Cellular-Mediated Immunity Flashcards
MHC Molecules
Class 1 MHC:
Contains 3 polymorphic genes: A, B, C.
Short antigen peptide (8-11 amino acids)
Class 2 MHC:
Contains 1 polymorphic gene: D.
Longer antigen peptide (10-30 amino acids)
Features of MHC:
Codominant Expression:
- -Both parental alleles of each MHC gene are expressed.
- -Increases number of different MHC molecules that can present peptides to T cells.
Polymorphic Genes:
- -Many different alleles are present in the population.
- -Ensures that different individuals are able to present and respond to different microbial peptides.
MHC-Expressing Cell Types:
–Class I MHC: Expressed on All Nucleated cells.
CD8+ CTLs can kill any virus-infected cell.
–Class II MHC: Expressed on Dendritic cells, Macrophages, B cells, and Endothelial cells.
CD4+ Helper T cells can interact with them.
Broad Specificity:
–Many different peptides can bind to the same MHC molecule.
Each MHC molecule displays one peptide at a time:
–Each T cell responds to a single peptide bound to an MHC molecule.
MHC molecules only bind peptides:
–MHC-restricted T cells respond only to protein antigens.
Peptides are acquired during Intracellular assembly:
–Class 1 and Class 2 display peptides from different cellular compartments.
Stable surface expression of MHC molecules requires bound peptide:
–Only MHC molecules that are displaying peptides are expressed for T cell recognition.
Very Slow Off-Rate:
–Displays bound peptide for long enough to be located by T cell.
Class 1 MHC Intracellular Antigen Pathway
Production of Protein Antigens in the Cytosol:
–May be produced in the cytoplasm from
Viruses living inside infected host cells.
–May be from Phagocytosed microbes that may leak from or be transported out of phagosomes into the cytoplasm.
–May be from mutated genes that encoded cytosolic or nuclear proteins, as in the case of Tumor cells, resulting in misfolded cytoplasmic or nuclear proteins.
1) All of these proteins are targeted for destruction by Proteolysis by the Ubiquitin-Proteasome Pathway:
a) Targeted proteins are unfolded and covalently tagged with ubiquitin
b) They are then threaded through the Proteasome where they are degraded by proteolytic enzymes into short peptides of 8-11 amino acids.
2) Transport of Peptides from Cytosol to ER:
a) TAP is a transport molecule embedded in the ER membrane. (Transporter associated w/ antigen processing)
b) TAP actively pumps them into ER
3) Assembly of Peptide-Class 1 MHC complexes in ER:
a) Newly synthesized Class 1 MHC molecules are linked to TAP (by tapasin, a bridging protein)
4) Cell Surface Expression:
a) Tight binding of the appropriate peptide Stabilizes the complex and it is expressed on the cell surface.
Class 2 MHC Extracellular Antigen Pathway
1) Uptake into Vesicles:
a) APCs (Dendritic cells, Macrophages, B cells, Endothelial cells) ingest Extracellular microbes or microbial proteins.
b) The proteins enter Endosomes or Phagosomes, which may fuse with Lysosomes.
2) Proteolysis:
a) Proteolytic enzymes degrade the proteins into peptides of 10-30 amino acids.
3) Biosynthesis of Class 2 MHC:
a) Occurs in the ER.
b) (CLIP on invariant chain prevents them from accepting peptides in the ER destined for class 1.
4) Assembly of Peptide-Class 2 MHC complex:
a) Class 2 MHC molecule is sent to late endosomal or late lysosomal vesicles
b) Binding of the appropriate peptide to the MHC stabilizes the complex and is delivered to the cell surface.
Cross-Presentation of Intracellular Antigens
Problem: Non-APCs may not express costimulators for T cell activation.
Solution: APC (e.g. Dendritic cell) ingests infected cell and presents Antigen:
Antigen peptide generated from Phagosome is presented to Naive CD4+ T cell on Class 2 MHC.
Antigen peptide from within Cytosol is presented to Naive CD8+ T cell on Class 1 MHC.
**Both CD4+ and CD8+ T cells specific for same Antigen (although different antigen peptides) are Activated Simultaneously by the same APC through Class 1 MHC and Class 2 MHC in close proximity.
Naive T cells always need costimulatory signals for activation whereas Effector CTLs are activated and kill without costimulation.
Thus, Antigen-stimulated Naive CD8+ T cells require help from CD4+ T cells to become Effective CTLs.
CD4+ T cells are Helpers to Both B cells and CD8+ T cells.
**Once Naive CD8+ T cells have differentiated into Effector CD8+ CTLs, they can kill infected cells WITHOUT Costimulation.
Summary of Class 1 MHC and Class 2 MHC Pathways
Composition of Stable Peptide-MHC Complex:
Class 1) Polymorphic Alpha chain, Beta2 microglobulin, and Peptide
Class 2) Polymorphic Alpha and Beta chains + peptide.
Types of APCs:
Class 1: All Nucleated cells
Class 2: Dendritic cells, Macrophages, B cells, some Endothelial cells
Responsive T cells:
Class 1: CD8+ T cells (CTLs)
Class 2: CD4+ T cells (Helper)
Source of Protein Antigens:
Class 1) Cytosolic proteins synthesized in the cell or leaked into Cytosol from phagosome.
