T Cell Development Flashcards
What are the different cell types that HSCs can develop into in the thymus?
Thelper (CD4+), cytotoxic (CD8+), NKT, Treg
What pharyngeal pouch is the thymus derived from, and at what stage of gestation?
3rd, at the 4th week of gestation
At what week is the thymus populated by HSCs?
7th week
At what week does the thymus begin to produce T cells?
12-13th week, mature T cells egress at 13-14th week
Will a thymectomy of a newborn cause an immediate immune deficiency?
No, as by the 13th week of gestation T cells have egressed the thymus and migrated to the periphery
What is DiGeorge Syndrome?
Genetic absence of Thymus, and thus unable to produce T cells, resulting in a sever immunodeficiency
What gene is implicated in DiGeorge syndrome?
mutation in FOXN1 gene
What can help ameliorate DiGeorge syndrome?
Thymic implant into muscle
What cells are predominant in the thymic stroma?
thymic epithelial cells and fibroblasts
Where are fibroblasts found?
in the thymic capsule and septa
What are the three TECs and what are they derived from?
Derived from endoderm, named for location:
- Cortical TEC
- Medullary TEC
- Hassall’s TEC
Thymic epithelial cells provide three functions for the development of T cells. What are they?
Produce cytokines, such as IL1, IL6, IL7, SCF (stem cell factor), TSLP (thymic stroma lymphopoietin) required for the growth and differentiation of immature T cells
Express ligands DL4 and DL1 for the Notch receptor on progenitor cells - this signaling is required for T cell lineage commitment
Expression of self: MHCI: and MHCII:self antigen complexes that help in the selection of maturing T cells, and also the expression of peripheral Ag (eg insulin)
How do macrophages and dendritic cells come to be found in the thymus?
They mature from bone marrow and migrate to the thymus
Where are macrophages and DCs found in the thymus?
scattered throughout the cortex and medulla, but are highly concentrated at the cortico-medullary junction
What are the functions of macrophages and DCs in the thymus?
Ag presentation
Negative selection (deletion of autoreactive T cells)
Phagocytosis of apoptotic thymocytes
What are thymocytes derived from?
Progenitor cells in the bone marrow
What is the predominant lymphoid cell of the thymus?
Thymocytes!
What is a good way to determine thymocyte subsets?
Flow cytometry analysis
What are the four subsets that T cells can be divided into?
double negative, CD4+, CD8+, CD4+/CD8+ (double positive)
What do HSC progenitor cells from the bone marrow express, and what can they develop into?
CD34+, can develop into T cells, as well as B cells, NK cells, or dendritic cells
Once in the thymus, this cell population is restricted to the T cell lineage
T cell development declines as we age. What can be found in an adult thymus?
loss of cortico-medullar distinctions and presence of abundant adipose tissue
What are the 4 developmental events in T cells?
T lineage commitment: restriction of lineage choices
proliferation: expansion of committed cells
differentiation: gaining of new cell surface markers
Maturation: positive and negative selection and gaining of immune functions
What is required for T cell lineage commitment?
Notch signaling
Notch signaling terminates the potential to commit to B cell or myeloid (DC and macrophage) lineages. These cells can still become NK cells, however, continued Notch signaling terminates NK development
What is the ligand for Notch, and what occurs upon binding?
DL4 and DL1, which commits the cell to the T cell lineage
What occurs after Notch binding, and what are these cells called?
Cells commit to T cell lineage and begin to rearrange TCR gamma, delta, and beta genes.
At this stage they are Pre T-cells
What can Pre T cells develop into?
TCRgamma/delta T cells or TCRalpha/beta T cells
What is required for TCR gene rearrangement?
RAG1/RAG2
What cytokine is required at the pre T cell stage for rearrangement?
IL7
What happens after TCR gene rearrangement?
the cells become immature single positive (ISP) by expressing CD4 (CD4ISP)
What else do CD4ISPs express, and what does that expression induce?
CD4ISPs also express preT-alpha, which induces the expression of CD3
What forms the preTCR complex?
preT-alpha, together with CD3 and TCR-beta
What signaling does the preTCR participate in, and what does it do?
signaling via preTCR stimulates ERK which terminates the rearrangement of gamma/delta TCR genes
What is beta selection, and what does it do?
Beta selection allows for the selection of cells that have successfully rearranged their beta-TCR genes, via signaling by the preTCR
Cells that have produced a non-functional preTCR die by apoptosis
Cells that survive beta selection proliferate and expand; this step is responsible for the massive generation of thymocytes with alpha:beta TCR in the thymus
What do cells that have survived beta selection do next?
These ISPs begin to express CD8 and thus become CD4+/CD8+ double positive cells
What type of rearrangement will DP cells undertake
When ISPs start expressing CD8 and become DP cells, they start to rearrange TCR-alpha genes
How do DP cells select one CD to express?
