Lecture # 13Molecular ImmunologyProf. Coscoy
T Cell Development and Selection, Part I
I. Origin of T cells and where they develop.
1) Hematopoietic stem cell in bone marrow.
2) Migrate to thymus. Structure of thymus. Components.
3) Involution of thymus with aging.
4) DiGeorge’s syndrome (human). Nude mice.
II. TCR gene rearrangement.
1). Four loci, , , , (but is contained completely within -- this has interesting implications).
2) Diversity via V(D)J recombination. RAG proteins, dsDNA repair proteins, scid mutation, etc. N-regions, P-nucleotides.
3) Distinctive features—TCR locus with ~ 50 J gene-segments; TCR locus with two D-J clusters.
4) rearrangement predominates in fetal thymus, especially on early days. becomes more frequent in late gestation, then throughout adult life.
III. Regulation of antigen receptor gene assembly.
1) Following T cell development by using surface markers.
2) Cells migrate to the thymus at the DN1 stage; all loci germline. Rearrangement of TCR locus begins in DN2/DN3 transition (RAG mutants arrest at DN3). Some DN cells (~20% in WT mouse) are T cells.
3) D-to-J rearrangement precedes V-to-DJ.
4) In-frame VDJ leads to formation of pre-TCR. pre-T (pT) is similar to 5 in pro/pre B cells. Pairs with chain.
5) Pre-TCR must get to cell surface. It then signals: striking cell division, cessation of chain rearrangement (allelic exclusion), followed by activation of TCR locus rearrangement. Also plays a role in vs developmental decision.
6) How does pre-TCR signal-- apparently without need for a ligand!
7) The role of the CD3 complex. There are CD3 components on the pro-T cell surface even in the absence of pre-TCR.
8) The role of protein tyrosine kinase Lck in pro-to-pre T transition. The mechanism of pre-TCR signaling.
-- note that a VDJ transgene promotes the DN to DP transition in RAG-/- thymocytes. In the absence of Lck, still see DP's but little proliferation. Lck transgene alone can promote DN to DP in RAG-/- background. Lck also signals TCR locus allelic exclusion in RAG+/+ thymocytes. “ selection.”
9) Influence of the cytokine Il-7. Phenotype of Il-7R chain knockout-- diminished numbers of thymocytes, and genetic interaction with pT knockout. Il-7 is necessary for survival and proliferation of early T cells.
10) cell development. Predominates in fetal life.
-- “programmed” rearrangements at different days gestation. Little or no TdT in fetal thymocytes, limited diversity of receptors.
-- cells home to distinct locations based on nature of receptor—V5 goes to epidermis where they are called dendritic epidermal T cells. V6 goes to reproductive tract epithelium. Very limited diversity, but antigenic target unknown. Analogy with B-1 (CD5+) B cells.
11) The vs. lineage decision.
-- instructive model--- if in-frame , then fate, if in frame then
-- stochastic model--- random fate determination, subsequent gene rearr.
data in favor of instructive model: preponderance of out-of frame VDJ in TCR+ cells; vice versa with VDJ in TCR+ cells.
Data in favor of stochastic model: IL-7R+ DN2 cells become cells upon IL-7 stimulation
IV. Selection.
1) Final stage of development is surface expression of complete TCR (in CD3 complex). DP cells then become either CD4+ class II MHC restricted or CD8+ class I MHC restricted single positive (SP) T cells. RAGs get shut off.
2) ~98% of all T cells never leave the thymus.
3) T cells must undergo both positive and negative selection to exit the thymus as mature T cells.
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