DEVELOPMENTAL BIOLOGY OF T LYMPHOCYTES 
Dennis Y. Loh, M.D., Associate Investigator 
Thymus-derived lymphocytes (T cells) possess 
three characteristic features. 1) Each T cell has a 
unique pair of cell surface receptors, the T cell re- 
ceptors (TCRs), by which antigens are recognized. 
2) Unlike the immunoglobulin molecule, the anti- 
gens can be recognized only in the context of the 
self MHC (major histocompatibility complex) prod- 
ucts, a phenomenon known as MHC restriction. 3) 
Self-reactive T cells must be eliminated or at least 
rendered nonfunctional if the organism is to avoid 
autoimmunity and self organ rejection. The devel- 
opmental processes by which a mature T cell arises 
are the focus of Dr. Loh's research. This includes 
the analysis of TCR gene rearrangements and ex- 
pression and subsequent cell fate determination as 
a result of TCR-MHC interaction in the thymus. 
L Positive-Selection Model of MHC Restriction. 
MHC restriction can be described as skewing of 
the peripheral T cell repertoire toward recognition 
of antigens in the context of self MHC molecules; 
this process has been shown to occur in the thy- 
mus during development. To analyze how skewing 
occurs, Dr. Loh introduced a pair of TCR genes 
from an allogeneic cytotoxic T cell clone (2C) into 
mouse embryos to create TCR transgenic mice. 
Clone 2C was originally derived from an H-2'' ani- 
mal, and its specificity is directed against an ele- 
ment in H-2''. In H-2'' transgenic mice, the majority 
of peripheral T cells expressed the 2C TCR, and 
its expression was predominantly restricted to the 
CD4"CD8''^ population. Surprisingly, when these 
mice were bred to H-2* or H-2'' backgrounds, the 
transgenic TCR-bearing cells failed to emerge in the 
periphery In Fl animals such as H-2'''" or H-2'"*, 
T cells bearing transgenic TCR were allowed to 
emerge into the periphery, eliminating clonal dele- 
tion as a mechanism for the lack of transgenic 
T cells in homozygous H-2* or H-2'' mice. Thus an 
element present in the H-2'' mice may be responsi- 
ble for positively selecting the 2C TCR in conjunc- 
tion with the CDS molecule in the thymus. 
Subsequently, using recombinant inbred mice, 
the positively selecting element in H-2'' has been 
mapped to the H-2K'' locus. To prove that the mo- 
lecular basis of positive selection depends on the 
K*" molecule itself, the TCR transgene-bearing mice 
were backcrossed to bml, a congenic mouse strain 
bearing a mutant K*' molecule. It has been shown 
that the three amino acid differences between K*' 
and K*""^ molecules is sufficient to abrogate positive 
selection. These data strongly suggest that a T cell 
bearing a surface TCR must interact productively 
with the MHC molecules displayed in the thymus 
before they are allowed to emerge as functionally 
and phenotypically mature T cells. This system will 
allow Dr. Loh to study T cell development as a 
function of molecular interactions between the 
TCR/CD8 and MHC in the thymus. Incidentally 
these data are directly relevant to transplantation 
rejection phenomenon, since they explain the 
origin of alloreactive T cells in terms of cross- 
reactivity resulting from positively selected self 
MHC-restricted T cells. 
II. Mechanism of Self-Tolerance to Antigens Known 
to be Present in the Thymus. 
To test the mechanism of self-tolerance develop- 
ment, the 2C transgenic mice were backcrossed to 
H-2''^'* and H-2'' backgrounds. In the periphery of 
these mice, functional T cells bearing the 2C TCR 
were deleted. Analysis of the thymocytes revealed 
that this elimination of the autoreactive T cells was 
taking place at or before the CD4^CD8^ stage in 
thymocyte development. These findings are consis- 
tent with the clonal deletion model of tolerance de- 
velopment against antigens known to be present in 
the thymus. In addition, immunocytochemical anal- 
ysis of the thymus undergoing clonal deletion re- 
vealed that the deletion was taking place in the 
thymic cortex. These data also suggest that a dis- 
tinct positive-selection step is not required for neg- 
ative selection to mediate clonal deletion, as exem- 
plified in the H-2'* mouse. 
III. Mechanism of SeLf-Tolerance to Antigens Whose 
Expression is Restricted to be Extrathymic. 
To examine the effects of aberrant expression of 
class II MHC proteins expressed exclusively in the 
periphery on tolerance development. Dr. Loh and 
his colleagues produced transgenic mice expressing 
the I-A'* genes under control of the pancreatic 
elastase promoter. Such transgenic mice express 
I-A"^ exclusively on exocrine pancreas, without ex- 
pression in thymus or by lymphocytes. No spon- 
taneous development of autoimmune reactivity 
toward exocrine pancreas was found in transgene- 
Continued 
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