ABBAS 3 Flashcards
How does Ags induce immune response in B lymphocytes?
o Ag receptors: membrane-bound antibodies
o Can recognise wide variety of macromolecules
Protein
Polysaccharides
Lipids
Nucleic acids
Small chemicals in soluble or cell surface-assoc form
How does Ags induce immune response in T lymphocytes?
o Can only recognise peptide fragments of protein Ags
o Peptides must be presented by specialised peptide display molecules on host cells
What are the barriers in mounting immune response?
o Low freq of naive lymphocytes specific for any 1 Ag
o Diff kinds of microbes need to be combated by diff types of adaptive immune responses
Antigens recognised by T lymphocytes
Most T cells recognise peptide Ags bound to and displayed by Major Histocompatibility Complex (MHC) molecules of APCs
Each T cell has a dual specificity: TCR recognises peptide Ag and MHC molecule displaying Ag
Some T cells recognise lipid and other nonpeptide Ags displayed by nonpolymorphic class I MHC-like molecules
Naive T cells need to see protein Ags presented by DC to initiate clonal expansion and effector cell differentiation
Differentiated effector T cells need to see Ags presented by APCs to activate effector fxns of T cells in humoral and cell-mediated immune responses
MHC and T cell relationship
MHC: genetic locus whose products fxn as peptide display molecules of immune system
MHC restriction: characteristic of T lymphocytes that they recognise a foreign peptide Ag only when it is bound to individual’s MHC molecules
Dendritic cells
Network of DC present in
o Epithelia and subepithelial tissues
o T cell-rich areas of peripheral lymphoid organs
o Other organs (less DC)
Epidermal DC in skin: Langerhans cells
Epithelial DC
o Immature due to inefficiency at stimulating T cells
o Express membrane receptors to bind microbes – capture and endocytose microbial Ags
o Soluble microbes can enter DC by pinocytosis
What happens when microbes bind to TLRs in DCs
Innate system is stimulated.
Production of inflammatory cytokines TNF adn IL-1 is stimulated
What molecules are used to activate immature DC?
TLR signalling
Cytokines
What happens to Activated DC after activation?
o Lose adhesiveness for epithelia
o Begin to express surface receptor CCR7 – specific for chemoattracting cytokines (chemokines) produced in T cell zones of lymph nodes
o Chemokines direct DCs to exit epithelium and migrate thru lymphatic vessels to lymph nodes
o DCs mature during migration, from Ag-capturing cells to APCs that can stimulate T cells
o Maturation of DC reflected in
Increased synthesis and stable expression of MHC
Production of costimulators (req for full T cell response)
Where does DC pick up antigens?
Soluble Ags in lymph picked up by DCs in lymph node
Blood-borne Ags picked up by DCs in spleen
Protein Ags are transported and concentrated in regions of lymph nodes where Ags are most likely to encounter T cells
Naive T cells
o Continuously recirculate lymph nodes
o Express CCR7 – promote entry of T cells into T cell zones of lymph nodes where they meet DCs carrying captured Ags
T cell response to Ags introduced to body within 12-18 hours
APCs
o DCs
Most potent APC for activating T lymphocytes
o Macrophages
Abundant in all tissue
Cell-mediated immunity: phagocytose microbes and display Ags of microbes to effector T cells
Effector T cells then activate macrophages to kill microbes
Mediation of antigens
A) B cells ingest protein Ags, then display them to helper T cells within lymphoid tissue
B) All nucleated cells can present antigen derived from microbes in cytoplasm in CTLs
C) DC initiate responses of CD8+ T cells to Ags of intracellular microbes
o Cross-presentation/cross-priming
Professional APC displays the Ags of another cell
Activates/primes a naive CD8+ CTL
Infected cell ingested by APC -> microbial Ags processed -> presented in assoc with MHC molecules
APC also provides costimulation for T cells
D) DC tt ingest infected cells present microbial Ags to CD4+ helper T cells
E) CD8+ T cells diff into CTLs -> kill infected host cells w/o need for DCs or signals other than Ag recognition
MHC
Membrane proteins on APCs
Display peptide Ags derived from protein Ags for recognition by Ag-specific T lymphocytes
Individuals identical at MHC locus (inbred animals, identical twins) -> can accept graft from one another
