Blood, 1 June 2001, Vol. 97, No. 11, pp. 3417-3423
GENE THERAPY
Induction of stable prenatal tolerance to 
-galactosidase by in
utero gene transfer into preimmune sheep fetuses
Nam D. Tran,
Christopher D. Porada,
Graça Almeida-Porada,
Hudson A. Glimp,
W. French Anderson, and
Esmail D. Zanjani
From the Department of Veterans Affairs Medical Center,
Reno, NV; Fleischmann Agriculture, University of Nevada, Reno, NV; and
Gene Therapy Laboratories, University of Southern California School of
Medicine, Norris Cancer Center, Los Angeles, CA.
The successful transduction of hematopoietic stem cells and
long-term (28 months) transgene expression within the hematopoietic system following the direct injection of high-titer retroviral vectors
into preimmune fetal sheep was previously demonstrated. The present
studies extended these analyses for 40 months postinjection and
evaluated whether the longevity of transgene expression in this model
system was the result of induction of prenatal tolerance to the
transgene product. The intraperitoneal injection of retroviral vectors
into preimmune sheep fetuses transduces thymic epithelial cells thought
to present antigen and thus define self during immune system
development. To directly demonstrate induction of tolerance, postnatal
sheep were boosted with purified 
-galactosidase and showed that the
peripheral blood lymphocytes from in utero-transduced sheep exhibited
significantly lower stimulation indices to transduced autologous cells
than did control animals and that the in utero-transduced sheep had a
reduced ability to mount an antibody response to the vector-encoded
-galactosidase protein compared with control sheep. Collectively,
our results provide evidence that the direct injection of retroviral
vectors into preimmune sheep fetuses induces cellular and humoral
tolerance to the vector/transgene products and provide an explanation
for the duration and stability of transgene expression seen in this
model. These results also suggest that even relatively low levels of
gene transfer in utero may render the recipient tolerant to the
exogenous gene and thus potentially permit the successful postnatal
treatment of the recipient.
