FIGURE 3.

Persistent tolerance to Sakai infection by preinjection of heat-killed Sakai cells in silkworms. A, the persistent immunization effect of heat-killed Sakai cells was examined. The vertical axis shows silkworm viability after 1 × 10⁷ cfu of Sakai infection. The horizontal axis shows the interval between preinjection of heat-killed Sakai cells and injection of live Sakai cells. The preinjection of heat-killed Sakai cells was conducted on the second day of the fifth instar larval stage. Ten silkworms were used for each time point. The point at 240 h on the horizontal axis shows the viability of the pupae after Sakai infection. Data are representative from three independent experiments. B, lethal Sakai infection in silkworm pupae and their acquisition of infection tolerance by preinjection of heat-killed Sakai cells were examined. We preinjected saline (left) or heat-killed Sakai cells (middle and right) into silkworms on the second day of their fifth instar larval stage and maintained those silkworms until they pupated. The pupae (n = 14–24) were then injected with 1 × 10⁷ cfu of live Sakai cells. The pupae killed by Sakai infection were blackened by melanization. The survival curve is presented in the bottom graph. The vertical and horizontal axes show the silkworm pupae viability after Sakai infection and time after Sakai infection, respectively. Asterisks indicate log-rank test p values less than 0.05. (*, p = 0.0338; **, p = 0.0094).