Biology Reference
In-Depth Information
deployed clinical gene therapy vehicle, accounting for almost 25 %
of ongoing clinical trials ( http://www.abedia.com/wiley/vectors.
php ) . However, their effi cacy in clinical trials have been hindered by
the host infl ammatory response against the vector that results pri-
marily from accumulation of the vector within non-target cell types
and organs, primarily in the liver and spleen, resulting in signifi cant
dose limiting toxicities associated with the vector.
Both clinically and experimentally, the most studied and best
understood serotype remains the species C adenovirus 5 (Ad5). In
vitro, and for certain local applications in vivo (e.g., following
intramuscular injection), Ad5 engages the Coxsackie and
Adenovirus Receptor (CAR) as its primary attachment receptor
[ 2 ], via interaction with specifi c key amino acids at positions within
the globular fi ber knob domain AB loop (Ser408 and Pro409),
DG loop (Tyr477), and
strand F (Leu485) [ 3 ]. Where the virus
is introduced systemically, it has been suggested the expression of
CAR within platelets and red blood cells can cause “off target”,
toxicity inducing interactions, resulting in hemagglutination and
thrombocytopenia respectively [ 4 - 6 ]. Thus, for systemic applica-
tion of Ad5, genetic modulation of the fi ber protein to ablate CAR
mediated cell association may be advantageous in preventing dose
limiting toxicities associated with platelet and red blood cell deple-
tion (plus potential downstream effects on deposition within the
liver). Following CAR engagement, the classical in vitro model of
Ad5 cellular infection suggests that the virus then engages with
α
β
5 integrins at the cell surface to mediate cellular inter-
nalization, via Arg-Gly-Asp motifs in the penton base protein
(positions 340-342) which stimulates internalization into clathrin
coated vesicles [ 7 ]. However, for intravascular applications, recent
studies have attributed a new and important role of this integrin-
penton base interaction in mediating uptake of virus to the spleen
with a corresponding induction of number of cytokines that pro-
mote a robust induction of innate immune responses [ 8 , 9 ]. Thus
it is clear that the genetic modulation of this interaction could
profoundly improve both the safety and dose limiting toxicity of
the virus for systemic applications. Further still, recent evidence
has confi rmed a compelling new role for the hexon protein in
mediating cellular uptake via recruitment of blood clotting factors
which coat the virus and “bridge” the virus: protein complex to
highly sulfated heparan sulfate proteoglycan (HSPG) receptors
expressed abundantly in liver hepatocytes [ 10 - 14 ]. Sequestration
within liver hepatocytes not only serves to deplete the pool of vec-
tor available for therapeutic purposes but also further stimulates
anti-vector immunity, and thus dose limiting toxicities. The hexon-
factor (F) X interaction has been carefully modelled and critical
interacting motifs located within and distal to the 5th and 7th
hypervariable regions (HVRs) have been identifi ed and genetically
modifi ed to preclude FX binding and hepatic transduction of the
v
β
3 and
α
v
β
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