Individuals with different MHC loci will reject grafts
Human Leukocyte Antigens (HLA): human MHC proteins
MHC locus contains 2 sets of polymorphic genes – class I and class II MHC genes – encode class I and II MHC molecules
MHC locus also contains nonpolymorphic genes – code for proteins involved in Ag presentation
Class I MHC molecule
o α chain noncovalently attached to β2-microglobulin (protein encoded by gene outside MHC)
o Peptide-binding cleft/groove
Formed by amino-terminal α1 and α2 domains
Peptides 8-11 AA long
Floor of cleft binds peptides for display to T cells
Sides and tops of cleft come into contact with TCR
o Polymorphic residue – AA that differ among diff individual’s MHC molecules
Located in α1 and α2 domains of α chain
Contribute to variations in floor of cleft – influence ability of diff MHC molecules to bind peptides
Contribute to variations in tops of clefts – influence recognition by T cells
o α3 domain
invariant
contains binding site for T cell co-receptor CD8
o T cell activation needs recognition of MHC-assoc peptide Ag by TCR and simultaneous recognition of MHC by co-receptor
o Thus CD8+ T cells can only respond to peptides displayed by class I MHC (MHC molecules to which CD8 co-receptor binds to)
Class II MHC
o Consists of 2 chains – α and β
o Amino-terminal regions of both chains – α1 and β1 domains
Contain polymorphic residues
Form cleft tt can accommodate peptides of 10-30 residues
o Β2 domain
Nonpolymorphic
Contains binding site for T cell coreceptor CD4
CD4 binds to class II MHC
Thus CD4+ T cells can only respond to peptides presented by class II MHC molecules
Class I vs Class II MHC
o Mb proteins
o Each contain a peptide-binding cleft at amino-terminal end
o Differ in subunit composition but similar in overall structure
Features of MHC - MHC genes are codominantly expressed
o Alleles inherited from both parents are expressed equally
o Class I
3 polymorphic class I genes (HLA-A, HLA-B, HLA-C)
Each person inherits 1 set of genes from each parent
Any cell can express 6 diff class I molecules
o Class II
Each person inherits 1 pair of HLA-DP, 1 pair of HLA-DQ, and 1 or 2 of HLA-DR
Heterozygous individual can inherit 6 or 8 class II MHC alleles, 3 or 4 from each parent
Number of expressed class II molecules can be more than 6 due to extra DR genes
o MHC haplotype
Set of MHC alleles present on each chromosome
All heterozygous individuals have 2 HLA haplotypes, 1 from each chromosome
MHC genes are highly polymorphic - Features of MHC
o Diff alleles are present among diff individuals in population
o Ensures tt a population can deal with diversity of microbes
o Variations in MHC molecules (thus polymorphism) result from inheritance of distinct DNA sequences, not induced by gene recombination (as they are in Ag receptors)
Expression of MHC - Features of MHC
Class I MHC expressed on all nucleated cells (ie. All cells except RBC)
Class II MHC expressed mainly on DCs, macrophages and B lymphocytes. Also expressed on thymic epithelial cells and endothelial cells. Can be induced on other cell types by cytokine interferon-γ
Peptide-binding cleft of MHC - Features of MHC
o Bind peptides derived from protein Ags
o Display these peptides for recognition by T cells
o Pockets in the floors of clefts
Place where side chains of AA in peptide Ags fit
Side chains (anchor residues) in pockets anchor peptides in cleft
Peptides anchored in cleft by anchor residues contain some residues tt bow upward and are recognised by Ag receptors of T cells
Features of interaction of peptides Ags with MHC
- Each MHC molecule can present only 1 peptide at a time since there is only 1 cleft
- Each molecule can present many different peptides – MHC has broad specificity for peptide binding
- MHC molecules only bind peptides
- MHC acquire peptide cargo during biosynthesis and assembly inside cells
- Only peptide-loaded MHC molecules are stably expressed on cell surfaces
- MHC cannot discriminate bt foreign Ags (peptides derived from foreign/microbial proteins) and self Ags(peptides from individual’s own proteins)
- Body does not react to self-Ags (autoimmune response)
- Each molecule can present many different peptides – MHC has broad specificity for peptide binding
