Students should know: Structure of TCRs CDRs Differences of recognized antigen between TCR and immunoglobulins (Ig) TCR germ line configuration and rearrangements TCR specificity Clonality of T cells Differences between TCR a and b chains T cell receptor gene rearrangement and lineage commitment preTCR Positive selection Negative selection Changes in CD4/CD8 expression during T cell maturation in thymus. The order and location of T cell selection Cell types involved in T cell selection Why it is important to match MHC molecules between donor and recipient during bone marrow transplantation for donor-derived T cells to be functional in recipient?
Figure 3-4 If you don’t not make antigen receptors (as in SCID patients), Oral Thrush you can not make T and B cells and are susceptible even to opportunistic pathogens (e.g. C. albicans)
T lymphocytes • T 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 28 CORE
CORE 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)e2z2gd
Similarity between TCR and Ig Both: • Bind antigen • Have variable region • Constant region • Each binding site is a heterodimer (composed of 2 different chains) TCRs act only as receptors Igs act as receptors and effector molecules (soluble antigen-binding molecules)
Figure 3-6 TCR complex CD3 chains transmit signals
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 5-2 Thymic 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 2 DN (CD4-CD8-) and DP (CD4+CD8+) Immature thymocytes are here Differentiation More mature SP (CD4+CD8-or CD8+CD4-) thymocytes are here
TCR genes undergo DNA rearrangement in thymus *No Ds in Va gene; DJ first then VDJ in b gene rearrangement Un-rearranged rearranged expression rearranged Un-rearranged
TCR gene rearrangement generates the TCR repertoire Pre-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 chain Successful rearrangement at one b copy blocks that at the other chromosome.
Multiple chances for productive (=correct reading frame) rearrangement in a chain Successful 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 cells • Successful 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 2 • gd 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 proteins • Do not need antigen processing and presentation on MHC molecules.
CD8 binds MHC class ICD4 binds MHC class II Most mature T cells are either CD4+ or CD8+. CD8 T cells kill cells infected with intracellular pathogens or tumor cells while CD4 T cells regulate (activate or suppress) other immune cells’ function (e.g.B cells and mac).
Figure 3-10 The structures of CD4 and CD8
CD4+CD8+ DP cells: To be CD4 or CD8? Interaction of DP cells with Ag:MHC I CD8+ T cells Interaction of DP cells with Ag:MHC II CD4+ T cells Thus, the antigen-specificity of TCRs determines the fate.
To survive in thymus, T cells need to bind self MHC (but not too strongly). Positive selection (DP stage) Negative selection (SP stage) Self MHCs shape the TCR repertoire. Individuals with different MHCs will have different TCR repertoire. Most DP thymocytes don’t survive to become SP cells.
Positive selection selects T cells that recognize peptides on self MHCThis 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 don’t 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 present outside of thymus (These cells enter circulation, but soon to be rendered anergic or unresponsive by other mechanims).
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) Is this a positive or negativeselection?
The number of MHC molecules and size of T cell repertoire deletion rate by negative selection As 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.
Figure 5-10 Bone marrow transplantation therapy in leukemia patients What happens if there is a complete mismatch in MHC I/II TYPE? See the next slide.
What happens if you do not have the thymus? DiGeorge’s 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.
Generation of naïve T cells in thymus T cell progenitors TCR gene rearrangement TCRab 100% TCRgd not selected by MHC I/II Selections for T cells that are MHC-restricted and not self reactive 2% Blood CD4 or CD8 TCRabT cells
Generation of T cell clones: clonality A G: TCR in germ line configuration A, B, C: rearranged TCRs with different specificities A A A Ag For TCR A A G G B B C C C Ag For TCR B C Secondary Lymphoid tissues. Ag-dependent expansion of clones. Thymus Selection for The T cells with good TCR Thymus TCR recombination C Stem cells
Figure 5-18 Summary of T cell development in thymus
Figure 5-19 Summary of T cell development in thymus Genes