9)
How are B and T cell receptors similar?
Structurally they are similar. (The edge of the T cell receptor looks like one of the arms of an immunoglobulin)
Both T and B cells both use additional membrane proteins for signal transduction . (the combined structures between the TCR/BCR and the other proteins form complexes.
Antigen/MHC binding results in clonal expansion response in both BCR and TCR responses.
Both BCRs and TCRs have polymorphisms with antigen-recognizing domains that are variable between clones. They also have constant regions required for structural integrity and effector functions which are conserved among all clones.
What is a complementarity determining region?
A region within each Variable region which forms part of receptor that binds to antigens.
Why is variation concentrated in small regions of receptor?
It maximises variability of antigen-binding while maintaining basic structure
What regions are variable in BCRs?
Heavy and light chains of membrane Ig.
What regions are variable in TCRs?
Variable regions of alpha and beta chains.
What are the 2 important functions of antigen receptors?
Specific antigen recognition by receptor and signal transduction mediated by different polypeptides.
What do receptor chains associate with for signal transduction?
Invariant membrane proteins
What do invariant membrane signalling proteins do?
Transmit signal to cytosol and nucleus which causes lymphocyte to divide, differentiate, or die.
What is a combined BCR/TCR with signalling molecule called?
BCR/TCR complex
What happens when antigen binds to antigen receptors on the molecular level?
Signalling proteins of receptor complex bundle together at the receptor and phosphorylation triggers complex signalling cascades activating transcription of genes and lymphocyte response.
What happens when antigen binds to antigen receptors on the molecular level?
Signalling proteins of receptor complex bundle together at the receptor and phosphorylation triggers complex signalling cascades activating transcription of genes and lymphocyte response.
What is an antibody molecule composed of?
4 polypeptide chains (2 heavy 2 light)
The heavy chains form the long middle chains and the light chains sit on top of the protein
How are the heavy chains attached to each other?
Disulfide bonds
How many variable (V) and constant domains do light and heavy chains have?
Light chain has 1 V and 1 C.
Heavy chain has 1 V and 3 - 4 C.
How are membrane bound immunoglobins (IgM) different to secreted immunogobins (IgG)?
Membrane bound IgM have 4 constant domains and a hydrophobic chain for membrane binding.
Secreted IgG antibodies have 3 constant chains
What do immunoglobin domains consist of?
2 layers of beta-pleated sheet held together by disulfide bridge.
How are adjacent Ig strands held together?
They are connected by short protruding loops
What regions of the variable domains are responsible for antigen recognition?
CDR (Complementarity Determining Regions) regions; CDR1, 2, and 3. 3 is the most variable.
What does the hinge region of antibodies do?
Allows the antigen to bend.
What are the types of light chains that can be present on antibodies?
2 types of light chains: κ and λ. Each B cell expresses one but never both.
What are the types of heavy chains on antibodies and how do they differ?
5 types: μ, δ, γ, ε, and α
They differ in C regions.
Does the type of light chain and heavy chain matter with the formation of antibody complexes?
No, each light chain can complex with any heavy chain
What does heavy chain divide antibodies into?
Isotypes/classes
What are the isotypes of antibodies?
IgA, IgD, IgE, IgG, IgM
What does the isotype say about the antibody?
Provides us with information about physical and biological properties.
Which classes of antibody get secreted as dimers?
IgA (form both monomers and dimers)
Which class of antibodies gets secreted as a pentamer?
IgM (they are usually membrane bound so this means they typically have a hydrophobic tail region meaning they get together prior to secretion due to the non-polar affinity)
Do IgD antibodies get secreted?
No, they are found on the membrane of naive B cells so they have low affinity (pointless secreting them)
What antibodies do naive B cells express?
IgM and IgD
{like MD students they are very naive to the world of medicine}
What stimulates B cell clones to expand?
Co-stimulation by antigen and helper T cells. They differentiate into antibody secreting progeny which can produce IgM and others which can produce other heavy-chain classes.
What is change in Ig isotype production called?
Class switching
What are the common functions of IgE?
Mast cell activation in hypersensitivity
Defense against helminthic parasites
What are IgG functions?
Opsonization of pathoges
Complement activation
Antibody dependent Cell-mediated immunity
Neonatal immunity
Feedback inhibition of B cells
What are IgM antibodies important for?
