Kinetic study of the inflammatory response in Streptococcus pneumoniae experimental pneumonia treated with the ketolide HMR 3004

Abstract

Patients still die from Streptococcus pneumoniae pneumonia after initiation of antibiotic therapy, when tissues are sterile and the pneumonia is clearing. There is growing evidence that overwhelming inflammation resulting from toxin release contributes to tissue injury, shock, and death. Monitoring host response may help us understand the consequences of antibiotic therapy for the inflammatory processes that occur in bacterial pneumonia. HMR 3004 is a ketolide that displays excellent in vitro activity against S. pneumoniae. In the present experiment, we investigated the chronology of inflammatory events that occur during pneumococcal pneumonia in mice treated with HMR 3004. Infection of mice with 10(7) CFU of living S. pneumoniae resulted in 100% mortality within 5 days. HMR 3004 given at 12.5 mg/kg of body weight/dose twice daily from 48 h postinfection achieved complete bacterial clearance from lungs and blood within 36 h and ensured survival of mice. Recruitment of neutrophils and monocytes from blood to lungs was significantly reduced, and nitric oxide release was totally prevented. Interleukin-6 secretion in lungs and blood became rapidly undetectable after initiation of therapy. Histological examination of lung tissue showed protection of interstitium against edema. By controlling bacterial invasion, HMR 3004 led to rapid and profound modifications of the host response in lungs, which may protect mice from deleterious inflammatory reactions.

FIG. 1

Number of survivors in different groups of untreated and HMR 3004-treated infected mice. HMR 3004 was delivered by gavage at a dose of 25 mg/kg (12.5 mg/kg/day given twice a day), started 24, 48, or 72 h after intranasal inoculation of animals with 10⁷ S. pneumoniae bacteria. Twelve mice per group were infected to determine survival rate.

FIG. 2

Quantitative lung tissue cultures (A) and percentage of animals developing bacteremia (B) in experimental pneumococcal pneumonia. Tissue burden of S. pneumoniae was rapidly reduced in mice treated with HMR 3004 at a dose of 25 mg/kg/day, started 48 h after infection (n = 6 at each time point). ∗, P < 0.05, treated versus untreated mice.

FIG. 3

Mean (±SEM) blood leukocyte counts (A) and MPO levels (B) in lung homogenates of infected and healthy mice, treated or not with HMR 3004 at 25 mg/kg/day. Significant leukopenia was seen in the untreated infected mice from 24 to 36 h after initiation of treatment (which was started 48 h p.i.) compared to the healthy controls or the HMR 3004-treated infected mice. Recruitment of PMNs to the lung (MPO levels) was greatly reduced by HMR 3004. ∗, P < 0.05.

FIG. 4

Mean (±SEM) neutrophil (A) and monocyte (B) counts in BALF of infected and healthy mice, treated or not with HMR 3004 at 25 mg/kg/day. Significant decreases in both cell populations were seen by 30 h after initiation of therapy, which was started 48 h p.i. ∗, P < 0.05.

FIG. 5

Mean (±SEM) NO levels in lung homogenates (A) and BALF (B) of infected and healthy mice, treated or not with HMR 3004 at 25 mg/kg/day. A significant prevention in NO release was noted in infected treated mice in comparison to untreated infected mice. Therapy was initiated 48 h p.i. ∗, P < 0.05.

FIG. 6

Mean (±SEM) IL-6 levels in lung homogenates (A), BALF (B), and serum (C) of infected and healthy mice, treated or not with HMR 3004 at 25 mg/kg/day. Significant reduction in IL-6 was rapidly observed in blood and lung tissue after initiation of therapy, which was started 48 h p.i. IL-6 remained undetectable in uninfected animals. ∗, P < 0.05.

FIG. 7

Histology of lung tissue in infected mice treated with HMR 3004 (A), untreated infected mice (B), or healthy control mice (C). Pictures were taken 30 h after initiation of therapy, which corresponds to 78 h p.i. Diffuse edema with swelling of interstitium was noted in untreated infected animals, while HMR 3004-treated mice recovered very fast and had tissue profiles similar to those of healthy controls.