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Chapter 5

• Understand how the thymus is the site of development for T cells

• How do cells commit to a lineage during T cell development

• How do we eliminate self-reactive T cells without eliminating the ability to recognize self-MHC

The Development

of T Lymphocytes

Two types of TCR

1. αβ T cells

CD4

CD8

2. γδ T cells

T cells develop in the thymusT cells (T-lymphocytes) = thymus-dependent lymphocytes

The thymus contains

The thymus is a primary lymphoid organ because it is only involved in development, not fighting infection.

a) Thymocytes (immature T cells) b) Thymic stroma (epithelial cells)

The Thymus

2 areas of the thymus:

Cortex – outer, close-packed consists of ectodermal cells; can contain thymocytes and macrophages

Medulla – inner, less dense consists of endodermal cells; contains thymocytes, dendritic cells, and macrophages

Thymic anlage : The combination of the ectodermal and endodermal cells, colonized by progenitor cells from the bone marrow.

Thymus facts

• Fully developed at birth and increases in size until puberty

• Most active in the young

• Degrades after puberty (involution), being replaced with fat tissue

• Even after involution (~30 yrs. old) or a thymectomy immunity by T cells is not impaired significantly

• Mature T cell repertoire is long-lived and self-renewing

T cell markersT Cell receptor – αβ or γδ chains bind peptide antigens

CD3 – complexes of CD3γε, CD3δε, and 2 ζ chains for signaling to the interior of the cell

CD4 or CD8 – receptors that bind MHC molecules

Progenitor cells entering the thymus have NO TCR, CD3, or CD4/CD8 surface receptors = immature thymocytes called double-negative thymocytes

After β chain rearrangement, T cells express both CD4 and CD8 surface receptors = immature thymocytes called double-positive thymocytes

Mature T cells express either CD4 or CD8 surface receptors = thymocytes called single-positive thymocytes

Order of TCR gene rearrangements controls the distribution of T cell lineages:

95% αβ T cells

5% γδ T cells

TCR gene rearrangement

T cell lineages

Pre T cell (pT) - a place-holder

αβ TCR rearrangement

• β chain– rearranges first– has variable (V), diversity (D), and joining (J) gene segments– Can attempt gene rearrangements on both chromosomes or by a

second rearrangement on the same chromosome• Tandem DJ and V segments• 80% of T cells make successful rearrangements

– Successful rearrangement leads to expression of CD4 and CD8

• α chain – rearranges second– has several variable (V) and joining (J) gene segments– Can undergo several gene rearrangements– Both α chain loci can rearrange leading to the potential for 2 different

α chains and 2 different TCRs on a single cell

Gene rearrangement . . . In pictures

β chain rearrangement

α chain rearrangement

γδ T cells

• Express rearranged TCR’s, generate immunologic memory and induce dendritic cell maturation

• Effector functions similar to T cells making them part of the Adaptive immune system

• HOWEVER, they have limitied TCR gene usage, TCRs act as pattern recognition receptors, and respond fast

• Might be a conserved, primitive form of immunity that bridges INNATE with ADAPTIVE

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Proposed scheme to use T cells for immunotherapy

T cell generation in mice

The first γδ T cells express receptors based on the first V segment nearest the D segments (epidermis- yellow). Later the majority of γδ T cells express receptors with the other V segment (reproductive tract - red). After birth the αβ T cells dominate.

Positive and Negative Selection of T Cells

Where: cortex, cortico-medullary junction

(thymus)

Who: double positive, : T cells

What is being selected: the TCR

Purpose•Positive Selection: Select TCRs that recognize self MHCs

-In periphery, T cells recognize foreign peptide that is presented by self MHCs

-T cells must recognize (bind to) self MHCs to be activated by foreign peptide

•Negative Selection: Eliminate TCRs that recognize self

-T cells recognizing self MHC containing self peptides are

potentially autoreactive

Positive Selection (PS)• TCR physically binds to MHC on APC

• Select T cells that can recognize self MHCs

• Primary thymic repertoire has bias toward general MHCs

-select those TCR that recognize inherited self MHCs

(6 MHC I and minimally 6 MHC II)

• Only 2 % of a given thymic repertoire can recognize self

MHCs; PS: process of stimulation those T cell to mature

POINT: Thymic T cells undergoing positive selection are pre-programmed to die unless they receive a signal to live and mature (98% DIE)

How are thymocytes (T cells in the thymus) positively selected?

