Characterization of a murine model of Ureaplasma urealyticum pneumonia

Ureaplasma urealyticum respiratory tract colonization in preterm infants has been associated with a high incidence of pneumonia and the development of bronchopulmonary dysplasia. However, study of this human pathogen has been hampered by the absence of animal models. We have developed the first juvenile mouse model of Ureaplasma pneumonia and characterized the histopathology during the month following inoculation. C3H/HeN mice were inoculated intratracheally with a mouse-adapted clinical Ureaplasma isolate (biovar 2) or sham inoculated with 10B broth. Culture of lung homogenates and PCR of DNA from bronchoalveolar lavage fluid (BAL) confirmed the presence of Ureaplasma in 100% of inoculated animals at 1 day, 60% at 2 days, 50% at 3 days, and 25% at 7 and 14 days. Ureaplasma was undetectable 28 days postinoculation. There were marked changes in BAL and interstitial-cell composition with increased number of polymorphonuclear leukocytes 1 to 2 days and 14 days postinoculation and macrophages at 2 and 14 days postinoculation. The Ureaplasma infection caused a persistent focal loss of airway ciliated epithelium and a mild increase in interstitial cellularity. There were no differences in BAL protein concentration during the first 28 days, suggesting that pulmonary vascular endothelial barrier integrity remained intact. Comparison of BAL cytokine and chemokine concentrations revealed low levels of tumor necrosis factor alpha (TNF-alpha) at 3 days and monocyte chemoattractant protein 1 at 7 days in Ureaplasma-infected mice but a trend toward increased TNF-alpha at 14 days and increased granulocyte-macrophage colony-stimulating factor and interleukin-10 at 28 days. These data suggest that Ureaplasma alone may cause limited inflammation and minimal tissue injury in the early phase of infection but may promote a mild chronic inflammatory response in the later phase of infection (days 14 to 28), similar to the process that occurs in human newborns.