Class 2) Endosome/Lysosome proteins internalized from Extracellular environment.
Enzymes Responsible for Peptide Generation:
Class 1) Cytosolic Proteasome
Class 2) Proteases in Endosome/Lysosome
Site of Peptide Loading on MHC:
Class 1) ER (aided by TAP)
Class 2) Vesicles (aided by invariant chain)
TCR (T cell receptor)
Membrane-bound heterodimer of Alpha chain and Beta chain.
Each chain contains 1 Variable region and 1 Constant region.
Each Variable region contains 3 Hypervariable regions: CDR3 is the most variable and is encoded by Junctional sequences.
TCR genes consist of Variable, Diversity, Joining, and Constant gene segments that undergo somatic recombination by VDJ Recombinase to produce variability.
D and J are located between the V and C genes.
All antigen receptor gene loci contain V, J, and C genes.
Only Ig Heavy chain and TCR Beta chain also contain D.
VDJ Recombinase
Expressed only in immature B and T cells.
Does somatic recombination of V and J or V, D, J genes.
RAG1 and RAG2 are part of the VDJ enzyme.
Combinatorial Diversity
The use of different combinations of V, D, J gene segments in different clones of lymphocytes.
Limited by the number of available V, D, J gene segments.
Junctional Diversity
Changes in nucleotide sequences introduced at the junctions between V, D, J gene segments.
Virtually unlimited.
- Exonucleases randomly remove nucleotides from these junctions.
- Terminal deoxyribonucleotidyl transferase (TdT) randomly adds nucleotides to these junctions, forming N-regions.
T cell Maturation
Progenitor T cells in bone marrow are sent to Thymus.
Pro-T cell is Double Negative.
- –Proliferate in response to IL-7 produced in Thymus.
- –VDJ Recombinase does TCR Beta gene recombination.
Pre-T cell is one that expresses pre-TCR complex on cell surface = TCR Beta protein with pre-T alpha.
- –Pre-TCR complex signals survival and proliferation.
- –TCR Alpha gene recombination.
Failure to express TCR Alpha chain and complete TCR results in cell death.
Double Positive T cells: Express complete TCR and both CD4+ and CD8+ Coreceptors.
Positive selection of Self-MHC Restricted T cells.
Negative selection of Strong Recognition: apoptosis.
Single Positive T cells: Recognize Class 1 MHC, become CD8+ only.
Recognize Class 2 MHC, become CD4+ only.
T cell Activation and Costimulation
TCR Complex = TCR alpha-beta heterodimer, CD3 and Zeta signal transduction proteins.
1) T cell Receptor (TCR) recognizes the Peptide Antigen and the MHC @ the peptide-binding cleft of the MHC.
2) Coreceptors CD8 or CD4 bind and recognize the MHC
3) CD3 and Zeta proteins transduce signals inside the T cell. (They are transmembrane proteins invariant among all T cells.)
4) Costimulatory B7 proteins (B7.1 and B7.2) are produced by the APC following microbial recognition.
5) B7 costimulators bind to CD28 on T cell surface during Antigen Presentation.
(Only when T cells are contacted by APC that present Antigen and also express B7 costimulators will T cell Activation occur.
6) APC secretes Cytokines, such as IL-12, that stimulate T cells.
7) CD40 Ligand on T cell surface binds to CD40 on APC, enhancing B7 expression and IL-12 production.
8) Chemokines + Antigen recognition also trigger conversion of
Low-Affinity State Integrins (expressed on Naive T cells) –>
to High-Affinity State Integrins on T cell surface
9) High-Affinity Integrins (LFA-1) on T cell adhere strongly to Ligand (ICAM-1) on APC.
Summary: Effective T cell Activation Requires
- –Peptide Antigen presented by MHC.
- –Coreceptors CD8 or CD4 recognize MHC.
- –Costimulators B7 to CD28; CD40 to CD40L.
- –Accessory Signaling: CD3 and Zeta.
- –Adhesion Molecules: Integrin (LFA-1) to ICAM-1.
Biochemical Pathways of T cell Activation:
Immunologic Synapse
Immunologic Synapse: formed by reorganization on cell surface of APC and T cells of TCR and Costimulators.
Reorganized TCR and accessory molecules recruit adaptor proteins that signal activation of transcription factors.
Cyclosporine
One biochemical pathway of T cell activation is *Calcium dependent.
Calcium-dependent Calmodulin activates *Calcineurin.
*Cyclosporine binds and blocks Calcineurin, inhibiting cytokines production and blunting the immune response.
Cyclosporine is, thus, useful in preventing Transplant Rejection.
IL-2 Autocrine Loop
In response to Antigen and Costimulators,
CD4+ T cells secrete several cytokines.
IL-2 is the first one they produce.
At the same time, Activated T cells develop IL-2 Receptor.
This creates an Autocrine Loop that *Stimulates T cell Survival and Proliferation, leading to Clonal Expansion.
Clonal Expansion of T cells
- 1-2 days after activation, T cells proliferate and peak in about one week.
- One week after the start of the infection, the Antigen-specific Clone is about 10-20% of the Entire T cell Population.
The Expansion of CD8+ T cells is much Higher than the expansion of CD4+ T cells.
**Differentiated Effector T cells
Appear 3-4 Days after exposure to the microbe and Peak around Day 7.
