Lentiviral Vectors for Gene Delivery to the Nervous System - Gene Delivery and Therapy for Neurological Disorders - page 33

Cell type-specifi c lentiviral vectors can be generated through
surface engineering, by incorporating cell type-specifi c ligands or
antibodies on a mutated viral envelope.
Several attempts have been made to alter the receptor-
recognition attachment function in the envelope glycoprotein,
without affecting membrane fusion [ 64 ]. Such modifi cations ablate
the native tropism of the vector, so it is no longer able to recognize
and bind to its cognate receptor, and redirect its tropism to the
corresponding cellular receptors recognized by the ligand or anti-
body. Targeting systems available at the moment involve, among
others, antibody specifi c binding methods based on (1) molecular
events of receptor binding and endocytosis followed by fusogen-
induced pH-dependent membrane fusion [ 65 - 67 ] or (2) active
membrane fusion [ 68 - 70 ].
In 2006 Yang et al. have proposed an effi cient targeting method.
The method involves the incorporation of an antibody and a fuso-
genic protein as two distinct molecules on the lentiviral surface,
separating the recognition from the fusion function by providing
them as separate molecules. The fusogen is modifi ed so it lacks the
ability to bind to cognate receptors, but retains the ability to trigger
pH-dependent membrane fusion, thus is binding defi cient but
fusion competent [ 65 , 67 , 71 , 72 ]. The specifi city of this LV is
solely determined by the antibody chosen to recognize a specifi c
antigen on the surface of the target cells. Recently these targeted
LVs have been optimized by using single chain antibodies (scFV)
genetically fused to either Sindbis virus or Measles virus (MV) hem-
agglutinin envelope [ 69 , 73 ]. Modifi cations applied to optimize
this method, aimed to overcome limitations such as receptor inter-
nalization and endosomal release [ 66 ], which are necessary when
vectors are pseudotyped with certain envelopes.
An additional targeting method involves coating of the viral
surface with polymers such as polyethylene glycol, poly-[ N -(2-
hydroxypropyl) methacrylamide], or biodegradable alginate mic-
roparticles [ 74 - 76 ]. All these methods are based on the same
principle: ablating native tropism and retargeting through surface
display of ligands (peptide, proteins, or antibodies). One disadvan-
tage of the method is that the two-component nature of these
bispecifi c molecules adds complications in vector production and
in maintaining stable batch-to-batch homogeneity, as the number
of plasmids required for production of these molecules targeted
vectors increases (usually 5-6 plasmid co-transfection is required).
Other approaches involve the use of a ligand protein or antibody as
a bridge to attach the virus to specifi c cells [ 67 , 71 , 77 , 78 ].
However, this approach requires endocytosis for the pH-dependent
fusion in addition to the fact, that once the envelope protein is
connected to the one end of the bridge molecule, it fuses ineffi -
ciently. Additionally to targeting through surface engineering, tis-
sue specifi c promoters [ 79 , 80 ] and or tissue-specifi c regulatory