TABLE 4.

Lung virus titers at day 4 postchallengea

Vaccine group (μg)bLung virus titer (104 PFU/ml) (±SD)
Single immunizationSecond immunizationFold decreasec
HA (0.5)+mLT6 ± 0.50d
HA (0.5)50 ± 40.97 ± 0.152
HA (0.1)+mLT6.5 ± 10d
HA (0.1)100 ± 201 ± 0.2100
HA (0.02)+mLT75 ± 80.3 ± 0.04250
HA (0.02)280 ± 304 ± 0.570
HA (0.5)+CTB20 ± 30d
Naïve500 ± 60500 ± 60
  • a Lung samples from individual mice in each group (n = 6) were collected on day 4 postchallenge with a lethal dose of mouse-adapted A/PR/8/34. Each lung sample was diluted to 1 ml with Dulbecco's modified Eagle's medium.

  • b Groups of mice are as described in the legend of Fig. 1. mLT, mutant LT(R192G).

  • c Fold decrease in lung viral titers after the second immunization compared to that observed after the single immunization.

  • d −, No virus in lungs was detected after the second immunization, indicating a complete inhibition of viral replications.