Nucleic-acid- and cell-based therapeutics - Pharmaceutical Biotechnology: Concepts and Aapplications - page 432

An additional virus that has more recently gained some attention as a possible vector is that of
the sindbis virus. A member of the alphavirus family, this ssRNA virus can infect a broad range
of both insect and vertebrate cells. The mature virion particles consist of the RNA genome com-
plexed with a capsid protein C. This, in turn, is enveloped by a lipid bilayer in which two additional
viral proteins (E1 and E2) are embedded. The E2 polypeptide appears to mediate viral binding
to the surface receptors of susceptible cells. The major mammalian cell surface receptor it targets
appears to be the highly conserved, widely distributed laminin receptor.
The sindbis virus is simple, robust, capable of infecting non-dividing cells and generally sup-
ports high levels of gene expression. However, it does display a broad host range and, hence, lacks
the inherent targeting specifi city characteristic of an idealized viral vector.
Recently, a novel recombinant sindbis virus, displaying altered host cell specifi city, has been
generated. Scientists inserted a nucleotide sequence coding for the IgG binding domain of Sta-
phyloccus aureus into the E2 viral gene. Disruption of the E2 gene renders its protein product
incapable of binding laminin (hence, destroying the natural viral tropism). However, the protein
A domain allows the chimaeric E2 product to bind monoclonal antibodies. This altered virus may
prove to be a useful generic or 'null' vector, potentially capable of being specifi cally targeted to any
desired cell type. This would simply necessitate pre-incubation of the virus with monoclonal anti-
bodies raised against a surface antigen unique to the proposed target cell population (Figure 14.6).

Modified E protein, now containing the
IgG binding domain of Protein A

2

E 2

(a)

(b)

(c)

Figure 14.6 Generation of engineered sindbis virus capable of being targeted to bind specifi c cell types. (a) A
simplifi ed depiction of the virus, displaying the surface E2 protein. (b) Genetic engineering facilitates disruption
of the E2 gene by incorporation of the IgG binding domain of protein A. (c) Incubation of such engineered viral
particles with most monoclonal antibody types results in effective immobilization of the antibody on the viral
surface. Thus, the engineered viral vector should be targetable to any specifi c cell type simply by its pre-incuba-
tion with monoclonal antibodies that selectively bind a surface antigen uniquely associated with the target cell