TCR and T lymphocyte development in thymus

Students should know:Structure of TCRsCDRsDifferences of recognized antigen between TCR and immunoglobulins (Ig)TCR germ line configuration and rearrangementsTCR specificityClonality of T cellsDifferences between TCR a and b chains

T cell receptor gene rearrangement and lineage commitmentpreTCRPositive selectionNegative selectionChanges in CD4/CD8 expression during T cell maturation in thymus.The order and location of T cell selectionCell types involved in T cell selectionWhy is it important to match MHC molecules between donors and recipients for bone marrow transplantation (for donor-derived T cells to be functional in recipients)?

Figure 3-4If you dont not make antigen receptors (as in SCID patients),you can not make T and B cells and are susceptible even to opportunistic pathogens (e.g. C. albicans)Oral Thrush

T lymphocytesT cell origin (bone marrow) and events associated with maturation in the thymus (cytokines:IL1, 2, 3, 6, 7, GM-CSF; thymic hormones)

(1) Stem cells (multipotent) migrate to thymus and move from cortex to medulla while interacting with MHC Class II-bearing nurse, epithelial and interdigitating cells.

(2) Maturation progression:

(a) Early thymocyte (CD4-8-, T cell receptor (TCR) gene rearrangements)(b) Common thymocyte (CD4+8+, T cell receptor gene rearrangements; low TCR and CD3 surface expression)(c) Mature thymocyte (CD4+ or CD8+ subsets; high TCR and CD3 surface expression; somatic recombination of TCR genes)(d) Positive and negative selection occurs and most self-reactive T cells eliminated.(e) All T cells positive for TCR, CD2, 3, and 28CORE

T cell antigen receptor (genetics, structure, accessory proteins and signal transduction)

(1)Antigen-specific TCR dimers: ab (90-95% of all T cells) or gd; V, D, J, and C (constant) genes for b and g chains; V, J, and C genes for a and d chains.

(2)CD3 complex e2z2gd associated with TCR has a signal transduction role.

(3)TCR/CD3 overall stoichemistry: (ab)e2z2gdCORE

Similarity between TCR and IgBoth:Bind antigenHave variable regionConstant regionEach binding site is a heterodimer (composed of 2 different chains)

TCRs act only as receptorsIgs act as receptors and effector molecules (soluble antigen-binding molecules)

Figure 3-6TCR complexCD3 chains transmit signals

Figure 3-7

Origin, generation and differentiation of T cells T cell progenitors migrate from bone marrow and seed thymus. T cell progenitors undergo differentiation to CD4, CD8 and NKT cells in thymus. Mature CD4 and CD8 T cells circulate between blood and lymphoid tissues until they meet antigens presented on dendritic cells in lymphoid tissues. T cells further undergo maturation to become functional memory or effector T cells in LT

Figure 5-2Thymic involution: Human thymus is fully developed before birth and increases in size until puberty. It then progressively shrinks during adult life.

Most thymectized adults have no problem in T cell immunity because they have enough memory T cells in the periphery, and these T cells are long-lived.

Figure 5-3 part 1 of 2DifferentiationDN (CD4-CD8-) and DP (CD4+CD8+) Immature thymocytes are hereMore mature SP (CD4+CD8-or CD8+CD4-) thymocytes are here

Figure 5-3 part 2 of 2

TCR genes undergo DNA rearrangement in thymusUn-rearrangedUn-rearrangedrearrangedexpressionrearranged*No Ds in Va gene; DJ first then VDJ in b gene rearrangement

TCR gene rearrangement generates the TCR repertoirePre-TCR complex stops further gene rearrangement at b locus, and induces thymocyte proliferation

Finally TCR+ DP cells are made

Two chances for productive (=correct reading frame) rearrangement: b chainSuccessful rearrangement at one b copy blocks that at the other chromosome.

Multiple chances for productive (=correct reading frame) rearrangement in a chainSuccessful rearrangement at one a copy does not block that at the other.Therefore, many T cells express two different a chains.

Lineage commitment to a:b or g:d T cellsSuccessful gene rearrangement in g and d before b g:d T

Successful gene rearrangement in b before g or d pTa:b T (not committed yet). This signals to halt rearrangement of the b, g and d-chain genes and to enter a phase of proliferation.

