In vivo N. meningitidis sepsis in WT versus C5ar1−/− mice. (A) Surface expression of C5aR1 as measured by flow cytometry on neutrophils (gated as Ly6Ghi) in WT mouse lepirudin-treated whole blood after 1 h of infection with 107 CFU of N. meningitidis MC58 versus uninfected control (ctrl.) and C5ar1−/− neutrophils as a staining control. Plotted is the mean fluorescence intensity (MFI) of results from three independent experiments. *, P < 0.05 (in paired matched observations per mouse; two-tailed Student’s t test). (B) Survival curves of WT and C5aR1−/− mice after intraperitoneal infection with 105 CFU of N. meningitidis MC58. ****, P < 0.0001 (in Mantel-Cox test). (C) N. meningitidis counts in blood of infected mice at indicated time points. The 18/24 h data comprise 18-h values from mice not surviving until 24 h plus 24-h values from the mice surviving until then. ns, not significant; **, P < 0.001; ***, P < 0.0001 (in unpaired, two-tailed Mann-Whitney test). (D) Plasma levels of inflammatory mediators at 12 h after intraperitoneal infection with 105 CFU N. meningitidis MC58 of WT versus C5ar1−/− mice (means ± SEM; n = 5 per genotype). ns, not significant; **, P < 0.01 (in unpaired, two-tailed Student’s t test).