FIG. 4.
In vitro induction of LLO-specific CTL responses by the ΔclpP mutant. Mice were immunized i.v. with 5 × 103 wild-type strain LO28 (▪), 5 × 108 LO28Δhly (●), 1 × 106 LO28ΔclpP (▵), or 5 × 108 LO28ΔclpP (▴) bacteria. Six days after infection, splenocytes were recovered, restimulated, and subsequently tested for cytotoxic activity against LLO-expressing target cells at different effector/target (E:T) ratios. Results are expressed as the mean percent specific lysis of duplicate cultures; error bars show standard deviations. An LLO-specific CTL response was generated from mice infected with the wild-type strain. No CTL activity was detected with LLO-deficient bacteria or with 1 × 106 clpP mutant bacteria. In mice infected with 5 × 108 clpP mutant bacteria, the CTL response was similar to that in mice infected with wild-type bacteria. Lysis produced by effector cells from infected mice sensitized with PBS was <2% and is not represented.