Naive B cell antigen receptor and complement activation
Which antibody classes have the highest concentration in blood?
IgG
How do antigens and antibodies find each other?
Antigen-binding CDR loops come together to form clefts for accomodating small molecules and flat surfaces for larger molecules (portions of proteins)
What are epitopes recognized by?
Shape (conformational epitopes)
Sequence of amino acids (linear epitopes)
What are epitopes recognized by?
Shape (conformational epitopes)
Sequence of amino acids (linear epitopes)
Are all epitopes easy to access?
No some epitopes are hidden within antigen molecules and are exposed as a result of physicochemical change
Is binding between antibody and antigen reversible?
Yes it is not covalent
How is affinity measured?
Dissociation constant (Kd)
Why is affinity maturation necessary?
Initial binding between naive lymphocyte and antigen is weak due to lack of complete specificity.
What is avidity?
Total strength of complex to bind antigen
What is an antibody-cross reaction?
Antibodies produced against 1 antigen may bind other structurally similar Ags
How many hypervariable regions are present in T cell receptor molecules?
3 each corresponding to a loop in V domain. (CDR1, 2, 3 like BCRs)
What chains do normal T cell receptors have?
an α and a β chain.
What chains do γδ T cells have?
γ δ instead of α and β
What is the function of γδ T cells?
Recognize variety of protein and non-protein antigens that aren’t displayed by normal MHC and are abundant in epithelia (they recognize microbes usually encountered at epithelial surfaces)
What are NK-T cells and what do they do?
Express markers of NK cells and α+β TCRs with limited diversity and they recognize lipid antigens displayed by non-polymorphic class I MHC-like molecules
What molecules are membrane bound immunoglobulin molecules associated with?
Igbeta and Igalpha
What molecules are TCR receptors associated with?
CD3 and zeta proteins
What do CD3 and zeta molecules do?
They transmit some of the signals that are initiated when TCR recognizes antigen.
What is required for T cell activation besides the TCR MHC complex?
Co-receptor binding (CD4 or CD8 with non-polymorphic portions of MHC)
How is a mature T/B cell produced in early lymphocyte development?
Lymphocytes must express a viable antigen to survive haematopoeisis.
Cells that can bind to MHC weakly are selected for. Cells that bind MHC strongly are killed. This selection produces useful T and B cells. (B cells this happens in bone marrow and T cells in thymus)
How are diverse receptors produced?
Complex genetic rearrangement takes place at 4 gene loci:
Which chromosome is the Ig H chain locus?
14
Which chromosome is Ig Kappa chain locus located?
Chromosome 2
Which chromosome is TCR beta located on?
7
Which chromosome is TCR alpha located on?
14
What do the C regions of the gene loci select for?
The class of antibody that is produced
Which chains have D regions?
Ig heavy chain and TCR beta-chain loci
Which chains contain V, J, and C regions?
All gene loci that select BCR/TCR components
How does somatic recombination take place?
1 D gene is joined with 1 J segment and V segment is rearranged and fused to D-J element and the intervening introns are cleaved off
VDJ recombinase recognises DNA sequences that flank all V, D, and J gene segments and brings 2 Ig or TCR segments together and cleaves DNA at specific sites. This protein also combines VDJ into one exon via ligases.
Resulting gene is transcribed and then spliced bringing the C-regions as well into the mRNA resulting in mRNA that can be transcribed.
What are the components of VDJ recombinase?
Recombinase-Activating Gene protein 1 and protein 2. (RAG-1 and RAG-2)
What does VDJ recombinase do?
Recognises DNA sequences that flank V, D, and J gene segments.
It brings 2 Ig or TCR gene segments together and cleaves DNA at specific sites
DNA breaks created are repaired by DNA ligases producing full length VJ or VDJ exon without any intervening segments.
Which cells express VDJ recombinase?
Only immature B and T lymphocytes
What is the combining of VDJ in different clones called?
Combinatorial diversity
What are changes in nucleotide sequences introduced at junctions of recombining V, D, and J gene segments called?
Junctional diversity
What limits combinatorial diversity?
Available V, D, and J gene segments
What makes junctional diversity so much more profound?
It is almost unlimited due to it maximising variations in CDR3 regions of antigen receptors.
Errors are introduced creating more diversity
How is junctional diversity created?