• thymic cortical epithelial cells express MHC I and MHC II on their surface

MHC molecules not stable in absence of peptide; all contain self peptides

• : chains of TCR test all MHC complexes for ones they recognize (bind to)

-if bound with 3-4 days: signal to live; if not, DIE by default

-120,000 self peptides presented by 12 different MHC molecules; most involved

in positive selection

Bone Marrow Transplant: Share HLA Allotypes

•Bone marrow transplant: destroy

recipient bone marrow cells including

hematopoietic stem cells

•Reconstitute all blood cells including

lymphocytes with donor cells

•T cells selected on self (recipient)

MHC (thymus)

•APCs developing in bone marrow are

donor

• Recipient T cells can not recognize

donor APCs, unless donor and

recipient have some common (share) HLA (MHC) molecules

• Minimum: share one common MCH I molecule and one common MHC II molecule

Transition from Double Positive to Single Positive

•Choice of whether to become a CD4+ T cell

or CD8+ T cell is determined during

positive selection

•Single positive T cells: express either CD4

or CD8, but NOT both

•Developing T cell interacts with MHC I on

thymic stromal epithelial cells: CD8+

•Developing T cell interacts with MHC II on thymic epithelial cells: CD4+

•Bare lymphocyte syndromes

-patients lacking MHC I have only CD4+

cells

-patients lacking MHC II have only CD8+

cells

Chain Gene Rearrangement

During Positive Selection

• chains of the TCR can continue to rearrange to enhance chances of passing

positive selection (reminiscent of light chain rearrangement: BCR)

• Permits development of

single T cell with two

different chains

•Probability of both chains

passing positive selection

is extremely remote

•Result: only one functional

TCR per T cell

Negative SelectionSelect cells for elimination that express

TCRs that strongly recognize self MHC

presenting self peptides

Where: cortico-medullary junction

Who is selecting: macrophages and

dendritic cells

How: strong binding of the TCR sends

signal to DIE (apoptosis)

Two individuals of completely different haplotypes have non-overlapping T cell repertoires: the T cell repertoire is highly personalized.

Mature, Naïve T Cells Meet Ag in Secondary Lymphoid Tissue

•Only 1-2% of immature T cells survive selection in the thymus

•Surviving cells leave, enter the periphery and circulate through secondary

lymphoid tissue

•Naïve T cells are long lived and can circulate for years in the absence of

Ag (antigen)

•Meet Ag in the T cell area of the secondary lymphoid tissue

Ag stimulates naïve T cells Effector T cells

CD8+ Cytotoxic T cells CD4+ Helper T cells CD4+ Helper T cells

TH1 TH2

Thymus: Leave Leave Stay

•Twice as many CD4+ T cells in the circulation than CD8+ T cells

Alloreactive T Cells

•5-10% of T cell repertoire reacts strongly to allogenic

cells (non-self MHC)

•Allograft rejection (kidney)

•T cell repertoire biased to recognize MHC molecules in

general

•Elimination of those T cells that recognize self MHCs

(negative selection) leaves an increased proportion of

T cells recognizing non-self MHC (allogenic)

12 Different MHC Isotypes is Optimal

•Double MHC isotype number; double those passing

positive selection

•Double number of isotype number; geometrically

increase those deleted by negative selection

•Too many MHC isotypes will significantly limit the T

cell repertoire

•12 isotypes appear optimal

Note: Skip Fig. 5.15

Development of T Cell Tumors

T cell tumors represent different stages of T cell development (like B cell tumors)

T Cell Development in the Thymus

Stages of : T Cell Development