Pathogenesis of septic shock in Pseudomonas aeruginosa pneumonia

Abstract

The pathogenesis of septic shock occurring after Pseudomonas aeruginosa pneumonia was studied in a rabbit model. The airspace instillation of the cytotoxic P. aeruginosa strain PA103 into the rabbit caused a consistent alveolar epithelial injury, progressive bacteremia, and septic shock. The lung instillation of a noncytotoxic, isogenic mutant strain (PA103DeltaUT), which is defective for production of type III secreted toxins, did not cause either systemic inflammatory response or septic shock, despite a potent inflammatory response in the lung. The intravenous injection of PA103 did not cause shock or an increase in TNF-alpha, despite the fact that the animals were bacteremic. The systemic administration of either anti-TNF-alpha serum or recombinant human IL-10 improved both septic shock and bacteremia in the animals that were instilled with PA103. Radiolabeled TNF-alpha instilled in the lung significantly leaked into the circulation only in the presence of alveolar epithelial injury. We conclude that injury to the alveolar epithelium allows the release of proinflammatory mediators into the circulation that are primarily responsible for septic shock. Our results demonstrate the importance of compartmentalization of inflammatory mediators in the lung, and the crucial role of bacterial cytotoxins in causing alveolar epithelial damage in the pathogenesis of acute septic shock in P. aeruginosa pneumonia.

Figure 1

Cytotoxicity of PA103 and its isogenic mutant, PA103ΔUT. BEAS-2B cells (2 × 10⁴ cells per well) were cultured with (a) 10⁸ CFU/mL PA103 or PA103ΔUT, or (b) 10⁷ CFU/mL PA103 or PA103ΔUT, for 4 hours. Triplicate measurements; mean ± SEM of percent change in cell viability. *P < 0.05 vs. group incubated with PA103.

Figure 2

MAP, cardiac output, and base excess for 7 hours. (a) Airspace instillation of PA103, PA103ΔUT, or L/R without bacteria (control group). (b) Pretreatment with intravenously administered anti–TNF-α serum or rhIL-10 followed by airspace instillation of PA103. (c) Intravenous infusion of PA103 and airspace instillation of vehicle. Mean ± SEM. Cardiac output is expressed as percent change from initial measurements. ^‡P < 0.05 vs. control group. *P < 0.05 vs. group instilled with PA103.

Figure 3

Airspace-protein tracer in blood. The quantity of ¹²⁵I-albumin that left the lungs and entered the circulation was calculated and shown as a percentage of the initial dose. (a) Airspace instillation of PA103, PA103ΔUT, or L/R without bacteria (control group). (b) Pretreatment with intravenously administered anti–TNF-α serum or rhIL-10 followed by airspace instillation of PA103. Mean ± SEM. ^‡P < 0.05 vs. control group. *P < 0.05 vs. group instilled with PA103.

Figure 4

Quantity of bacteria in blood. (a) Airspace instillation of PA103 or PA103ΔUT. (b) Pretreatment with intravenously administered anti–TNF-α serum or rhIL-10 followed by airspace instillation of PA103. (c) Intravenous PA103 and airspace instillation of vehicle. Mean ± SEM. *P < 0.05 vs. group instilled with PA103.

Figure 5

Concentrations of plasma cytokines. (a) Airspace instillation of PA103 or PA103ΔUT. (b) Pretreatment with intravenously administered anti–TNF-α serum or rhIL-10 followed by airspace instillation of PA103. (c) Intravenous PA103 and airspace instillation of vehicle. Mean ± SEM. *P < 0.05 vs. group instilled with PA103.

Figure 6

Concentrations of cytokines in BALF and plasma. Comparison of instillation with PA103 and instillation with PA103ΔUT. Mean ± SEM. *P < 0.05 vs. group instilled with PA103.

Figure 7

¹²⁵I–TNF-α tracer studies. (a) ¹²⁵I–TNF-α was instilled with PA103, PA103ΔUT, or vehicle. The quantity of ¹²⁵I–TNF-α in the circulation is shown as a percentage of the initial dose instilled into the airspaces. Mean ± SEM. *P < 0.05 vs. group instilled with airspace PA103. (b) Plasma concentration curves of intravenously injected ¹²⁵I–TNF-α and ¹³¹I-albumin. Data are percentages of the peak values at 1 minute after the injection.

Figure 8

Number of bacteria in the right lung at the end of experiments (7 hours). The quantity of bacteria in the lungs was determined by multiplying colonies by the dilution ratio. Mean ± SEM.