They will receive stimulation either via CD4 or CD8 that will result in them down-regulating the non-stimulated co-receptor, thus becoming single positive cells.
What is positive selection?
In order to survive, the DP thymocytes with a functional alphabeta-TCR/CD3 complex, must recognize self antigen presented via MHCI and II presented by cortical TEC
In positive selection, what affinity must DP cells have in order to survive?
the interaction between TCR/CD3 complex and self-Ag/MHC must be of low affinity in order for the cells to be rescued from apoptosis
What happens to cells that survive positive selection?
They downregulate RAG and undergo 1-2 rounds of proliferation
What happens to DP Tcells that do not recognize self Ag?
They undergo apoptosis
What is a consequence of positive selection?
skews cell population towards self-Ag, increasing the chance of autoreactive T cells
What has BMT told us about the MHC that T cells utilize during development?
after a BMT, T cells from the patient recognize foreign Ag in the context of their own MHC, not the donor’s, suggesting that development of donor T cells occurs on host TEC
What is negative selection, and what does it result in?
The deletion of mature T cells that react strongly to self MHC:antigen (autoreactive T cells)
This results in central tolerance
Where does negative selection predominantly occur?
The cortico-medullary junction
The cortico-medullar junction has a large number of thymic DCs, which are also responsible for the phagocytosis of apoptotic cells
How do TEC participate in the deletion of cells that are reactive to organ specific Ag?
the AIRE complex (Auto Immune Regulator Element) is a gene that produces a large number of peripheral-tissue Ag
AIRE is expressed by thymic medullary epithelial cells
What do defects in AIRE cause?
autoimmune polyendocrinopathy candidiasis ectodermal dystrophy (APECED) or Autoimmune polyendocrinopathy syndrome type 1 (APS1)
Affected tissues: adrenal, thyroid, parathyroid, and pancreas.
Where do mature SP cells predominantly reside?
Thymic medulla
Describe some features of delta-gamma T cells, and the two major types
1-5% of thymic T cell population
CD4 and CD8 negative
bind Ag directly (don’t need MHC)
TCRgamma9delta2: majority, and circulating
TCRgammadelta1: first cells to emerge from fetal thymus, populate skin and intestine, recognize Ag presented by CD1B or CD1C
What are gamma/delta T cell functions?
delta1: lyse stressed/transformed epithelial cells
gamma9delta2: recognize non-peptide Ag
Discuss the development of NKT cells
develop in the thymus from DP cells
DP cells that recognize glycolipid presented by CD1D+ cortical thymocytes will develop into NK cells
NKT are either CD4+ or DN
NKT express both CD56 and alphabeta-TCR/CD3 complex
NKT populate the lymph nodes, bone marrow, liver, and spleen
What is the function of NK cells?
produce both Th1 (IFN-gamma, IL2) and Th2 (IL4, IL13, IL10) cytokines
What are Tregs?
CD4/CD25 positive suppressor cells that are specific for autoreactive T cells (dominant tolerance)
Discuss T reg development
develop in the thymus from DP thymocytes
Require TSLP (thymic stroma lymphopoietin) expressed by TEC of Hassall’s body
expression of FOXP3 is essential
Tregs can also be induced in the periphery from mature CD4+ T cells by TGF-beta –> these can either be Tr1 or Th2 cells
Treg in human disease –>
Immune dysregulation, polyendocrinopathy, enteropathy, and X-linked inheritance (IPEX) is a clinical syndrome that presents with multisystem autoimmune disease. Clinically, patients with IPEX manifests most commonly with diarrhea, insulin-dependent diabetes mellitus, thyroid disorders, and eczema. FOXP3, the gene responsible for IPEX, maps to chromosome Xp11.23- Xq13.3 and encodes a transcription factor. Because patients with IPEX lack Treg, it was determined that expression of FOXP3 in CD4Pos T cells is required for the development of Treg in the thymus.
Where do HSCs come from before vs after birth?
before: yolk sac and liver
after: bone marrow
What cell surface markers are present at the different stages of T cell development?
Progenitor: CD34+
T/NK: CD34+, CD7+
Pre-T: CD1a+
ISP: CD1a+, CD3+ CD4+
DP: CD1a+, CD4+, CD8+
SP: CD8+ OR CD4+
Is there allelic exclusion in the TCR-Valpha gene?
No! Probably there are two pre-TCRs that are made up of a V beta chain and the two alleles of the V alpha chain. positive selection will ensure functional specificity
How do TCRs signal intracellularly?
Zeta chain signaling
What is Bare Lymphocyte Syndrome (BLS)?
development of CD4+ T cells is affected - defect in MHCII expression
What cell types induce negative selection?
Bone marrow derived macrophages and dendritic cells, but also some thymic epithelial cells (important for BMT)
