T Cell Surface Glycoproteins in Development and 
Viral Infections 
Dan R. Littman, M.D., Ph.D. — Assistant Investigator 
Dr. Littman is also Associate Professor of Microbiology and Immunology and of Biochemistry and Bio- 
physics at the University of California, San Francisco. Following undergraduate studies on the structure 
of microtubules in Marc Kirschner's laboratory at Princeton University, he completed his M.D./Ph.D. 
program at Washington University in St. Louis, working with Benjamin Schwartz and Susan Cullen on 
the function of histocompatibility molecules in antigen presentation. After a postdoctoral fellowship in 
Richard Axel's laboratory at Columbia University, where he isolated the genes for CD4 and CDS, he as- 
sumed his present position at the University of California, San Francisco. 
MATURATION of T lymphocytes involves the 
interaction of surface proteins on these 
cells with components of the thymic microen- 
vironment. Thymocyte precursors (immature 
thymus-generated cells) bearing appropriately 
rearranged and assembled T cell receptors are se- 
lected through this process of cell-cell interac- 
tion to mature and migrate to peripheral lym- 
phoid organs. Selection involves elimination of 
self-reactive T cells and preferential outgrowth of 
cells capable of reacting to foreign antigen com- 
plexed to major histocompatibility complex 
(MHC) molecules. 
Aside from the T cell receptors, two other cell 
surface glycoproteins, CD4 and CDS, play central 
roles in this selection process. Both CD4 and CDS 
molecules are expressed on immature thymo- 
cytes, but upon maturation the gene for one or 
the other is shut oflf. Cells that have receptors for 
class I MHC molecules continue to express CDS, 
but shut off CD4; cells with receptors for class II 
MHC molecules express CD4 and shut off CDS. 
Our laboratory is studying the mechanism of regu- 
lation of the CD4 and CDS genes as well as the 
molecular basis of the cell-cell interaction result- 
ing in the selection of cells having appropriate 
specificity. 
We have previously used a cell-cell adhesion 
assay to demonstrate that the CDS molecule binds 
to the membrane-proximal domain of the class I 
molecule. Since the T cell receptor binds to an- 
other region of this molecule, it is likely that the 
MHC molecule serves as a bridge to bring the re- 
ceptor complex and CDS into close contact. 
We have now tested the hypothesis that bind- 
ing of CDS to class I MHC during thymic differen- 
tiation is required for the maturation of class I- 
restricted T cells. Transgenic mice that express a 
mutant form of MHC-I that cannot bind CDS, but 
can interact with T cell receptors, were shown to 
be defective in developing a mature T cell reper- 
toire. Analysis of T cells in these mice provides 
evidence that CDS-MHC binding is required both 
for intrathymic deletion of self-reactive T cells 
and for positive selection of useful T cells. 
To study the roles of CD4 and CDS in greater 
detail, we are preparing animals that are defec- 
tive in the expression of these molecules. 
Namely, we have utilized gene-targeting technol- 
ogy to mutate the CD4 gene in embryonal stem 
cells and have injected these cells into mouse 
blastocysts, generating chimeric animals. We are 
attempting to propagate the mutant gene in the 
mouse germline, and the immune system of mice 
lacking CD4 expression will be studied. We have 
also prepared transgenic mice expressing mutant 
forms of CD4 that are predicted to be defective in 
signal-transducing functions, and these will be 
used to analyze CD4 function in the absence of 
endogenous CD4 expression. 
The CD4 and CDS glycoproteins have been 
shown to function importantly in the activation 
of peripheral T lymphocytes. For example, artifi- 
cial crosslinking of CD4 or CDS to the T cell re- 
ceptor complex results in T cell activation. More- 
over, T cells that have lost expression of either 
CD4 or CDS but continue to express the T cell 
receptor are usually no longer responsive to 
antigen. 
We are performing a variety of structure/func- 
tion studies to determine the mechanism through 
which CD4 and CDS facilitate signal transduction 
in T cells. It is known that CD4 and CDS are asso- 
ciated with a cytoplasmic tyrosine kinase, p56'''*, 
a member of the src kinase family. The sites of 
interaction between these molecules map to cys- 
teine-containing regions in the cytoplasmic 
domains of CD4 and CDS and in the unique 
amino-terminal domain of p56'''*. We have dem- 
onstrated that only CD4 molecules that can asso- 
ciate with the kinase are functional in antigen- 
specific T cell hybridomas, which normally 
produce interleukin-2 upon stimulation with the 
appropriate antigen. Since interaction of CD4 
with the Ick kinase is essential for T cell activa- 
tion, we are now beginning to characterize the 
proteins that serve as substrates for tyrosine 
phosphorylation . 
The CD4 glycoprotein is doubly important be- 
cause it is the receptor for the human immunode- 
ficiency virus (HIV) . We have completed a muta- 
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