T Cell Development Flashcards
What is the goal of T cell development?
generate a mature pool of T lymphocytes, each w a single specificity, that recognises foreign but not self antigen when complexed w self MHC molecules
– self tolerant and self restricted
challenge
– Ag recognition occurs via a specific TCR; generated by random rearrangement of segmented genes
– not all arrangements are productive, many of the TCRs don’t recognise self MHC plus peptide and some will be self-reactive
What evidence shows that T cell development occurs in the thymus?
Nude mice lack a functional thymus due to a mutation in the Foxn1 gene, a TF which is essential for thymus development
this results in the absence of mature T cells, and the mice are severely immunocompromised
– able to accept skin graft from other mice, and even rats
– no cell mediated immunity, no Ab response to most Ags
What are the components of the thymus?
defined by histology: cortex and medulla
stroma composed of BM derived and non-BM derived elements
epithelial component of stroma provides specialist functions (cTEC and mTEC)
– cortical and medullary thymic epithelial cells
macrophages clear dead cells
hassall’s corpuscles: secrete TSLP - activated thymic DCs which induce and generate Tregs
How does colonisation of the thymus occur?
there is no self-renewing intrathymic lymphocyte population under normal conditions
– colonisation thought to occur in waves
- vasculature-independent (fetal thymus prior to establishment of vascular network)
– migration through connective tissue, mediated by chemokines eg CCL21 and 25 - vasculature dependent (late fetal and postnatal thymus)
– entry at cortico-medullary junction, mediated by chemokine receptors eg CCR7 (CCL19 and 21) and CCR9 (CCL25)
– requires seeding cells to adhere to P-selectin glycoprotein (PSGL1), V-CAM, and I-CAM on thymic endothelial cells
What are the seeding cells?
the progenitor cells that migrate from the bone marrow to the thymus to initiate the development of T cells
phenotype of earliest intrathymic progenitors, early thymocyte progenitors (ETP)
- cKit+ within DN1 population
- Flt3+ cells within ETP gate shown to be canonical T cell progenitor
- single Flt3+cKit+ ETP can generate T, B, GM, and macrophages in in vitro assays
- EPT most closely related to lymphoid-primed multipotent precursor (LMPP)
Outline the commitment in T cell development.
progress down T cell pathway begins when DN1 (CD44+25-) cells make a transition to DN2 stage (CD44+25+)
– DN2 can still make macrophages, NK cell, DC in vitro
non-T cell fate choices lost by DN3
How does Notch regulate gene expression?
family of TM receptors (1-4)
5 ligands (mammals)
– delta-like 1, 3, 4
– jagged 1, 2
upon ligand binding, intracellular portion of Notch, ICN, is proteolytically cleaved and translocated to nucleus -> interacts w CSL (RBPJk) which binds to co-factors to induce gene transcription
What experiments show that Notch is critical in T cell development?
creation of Notch I knockout using Cre-loxP system
– cross w MxCre mice; Cre expression induces in response to IFNa
conditional deletion of Notch I abrogates intrathymic T cell development
– specific block in T cell development at DN1 stage
– all cells end up becoming B cells as shown by B220 expression
What cells produce ligand for Notch1, and what evidence shows this?
delta-like 4 and delta-like 1 produced by cTECs and mTECs
FoxN1 deletion in only TECs
- again, progenitors become B cells as they cannot enter T cell development
How do Notch, GATA3, and TCF7, induce commitment to T cells?
TFs GATA3 and TCF1, both required for T cell development
GATA3 represses B cell lineage potential in ETPs
TCF7 mainly acts as a positive regulator of T cell specification
Notch, GATA3, and TCF7 upregulate expression BCL11B
– allows full commitment to T cell lineage as it is expressed during DN2 (specifically DN2b)
What is beta-selection?
a checkpoint during T cell development at DN3
expression of surrogate light chain and formed heavy chain (pre-TCR) on cell surface signals through CD3 and rescues cells from apoptosis and initiates further development
If preTCR binds p:MHC, proliferation of DN3 is enhanced
Along with Notch, and the pre-TCR, what other receptor is required at the beta-selection checkpoint?
CXCR4 binding SDF-1α (aka CXCL12)
causes downstream signalling which upregulates BcI-2A1, inhibits caspase 3-mediated apoptosis
What happens after beta-selection (formation and signalling through pre-TCR)?
failure to pass beta-selection (or make gamma-delta receptor) causes death by apoptosis
after b-selection:
– lose pre-TCR
–DN4 cells undergo a burst of proliferation, then upregulate CD8 and CD4 = DP
DP then enter quiescent phase
– Rag genes reactivated
– Valpha genes begin to rearrange
TCR alpha loci undergo successive rearrangements until cell is positively selected or die
What are two models of TCR receptor lineage commitment?
Stochastic selection
- cells pre-committed to alpha-beta or gamma-delta before TCR rearrangement
- cells that develop wrong receptor die
signal strength
- ligand engagement of the gd-TCR elicits strong signals that result in commitment to this lineage
- transduction of weak signals can result in commitment to ab-TCR lineage and the generation of DP thymocytes
– this is the favoured model
How does the thymic stroma regulate thymocyte development?
- early stages take place in the cortex
- thymocytes at different stages are found in different intracortical organs
- outward migration can be partly mediated by CXCL12/CXCR4 and CCL19/21 - CCR7
- but still not understood
