Normal and Abnormal Lymphocyte Growth Regulation 
lyze such mutant strains for their biological po- 
tential and test for analogous mutations in natu- 
rally occurring human leukemias. 
The activation of the cellular ABL gene to on- 
cogenic forms correlates with the stable in vivo 
modification of the protein with phosphotyro- 
sine. The normal ABL proteins are not detectably 
modified with phosphotyrosine in vivo, al- 
though they can autophosphorylate themselves 
in vitro. We have recently demonstrated that a 
specific intracellular inhibitor suppresses the ty- 
rosine kinase activity of the normal cellular ABL 
proteins, but does not effect the oncogenic 
forms. If normal ABL proteins are hyperex- 
pressed, they can overcome this inhibitor and 
modify themselves on tyrosine in vivo. In further 
studies on this inhibitor, we will try to define its 
mechanism of differential action on the normal 
and oncogenic forms of the ABL proteins. 
Stem and Progenitor Cell Growth Regulation 
One possible mechanism of action of onco- 
genic forms of ABL would be a transcriptional 
activation of specific growth factor genes, lead- 
ing to an autocrine stimulation of receptors in 
different cell types. We have tested this idea in a 
pre-B cell line that is interleukin-7 (IL-7) depen- 
dent. Although all oncogenic forms of ABL could 
relieve the cell of growth factor dependence and 
lead to tumor progression, there were interesting 
differences between Abelson virus and the BCR/ 
ABL forms. A subset of the lines derived with 
Abelson virus showed activation of a low level of 
transcription from the IL-7 locus, but none of the 
BCR/ABL lines showed this activation. When 
forced autocrine expression of IL-7 was engi- 
neered in the same indicator line, it could render 
the cells growth factor-independent, but the 
cells did not progress to tumorigenic behavior. It 
is likely that a bypass of the growth factor require- 
ments and additional signals initiated by the on- 
cogenic forms of ABL determine tumor progres- 
sion for immature B lineage cell types. 
In collaboration with Naomi Rosenberg of 
Tufts Medical School, we have developed a sys- 
tem for retroviral infection of murine bone 
marrow stem cells with the BCR/ABL oncogene 
and reimplantation into syngeneic hosts. This 
procedure leads to tumors with the characteris- 
tics of human chronic myelogenous leukemia. 
Interestingly, animals infected with the PI 85 
BCR/ABL forms show more aggressive tumors 
that invade nonhematopoietic organs like liver or 
kidney and show shorter latency than animals in- 
fected with the P2 1 0 BCR/ABL form of the gene. 
This system should be valuable for analyzing new 
therapies directed at the BCR/ABL oncogene and 
for defining the growth regulation of primitive 
hematopoietic stem cells. 
One long-range goal of our group is to develop 
effective in vitro culture techniques for the prop- 
agation and enrichment of stem and progenitor 
cells for different lineages. Previously we used 
the grovvT:h stimulatory properties of the BCR/ 
ABL oncogene in concert with selected bone 
marrow-adherent stromal lines to grow clonal 
lines of B lymphoid progenitor cells that could 
repopulate the B cell lineage of immunodefec- 
tive (SCID) mice. By modifying the culture con- 
ditions, we have now been able to cultivate such 
progenitor cells without the need for co-stimula- 
tion by the oncogene. These populations are very 
effective in reconstituting the B cell lineage in 
vivo and, in preliminary analysis, also show evi- 
dence of reconstituting other lineages. We hope 
to develop clonal lines that will retain these 
functional properties. 
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