Studies on T Lymphocytes and Mammalian Memory 
Susumu Tonegawa, Ph.D. — Investigator 
Dr. Tonegawa is also Professor of Biology at the Massachusetts Institute of Technology. He received a B.S. 
degree in chemistry from Kyoto University in Kyoto, Japan, and a Ph.D. degree in biology from the 
University of California, San Diego. His postgraduate training and research were at UCSD in the 
laboratory of Masaki Hayashi and at the Salk Institute with Renato Dulbecco. Dr. Tonegawa was a 
member of the Basel Institute for Immunology in Basel, Switzerland, before joining the Department of 
Biology and Center for Cancer Research at MIT. He was awarded the Nobel Prize for physiology or 
medicine in 1987. 
T lymphocytes play a pivotal role in the body's 
defense against a variety of infectious agents 
and malignant tumors as well as in the rejection 
of grafted foreign tissues. Our laboratory contin- 
ues to study the development and functions of T 
lymphocytes, with a particular focus on more re- 
cently discovered and less-characterized yb T 
cells. In addition, outwork on mammalian mem- 
ory has progressed significantly during the past 
year. 
Our study on yb T cells could be divided into 
three categories: development and selection, 
specificities, and function. As for the early devel- 
opment of these cells, one major issue that we 
continued to pursue is the mechanism by which 
the yb lineage segregates from the a.0 lineage 
from common progenitor cells. Following our 
earlier observations that differential activation of 
the transcriptional silencer associated with the y 
genes of the T cell receptor (TCR) plays a pivotal 
role in the cell lineage segregation, we character- 
ized the silencer DNA elements in detail and 
identified proteins that bind to these elements. 
We intend to clone the genes encoding the 
silencer-binding proteins and produce mice with 
mutations in these genes. Analysis of the mice 
should be highly informative in the dissection of 
the lineage segregation mechanism. 
a/? T cells are known to undergo in the thymus 
a critical maturation step called positive selec- 
tion. This depends on an appropriate interaction 
between the TCRs and a self determinant en- 
coded in part by the genes of the major histocom- 
patibility complex (MHC). Following our earlier 
study indicating that epithelium-associated yb T 
cells (called s-IEL and vut-IEL) undergo a similar 
positive selection, we have now obtained evi- 
dence of positive selection for circulating yb T 
cells. Thus the development of a transgenic yb T 
cell clone (KN6) specific for an MHC class I prod- 
uct (encoded by the T2^ gene) was shown to be 
blocked at an immature state in a mouse geneti- 
cally deficient in the expression of MHC class I 
gene products. 
We also studied T cell development by exploit- 
ing the embryonic stem (ES) cell gene-targeting 
method. We produced four types of mutant mice 
relevant for the analysis of T cell development 
and functions. The first and second types of mice 
have mutations in the TCRa and TCRjS genes, re- 
spectively, and are deficient in mature T cells 
but not in mature yb T cells. The third type of 
mouse has a TCR6 gene mutation and is deficient 
in yb T cells but not in afi T cells. The fourth type 
was produced by mutating the RAG-1 gene, 
whose product is required for somatic rearrange- 
ment of immunoglobulin and TCR genes. Neither 
mature T cells nor B cells are present in the 
RAG-1 mutant mice. 
The initial analysis of these mice indicated that 
there is no major interaction between the devel- 
opment of and yb T cells and that the muta- 
tions in the various genes block lymphocyte de- 
velopment at distinct stages. Detailed analysis of 
the immature lymphocytes accumulating in these 
mutant mice is expected to augment our under- 
standing of lymphocyte development. 
Specificities of yb T cells were studied by char- 
acterizing the self-reactive yb T cell subset 
(V4C1) on the one hand, and by testing our hy- 
pothesis that TL class I molecules have evolved to 
present peptides to yb TCRs. In the V4C1 subset 
study, we produced three types of monoclonal 
antibody (mAb), each of which blocks self- 
reactivity of these TCRs in vitro. These mAbs rec- 
ognize 1) a determinant specifically present on 
the 75 TCR utilized as the immunogen (called 
clonotypic), 2) determinants present on all 
V74C71 -expressing TCR, or 3) determinants pres- 
ent on the receptor for vitronectin, a family of 
cell adhesion molecules previously implicated as 
coreceptors for this T cell subset. These mAbs 
will be extremely useful in the elucidation of the 
ligand for the self-reactive yb T cell subset, as 
well as for the analysis of its development and 
tissue localization. 
We are studying the general peptide-present- 
ing role of TL class I molecules by producing 
mouse mutants lacking a cluster of these mole- 
cules. We have thus far identified ES cell clones 
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