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1 Interactions between innate immunity & adaptive immunity 2 Naïve T cells exit the thymus and enter the bloodstream. If they remain in the bloodstream, then they will pass through the spleen. Naïve T cells can also enter lymph nodes by crossing high endothelial venules (HEVs). In these 2 ˚ lymphoid organs, naïve T cells encounter thousands of APCs. If they are not activated, then they reenter the bloodstream and start all over.
What happens to T cells after they leave the thymus?
3 T-cell activation T cells migrate to secondary lymphoid tissues where they interact with antigen, antigen-presenting cells, and other lymphocytes: 4 Naïve T cells 5 Lymphoid Tissues 6
Naïve T cells enter lymph nodes via
high endothelial venules (HEVs)
7 What happens when a naïve T cell is activated? Proliferation Cell cycle entry and cell division Clonal expansion Differentiation Secretion of cytokines (helper cells) Activation of killer functions (cytotoxic cells) Acquisition of effector function Memory Death Important for down-regulation of immune response 8 Response of CD4 T cells 9 After activation, T cells remain in lymph nodes for 5-6 days 10 2 ˚ lymphoid organs bring cells of the immune system together Facilitate the complex cellular and molecular interactions necessary to initiate the immune response How do T cells and antigen meet in the lymph node? There are 3 types of professional antigen presenting cells Professional Antigen Presenting Dendritic cells^ Cells 12 Note: B cells are poor APCs for primary, naive T cells. They are selfish. They really only function as APCs in order to direct activated, antigen-specific T cells to provide them with help. Dendritic cells are by far the strongest activators of naive T cells Why?
Poor T cell activators Great T cell activators 19 Dendritic cells undergo “maturation” to become potent antigen-presenting cells in lymphoid tissue I. Mellman, P. Pierre, and S. Turley. Nature 1997, 388:787-792.^20 Immature phenotype: DCs from bone marrow cultures Intermediate phenotype: 6 d culture with GM-CSF Mature DCs: Day 8 of culture with GM-CSF Dendritic cells undergo “maturation” 21 Immature DC Mature DC DC maturation is associated with migration to the lymph node 22 Why are DCs such good APCs?
- The express both MHC class I and MHC class II
- They start in the right place (tissues)
- They end up in the right place (lymph node)
- Immature DCs are highly efficient at taking up antigens in the tissues.
- After maturation and migration, mature DCs present these antigens to T cells in the lymph node.
- They express costimulatory molecules 23
Mature dendritic cells express key
costimulatory molecules
B7.1 (CD80) and B7.2 (CD86) are structurally related glycoproteins expressed on mature DCs. Collectively called B molecules, they bind CD28 on T cells 24
B7 molecules are ligands for CD28 on T cells
25 Naive T cells require two signals to become activated: TCR/MHC and costimulation How does CD28 signaling enhance T cell activation? 26
RECEPTOR
Protein tyrosine kinase Phospholipase C Intracellular Ca2+ Protein kinase C Ras (G-proteins) MAP kinases The IL-2 gene NF-AT NF- κ B AP-1 promoter Remember how TCR signaling leads to IL-2 transcription 27
RECEPTOR
Protein tyrosine kinase Phospholipase C Intracellular Ca2+ Protein kinase C Ras (G-proteins) MAP kinases The IL-2 gene NF-AT NF- κ B AP-1 promoter CD28 signaling activates AP-1 and NF- κ B CD 28 CD28 signaling stabilizes IL-2 mRNA
- Most cytokine mRNAs are very short-lived because of “instability” sequences in their 3’ untranslated regions.
- Instability is important for tight regulation.
- CD28 signaling stabilizes IL-2 mRNA leading to a 20-30 fold increase in protein secretion.
- Stabilization combined with the activation of NF-κB and AP-1 increases IL-2 secretion 100-fold. 29 The costimulatory signal is necessary for the synthesis and secretion of IL 30 Summary I
- Naïve T cells leave the thymus and home to secondary lymphoid organs.
- Dendritic cells are the most potent activators of naïve T cells.
- DCs carry antigen from the periphery to the draining lymph nodes.
- This migration is associated with “maturation”.
- Mature DCs express high levels of MHC as well as B7 costimulatory molecules.
- B7 molecules bind CD28 on T cells. CD28 signaling provides “Signal 2” to T cells and is necessary for T cell activation.
- CD28 costimulation is mediated, in part, by increasing IL- secretion.
37 Summary II Naïve T cells require 2 signals to be activated: 1 - TCR 2 - CD28 (costimulation) Signal 1 in the absence of signal 2 leads to anergy Anergic T cells are unresponsive to further stimulation, even in the presence of costimulation. Anergy is thought to be a mechanism of ensuring tolerance to peripheral antigens not expressed in the thymus. 38 To generate cytotoxic killer cells, which have the capacity to eliminate infected cells and attack transplants and tumour cells, DCs have to present antigenic peptides complexed to MHC class I molecules to CD8-expressing T cells.
- Straightforward if the DC is infected itself (ie. influenza virus).
- How DCs could process and present antigens that have no access to the cytosol in an MHC class-I-restricted manner (for instance transplant- and tumour-derived antigens, or antigens from viruses that cannot infect DCs)? Cross-priming 39 40 41 42
Once the T cell response is started, how is it turned off?
T cell activation is linked to signals that eventually limit activation and remove most activated cells. CTLA-4 blocks costimulation Fas induces apoptosis Naive Memory
43
- CTLA-4, also expressed by T cells, is a CD28 homologue
- CD28 sends a positive co-stimulatory signal, enhancing T cell activation
- CTLA-4 sends an inhibitory signal
- TCR + CD28 signaling leads to upregulation of CTLA-
- CTLA-4 has a much higher affinity for B7 than does CD28, so it outcompetes CD28 for B7 binding.
CD28 and CTLA-
44
T-cell activation through TCR and CD
leads to surface expression of CTLA
45
CD28 CTLA-
MCC
(specific antigen)
Antigen Induced Protein Translocation
in 5C.C7 T Cells
HB
(control antigen) Green-Tubulin Blue-Nucleus 46
Manipulating signal 2
CTLA-4 null mouse. Dies of massive lymphadenopathy at several weeks of age. Idea is that there is nothing to counteract T cell activation CD28 null mouse. Hyporesponsive immune system, but normal T cell development. Transfect B7 into non-APC and convert it into effective APC. Consider implications for vaccine production CTLA-4-Ig can inhibit immune responses (consider mechanism and potential uses). Anti-CTLA-4-antibody can contribute to a more vigorous immune response. 47 Cancer regression and autoimmunity induced by CTLA4 blockade in patients with metastatic melanoma 48
Activated T cells also upregulate Fas
Both Fas and Fas ligand (FasL) are upregulated on activated T cells Signaling through Fas leads to activation of caspases and programmed cell death (apoptosis). Mutations in Fas or FasL lead to lympoproliferative and autoimmune disorders FasL Fas Naïve T cell Activation
