Wk 1 TBL 1 Adaptive Immunity Flashcards
How do dendritic cells recognize bacteria?
Via expression of toll-like receptors (PRRs) on the DC membrane, which induces activation
How are dendritic cells activated?
When their toll-like receptors recognize bacteria
-they live in an inactive state in skin and other tissues
What do dendritic cells do once activated?
Migrate to lymph nodes where they can activate T cells
What are the 2 ways dendritic cells can become activated?
- recognition of pathogens directly via toll-like receptors
- exposure to cytokines in inflammatory environment (IL-1, IL-6, TNF or viral interferon alpha and beta)
Where are dendritic cells in their inactivated state?
Skin and other tissues
Once activated, what are 2 effector fxns of dendritic cells?
- increase expression of MHC, costimulatory molecules, production of cytokines (ie. IL-12) -> T cell activation
- increase CCR7 (CC-chemokine receptor): chemokine-mediated trafficking to draining lymph node
What is necessary to activate naive T cells?
APCs - activated dendritic cells (or others)
What are 2 notables regarding APC and T cell interactions?
- interactions can last hours - a couple of days
- adhesion molecules (like integrins LFA-1 and ICAMs (ICAM-1) stabilize the interaction
What happens after T cell activation?
Clonal expansion
Develop effector functions (Helper T, cytotoxic lymphocytes)
What are 3 signals for T cell activation?
- TCR signaling - includes TCR + pMHC & co-receptor + CD3 (on T cell))
- costimulation
- cytokine-induced effector differentiation
Where are MHCs?
on dendritic cells
Why are co-stim receptors necessary?
T cells don’t have intracellular signaling from the TCR
-co-receptor stabilizes the TCR-pMHC binding
-brings tyrosine kinase into close prox of TCR to initiate TCR signaling
-also a protective mechanism, like a checkpoint
What is CD3?
Another aspect of Signal 1 (TCR + pMHC + co-receptor + CD3)
-CD3 has cytoplasmic signaling domains (TCR does not)
-CD3 associates w/ TCR and transmits TCR signal
-has 3 subunits: alpha, gamma epsilon
-each subunit has multiple immunoreceptor tyrosine activation motifs (ITAMs) to allow for tyrosine activation
What is ITAM?
immunoreceptor tyrosine activation motif
-phosphorylation of this on CD3 subunits initiates tyrosine kinase signaling cascade
What is Signal 2?
costimulation (B7 on APC:CD28 on T cell) - there are others, but this is the focus
For signal 2, what are the 2 B7 co-stim molecules and their pathways?
B7.1 (CD80)
B7.2 (CD86)
Each binds on receptors of CD28 -> survival activation pathway w/in cell
What are 4 events that occur for costimulation to happen and what does it cause?
- activation of DCs induces B7 expression
- Naive T cells express CD28
- Binding to B7.1 or B7.2 induces CD28 to deliver survival and activation signals to the T cell (There are multiple costim pathways, but B7:CD28 is best characterized)
- costim is required for productive T cell activation
What happens if signal 1 happens w/o signal 2?
It induces T cell tolerance or death
What are 2 negative co-stimulatory molecules?
- PD-1
- CTLA-4
When are CTLA-4 and PD-1 expressed?
A few days after T cell activation, so they are not on naive T cells
Where does CTLA-4 bind?
Binds B7 w/ higher affinity than CD28, so it competitively inhibits CD28
What do CTLA-4 and PD-1 induce?
They deliver signals that suppress T cell activation, attenuating TCR signals
Analogy for CD28, CTLA-4 and PD-1
CD28 is the accelerator
CTLA-4 and PD-1 are the brakes
What happens if CTLA-4 or PD-1 become absent?
Can -> auto-reactive T cell response
IMP responses for maintaining tolerance and preventing T-cell mediated immunopathology
What are checkpoints?
CTLA-4 and PD-1
both are targets for cancer immunotherapy
What is signal 3?
Cytokines (incl IL-12)
-induce effector differentiation
What cells produce signal 3 cytokines?
APCs or other cell types
-induce the acquisition of effector functions
How does TCR signaling initiate the cascade?
- co-localization of TCR and co-receptor
- Lck-mediated phosphorylation of CD3 ITAMs
- Activation of TKs, including ZAP-70
- Activation of phospholipases, induction of Ca2+ flux from ER and outside the cell
- Ca2+ dependent activation of NFAT
- TK-dependent activation of NFkB, AP-1
- NFAT, NFkB and AP-1 induce a transcriptional program that -> T cell expansion, differentiation and survival
What are 2 transcription factors relevant to TCR signaling?
AP-1 and NFkB
What is the TCR signaling cascade?
- TCR and co-receptor activated on outer domain
- Lck phosphorylates CD3 ITAMs and the tyrosine kinase ZAP-70
- ZAP70 activates phospholipases
- activated TKs and phospholipases activate NFkB
- TKs activate MAPK
- MAPK activates AP-1
- AP-1 and NFkB activate T cell clonal expansion, differentiation and survival
- phospholipases also activate Ca2+ channels
- Ca2+ enters cell and activates calcineurin
- Calcineurin activates NFAT
- NFAT also activates T cell clonal expansion, differentiation, and survival
What is the NFAT pathway?
Ca2+ -> calcineurin -> NFAT -> T cell clonal expansion, differentiation, and survival
What happens when NFAT, NFkB and AP-1 get activated?
