T cell differentiation

Question 1

components of a TCR

Answer 1

• antigen specific TCR = alpha-beta heterodimer
• associated CD4 or CD8 co-receptor
• part of CD3 complex

Question 2

what is the function of CD4 and CD8 co-receptors?

Answer 2

• required for TCR cell to recognize the peptide bound to MHC on an APC
• CD8 recognizes MHC I
• CD4 recognizes MHC II

Question 3

MHC restriction

Answer 3

• T cells are selected in the thymus for their ability to weakly bind self-MHC molecules
• Key factor in self/non-self discrimination: T cells will only recognize peptide bound to self MHC and MHC are the major proteins recognized as foreign in transplant rejection

Question 4

what are the stages of T cell maturation in the thymus and where do they occur?

Answer 4

• T cells begin as double negative
• In cortex: positive selection => double positive
• In medulla: negative selection => single positive

Question 5

thymocyte

Answer 5

early T cell precursor (CD3-CD4-CD8-)

Question 6

how is the double positive T cell generated?

Answer 6

in the cortex:
RAG1/RAG2 gene expression => VJ recombination on alpha chain and VDJ recombination on beta chain => T cell with unique TCR/CD3+/CD4+/CD8+

Question 7

what is the next stage in maturation for a double positive thymocyte?

Answer 7

positive selection in the cortex:

• cortical epithelial cells express MHC class I and II
• if a TCR binds to a self MHC, it survives (if it binds MHC II -> becomes CD4+ and if it binds MHC I -> becomes CD8+)

Question 8

what is the next stage in maturation for a single positive thymocyte?

Answer 8

negative selection in the medulla:

• cortical epithelial cells express MHC class I and II
• autoimmune regulator (AIRE) transcription factor induces medullary epithelial cells to synthesize self proteins to be expressed in the MHC molecules
• if a T cell binds self-antigen/MHC => death or Treg
• if a T cell doesn’t bind self-antigen/MHC => survival

Question 9

how is T cell self-reactivity managed in the periphery (3)

Answer 9

• cells that react with antigen in the absence of co-stimulation (aka self antigen) become non-responsive
• clonal deletion when T cells repeatedly encounter high levels of self-antigen in periphery
• T regs suppress self-reactive T cells

Question 10

CD4+ helper T cells

Answer 10

• recognize MHC class II (extracellular)
• secrete cytokines to support other cells
• differentiate into Th1, Th2, and Th17 subsets

Question 11

CD8+ cytotoxic T cells

Answer 11

• recognize MHC class I (intracellular)

- directly kill other cells

Question 12

Th1 CD4+ T cells

Answer 12

• secrete IFN-gamma which promotes and activates macrophages (intracellular pathogens)
• secretes IL-2 which promotes T cells
• differentiation driven by IL-12 and IFN-gamma
• involved in autoimmune and chronic inflammatory diseases

Question 13

Th2 CD4+ T cells

Answer 13

• secrete IL-4, IL-5, and IL-13 which promote IgE, IgG, and IgA class-switching and differentiation in B cells (helminth defense)
• secrete anti-inflammatory IL-10
• differentiation driven by IL-2 and IL-4
• involved in allergies

Question 14

Th17 CD4+ T cells

Answer 14

• produce IL-17 to promote neutrophils in response to extracellular pathogens/fungi
• enhance mucosal immunity
• differentiation driven by TGFB, IL-6, IL-1, IL-23
• involved in autoimmune and other chronic inflammatory diseases

Question 15

Tregs

Answer 15

• inhibit CD4+ and CD8+ T cell functions
• CD4+CD25+
• express FoxP3 (decreased cytokine transcription) and CTLA-4 (inhibitor of T cell B7:CD28 co-stimulation)
• secrete anti-inflammatory IL-10 and TGFB

Question 16

CD8+ T cells

Answer 16

direct cellular killing:

• release perforins which form membrane channels
• release granzymes which are cell membrane proteases
• activate caspases => apoptosis
• express FasL which binds to Fas on target cells => apoptosis

Question 17

two signals in T cell activation (general)

Answer 17

1. antigen recognition in the context of MHC by the TCR
   - ITAM phosphorylation => recruitment and activation of ZAP70 tyrosine kinase => signaling cascade
2. binding of activating co-stimulatory molecules or cytokines to CD28 or cytokine receptors

Question 18

two signals in CD4+ T cell activation

Answer 18

1. TCR recognizes antigen on MHC II

2. APC B7 binds T cell CD28

Question 19

two signals in CD8+ T cell activation

Answer 19

1. TCR recognizes antigen on MHC I

2. cytokines (IL-2) from activated CD4+ T cell

Question 20

T cell inhibitory molecules

Answer 20

CTLA-4: blocks binding of B7 to CD28 and inhibits IL-2 synthesis

PD1: expressed on T cells and interacts with PDL1 receptors on APC’s => inhibited immune response

Question 21

T cell superantigens (examples and mechanism)

Answer 21

directly bind TCR and MHC class II without internal processing => activation of many T cells at once regardless of antigen specificity => uncontrolled cytokine release (IL-2, IL-1, TNF)

Question 22

memory T cells

Answer 22

antigen specific T cells that have gone through a first primary response to antigen and on subsequent antigen recognition, activated with lower level of stimulation and a more rapid/robust response