T Cell Surface Glycoproteins in Development 
and Viral Infections 
Dan R. Littman, M.D., Ph.D. — Associate Investigator 
Dr. Littman is also Associate Professor of Microbiology and Immunology and of Biochemistry and 
Biophysics 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. His postdoctoral research 
was done in Richard Axel's laboratory at Columbia University, where he isolated the genes for CD4 
and CDS. 
THE shaping of a mature repertoire of T lym- 
phocytes capable of responding to patho- 
genic microorganisms involves a complex pro- 
cess of differentiation within the thymus. In this 
process, self-reactive cells are eliminated and 
cells that can react to foreign antigen complexed 
to molecules of the major histocompatibility 
complex (MHC) are selected to survive and mi- 
grate to peripheral lymphoid organs. 
The signals involved in the different phases of 
thymic selection require the interaction of sev- 
eral thymocyte surface proteins with components 
of the thymic microenvironment. These cell sur- 
face molecules include the clonally restricted T 
cell receptors and the CD4 and CDS glycopro- 
teins. Both CD A and CDS are expressed on imma- 
ture thymocytes, but the gene for one or the other 
is shut off upon maturation. Cells that have re- 
ceptors for self-MHC class I molecules continue 
to express CDS, but shut off CD4; cells v^^ith re- 
ceptors for MHC class II molecules express CD4 
and shut off CDS. Our laboratory is studying the 
mechanism of regulation of the CD4 and CDS 
genes as well as the molecular basis of the cell- 
cell interaction resulting in the selection of cells 
that have appropriate specificity. 
The CD4 and CDS molecules interact directly 
with MHC class II and class I molecules, respec- 
tively. Effective T cell activation requires co- 
recognition of MHC by both the T cell receptor 
and either CD4 or CDS. To test whether corecog- 
nition is also required in transmembrane signal- 
ing during T cell development, we have analyzed 
transgenic mice that express a mutant form of 
MHC class I incapable of binding to CDS but in- 
teractive with T cell receptors. These mice were 
shown to be defective in developing a mature T 
cell repertoire specific for the mutant MHC class 
I molecule. 
This study indicates that CDS:MHC binding is 
required for both intrathymic deletion of self- 
reactive T cells and positive selection of useful T 
cells. Moreover, since endogenous MHC class I 
molecules were competent to bind CDS but un- 
able to rescue the defect, coordinate recognition 
of antigen/MHC by a complex of the T cell recep- 
tor and CDS is required for both positive and neg- 
ative selection. 
To study the roles of CD4 and CDS in greater 
detail, we are preparing animals defective in the 
expression of these molecules. We have utilized 
gene-targeting technology to mutate the CD4 
gene in embryonal stem cells. These have been 
injected into mouse blastocysts, resulting in the 
birth of chimeric animals. The mutant CD4 gene 
has been propagated in the mouse germline, and 
we are currently studying the immune system of 
mice lacking CD4 expression. These mice lack 
helper T cells and have essentially no antibody 
response to T-dependent antigens and no T cell 
response to antigen presented by MHC class II 
molecules. 
Transgenic mice expressing mutant forms of 
CD4, predicted to be defective in signal- 
transducing functions (as a result of defective in- 
teraction with cytoplasmic tyrosine kinases or 
with the T cell receptor complex), have been 
prepared and are being used to analyze the role of 
CD4 signaling during development in the ab- 
sence of endogenous CD4 expression. We have 
also prepared transgenic mice that express the 
human CD4 gene and will examine the ability of 
the human molecule to function during T cell 
development in mice that lack murine CD4. Such 
animals may be especially useful for studies of 
pathogenesis and therapy in autoimmune dis- 
eases and AIDS. 
The CD4 and CDS glycoproteins have been 
shown to be involved in the activation of periph- 
eral T lymphocytes. For example;, artificial cross- 
linking of CD4 or CDS to the T cell receptor com- 
plex results in T cell activation. Moreover, 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/function stud- 
ies to determine the mechanism through which 
CD4 and CDS facilitate signal transduction in T 
cells. 
It is known that CD4 and CDS are associated 
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