Lecture 8 The Development of Lymphocytes

Core content

Students should know: T cell receptor gene rearrangement and lineage commitment preTCR Positive selection Negative selection Changes in surface phenotype 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?

100% 2% CD4 or CD8 TCR  T cells TCR  Selections for T cells that are MHC- restricted and not self reactive Generation of naïve T cells in thymus T cell progenitors TCR gene rearrangement TCR  not selected Blood

Generation of T cell clones: clonality Thymus TCR recombination G G A B C A B C A A A C C C Thymus Selection of T cells with Good TCR Ag For TCR A Ag For TCR B Secondary Lymphoid tissues Ag-dependent expansion of clones Stem cells G: TCR in germ line configuration A, B, C: rearranged TCRs with different specificities

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. Thymectized adults have no problem in T cell immunity because enough T cells are present in periphery, and these T cells are long-lived.

Figure 5-3 part 1 of 2 Differentiation

Figure 5-3 part 2 of 2

Lineage commitment to  or  T cells Successful gene rearrangement in  and  before    T Successful gene rearrangement in  before  or   pT  T (not committed yet). This signals to halt rearrangement of the  and  -chain genes and to enter a phase of proliferation. Further rearrangement in  and . Lineage commitment now depends on whether a functional  or  T-cell receptor is made first. More  T cells are made than  T cells

Figure 5-5

TCR gene rearrangement generates the TCR repertoire

Gene rearrangement at 

Gene rearrangement at  Pre-TCR complex stops further gene rearrangement at  locus, and induces thymocyte proliferation Finally DP cells are made

Two chances for productive (=correct reading frame) rearrangement:  chain

Multiple chances for productive (=correct reading frame) rearrangement:  chain Successful rearrangement at one  copy does not block at the other. Therefore, many T cells express two different  chains.

 T cells with different TCRs are generated at different time points during life time Epidermis homing Reproductive tract homing Intestinal epithelium or lymphoid tissues

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 other immune cells’ function to respond to pathogens

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

Positive selection Negative selection Both selections occur at DP stages To survive, 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 don’t 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. After positive selection, rearrangement at remaining  locus stops. 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).

Is this a positive or negative selection? Step 1: Selected people for the show by CBS (=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 APC)

Figure 5-15 part 1 of 2 In this example, the child has twice more positively selected TCR repertoire but 4 times more negatively deleted TCR repertoire The effects of pos. and neg. selections on TCR repertoire size

The number of MHC molecules changes selected T cell repertoire 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 (N 2 times). N= number of MHC isotypes a person expresses Therefore the magic N to result in maximum T cell repertoire is around 12.

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

Figure 5-11

Figure 5-18

Figure 5-19

Figure 5-14 Person A Person B White blood cells Mixed Lymphocyte Reaction (MLR) is used to test for HLA compatibility between individuals The higher the response The higher the mismatch

What happens if you do not have the thymus? It depends on age. DiGeorge’s syndrome No or few T cells Symptoms similar to SCID patients