III. Selection of TCRs In Vitro. 
Classical experiments in cellular immunology 
have long suggested that the thymus exerts consid- 
erable control over the specificity of T cells that are 
permitted to leave. In particular, it is believed that 
self-reactive T cells are eliminated (negative selec- 
tion) and that only those T cells that are somehow- 
compatible with one or the other self-MHC mole- 
cules are permitted to leave (positive selection). A 
very promising route toward examining these phe- 
nomena is the use of transgenic mice bearing TCR 
genes of known TCR specificity. These animals have 
large numbers of T cells bearing the appropriate re- 
ceptor, and thus the selective forces operating on 
those receptors are much easier to study. Lines of 
mice have been established that express two differ- 
ent TCR heterodimers, both of which should be 
subject to positive selection based on the I-E mole- 
cule and one of which should be negatively se- 
lected in I-A*-bearing mice. Results indicate that 
these phenomena are occurring and that expres- 
sion of the original restricting element (I-E) on 
thymic epithelial cells is all that is necessary for 
positive selection to occur. 
IV The XLR Locus and a New Methodology for Sub- 
tractive cDNA Cloning. 
Another area of interest involves the isolation 
and characterization of genes that might control 
differentiation in lymphocytes. One such gene is 
the XLR gene, which encodes a small (25 kDa) pre- 
viously unidentified nuclear protein, which is 
weakly homologous (—15%) to intermediate fila- 
PUBLICATIONS 
ment proteins and is specifically turned on in late- 
stage B cells and medium-to-later stage T cells. The 
XLR protein is stabilized in the nucleus by zinc 
ions. This phenomenon has not been described 
previously and seems characteristic of a number of 
other nuclear proteins as well. The XLR protein 
does not have a zinc-finger motif but may bind zinc 
directly in some other fashion. Alternatively, there 
may be some structure in the nucleus that is stabi- 
lized by zinc, which in turn binds XLR. In either 
case, the possibility exists that intracellular free 
zinc concentrations may regulate the nuclear local- 
ization of a novel class of polypeptides. 
A major effort is also under way to isolate other 
potential regulatory genes, using a subtractive hy- 
bridization and cloning scheme involving the vector 
XZAE This vector has the ability to express a cDNA 
insert as a single-stranded plasmid circle and, by se- 
lecting such circles directly by hybridization with 
the relevant RNAs and hydroxyapatite chromatogra- 
phy, species of interest can be enriched and charac- 
terized much more quickly than was previously 
possible. Dr. Davis and his colleagues are particu- 
larly interested in genes that are turned on late in 
B cell differentiation and also in those specific for 
early thymocytes. Nuclear localizing proteins in ei- 
ther category may provide important clues about 
the regulation of differentiation in these cells, clues 
that may be applicable to cellular differentiation in 
general. 
Dr. Davis is also Associate Professor of Microbiol- 
ogy and Immunology at the Stanford University 
School of Medicine. 
Articles 
Berg, L.J., Fazekas de St. Groth, B., Ivars, F., Goodnow, C.C., GilfiUan, S., Garchon, H.-J., and Davis, M.M. 
1988. Expression of T-cell receptor alpha-chain genes in transgenic mice. Mol Cell Biol 8:5459-5469. 
Berg, L.J., Fazekas de St. Groth, B., Pullen, A.M., and Davis, M.M. 1989- Phenotypic differences between aP 
versus (3 T-cell receptor transgenic mice undergoing negative selection. Nature 340:559-562. 
Davis, M.M. , Chien, Y-H., Bjorkman, PJ. , Elliott, J.F., Iwashima, M., Rock, E.P, and Patten, PA. 1989- A possi- 
ble basis for major histocompatibility complex-restricted T-cell recognition. Philos Trans R Soc Lond 
323:521-524. 
Garchon, H.-J., and Davis, M.M. 1989. The XLR gene product defines a novel set of proteins stabilized in the 
nucleus by zinc ions. / Cell Biol 108:779-787. 
Iwashima, M., Green, A., Davis, M.M., and Chien, Y-H. 1988. Variable region (Vg) gene segment most fre- 
quently utilized in adult thymocytes is 3' of the constant (Cg) region. Proc Natl Acad Sci USA 85:8161- 
8165. 
Continued 
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