Table 1.

Possible strategies to overcome humoral immunity to AAV in systemic gene transfer

Strategy	Advantages	Disadvantages
Select subjects with low-to-undetectable anti-AAV NAb.	Enrollment of NAb-negative subjects has been effective in allowing successful gene transfer.	Antibody assays are relatively insensitive (risk of false negatives) particularly for AAV serotypes performing poorly in vitro56;
		More than 50% of humans develop anti-AAV humoral immunity after the age of 3,53,55 and thus exclusion of those with antibodies restricts the patient population.
Administer high vector doses.	Simple approach that may be effective in the presence of low-titer NAb.	High vector doses may trigger anticapsid CTL responses4,15,95,128;
		Relatively low-titer antibodies (1:5-1:17) can neutralize large doses of vector.15,39
Use empty capsids to adsorb anti-AAV antibodies, thus allowing for vector transduction.	Effectively overcomes humoral immunity to AAV119;	Increase antigen load in target organ, thus potentially triggering anticapsid T-cell immunity.95
	Does not require any pharmacologic intervention;
	Empty capsids are an easy-to-manufacture by-product of AAV vector production.
Administer IS to prevent or eradicate humoral immune responses to AAV.65	Drugs selectively targeting B cells and plasma cells are approved for use in humans66;	Potential risks associated with systemic IS; Risks associated with blocking the induction of regulatory T cells with IS 67,91;
	The approach has been shown to be at least partially effective.40
		Many IS drugs, particularly biologics, are not effective in animal models of gene transfer, making modeling IS strategies in animals difficult;
		Not effective in the complete eradication of preexisting high-titer NAb.40
Switch AAV serotype62 or engineer AAV capsids that are less susceptible to NAb.68,69	This approach has been shown to be effective in some instances.70	Switching or altering the capsid may modify significantly the AAV vector tissue tropism;
		Anti-AAV capsid antibodies are highly cross-reactive among serotypes.21
Use repeated plasma exchange cycles to adsorb immunoglobulins and therefore reduce the anti-AAV antibody titer.	Does not require IS;	Requires several cycles of plasmapheresis to achieve significant drop in antibody titer;
	Relatively safe, noninvasive procedure;
	Effective in dropping anti-AAV antibody titer.71	Does not completely eradicate high-titer anti-AAV antibodies.71
Use delivery techniques such as balloon catheters followed by saline flushing to isolate the target tissue from the systemic circulation to avoid vector dilution in blood and exposure to NAb.72	Limits systemic exposure to the vector.	Relatively invasive, safety in diseased target organs/tissues needs to be assessed;
		Not useful if systemic gene transfer is required for therapeutic efficacy, not feasible for all target tissues.

CTL, cytotoxic T lymphocyte; IS, immunosuppression.