o As long as pockets of MHC can accommodate the anchor residues of the peptide – can be displayed on MHC molecule
o Only 1 or 2 residues of peptide have to fit into cleft
o Each molecule can bind many but not all possible peptides
o Each individual has only a few diff MHC molecules that can present a vast number and variety of Ags
- MHC molecules only bind peptides
o MHC-restricted CD4+ and CD8+ T cells can recognise and respond to protein Ags (source of peptides)
- MHC acquire peptide cargo during biosynthesis and assembly inside cells
o MHC display peptides derived from microbes inside host cells
o Thus MHC-restricted T cells recognise cell-assoc microbes
o Class I acquire peptides from cytosolic proteins
o Class II acquire peptides from proteins in intracellular vesicles
- Only peptide-loaded MHC molecules are stably expressed on cell surfaces
o “empty” MHC are degraded inside cells
o Ensure that only “useful” MHC molecules (those displaying peptides) are expressed on cell surfaces for recognition by T cells
o Once peptides bind to MHC and are displayed on cell surface, stay bound for a long time (days) – slow off-rate ensures tt after an MHC acquires a peptide, it’ll display it long enough to maximise chance tt it’s specific T cell will recognise it and initiate response
- MHC cannot discriminate bt foreign Ags (peptides derived from foreign/microbial proteins) and self Ags(peptides from individual’s own proteins)
o MHC can display both foreign and self Ags
o Although self Ags is always more than foreign Ags, MHC are not all occupied by self Ags as MHC are constantly being synthesised and ready to accept peptides -> adept at capturing any peptides present in cells
o T cells also only need to see a peptide presented by a few MHC molecules to initiate immune response
- Body does not react to self-Ags (autoimmune response)
o T cells specific for self-Ags are killed or inactivated
o MHC molecules presenting self Ags is key to normal surveillance fxn of T cells
Antigen processing
Class I MHC
o Proteins in cytoplasm of any nucleated cell processed in cytoplasm
o Display endogeneously produced peptides
o Usually presents self-Ags
o Present viral Ags when host cell is infected by virus and produces viral proteins
Class II MHC
o Extracellular proteins internalised by specialised APCs (DCs, macrophages, B cells) processed in vesicles
o Displayed engulfed proteins
2 different pathways of Ag processing allows sampling of all the proteins present in extracellular and intracellular envs
Processing of internalised Ags for display by Class II MHC
- Mechanisms of internalisation of extracellular microbes/proteins by APCs
o Microbes bind to surface receptors specific for microbial products
o Microbes bind to receptors tt recognise Abs
o Microbes bind to products of complement activation attached to microbes
o B lymphocytes: internalise protein tt specifically binds to cells’ Ag receptors
o Pinocytosis / Phagocytosis (w/o specific recognition) - After internalisation into APCs
o Microbial proteins enter acidic intracellular vesicles – endosome/phagosome
o Vesicles may fuse with lysosomes
o Proteins are broken down in vesicles by proteolytic enzymes into peptides of varying lengths and sequences
3. Synthesis of class II MHC by APCs o Class II MHC synthesised in ER o Each molecule carries an attached protein – invariant chain – contains sequence Class II Invarian Chain Peptide (CLIP) that binds tightly to peptide binding cleft of MHC o “occupied” MHC molecule is then transported to cell surface in an exocytic vesicle which fuses with endosomal vesicle containing peptides derived from ingested extracellular proteins o Endosomal vesicle contains DM protein – remove CLIP from MHC thus making peptide binding cleft available
4. Fate of class II MHC molecule after CLIP is removed o If MHC can bind peptide generated from ingested proteins -> complex becomes stable -> delivered to cell surface o If MHC does not bind peptide -> empty molecule is unstable -> degraded by proteasomes in endosomes o 1 protein Ag can give rise to many peptides but only a few can bind to MHC o Peptides that can bind to MHC and stimulate immune responses: immunodominant epitopes of the Ag
Processing of cytosolic Ags for display by Class I MHC
- Ag proteins produced in cytoplasm from
o Viruses living inside infected cells
