Table 2.

Influence of maternal antibodies on protective effect of offspring immunized with the same DNA vaccine as that for their mothers^a

		Serum IgG titersb in offspring
Plasmid		ELISA (2n)c		NI assay (2n)c		Lung virus titersb (log10 TCID50)	Survival offspring/Tested offspring (3 weeks)
Plasmid for female mice	Plasmid for offspring	21 days after primary immunization	7 days after booster	21 days after primary immunization	7 days after booster
30 μg HA	30 μg HA	15.0 ± 0.00	13.3 ± 0.60			4.3 ± 0.35	0/7
Unimmunized	30 μg HA	11.3 ± 0.50	14.3 ± 0.50			4.0 ± 0.58*	5/7*
30 μg NA	30 μg NA			5.6 ± 0.90	3.3 ± 0.60	4.0 ± 0.70	2/7
Unimmunized	30 μg NA			4.6 ± 0.60	8.5 ± 0.70	2.8 ± 0.35*	7/7*
Unimmunized	Unimmunized	<1	<1	<3	<3	5.7 ± 0.00	0/7

^a Female mice were immunized twice, 3 weeks apart, with 30 μg HA DNA or 30 μg NA DNA. The offspring were immunized at ages of 1 and 4 weeks, respectively, with the same vaccine as their mothers. Serum samples from offspring were collected 3 weeks after primary immunization and 1 week after booster. The anti-HA antibody titers were measured by ELISA. The anti-NA antibody titers were measured by NI assay. One week after booster, the offspring were challenged with a lethal dose of A/PR/8/34 (20 × LD₅₀). Lungs were taken out from at least three mice in each group 3 days after challenge for virus titration by standard MDCK assay. Survival rates of mice were measured 3 weeks after challenge.

^b Values represent mean ± S.D. of each group.

^c The serum samples were diluted 2-fold serially and "n" represents the dilution factor.

*Significant difference (p < 0.05)