Lecture # 13
Molecular Immunology
Prof. 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.
1
Lecture # 13
Molecular Immunology
Prof. Coscoy

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!
2
Lecture # 13
Molecular Immunology
Prof. Coscoy
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-topre 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.”
3
Lecture # 13
Molecular Immunology
Prof. Coscoy
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—V5 goes
to epidermis where they are called dendritic epidermal T cells. V6 goes to
reproductive tract epithelium. Very limited diversity, but antigenic target
unknown. Analogy with B-1 (CD5+) B cells.
4
Lecture # 13

Molecular Immunology
Prof. Coscoy
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.
5