SIGNAL TRANSDUCTION IN HEMATOPOIETIC CELLS 
Roger M. Perlmutter, M.D. Ph.D., Investigator 
Although substantial progress has been made in 
defining the cell surface molecules responsible for 
specific recognition of antigens by B and T lympho- 
cytes, the signal transduction pathways that couple 
ligand binding by these receptors to changes in cell 
behavior remain largely enigmatic. Members of Dr. 
Perlmutter's laboratory have employed molecular 
genetic strategies to identify important constituents 
of these signaling pathways and to manipulate lym- 
phocyte signaling experimentally in vivo through 
the generation of transgenic mice. 
Gj Proteins Regulate Lymphocyte Migration 
Guanine nucleotide-binding (G) proteins are 
components of many signal transduction pathways 
in nonlymphoid cells and have previously been im- 
plicated in signaling from the T cell antigen recep- 
tor. Members of Dr. Perlmutter's laboratory have 
employed a novel strategy to investigate the impor- 
tance of one class of G proteins (the Gj proteins) in 
T cell signaling. These experiments take advantage 
of the fact that Gj proteins can be specifically and 
irreversibly inactivated by pertussis toxin-mediated 
ADP-ribosylation. Transgenic animals were gener- 
ated in which essentially all thymocytes express the 
catalytic subunit of this toxin under the control of a 
T cell-specific promoter derived from the Ick gene. 
The pertussis toxin-bearing thymocytes exhibit 
no defects in T cell receptor-mediated stimulation, 
but instead manifest a remarkable inability to colo- 
nize peripheral lymphoid tissues correctly. These 
experiments demonstrate that a pertussis toxin- 
sensitive process, almost certainly involving a Gj 
protein, assists in regulating lymphocyte trafficking. 
Detailed studies have excluded most of the known 
cell adhesion phenomena as potential targets of per- 
tussis toxin. Hence current experiments focus on Gj 
protein-mediated chemotaxis to explain the migra- 
tory behavior of normal lymphocytes. More recent 
experiments indicate that the earliest migration 
events, which establish the organization of cortical 
and medullary elements in the thymus, also proceed 
via a pertussis toxin-sensitive process. Current ef- 
forts are focused on the receptor and effector mecha- 
nisms that regulate the migratory behavior of imma- 
ture thymocytes. 
Control of Thymocyte Signaling 
by Protein-Tyrosine Kinases 
Members of Dr. Perlmutter's laboratory have ad- 
duced persuasive evidence positioning two mem- 
brane-associated protein-tyrosine kinases, the prod- 
ucts of the Ick and fyn genes, in the signaling 
pathway that is activated by ligand occupancy of the 
T cell antigen receptor. The Ick gene product, 
p56'''*, is expressed in a lymphocyte-specific fash- 
ion and was previously shown to interact with the 
CD4 and CDS co-receptor components of the T cell 
antigen receptor complex. A comprehensive series 
of experiments performed in transgenic mice has 
provided evidence linking p56'''* activity to the con- 
trol of thymocyte maturation. Moreover, p56'''* also 
interacts with the signaling component of the inter- 
leukin-2 receptor. Hence this kinase participates in 
the regulation of multiple T cell signaling pathways. 
One consequence of overexpression of p56'''* in 
otherwise normal thymocytes is the rapid develop- 
ment of thymic tumors. Indeed, several lines of ani- 
mals in which tumors reproducibly develop in the 
first 8 weeks of life have now been propagated for 
more than 10 generations. Tumorigenesis occurs 
as a consequence of the amount of p56'''* activity 
present in individual cells. Introduction of an 
"activating" mutation (which eliminates a carboxyl- 
terminal phosphorylation site) into the /cfe-coding 
sequence produces an even more potent oncogene. 
The tumors that arise in Ick transgenic mice resem- 
ble lymphoblasts in some cases of human acute lym- 
phocytic leukemia, where translocation of the Ick 
gene has been observed. Although the frequency of 
such activation events is unknown, Ick transgenic 
mice provide a valuable test system for the study of 
novel therapies for at least some forms of human 
malignant disease. 
A second protein-tyrosine kinase, p5S>^"^, is also 
expressed exclusively in hematopoietic cells and 
appears to transmit activating signals directly from 
the T cell antigen receptor. Transgenic mice in 
which augmented lymphocyte-specific expression 
of p59*" has been achieved contain thymocytes that 
respond too vigorously to ordinary stimuli. This ob- 
servation permitted application of an especially 
powerful strategy to the analysis of p59^". In this 
case a mutation was introduced into the fyn trans- 
gene that abrogated catalytic activity, producing a 
"dead" kinase. Overexpression of this inactive mu- 
tant protein provoked a dramatic reduction in the 
response of otherwise normal thymocytes to T ceil 
receptor-specific stimuli. This result almost cer- 
tainly reflects competition between active and inac- 
tive forms of p59^" for appropriate cellular targets 
(either receptor structures or substrates) . 
IMMUNOLOGY 353 
