T cell development and effector function 9/5

Question 1

T cell receptor (TCR)

Answer 1

• similar to BCR: consists of alpha chain and Beta chain, and contains hypervariable loops which contact the MHC molecule
• CDR3 is most hypervariable because it is contacting the peptide
• Has V(D)J recombination using RAG proteins: V region has hypervariable loops
• IN CONTRAST:
• membrane bound
• no class switching
• no affinity maturation (don’t want T cell to recognize self)
• Gamma/Delta T cells exist in epithelial compartments (comprise 5-10% of the T cell population)

Question 2

TCR-MHC Interaction

Answer 2

• TCR’s recognize as few as one to three residues of the MHC-peptide complex
• In comparison to antibody-antigen interactions, the binding of TCR to MHC molecules is weak
• Additional cell surface molecules are necessary for TCR activation

TCR must recognize the MHC and the peptide specifically

Question 3

Positive and Negative selection governs T cell developlment

Answer 3

Steps in selection: (only 1-3% will be selected)

1. Entry of progenitor cells (double negative) into the thymuc
2. Thymal interaction results in generation of CD4+CD8+ double-positive (DP) thymocytes at the outer cortex
3. As T cell moves from cortex –> medulla becomes single positive: Positive and negative selection of DP thymocytes is in the cortex by cortical epithelial cells
4. Interaction with medullary thymic epithelial cells (mTECs) to ensure central tolerance
5. Export of mature T cells from the thymus

Positive selection = cortical epithelial cells (if binds MHC it will survive)
Negative selection = dendritic cell, macrophage, and medullary epithelial cells: (if acts too strongly with self, it will die)

Question 4

Naive T cells in the peripheral lymph organs…

Answer 4

• Parafollicular cortex region of the lymph node contains T cells
• Lymphoid follicle region of the lymph node contains B cells
• T cells circulate in the blood and are activated in lymph organs (i.e. lymph nodes)
• Innate immune response aid in initiation of T cell activation through the generation of inflammation
• DC’s alone are capable of activating naïve T cells; B cells and macrophages activate memory T cells
• CD4 T cells are responsible for activating B cells and macrophages

T cells are activated in lymph node, can also be activated in the spleen. Important for people to get vaccinations, if they are asplenic.

Question 5

Molecules important for T cell activation: TCR complex

Answer 5

• HLA binding with TCR is important for Ag recognition, but this is not the only thing binding. This is not a very high affinity bond.
• There is the CD3 marker and CD4 on T cells which along with TCR forms the TCR complex (first signal)
• There is also CD28 which interacts with B7 (on APC) (second signal)
• adhesion molecules: LFA1 interacts with ICAM1 (on APC): important for activation of lymph nodes
• VLA-4 and VCam-1: important for adhesion and getting cells out of blood stream

Question 6

CD28

Answer 6

• interact with B7 molecules. CD28 provides the second signal for T cell activation Once T cells receive second signal it results in IL2 production and changed affinity of IL-2 receptor.
• after T cells have been on for a period of time we have CTLA-4 which starts to shut down IL-2 production. It outcompetes CD28

Question 7

Cross-presentation and Activation of CD8+ T cells

Answer 7

• The DC is infected with the microbe. The DC can then present using both HLA I and HLA II classes in order to stimulate both the CD4+ T cell and CD8+ T cell.
• the activated CD4+ cell will then release cytokines that will in turn further sitmulate the CD8+ T cell
• The stimulation of CD8+ cell along with costimulator factors will result in clonal expansion of effector CTL’s and differentiation of CD8+ T cells

Question 8

IL-2

Answer 8

• promotes survival, proliferation and differentiation of effector and regulatory T cells
• source: CD4+ and CD*+ T cells

Question 9

IL-4

Answer 9

• triggers B cell switching to IgE
• source: CD4+ cells, mast cells

Question 10

IL-5

Answer 10

• activation of eosinophils
• source: CD4+ cells and mast cells

Question 11

Interferon- gamma

Answer 11

• results in activation of macrophages
• source: CD4+ and CD8+ T cells, NK cells

Question 12

TGF-Beta

Answer 12

• inhibition of T cell activation; differentiation of regulatory T cells
• source: CD4+ regulatory T cells

Question 13

T cell Subsets

Answer 13

• Tp (T cell precursor in thymus) differentiates into different T cell subsets (SP) in periphery based on the cytokine environment present
• CD8+ = cytotoxic T cell: kills infected target cells, activates macrophages and releases perforin and granzymes
• CD4+: Th1, Th2, Th17, T reg
• Th1 cell: produces IFNgamma, promotes cellular activity, and activates macrophages
• Th2 cell: produces IL-4/5/13 and promotes humoral immunity
• Th17 cell: produces IL17 and is involved in host defense and pathogenesis of autoimmune diseases
• T regulatory Cell: suppresses T cell function

Question 14

TH1 Cells

Answer 14

• characterized by IFN-gamma secretion
• activates macrophages and enhanced microbial killing
• activates B cells: Stimulates complement binding on B cells and opsonizing, along with production of IgG Abs
• results in Class II HLA and B7 expression
• TH1 cells also secrete TNF-alpha

Question 15

TH2 cells

Answer 15

• mediate Phagocyte independent immunity
• secrete IL-4, IL-13, IL-5:
• IL-4: stimulates B cell to make antibody production and production of IgE
• IL-4/IL-13: result in intestinal mucus secretion and peristalsis
• IL-5: activates eosinophils to destroy helminith worms
• IL-4/IL13: alternatively activate macrophage (enhanced fibrosis/tissue repair)

Question 16

TH17 Cells

Answer 16

• Secrete IL17:
• IL17 acts on leukocytes and endothelial tissue cells
• IL17 is a chemoattractant for neutrophils and monocytes
• neutrophils and monocytes release TNF and IL-1 to stimulate inflammation
• functions to maintain epithelial barrier function, and implicated to play a role in maintatining gut tolerance

Question 17

How do T cells move to where they need to go?

