Development and Function of T Cells 
known determinants expressed in fetal thymuses 
(called positive selection). While neither the 
role of s-IEL or vut-IEL or the nature of the select- 
ing determinants (called ligands) is known, such 
strong cellular selection of T cells with a particu- 
lar TCR structure (and hence specificity) sug- 
gests its pivotal role in the functioning of these T 
cells in the respective peripheral site. 
We previously showed that at least part of the 
determinant recognized by a spleen-type yd TCR 
(KN6 76 TCR) is encoded by a novel gene (722*") 
that maps in the major histocompatibility com- 
plex (MHC) and has structural similarity with the 
MHC class I genes (the products of which are 
known to present antigen-derived peptides to a/? 
TCR). This and related findings by others sug- 
gested that the products of a few dozen class I 
genes clustered in the so-called Tla subregion of 
MHC, whose function has been unknown, may 
generally serve as peptide-presenting molecules 
for 76 TCR. Our recent finding that the product of 
one of these class I genes, 73*, is expressed, ap- 
parently exclusively, on the surface of gut epithe- 
lia cells supports this hypothesis. (Note that a par- 
ticular 75 T cell subset referred to as i-IEL is 
localized in gut epithelium.) 
While we have accumulated a substantial 
amount of information with respect to develop- 
ment, diversity, and tissue localization of 7^ T 
cells, their role in the immune system remains 
elusive. To help shed light on this most funda- 
mental issue, we have been producing mutant 
mice deficient in a/3 or 76 T cells. We accomplish 
this by manipulating a TCR gene in a test tube, 
injecting it into the nuclei of a cultured embry- 
onic stem (ES) cell, returning the DNA-injected 
stem cell into an early embryo (a blastocyst), and 
finally injecting the embryo into a pseudopreg- 
nant mother. The chimeric newborn are tested by 
breeding them further for germline transmission 
of the engineered TCR gene, and the germline- 
transmitted mice are bred by sister-brother mat- 
ing to generate mice with homozygous mutation. 
(The entire procedure is called the ES cell gene- 
targeting technique.) 
To date we have succeeded in obtaining mice 
in which a mutated TCR 13, a, or 8 gene is germ- 
line transmitted. We intend to analyze the im- 
mune system of these mutant mice before and 
after various immunizations, after grafting with 
normal and tumor cells, and after infecting them 
with a variety of pathogens. We expect these stud- 
ies to help not only in identifying the roles of 76 T 
cells but also in redefining the role of some 
T cells. 
Finally, we have also been applying the ES cell 
gene-targeting technique to a calcium/calmodu- 
lin-dependent protein kinase. This kinase is 
highly enriched in the postsynaptic density of 
hippocampus and is suspected to be involved in 
establishing and maintaining long-term memory 
in mammals. Our goal is to determine whether 
and how this kinase is involved in memory. 
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