Récepteur T pour l’antigène

B - Thymus  Thyméctomie néonatale (Miller – 1961) Site majeur de la lymphopoïèse T : - reçoit les progéniteurs T issus de la MO - fait la maturation (TCR spécifique) - coordonne le rélarguage des cellules T compétentes à la périphérie Cellules T matures vierges, avec TCR spécifique et non auto réactives Localisation Architecture et fonctions Trois compartiments distincts au plan phénotypique et de la maturation Zone sub-capsulaire : Arrivée des progéniteurs médullaires Localisation des premiers lymphoblastes Zone corticale : Lymphoblastes évoluant en petits lymphocytes Interactions avec épithélium corticale pour la sélection CMH dépendante Zone médullaire : Maturation terminale des lymphocytes Phase finale de la sélection

Organisation du thymus Zone sub-capsulaire Zone corticale Zone médullaire

Thymus enfant 2 ans Thymus adulte

Rôle fonctionnel du thymus Séléction positive et négative Production d’un répertoire de TCRs capable de reconnaître un nombre très important d’Ag étrangers, mais pas autologues Sélection positive : - Apartir du pool initial de TCRs, sélection des cellules exprimant des récepteurs reconnaissant le soi - Les peptides sont présentés en association à CMH I ou II par les cellules épithéliales corticales Sélection négative : - Elimination de ces cellules auto réactives qui, en cas de passage en périphérie, seraient responsables de phénomènes d’auto-immunité - Rôle des cellules réticulaires interdigitées dans ce phénomène

Similitudes TCR and Ig Both: Lient l’antigène Régions variables Région constante Chaque site de liaison est un hétérodimère TCRs sont des récepteurs Igs Récepteurs et solubles 8

Figure 3-6 Complexe TCR Chaines CD3 transmettent le signal 9

Figure 3-7 10

Origine génération et différenciation des lymphocytes T 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 11

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. 12

Figure 5-3 part 1 of 2 Differentiation DN (CD4-CD8-) and DP (CD4+CD8+) Immature thymocytes are here Differentiation More mature SP (CD4+CD8-or CD8+CD4-) thymocytes are here 13

Figure 5-3 part 2 of 2 14

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 15

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 17

Two chances for productive (=correct reading frame) rearrangement: b chain Successful rearrangement at one b copy blocks that at the other chromosome. 18

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. 19

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 20

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? 23

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. Figure 3-8 part 2 of 2 24

CD8 binds MHC class I CD4 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). 25

Figure 3-10 The structures of CD4 and CD8 26

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. 27

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. 28

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. 29

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 expressed outside of thymus (These cells enter circulation, but soon to be rendered anergic or unresponsive by other mechanims). 30

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? 32

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 negative selection? 33

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. 34

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. 35

Figure 5-11 What you learned at school should work in the real world ! 36

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. 37

Generation of naïve T cells in thymus T cell progenitors TCR gene rearrangement 100% TCRab TCRgd not selected by MHC I/II Selections for T cells that are MHC-restricted and not self reactive 2% Blood CD4 or CD8 TCRab T cells 38

Generation of T cell clones: clonality 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 Thymus Selection for The T cells with good TCR Secondary Lymphoid tissues. Ag-dependent expansion of clones. Thymus TCR recombination C Stem cells 39

Figure 5-18 Summary of T cell development in thymus 40

Figure 5-19 Summary of T cell development in thymus Genes 41