Protein-Tyrosine Phosphatases and the Control 
of Lymphocyte Activation 
Matthew L. Thomas, Ph.D. — Assistant Investigator 
Dr. Thomas is also Associate Professor of Pathology and Assistant Professor of Molecular Microbiology at 
Washington University School of Medicine, St. Louis. He received his Ph.D. degree from the University of 
Utah after completing his thesis research at Harvard Medical School. His postdoctoral training was done 
with Alan Williams at Oxford University and with Ian Trowbridge at the Salk Institute. Prior to his present 
appointment. Dr. Thomas was an Established Investigator of the American Heart Association. 
THE molecular mechanisms by which cells re- 
spond to their environment are a central 
theme in many areas of medical research. Our 
interests are in understanding how signals re- 
ceived by proteins of leukocyte surface mem- 
branes result in changes in a wide variety of cel- 
lular processes. In particular, we are interested in 
lymphocyte activation. 
Lymphocytes, by virtue of their ability to recog- 
nize an infinite variety of foreign antigens, play a 
key role in effecting and regulating an immune 
response. Upon binding antigen, lymphocytes 
undergo profound biochemical changes to en- 
gage the cellular machinery required for clonal 
expansion and to produce molecules needed to 
fight infection. A key regulatory mechanism by 
which many different cell types control the signal 
transduction process is phosphorylation and/or 
dephosphorylation of distinct protein-tyrosine 
residues of specific substrates. This process is 
controlled by families of enzymes that either add 
phosphate (protein-tyrosine kinases) or remove 
phosphate (protein-tyrosine phosphatases). The 
research interests of my laboratory are centered 
on how protein-tyrosine phosphatases control 
lymphocyte activation. 
The protein-tyrosine phosphatase family can 
be divided into two main branches: transmem- 
brane and intracellular. The leukocyte-common 
antigen CD45 is a major transmembrane protein- 
tyrosine phosphatase of lymphocytes and is ex- 
pressed by all nucleated cells of hematopoietic 
origin. To study the function of this molecule, we 
generated nontransformed T cell clones deficient 
in the expression of CD45. The deficiency re- 
sulted in the cells' inability to respond to antigen. 
CD45-deficient cells still respond to other prolif- 
erative signals, such as stimulation by growth fac- 
tors or plant lectin mitogens or direct activation 
of protein kinase C. However, these cells cannot 
proliferate or produce cytokines in response to T 
cell antigen receptor stimulus, and their ability to 
cytolyze target cells is also impaired. Therefore 
the protein-tyrosine phosphatase CD45 is re- 
quired for antigen-induced T cell activation. 
We are investigating the molecular mechanism 
of antigen-induced CD45-controlled activation 
by examining the differences in tyrosine phos- 
phorylation among the CD4 5 -expressing and 
-nonexpressing cell lines. We have observed in- 
creased tyrosine phosphorylation of members of 
the Src-tyrosine kinase family of proteins in the 
CD45-deficient cells, indicating that they are po- 
tential substrates for CD45. Three members of 
the Src family, p56'^\ p59'^, and ip62^^\ are ex- 
pressed by T cells, and recent studies have indi- 
cated that kinase activity for all three is decreased 
in the CD45-deficient T cells. Src family 
members are negatively regulated by phosphory- 
lation of a carboxyl-terminal tyrosine residue. 
CD45 may serve to dephosphorylate the car- 
boxyl-terminal tyrosine site and thus function to 
activate members of the Src family. The inability 
of antigen to activate CD45-deficient T cells im- 
plies that activation of Src family members is criti- 
cal to antigen-induced T cell activation and that 
CD45 is the phosphatase important in initiating 
this process. This work is supported in part by a 
grant from the National Institutes of Health. 
Immortalization of CD45-deficient T cells by 
fusion with a transformed thymoma cell line has 
permitted the development of a model system in 
which function can be reconstituted by cDNA 
transfection. We are currently analyzing regions 
of functional importance in the CD45 molecule 
by reconstituting the cells with altered CD45 
cDNAs. These experiments should allow precise 
definition of the regions of the molecule impor- 
tant in regulating T cell activation. 
To analyze further how protein-tyrosine phos- 
phatases control lymphocyte activation, we have 
isolated multiple cDNAs that encode protein- 
tyrosine phosphatases expressed by leukocytes. 
LRP is a transmembrane phosphatase whose exte- 
rior domain is predicted to be a highly glycosy- 
lated, elongated rod. This type of structure is also 
found at the amino terminus of CD45's exterior 
domain and may provide a novel means of regu- 
lating the phosphatase activity. To understand its 
function and regulation, we have developed a 
monoclonal antibody that recognizes the native 
protein. Use of this antibody has allowed us to 
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