Further rearrangement in a, g and d. Lineage commitment now depends on whether a functional a:b or g:d T-cell receptor is made first.More a:b T cells are made than g:d T cells

Figure 3-8 part 2 of 2

Most gd T cells do not express CD4 or CD8.

They are thought to be:First line of defense?Bridge between innate and adaptive responses?

Figure 3-8 part 2 of 2gd T cells recognize a limited set of unusual antigens:

Small microbial compound (E)-4-hydroxy-3-methyl-but-2-enyl pyrophosphate (HMB-PP, an essential metabolite in most pathogenic bacteria including Mycobacterium tuberculosis and malaria parasites, but is absent from the human host).

Host MHC class 1b: T10/22, MICA, MICB (structure similar to MHC I; expressed by transformed or stressed host cells)

Nonprotein alkylamines (derived from microbes and plants)

Bacterial products: Mycobacterial HSP (heat shock protein), superantigens (SEA)

Host heat shock proteinsDo not need antigen processing and presentation on MHC molecules.

CD8 binds MHC class ICD4 binds MHC class IIMost mature T cells are either CD4+ or CD8+.CD8 T cells kill cells infected with intracellular pathogens or tumor cellswhile CD4 T cells regulate (activate or suppress) other immune cells function (e.g.B cells and mac).

Figure 3-10The structures of CD4 and CD8

CD4+CD8+ DP cells: To be CD4 or CD8?Interaction of DP cells with Ag:MHC I CD8+ T cellsInteraction of DP cells with Ag:MHC II CD4+ T cellsThus, the antigen-specificity of TCRs determines the fate.

Positive selection (DP stage)

Negative selection (SP stage)To survive in thymus, T cells need to bind self MHC (but not too strongly).Self MHCs shape the TCR repertoire. Individuals with different MHCs will have different TCR repertoire. Most DP thymocytes dont survive to become SP cells.

Positive selection selects T cells that recognize peptides on self MHC

This is to assure that mature T cells can respond to antigen-presented on self MHC.

-Self MHC I and II harboring self peptides on thymic epithelial cells recognize and activate TCRs on some DP thymocytes.

-DP thymocytes should receive this signal within 3-4 days to survive.Otherwise they undergo apoptosis.

Negative selection eliminates T cells with TCRs that bind too strongly to self antigen/MHC complex. This is to assure that T cells dont react against self antigens. In other words, autoreactive cells are removed by this process. Dendritic cells and macrophages in cortico-medullary junction mediate it. Negative selection cannot eliminate T cells whose receptors are specific for self peptides that are expressed outside of thymus (These cells enter circulation, but soon to be rendered anergic or unresponsive by other mechanims).

Is this a positive or negative selection?Step 1: Selected people for the CBS show (=selected useful T cells by epithelial cells)

Is this a positive or negative selection?Step 2: Selected persons are eliminated(=eliminated harmful T cells by thymic dendritic cells)

The number of MHC molecules and size of T cell repertoireAs the number (N) of MHC molecules increases, the proportion of T cells that are positively selected (= # of the cells that survive) goes up arithmetically (N times), while that of negatively selected (= # of deleted cells) goes up geometrically (N2 times).

N= number of MHC isotypes a person expresses

Therefore, the magic N to result in maximum T cell repertoire is around 13.deletion rate by negative selection

Figure 5-10Bone marrow transplantation therapy in leukemia patientsWhat happens if there is a complete mismatch in MHC I/II TYPE?See the next slide.

Figure 5-11What you learned at school should work in the real world !

What happens if you do not have the thymus?

DiGeorges syndrome

No or few T cells due to very small or no thymus

Symptoms similar to SCID patients

Thymic involution: Thymus degenerates with age.If you are older than 60, your thymus is too small to produce T cells.

100%2%CD4 or CD8 TCRab T cellsTCRabSelections for T cells that are MHC-restricted and not self reactiveGeneration of nave T cells in thymusT cell progenitorsTCR gene rearrangementTCRgdnot selected by MHC I/II Blood

Generation of T cell clones: clonalityThymusTCRrecombinationGGABCABCAAACCCThymusSelection for The T cells with good TCRAg For TCR AAg For TCR BSecondary Lymphoid tissues.Ag-dependent expansion of clones.Stem cellsG: TCR in germ line configurationA, B, C: rearranged TCRs with different specificities

Figure 5-18Summary of T cell development in thymus

Figure 5-19Summary of T cell development in thymusGenes

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