SigE contributes to cytotoxicity to macrophages We further tested whether RB50ΔsigE interacts differently than RB50 with another major bactericidal component in the bloodstream, phagocytes. B. bronchiseptica is cytotoxic to macrophages, and this toxicity has been attributed to the activities of the type three secretion system (TTSS) [49]. To test

the role of SigE in macrophage cytotoxicity, RAW264.7 murine macrophages were incubated for 4 hours at an MOI of 10 with RB50, RB50 lacking sigE, or RB50 lacking a functional TTSS (WD3). In this experiment, both the RB50 and RB50ΔsigE strains contained the empty cloning selleckchem vector pEV to allow direct comparisons with the complemented strain, RB50ΔsigE pSigE. Cytotoxicity was determined by measuring LDH release from the treated macrophages. WD3 caused little cytotoxicity, similar to treatment with medium alone. RB50ΔsigE pEV caused approximately 50% less cytotoxicity than wild-type RB50 pEV (Figure 5). This defect in cytotoxicity was complemented by supplying the sigE gene on the plasmid pSigE (Figure 5), indicating that

loss of sigE negatively impacts the ability of RB50 to kill macrophages. Figure 5 RB50Δ sigE is less cytotoxic to macrophages than RB50. RAW 264.7 cells were incubated at an MOI of 10 with medium containing RB50 pEV, RB50ΔsigE pEV, RB50ΔsigE pSigE, TTSS-deficient RB50 MK5108 ic50 strain WD3, or medium alone for 4 hours in the presence of 1 mM IPTG to induce expression of sigE from the pLac promoter of pSigE. The average percent cytotoxicity of four wells in four separate experiments as measured by (LDH release from a well/LDH release from the positive control well) x100 ± SE is shown. The differences in percent cytotoxicity between RB50ΔsigE pEV and either RB50 pEV or RB50ΔsigE pSigE are statistically significant Epothilone B (EPO906, Patupilone) (** indicates P value < 0.01), while the cytotoxicities of RB50 pEV and RB50ΔsigE pSigE are not significantly

different. RB50ΔsigE is more efficiently phagocytosed and killed by PMNs To test if RB50ΔsigE is more susceptible to another bactericidal mechanism, phagocytosis by peripheral blood polymorphonuclear leukocytes (PMNs), RB50 and RB50ΔsigE were incubated with freshly isolated human PMNs and attachment to, phagocytosis by, and killing by these cells were measured. PMNs bound RB50ΔsigE more efficiently than RB50 (Figure 6A), and significantly more RB50ΔsigE than RB50 were phagocytosed by PMNs (Figure 6B). However, the number of viable BV-6 concentration intracellular RB50ΔsigE was ~50% of the numbers of viable RB50 (Figure 6C, left panel). When differences in attachment and phagocytosis were taken into consideration, significantly more internalized RB50ΔsigE were killed compared to RB50 (Figure 6C, right panel). Together, these data indicate that SigE contributes to B. bronchiseptica resistance to phagocytosis and killing by PMNs.