Exonucleases removes nucleotides from V, D, and J gene segments at sites of recombination
Terminal deoxyribonucleotidyl transferase (TdT) catalyses random addition of nucleotides between V-D and D-J segments forming the N regions.
During intermediate stage in V(D)J recombination overhanging DNA sequences may be generated and then filled in forming P-nucleotides
What else can be produced by junctional diversity that need to be selected against?
Genes with sequences that don’t code for any proteins.
What are the steps in maturation of B cells?
Stem cell (contains germline DNA)
Pro-B cell (destined to become B cell but only has germline DNA)
Pre-B (Has recombined H chain gene (VDJ); μ mRNA as well as μ and pre-B receptor associated μ)
Immature B
Mature B
What are pro-B cells?
Progenitors committed to B cell lineage that give rise to large number of pro-B cells beginning to rearrange Ig genes, initially in heavy-chain locus
Which pro-B cells develop into pre-B cells?
Cells that make productive VDJ rearrangements at Ig heavy-chain locus develop into pre-B cells
What forms the pre-BCR complex?
μ chain and surrogate light chains associate with Igα and β signalling molecules forming pre-BCR complex
What happens to pre-B cells that don’t produce functional μ chains?
They die by apoptosis
What are the stages of B cell maturation?
Progenitors that are committed to B cell lineage proliferate. (resulting cells are called pro-B cells)
Pro-B cells begin rearranging Ig genes in heavy region initially and then in variable genes and once VDJ rearrangements are productive at the Ig heavy-chain locus the cell is now a pre-B cell.
μ chain and surrogate light chains associate with Igα and β signalling molecules. This combined structure is the pre-BCR complex.
If the μ heavy chain is productive the pre-B cells are selected and expanded.
Once a complete heavy chain is formed there is no longer any need for heavy chain gene transcription and so heavy chain recombination is switched off and now light chain starts to be expressed.
Pre-BCR triggers recombination at the Ig kappa light chain locus and lamda light chain if the kappa chain fails to express a locus.
Functional light chain associates with μ chain to form IgM membrane antibody. (immature B cell) at this point the recombinase enzyme is switched off.
Cells that express the complete receptor receive the signal that promotes survival of the cells that express complete receptors.
Further maturation occurs in either bone marrow or spleen. The resulting mature B cell expresses both IgM and IgD through alternate splicing (the VDJ exon can be spliced onto either Cμ or Cδ mRNA.
What is allelic exclusion?
Each B cell can express Ig heavy chain from only 1 parent. The other allele does not get expressed.
When is the lamda chain used?
Only when the kappa chain fails to be expressed
What happens if immature B cell binds an antigen in bone marrow with high affinity?
It may reactivate VDJ enzyme which further edits the receptor by undergoing further V-J recombination and change in specificity.
B cell repertoire is further shaped by negative selection and deletion of self-reactive B cells.
Where are mature B cells located?
Mature B cells are follicular B cells found within lymph node and spleen follicles.
What cells do marginal zone B cells originate from?
Marginal-zone B cells develop from pro-B cells (progenitors identical to those of follicular B cells)
What are B-1 lymphocytes?
Distinct population in lymphoid organs and peritoneal cavity that develop earlier than lymph node/spleen B cells and from different precursors
What is the pathway of maturation of T cells?
Identical to that of B cells except most of the maturation process occurs in the thymus (following the pro-T cell stage or double T cells/thymocytes)
The T cells that head to the thymus do not contain CD4 or CD8
TCR beta gene recombination occurs in some double-negative cells.
If VDJ recombination is successful and a TCR Beta chain is made that chain gets expressed on the surface. along with the TCRbeta is a pre-Talpha
pre-TCR complex delivers intracellular signals that promote survival and TCR-alpha recombination and inhibit VDJ recombination at the TCR-beta locus (allelic exclusion)
Failure to express alpha chain and complete TCR results in death.
Surviving cells express complete alphabetaTCR and both CD4 and CD8 co-receptors. (these are double positive cells)
What stimulates expansion of T cells in thymus?
IL-7
How are TCRs tested?
They are bound to both class I and II MHC + self peptide and weak recognition is considered a positive result,
No recognition is considered failure and death by neglect results. Positive selection.
Strong recognition by either class I or II MHC results in negative selection
