Mouse Pneumonia Model of Sickle Cell Acute Chest Syndrome.

Increased pulmonary inflammation in response to bacterial pneumonia may underlie the pathogenesis and lethality of the acute chest syndrome (ACS) in patients with sickle cell disease (SCD). Although transgenic mice expressing exclusively human sickle hemoglobin are gaining wide use as models of severe SCD, due to their similarities to human SCD-SS (i.e., severe hemolytic anemia, sickle deformation, microvascular occlusion, and multi-organ infarcts; Pastzy 1997 Science 278:876), their applicability for the investigation of ACS has not been tested. Currently, there is no animal model of sickle ACS other than acute hypoxic death, which is not a clinically relevant scenario. HYPOTHESIS: SCD is associated with an extreme compensatory state that depletes anti-oxidant and endothelial functional reserves and results in a heightened susceptibility to physiologic challenges. We hypothesized that, similar to human SCD-SS, these transgenic mice would exhibit an increased sensitivity to the lethal effects of intrapulmonary bacterial challenge. METHODS: Age- and sex-matched SS mice and hemizygote colony controls were raised in pathogen-free breeding colonies. Randomly selected animals at baseline had lung histology and micro-computed tomography (CT) performed. E. coli was selected for intrapulmonary bacterial inoculation to avoid the confounding issues of immunologic susceptibility to encapsulated bacteria in SCD. Using isoflurane anesthesia and orotracheal intubation, E. coli was inoculated in low (0.15 x 10 ⁹ CFU/kg) or high (0.6 x 10⁹ CFU/kg) doses, each in 50 mcl saline. Following E. coli inoculation all mice were treated with fluids on day 1 and antibiotics on day 1 to day 4, and were observed until day 7. RESULTS: On lung histology at baseline, SS mice showed greater vascular congestion and capillary dilation than control mice (mean scores ± SD) and increased circulating neutrophils (mean cells/HPF ±SD) (6.0±0 vs 2.5±2.4, 5.5±0.6 vs 1.0±0.8, and 3.5±1.7 vs 0.8±0.5 respectively, all p<0.05). Findings on lung micro CT were consistent with these differences. Low dose E. coli was nonlethal in both controls and SS mice (n=5, respectively). With high doses of E. coli, although overall mortality rate at day 7 was greater in SS mice than in controls (7 of 10 SS died vs 6 of 10 controls) this was not significant ( p=ns). However, compared to control nonsurvivors, the time to death (mean ±SD) in SS nonsurvivors was shorter (52±47 vs 88±29, p=0.15). Furthermore, by 48 h although all controls were still alive, 3 of 10 SS mice had already died (p=0.004 by Chi-square, uncorrected). Micro CT showed patchy consolidation of lungs. DISCUSSION: Thus this transgenic mouse model of SCD was associated with baseline pulmonary changes on histology and micro CT and increased sensitivity to the lethal effects of intrapulmonary bacterial challenge, changes consistent with those observed clinically. The lethality difference between low- and high-dose E. coli demonstrates that the susceptibility of SS mice is not to the invasive procedures, but to the physiologic challenge of the infection. The present findings suggest that this mouse model of SCD and pneumonia should be explored further as a tool to investigate the pathogenesis and treatment of ACS.