Serotype Switching to Evade Vector- Induced Neutralizing Antibodies
One approach to evade neutralizing antibodies is to “serotype switch” the vector. For example, administration of Ad2 in mice generated potent neutralizing antibodies against Ad2 that reduced second transduction (78). However, if an Ad5 vector is used for the second round of gene delivery, this vector is not neutralized by Ad2-specific antibodies and it is effective (42,78). This serotype-switching approach has subsequently been repeatedly demonstrated as a robust approach for RD-Ad HIV vaccines by Drs. Barouch and Ertl’s groups and others (76-80).
The HD-Ad system is even better suited to serotype switching, since Ads in the same class can generally cross-package each other’s genomes. For example, an HD-Ad vector bearing a packaging signal and inverted terminal repeats (ITRs) from Ad5 can be cross-packaged by other type C Ads including Ad1, Ad2, and Ad6 ((42,78) and see below).
PEGylation to Evade Pre-Existing Neutralizing Antibodies and Reduce Adaptive Antibody and Cellular Responses against Ad Vectors
Polyethylene glycol (PEG) is a clinically approved conjugation agent used to improve the pharmacokinetics of a variety of protein therapeutics. In these applications, the hydrophilic PEG molecule is cross-linked to the therapeutic agent to “shield” or reduce interactions of it with proteins and cells that would normally decrease the therapeutic interactions with its target. PEG has also been applied to improve the pharmacology of Ad vector. 3-5 kDa PEG molecules bearing reactive groups are randomly chemically conjugated to free amine groups on the virion surface. By this approach, as many as 15,000 PEG molecules can be added to the virion surface to shield the virus. Previous work shows that PEG can protect Ad vectors from neutralizing antibodies to allow multiple administration into immune recipients (66,81,82). PEGylation also reduces the production of new antibody and cellular immune responses against Ad proteins (66).
In practice, most random PEGylation strategies have relative weak abilities to shield at least Ad5 from neutralizing antibodies (83). In our hands, random PEGylation does rescue Ad activity, but you may still lose 95% of activity relative to the same vector acting without antibodies. Also, random PEGylation reduces CAR binding, so PEGylated vectors are weaker by the intramuscular and intranasal route (83). PEG affects Ad5 vaccines less by the intranasal route (83), perhaps because the vectors may be relying on penton base-integrin interactions more than by fiber-CAR interactions. Using PEGs with glucose or galactose on their ends can rescue some of this lost activity by the intranasal route (83), perhaps by retargeting to receptors that bind these sugars.
