FUNCTION OF T CELL SURFACE GLYCOPROTEINS IN DEVELOPMENT AND RETROVIRAL ENTRY 
Dan R. Littman, M.D., Ph.D., Assistant Investigator 
The major interests of Dr. Littman's laboratory 
are 1) the mechanisms through which T cell surface 
molecules specify developmental pathways of anti- 
gen-specific T cells and 2) the cell surface receptors 
required for entry of human retroviruses into target 
cells. The studies with T lymphocytes are aimed at 
deciphering the functions of the CD4 and CDS gly- 
coproteins, cell surface molecules involved in T cell 
differentiation and T cell activation. Studies with 
retroviruses have focused on the molecular basis of 
the interaction between the envelope glycoprotein 
of the human immunodeficiency virus (HIV) and 
the viral receptor CD4. Ongoing studies are aimed 
at identifying a second cell surface molecule re- 
quired for HIV entry and the receptor for 
the human T cell leukemia viruses (HTLV-I and 
HTLV-II). 
I. Function of CD4 and CDS in Thymocyte 
Differentiation. 
The process of thymocyte differentiation yields 
mature T cells that are tolerant to self antigens yet 
react to foreign antigens complexed to host major 
histocompatibility complex (MHC) molecules. Dur- 
ing this process, clonally distributed T cell recep- 
tors (TCRs) interact with MHC molecules on spe- 
cialized cells within the thymus. These interactions 
result in elimination of clones with TCRs that react 
to self antigens complexed to host MHC molecules 
(thymic tolerance) and expansion of clones with 
TCRs specific for foreign antigens complexed to 
host MHC molecules (positive selection). In the 
course of clonal expansion, thymocytes that ex- 
press both the CD4 and CDS cell surface glycopro- 
teins (double positive cells) give rise to mature cells 
that express either one or the other molecule: the 
CD4''" T cells, primarily helper cells whose T cell 
antigen receptors are specific for class II MHC mol- 
ecules, and CDS^ cells, almost exclusively cytotoxic 
cells with receptors recognizing class I MHC mole- 
cules. There is mounting evidence that signals 
transmitted through the CD4 and CDS molecules 
are important in clonal deletion and expansion and 
in determining the developmental pathway of dou- 
ble positive thymocytes. Research in Dr. Littman's 
laboratory has focused on the ligand-binding prop- 
erties of the CD4 and CDS molecules and on the 
role of an intracellular lymphocyte-specific tyrosine 
kinase, p56''^*, in transducing signals received by 
CD4 and CDS. 
The CD4 and CDS molecules have been shown 
to bind directly to MHC-II and MHC-I molecules, 
respectively. In collaboration with Dr. Peter Parham 
(Stanford University), Dr. Littman's laboratory has 
shown that CDS binds to a membrane-proximal do- 
main of the MHC-I molecule. Because this domain 
is distinct from that recognized by the TCR, a single 
MHC molecule can crosslink CDS and the TCR, 
thus initiating a specific signal that may be impor- 
tant in development of the thymocyte. Point muta- 
tions within the membrane-proximal domain of 
MHC-I result in loss of binding of CDS but do not 
affect binding of peptides or recognition by the 
TCR. MHC-I molecules bearing such mutations 
have been introduced into transgenic mice, allow- 
ing analysis of the role of CD8-MHC binding in tol- 
erance induction and in selection of the TCR reper- 
toire. 
It has recently been demonstrated that both CD4 
and CDS are noncovalently associated with a 
cytoplasmic protein tyrosine kinase, p56''^^, a mem- 
ber of the src family of kinases. This interaction may 
be important for transmembrane signal transduc- 
tion in T cell activation, as well as in thymocyte dif- 
ferentiation. Recent work in Dr. Littman's labora- 
tory has shown that this association is dependent 
on cysteine residues within a sequence shared by 
the cytoplasmic domains of both CD4 and CDS 
and on a pair of cysteines in the amino-terminal do- 
main of the kinase. Binding is disrupted by divalent 
metal cation chelators, suggesting that a metal co- 
ordination complex is critical in the interaction. 
Functional studies with antigen-specific T cell 
hybridomas indicate that the interaction of CD4 
with p56''^* is required for effective activation of the 
T cell. To study the importance of the interaction of 
CD4 or CDS with p56'''* during development. Dr. 
Littman and his colleagues, in collaboration with 
Dr. Roger M. Perlmutter (HHMI, University of Wash- 
ington), have prepared transgenic mice expressing 
mutant CD4 molecules that do not bind the kinase. 
Development of thymocytes in these animals is 
being investigated. Since the absence of an intact 
endogenous CD4 or CDS gene would greatly facili- 
tate in vivo studies of mutant molecules. Dr. 
Littman's group is preparing mice that are defective 
in the expression of CD4 and CDS. The CD4 gene 
has been disrupted by homologous recombination 
in embryonal stem cells; these cells can now be in- 
jected into mouse blastocysts to yield chimeric ani- 
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