INTRODUCTION
The goal of gene therapy for inherited diseases is to provide lifelong
correction of the genetic defect. AAV has become the platform of choice
for most in vivo gene therapy applications due to its demonstrated safety
and efficacy in human clinical trials, highlighted by the approval of the first
two AAV gene therapeutics by the U.S. Food and Drug Administration (1-5).
However, questions about the durability of AAV-mediated gene therapy remain
because of the nonreplicating, episomal nature of AAV vectors. Colella et al.
recently highlighted several challenges in the ability to achieve and maintain
therapeutic levels of transgene expression. One issue is the potential waning
of transgene expression over time due to cell proliferation in the growing
target organ of pediatric patients, resulting in vector genome dilution or
to cell turnover as an outcome of damage to the target organ or a cellular
immune response against the transgene or viral capsid (7-11).
There is also evidence of rapid posttransduction degradation of
episomal AAV vector genomes in the neonatal liver . Now,
vector re-administration is prevented by the formation of high titers of
neutralizing antibodies following initial treatment that can persist for
many years (13). A second issue is improving vector potency to achieve more
efficient transgene expression at lower vector doses. Higher vector doses are
associated with increased immunogenicity and increased risk of liver inflammation
that can compromise liver-directed transgene expression (14, 15). In addition,
preclinical studies have shown that high vector doses have been associated
with acute liver failure and shock in nonhuman primates, proprioceptive deficits
and ataxia in pigs (16), and increased risk of hepatocellular carcinoma in mice (17).
Moreover, the high cost of manufacturing AAV vectors is a considerable obstacle
to systemic gene therapy (6). Thus, even a twofold decrease in vector dose could
have a meaningful impact on safety and cost of therapy.
We have previously reported development of ImmTOR nanoparticles
encapsulating rapamycin (also called SVP-Rapamycin) that have been
shown to induce a tolerogenic immune response to co-administered
biologic therapies via induction of tolerogenic dendritic cells and
antigen-specific regulatory T cells and reduction in antigen-specific B cell
activation (18–20). Moreover, the addition of ImmTOR to AAV gene therapy
vectors was recently shown to effectively and specifically inhibit adaptive
antibody and T cell immune responses against AAV capsid, thereby enabling
successful repeat administration of AAV vectors in mice and nonhuman primates (21).
Here, we further demonstrate that co-administration of ImmTOR with AAV-based
vectors also enhances transgene expression after the first dose of AAV vector
in naïve mice. This beneficial effect of ImmTOR on first dose transgene expression
appears to be independent of its immunomodulatory effects on adaptive
immunity and cannot be achieved by free rapamycin. Admixing ImmTOR and AAV
before injection is important for enhanced transgene expression after the first dose
but it is not required for inhibition of the antibody response to
AAV or for repeated administration of AAV vectors. Mechanistically, our data
suggest that ImmTOR can enhance trafficking of AAV to the liver and increase
transduction of hepatocytes in a manner that appears to be independent of the
AAV receptor (AAVR). This multipronged mechanism of ImmTOR action makes
it an attractive candidate to enhance systemic gene therapeutic applications,
particularly in those clinical indications where repeat vector dosing may be necessary.