They translocate to the nucleus and induce the expression of many genes, incl IL-2 and IL-2R
What is IL-2
It’s genes are activated by AP-1, NFkB, and NFAT
-it is a signaling molecule that gets secreted into the extracellular space
What is IL-2R?
-a receptor
-its genes get activated by AP-1, NFkB and NFAT in the nucleus
-it translocates to the T cell membrane to bind IL2 floating in the extracellular space
What and how is T cell clonal expansion driven?
IL-2 and IL-2R drive T cell clonal expansion
IL-2 can bind onto its receptors on the same cell that secreted it (autocrine) or neighboring cells (paracrine)
How many T cells can one activated T cell give rise to?
10k-100k w/ the same TCR specificity
What drives clonal expansion?
IL-2 binding to IL-2R
What does signal 3 induce?
Effector T helper subsets: Th1, Th2, Th17, Treg
What signal induces Th1?
Signal 3 = IL-12
What does Th1 secrete?
IFN gamma
What induces Th2?
signal 3 = IL-4
What does Th2 secrete?
IL-4, IL-5 and IL-13
What induces Th17?
signal 3 = IL-6
What does Th17 secrete?
IL-17 and IL-22
What induces Treg?
signal 3 = transforming growth factor beta (TGF-beta)
What does Treg secrete?
IL-10 and TGF beta
Diagram of T helper subsets
What is CXCR5?
=chemokine receptor type 5
A chemokine receptor expressed by T follicular helper cells that migrate to B cell zone and enhance B cell activation
What is the signal for CD8+ T cells?
Signal 3 = IL-12 and Interferon alpha/beta
-induce them to differentiate into cytotoxic T lymphocytes
What do cytotoxic T lymphocytes (CTLs) secrete?
Inflammatory cytokines: Interferon gamma and cytotoxic granules
Summary
What is the role of Th1 cells?
Migrate out of lymph nodes to injury/sit of inflammation to enhance macrophage
-macrophages have MHC class II molecules where they can present antigens from their phagocytic activity to Th1 cells
1. Th1 cells engage the CD40L - CD40 (on macrophage) costimulatory pathway
2. secrete interferon gamma, which binds to IFGR on macrophages to induce activation
What are 5 things activated macrophages do?
- enhanced phagocytosis
- enhanced phagolysosomes (increased NADPH and NO)
- enhanced antigen presentation (increased MHC Class II)
- enhanced cytokine production (increased IL-1, IL-6, TNF)
- Intracellular (phagocytosed) pathogens
What is the function of Th2 cells?
- Promote anti-helminth (parasitic worms) responses
- promote formation of alternatively activated (M2) macrophages
What do the Th2 secretions do?
- IL-5 activates eosinophils and mast cells & induces their degranulation
- IL-4 can induce the macrophages to change its activation status from M1 to M2
- IL-13 can induce the macrophages to change its activation status from M1 to M2
What does an activated M2 macrophage do?
Secretes more immunosuppressive cytokines like IL-10
-IMP in wound healing and resolution of inflammatory responses
What is the function of Th17 cells?
- promote anti-helminth responses by mobilizing neutrophils and releasing IL-17
- promote formation of M2 macrophages
- Release IL-22 to increase mucins, defensins, lysozymes in mucosa-associated tissue
Where are Th17 cells abundant?
Mucosa-associated tissues
-secretions act in lumen of barriers to prevent colonization
What is the fxn of Tregs?
- Suppress APC (dendritic cells, macrophages) and Th cell activation, induce negative signals to reverse their activation via direct contact and cytokine release
- secrete immunosuppressive cytokines (TGF beta, IL-10)
What are 2 types of Tregs?
- those that arise during thymic selection and enforce immune tolerance to self-antigens
- those that are induced during T cell responses to foreign pathogens to prevent excessive tissue damage
Fxn of CTLs
=cytotoxic T lymphocytes
-mediate cytolysis of virus-infected target cells by forming cytoplasmic bridge w/ target
How do CTLs cause death of target cells?
- form cytoplasmic bridge w/ target
2a. secrete cytotoxic granules that contain perforin and granzyme B
3a. Perforin forms pores in membrane
4a. Granzyme B induces apoptosis of target (primary killing mechanism of CTLs)
2b. CTL expresses Fas Ligand, which binds to apoptotic receptor Fas on target
3b. Fas signaling induces apoptosis of target (alternative killing mechanism of CTLs)
What are the 4 stages of immunological memory?
- Priming and expansion - pathogen exposure -> increased T cell response
- contraction - effector T cell response decreases
- memory
- recall - secondary challenge -> rapid response
Summary
What are the 3 T cell activation signals?
- anti- gen recognition
- costimulation
- cytokine- mediated differentiation and expansion
Bacterial infection, which of the following lymphocyte subsets likely plays the most important role in limiting spread of this infection:
A. Cytotoxic T lymphocyte
B. T helper 2 cell
C. T helper 17 cell
D. Regulatory T cell
E. Natural killer cell
C. T helper 17
Cytotoxic T cells and NK cells attack viral infections most often
bacterial infection - does not enter cytosol
Th2 and Treg bigger role to regulate T response
Th2 - anti-helminth
Th17 - bacterial
NK cells - subset of lymphocytes, part of innate
Th1 would help activate macrophages
Tfh - to help activate antibodies on B cells
What immune cell might be imp to counteratact a superantigen in Staph infection?
Tregs
Superantigens promote t cells -> cytokine storm, septic shock
Tfh role
Enhance Ab production