Answer 17

• Migration through Lymph node:
• L selectin binds L selectin ligand on the HEV, allowing for adhesion of Naive T cell to HEV
• LFA-1 binds to ICAM-1 on HEV allowing for stable arrest on HEV
• L selectin must be downregulated after T cell activation in order for T cells to leave the lymph node and move to the periphery
• Migration through peripheral tissue:
• TNF-alphaand IL-1 are released which act on endothelial cells and result in expression of E and P selectin and ICAM-1 and VCAM-1
• E/P selectin ligand on activated T cells binds the E/P selectin on endothelial cells, allowing for intial weak adhesion
• LFA-1 binds ICAM1 on Endothelial cell
• VLA-4 binds VCA1 on EC allowing for stable arrest of cytokine at peripheral tissue

Question 18

Activation of CD4+ Th1 cells

Answer 18

• Naive CD4+ T cell binds the macrophage that has a microbe via the CD40L/CD40 interaction.
• This results in the release of IL-12 from the macrophage
• IL-12 acts on Naive CD4+ T cell to differentiate into a TH1 cell
• The TH1 cell then secretes IFNgamma, which results in the activation of the macrophage and the killing of microbes

Question 19

Macrophages Activation by Th1 cells

Answer 19

• this is dependent on Ag-recognitionon on macrophage (The HLAII presents the Ag to the TCR), and the CD40L/CD40 interaction occurs
• this results in IFNgamma being secreted by the Th1 cell which activates the macrophage

1. killing of phagocytosed microbes: release of internal ROS/NO in macrophage
2. increased expression of MHC molecules and costimulators (B7)
3. Secretion of cytokines (TNF, IL-1, chemokines, IL-12)–> inflammation/adaptive immunity

• this is the basis for delayed-type hypersensitivity (DTH) reaction - which occurs 24-48 hours after challenge
• i.e. PPD (purified protein derivative) skin test/tuberculin test
• intradermal injection of microbial antigen - immune system will respond to Ag if it has already seen it –> resulting in a raised red area due to activated macrophages –> shows that you have been exposed to TB

Question 20

Balance Between Th1/Th2

Answer 20

• The balance between Th1 and Th2 cell activation determines the outcome of intracellular functions
• the two act in opposition of eachother and are inhibitory
• TH2 under IL4/10/13 inhibits macrophage activity. If patients have a defective Th1, this Th2 will be upregulated which will alllow for bateria living in phagosomes to escape –> resulting in lepromatous leprosy (high bacterial count)

Question 21

Mechanism of Action for CD8+ T cells

Answer 21

• Ag recognition bound to HLAI via the TCR occurs in costimulation with CD8, along with the binding of ICAM1 with VCAM1
• this results in CTL activation and granule exocytosis: Perforin facilitates granzymes entry into cytosol, and granzymes activate caspases –> activation of apoptosis

Question 22

Summary Points: TCR and T cell development

Answer 22

• TCRs consist of an α chain and a β chain and resemble BCRs except that they are not secreted and do not undergo somatic hypermutation.
• The genes encoding TCRs consist of multiple segments that are separate in the germline and brought together during the maturation of receptors.
• In the thymus T cells undergo both negative and positive selection. Negative selection removes strongly self-reactive T cells while positive selected cells weakly recognize self-peptide-MHC molecules.
• TCRs are MHC-restricted meaning antigens are only recognized in the context of self-MHC molecules.

Question 23

Summary Points: T cell activation

Answer 23

• TCRs recognize peptide in the context of MHC molecules. Co-receptors (CD4, CD8) are necessary for activation.
• The binding of T cells to APC’s is enhanced by adhesion molecules and co-stimulatory molecules (B7) are necessary for T cell activation.
• CD4+ T cells can differentiate into 3 subset, TH1, TH2, and TH17, each of which have distinct function.
• Cell-mediated immunity (TH1 and CD8) is specific for intracellular pathogens.
• CD8+ T cells may require help from CD4+ T cells for activation and are activated through cross-presentation.

Question 24

Summary Points: Effector Functions of T cell subsets

Answer 24

• Cell-mediated immunity can be carried out by CD4+ T cells that activate macrophages or CD8+ T cells that directed kill the infected cell.
• T lymphocytes are activated in peripheral lymph organs then transverse to the site of infection where they are preferentially retained.
• TH1 cells secrete IFN-γ, resulting in classical activation of macrophages. The response is characteristic of DTH reactions.
• TH2 cells secrete IL-4, IL-5, and IL-13 and trigger alternative activation of macrophages.
• TH1 and TH2 responses are inhibitory of each other.
• TH17 cells secrete IL-17 and IL-22 and are important for neutrophil activation.
• CD8+ T cells or CTLs kill infected cells via granzyme-perforin pathways or Fas-FasL interactions.