T Cell Surface Glycoproteins in Development and Viral Infections 
with a cytoplasmic tyrosine kinase, p56'''*, a 
member of the src kinase family. The sites of 
interaction between these molecules map to 
cysteine-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 activation, we are now begin- 
ning to characterize the proteins that serve as 
substrates for tyrosine phosphorylation. One of 
the substrates appears to be a tyrosine kinase as- 
sociated with the T cell receptor complex. Cross- 
linking of p56'''* to the receptor-associated kinase 
results in tyrosine phosphorylation and activation 
of phospholipase-C7 1 , an enzyme that is also as- 
sociated with the T cell receptor complex. Cleav- 
age of phosphoinositol-containing phospholipid 
by this activated enzyme generates second mes- 
sengers that activate the transcriptional machin- 
ery in the T cell. 
The laboratory is also studying the mechanism 
involved in the developmental switch from 
double-positive (CD4"^CD8'^) to single-positive 
thymocytes. The switch may be due to differen- 
tial signaling through CD4 or CDS (hence due to 
instruction) or to random (stochastic) shutoff of 
either CD4 or CDS, followed by selection of cells 
that have the appropriate T cell receptor and co- 
receptor. Several experiments are being per- 
formed to discriminate between these two basic 
mechanisms of differentiation. 
An understanding of the mechanism involved 
should also shed light on the transcriptional regu- 
lation that thymocytes employ, first to turn on 
both the CD4 and CDS genes and subsequently to 
shut off only one of the genes. We have character- 
ized a sequence upstream from the CD4 pro- 
moter with T cell-specific transcriptional en- 
hancing activity. The ability of this and other 
sequences to provide subset-specific regulatory 
signals in thymocyte differentiation is being stud- 
ied in transgenic mice. 
The CD4 glycoprotein is doubly important be- 
cause it is the receptor for the human immunode- 
ficiency virus (HIV) . We have found that several 
cell types that express CD4 can bind virus but 
cannot be infected, indicating that host factors 
other than CD4 are involved in viral entry. Resis- 
tance of these cells to infection is due to the in- 
ability of the viral envelope to fuse to the cellular 
plasma membrane. It is therefore likely that 
plasma membrane molecules other than CD4 are 
required for fusion of virus to target cells. We are 
using genetic and biochemical approaches to 
identify such molecules. 
Identification of additional molecules in- 
volved in HIV entry may permit design of novel 
agents to interfere with the spread of HIV. In ad- 
dition, we hope that this information will facili- 
tate the design of a mouse model system for HIV 
disease. The currently available mice that express 
the human CD4 transgene are resistant to infec- 
tion, but expression of additional genes involved 
in HIV entry may permit infection of these ani- 
mals with HIV. 
Using a genetic system for studying HIV entry, 
we have shown that the envelope glycoprotein of 
HIV can be replaced by that of another patho- 
genic human retrovirus, human T cell leukemia 
virus (HTLV), forming HIV(HTLV) pseudotypes. 
HTLV causes T cell leukemias and lymphomas 
and myelopathies. Individuals infected with both 
HIV and HTLV have more rapid progression of 
HIV disease than those infected with HIV alone. 
Since both viruses infect T lymphocytes, it is 
likely that mixed viral particles can form in vivo. 
We have demonstrated that mixed particles 
(pseudotypes) that form in vitro have an ex- 
panded host range — i.e., HIV particles, endowed 
with the HTLV-I envelope glycoprotein, can 
readily infect CD4-deficient cells. Such mixed 
particles may have an important role in HIV 
pathogenesis, particularly in infection of cells 
that do not have HIV receptors, such as cells of 
the central nervous system. In addition to investi- 
gating HIV infection, we are using the hybrid par- 
ticles to study the yet uncharacterized HTLV 
receptor. 
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