Table 1

Table 1. Properties of Genetic Vaccines

	Recombinant viruses	Bacteria	DNA	RNA^a
Access to class I MHC processing/ presentation pathway	Yes	Yes	Yes	Yes
Access to class II MHC processing/ presentation pathway	Yes	Yes	Yes	?
Ability to target APC	+	++	+	?
Built-in adjuvant	dsRNA, unknown PAMPs	LPS, lipoteichoic acid, unknown PAMPs	CpG motifs	Cell death induction, dsRNA
Immunogenicity	High	Moderate	Low	High
Response to vector determinants precluding repeated administrations	Yes	Yes	No^b	No
Efficacy reduced by existing antibodies (maternal or previous exposure) or memory T cells	Yes	Yes	No^b	No
Safety in immunocompromised hosts	No	No	Yes	Yes
Reversion to virulence	Possible	Possible	No	No
Heterologus gene expression	Transient	Transient/ prolonged	Transient/ prolonged	Transient
Vaccine manufacture and distribution	Not easy	Not easy	Easy	Not easy
Vaccine stability	Low	Low	High	Low

^a Replicon-based, self-replicating RNA

^b CTL response to a cryptic determinat encoded by an open reading frame in the backbone sequence of a plasmid expression vector was recently described [van Hall et al., 1998].

PAMP= Pathogen-Associated Molecular Patterns.