Regulation of Gene Expression During Cellular Differentiation and Activation 
novel T cell-specific transcription factor called 
hGATA-3 that binds to a third nuclear protein- 
binding site within the TCR a enhancer. Ongoing 
studies are designed to determine the role of each 
of these transcription factors in T cell tumors, as 
well as in normal T cell development and 
activation. 
Genetically Engineered Myoblasts 
as a Recombinant Protein Delivery System 
A variety of acquired and inherited diseases are 
currently treated by repeated intravenous or sub- 
cutaneous infusions of recombinant or purified 
proteins. In addition to hemophilia A, which is 
treated with intravenous infusions of human fac- 
tor VIII, these include diabetes mellitus, treated 
with subcutaneous or intravenous injections of 
insulin, and pituitary dwarfism, treated with sub- 
cutaneous injections of growth hormone. The de- 
velopment of cellular transplantation systems 
that could stably produce and deliver such recom- 
binant proteins into the systemic circulation 
would represent an important advance in our abil- 
ity to treat such diseases. 
The ideal recombinant protein delivery system 
would utilize a cell that could be easily isolated 
from the recipient, grown and transduced with 
recombinant genes in vitro, and conveniently 
reimplanted into the host organism. Such a cell 
should produce large amounts of secreted recom- 
binant protein, and following secretion, this pro- 
tein should gain access to the circulation. Finally, 
these implanted, genetically engineered cells 
should survive for long periods and continue to 
secrete the transduced protein product without 
themselves interfering with the function of the 
tissue into which they were implanted. Several 
different cellular systems have been used to pro- 
duce recombinant proteins in vivo. These in- 
clude keratinocytes, skin fibroblasts, hepato- 
cytes, lymphocytes, and bone marrow. Although 
some of these systems have yielded detectable lev- 
els of circulating proteins briefly, stable physio- 
logical levels of circulating proteins have proved 
difficult to produce in normal animals. 
Genetically engineered myoblasts represent a 
potentially useful system for the in vivo delivery 
of recombinant proteins into the circulation. 
Myoblasts can be readily isolated from a muscle 
biopsy and expanded in vitro to very large cell 
numbers. Cultured myoblasts can be transfected 
in vitro and will synthesize large amounts of re- 
combinant proteins. Most importantly, previous 
studies have demonstrated that cultured myo- 
blasts can be injected intramuscularly and will 
survive and fuse into adjacent normal muscle 
fibers at the site of injection. Finally, skeletal 
muscle is a highly vascular tissue. Thus proteins 
secreted from myoblasts should readily enter the 
circulation. 
We have recently explored the feasibility of us- 
ing genetically engineered myoblasts as a recom- 
binant protein delivery system. To this end, 
stable transfectants of the murine C2C12 myo- 
blast cell line were produced that synthesize and 
secrete high levels of human growth hormone 
(hGH) in vitro. These stably transfected myo- 
blasts were injected intramuscularly into normal 
syngeneic C3H mice, and serum and muscle lev- 
els of hGH were measured 5 days to 3 weeks after 
injection. 
Mice injected with the growth hormone- 
transfected myoblasts produced significant and 
physiological levels of hGH both locally in mus- 
cle and in serum, as compared with control mice 
injected with nontransfected myoblasts. Human 
growth hormone levels in both muscle and serum 
were stable for at least three months following 
injection and exceeded those measured in 
serum from normal human volunteers. Histologi- 
cal examination of muscles injected with /3- 
galactosidase-expressing C2C12 myoblasts dem- 
onstrated that many of the injected cells had 
fused to form multinucleated myotubes. Thus 
these studies demonstrated that genetically engi- 
neered myoblasts represent a novel and powerful 
system for the stable delivery of recombinant 
proteins into the circulation. 
Dr. Leiden is now Professor of Medicine and 
Pathology at the University of Chicago. 
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