FUNCTION OF T CELL SURFACE GLYCOPROTEINS IN DEVELOPMENT 
AND IN HIV PATHOGENESIS 
Dan R. Littman, M.D., Ph.D., Associate Investigator 
Research in Dr. Littman's laboratory has focused 
on understanding the molecular events involved in 
T lymphocyte differentiation and activation and in 
the processes through which human immunodefi- 
ciency virus (HIV) enters target cells and causes sys- 
temic depletion of helper T cells. Both areas of in- 
vestigation involve studies on the functions of T cell 
surface molecules and on their interactions with in- 
tracellular signal transducing components. 
Signal Transduction in Thymocyte 
Differentiation and T Cell Activation 
Mature T cells that emerge from the thymus are 
tolerant to self antigens yet react to foreign antigens 
complexed to host major histocompatibility com- 
plex (MHC) molecules. Selection of T cells bearing 
the appropriate T cell receptors (TCRs) involves de- 
letion of cells specific for self antigen and expan- 
sion (positive selection) of clones that react with 
host MHC molecules. 
In the course of development, precursor thymo- 
cytes that express both the CD4 and CDS cell sur- 
face glycoproteins (double-positive cells) give rise 
to mature CD4^ T helper cells specific for class II 
MHC molecules and CD8^ cytotoxic cells that recog- 
nize class I MHC molecules. The CD4 and CDS 
molecules have been shown to bind to membrane- 
proximal domains of class II and class I MHC mole- 
cules, respectively. It has been proposed that signals 
transmitted through the CD4 and CDS molecules, 
possibly via the associated cytoplasmic tyrosine ki- 
nase p56''^*, are important in clonal deletion, posi- 
tive selection, and specification of the developmen- 
tal pathways of double-positive cells. 
To study these problems, mice lacking expression 
of CD4 were generated by disruption of the CD4 
gene in an embryonal stem cell. Animals homozy- 
gous for the gene disruption have a dramatic reduc- 
tion in number and function of helper T cells. Re- 
constitution of these mice with a murine, wild-type 
CD4 transgene restores development of helper 
cells. Surprisingly, a mutant CD4 transgene whose 
product cannot associate with p56'''* also partially 
restores selection of helper cells. CD4 is therefore 
required for positive selection of helper cells, but 
this function does not require its association with 
p56'''*. Mice lacking expression of CD4 were also 
able to delete thymocytes reactive with self super- 
antigens, suggesting that CD4 signaling function 
may not always be essential for tolerance induction 
in the thymus. 
This result contrasts with earlier studies in Dr. 
Littman's laboratory indicating that corecognition 
of MHC by a class I-specific TCR and CDS was re- 
quired for deletion of thymocytes in the presence of 
self antigen. Taken together, these results indicate 
that the requirement for CD4 and CDS function is 
dependent on properties of individual TCRs. 
Dr. Littman's laboratory earlier showed that the 
interaction of CD4 and p56'''* is essential for the 
ability of T cell hybridomas to secrete interleukin-2 
upon stimulation with antigen. To study functions 
of individual domains of p56''"*, chimeric molecules 
containing the external and transmembrane do- 
mains of CD4 were fused to the p56'''* polypeptide 
and shown to reconstitute the response of the hy- 
bridoma to antigen. Other related cytoplasmic tyro- 
sine kinases, including p60''""''' and p59*", were also 
able to function in the context of CD4 fusion pro- 
teins, indicating that these molecules can all inter- 
act with TCR-associated signaling components to 
potentiate T cell activation. Most surprising was the 
finding that kinase-negative mutants of the CD4- 
Ick chimeric protein could also reconstitute the 
antigen-specific response in the T cell hybridoma. 
This result suggests that the CD4-associated p56'''* 
molecule has a distinct kinase-independent func- 
tion, possibly involving interaction with cytoskele- 
ton or recruitment of signaling components into the 
TCR complex. 
The developmental switch from double-positive 
to single-positive thymocytes involves the shut-off 
of either CD4 or CDS transcription. The signal for 
gene down-regulation could be either instructed 
(through the TCR) or stochastic. Results in Dr. Litt- 
man's laboratory support a stochastic mechanism 
for the loss of expression of CD4. Details of this 
mechanism are poorly understood, but a pan-T cell 
transcriptional enhancer has been identified up- 
stream from the CD4 promoter. It has recently been 
possible to demonstrate that, in transgenic mice, 
this enhancer directs appropriate developmentally 
regulated transcription from the CD4 promoter. 
Dr. Littman and his colleagues have used this sys- 
tem to introduce an appropriately regulated human 
CD4 transgene into mice and have shown that de- 
velopment and function of helper T cells in CD4- 
negative mice are restored by expression of the 
transgene. Efforts are under way to identify se- 
quences adjacent to the CD4 enhancer that control 
the shut-off of transgene transcription in CDS"^ cells. 
A gene ablation approach has also been used to 
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