o Phagocytosed microbes that may leak from/transported out of vesicles into cytoplasm
o Mutated/altered host genes in tumours - Proteins are targeted for destruction by proteolysis
o Proteins unfolded
o Covalently tagged with multiple copies of peptide ubiquitin
o Proteasome action on protein -> cleave and degrade protein into peptides with size and sequence capable of binding class I MHC - Binding of peptide (in cytoplasm) to MHC (synthesised in ER)
o Transporter associated with Antigen Processing (TAP)
Specialised transport molecule in ER membrane
Binds peptides from cytoplasm
Actively pumps peptides across ER membrane into interior of ER
o Newly synthesised class I MHC loosely attached to interior face of TAP
o As peptides enter ER, captured by Class I MHC molecules
o If binding is of right fit -> complex stabilised and transported to cell surface
o Class I MHC-peptide complex
Unavailable to bind other peptides
Can resist proteolysis by endosomal proteases due to stability
o If MHC does not bind peptide -> unstable -> degraded - Co-evolution of microbe and host
o Ways viruses use to block class I MHC pathway of Ag presentation
Remove newly synthesised MHC molecules from ER
Inhibit transcription of MHC genes
Block peptide transport by TAP
o By inhibiting class I MHC pathway
Viruses reduce presentation of their own Ags to CD8+ T cells
Evade adaptive immune system
o Counterbalanced by NK cells
Recognise and kill virally infected cells which have lost class I MHC exp
What is the Significance of MHC-associated Ag presentation?
- Restriction of T cell recognition to MHC-associated peptides
- Segregation of class I and II ag presentation pathways ensure the correct, specialised immune response against microbes in diff locations
- Restriction of T cell recognition to MHC-associated peptides
o Ensures that T cells see and respond only to cell-associated Ags (Ags of phagocytosed and intracellular microbes)
MHC are cell membrane proteins
MHC molecules can be loaded with peptides only inside cells, where Ags of phagocytes and intracellular pathogens are present
- Segregation of class I and II Ag presentation pathways ensure the correct, specialised immune response against microbes in diff locations (extracellular/intracellular)
o Extracellular microbes
Captured by APCs including B cells and macrophages
Presented by Class II MHC which are mainly expressed on APCs
CD4 is specific for class II -> class II assoc peptides recognised by CD4+ T cells
CD4+ T cells fxn as helper cells -> help B cells produce Abs, help phagocytes destroy ingested microbes
Eliminate extracellular and ingested microbes but not effective against viruses or other pathogens that replicate in cytoplasm of host cells
o Intracellular microbes
Cytosolic Ags processed and displayed by class I MHC
Class I associated peptides recognised by CD8+ T cells which diff into CTLs
CTLs kill infected cells and eradicate infection
o Nature of protective immune response to diff microbes optimised by linking features of Ag presentation and T cell recognition
Pathways of processing of vesicular and cytosolic Ags
Cellular exp of Class I and II MHC molecules
Specificity of CD4 and CD8 co-receptors for class II and class I molecules respectively
Functions of CD4+ cells as helper cells; CD8+ cells as CTLs
T cells cannot distinguish bt extracellular and intracellular microbes
Ag recognised by B cells and other lymphocytes
B cells use membrane-bound Abs to recognise Ags
Ags can be
o Expressed on microbial surfaces eg. capsular or envelope Ags
o In soluble form eg. secreted toxins
In response to Ags, B cells secrete Ab
Abs enter circulation and mucosal fluids -> bind to Ags -> neutralise and eliminate Ags
No need for Ag processing or display
Follicular Dendritic Cells (FDCs)
o Cells in B cell-rich lymphoid follicles of lymph nodes and spleen
o Display Ags to activated B cells
o Ags displayed by FDCs are coated with
- Abs
- Complement byproducts eg. C3b, C3d
o Use Fc receptors to bind Ag-Ab complexes
o Use receptors for complement proteins to bind Ags with these proteins attached
o B cells that bind Ags with high affinity are selected for
T cells (small number) that recognise non-peptide Ags
o Natural Killer T cells (NK-T cells): specific for lipids displayed by class I-like CD1 molecules
