Ureaplasma urealyticum induced pulmonary inflammation in the development of chronic lung disease of prematurity

Abstract

Chronic lung disease (CLD) of prematurity is a prolonged respiratory failure in very-lowbirthweight neonates. This disorder is multifactorial and associated with an early increase in the numbers of neutrophils and alveolar macrophages, together with. morphological abnormalities in epithelial and endothelial cells. Infection and associated inflammatory responses in the lungs play important roles in the development of CLD. Ureaplasma urealyticum has been postulated to be of importance in this context, but this is still a matter for debate. Treatment of neonates with steroids provides some improvement.

Proinflammatory cytokines have been implicated as being involved in the development of CLD. In the present study, production of TNF-alpha and IL-6 by both a human and a rat macrophage cell line was found to increase after stimulation with U. urealyticum antigen. This induction was down-regulated by dexamethasone, budesonide and recombinant IL-10 (rIL-10) in the human macrophage cell line. In tracheobronchial aspirate fluid (TAF) macrophages, U. urealyticum antigen enhanced the production, of TNF-alpha 14-84% and IL-6 46-268%. In the rat alveolar macrophage cell line, steroids inhibited the increases in IL-6 and TNF-alpha production caused by U. urealyticum antigen, whereas rat rIL- 10 was without effect.

Nitric oxide (NO) has been suggested to be an important mediator of inflammation associated with pulmonary infections. We discovered that U. urealyticum antigen stimulates alveolar macrophages directly to increase their production of NO in a dose- and time-dependent manner. This effect was further enhanced by IFN-gamma, while being attenuated by budesonide and dexamethasone. The NO was regulated at transcriptional levels as measured by inducible NO synthase (iNOS). The NO formed in response to U. urealyticum caused a 6-fold reduction in the growth rate of this organism itself after 10 hours of infection.

Vascular endothelial growth factor (VEGF) and intercellular adhesion molecule-1 (ICAM-1) may be involved in both early and later pathological changes in the lung during the development of CLD. We found that U. urealyticum antigen enhances expression of VEGF, cell surface and soluble ICAM-1 by human macrophages in a dose-dependent manner, an effect which is also inhibited by budesonide and dexamethasone. The extent of this up-regulation of ICAM-1 was reduced by 86% when TNF-alpha was prevented from exerting its action by an anti-TNF-alpha antibody. In addition, U. urealyticum antigen triggered activation of nuclear factor -kappaB (NF-kappaB). This observation suggests a possible mechanism for the increases in cytokine production, and expression of iNOS, growth factor and cellular adhesion molecule evoked by this antigen.

In our clinical study, levels of TGF-beta I in TAF from infants who developed CLD were found to be significantly elevated during the first week of postnatal life and remain elevated at 2 weeks and even beyond 4 weeks of age. These levels of TGF-beta I were not significantly decreased by treatment with steroids in 6 infants with CLD. IL-10 was detected in 12/44 (27%) TAF samples from 24 infants who developed CLD, compared to 6/57 (11 %) TAF samples from 22 preterm infants who did not develop this disease.

These findings indicate that infection by U. urealyticum may be an important causative factor in the development of CLD. NO is probably involved in host defenses against such infection. The down- regulatory effect by steroids might in part explain the beneficial results of treating neonates with CLD with these agents. TGF-beta1 might play an important role in the fibrotic response observed in the CLD lung.