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Despite effective antibiotic therapies, pathogen-host interaction in pneumonia may lead to excessive inflammation and endothelial hyperpermeability, which can result in lifethreatening lung failure and death. The treatment of acute lung injury is hitherto mainly supportive. To develop new therapeutic approaches and to identify pneumonia patients at high risk of mortality as early as possible, it is essential to elucidate pathomechanisms underlying pneumonia.
Ang-1 and -2 are ligands of the receptor tyrosine kinase Tie2. Their roles in angiogenesis and tumor vascularization are well known, but they are also centrally involved in the development of endothelial permeability and inflammation. Generally Ang-1 evokes Tie2 activation, which in turn leads to an inactive, quiescent vascular endothelium, whereas on the contrary Ang-2 as the competitive antagonist leads to endothelial activation, increasing endothelial permeability and initiation of other inflammatory cascades. It has been shown that Ang-2 blood levels and the Ang-2/Ang-1 ratio were increased in sepsis and particularly in sepsis with lung failure. However, the role of the Ang/Tie2 system in severe pneumonia has not been investigated so far.
The aim of this study was to analyze the role of the Ang/Tie2 system in pneumococcal pneumonia and to evaluate Ang-2 as a possible biomarker for risk stratification in severe pneumonia. Therefore, different experimental approaches were employed to assess the role and the therapeutic potential of the Ang/Tie2 system in pneumonia-induced acute lung injury.
In a pilot analysis decreased Ang-1 and increased Ang-2 serum levels were observed in pneumonia patients (n = 20) as compared with healthy blood donors. In the following study, these results were verified. Furthermore, a difference in Ang-2 serum levels between survivors (n = 36) and non-survivors (n = 64) with significantly higher Ang-2 serum levels in non-survivors (28-day mortality) was determined. However, this finding could not be reproduced in a larger cohort study (n = 75 per group). Next, serum of mechanically ventilated intensive care unit patients during the course of pneumonia-induced ARDS (VALID study, n = 38) was analyzed. There were increased Ang-1 serum levels at study entry and 24 hours later. During recovery, Ang-1 increased and Ang-2 decreased almost to serum levels of healthy control subjects within seven days. To further decipher the kinetics of Ang-2
liberation in acute inflammation, a prospective, observational study was conducted with subjects who participated in the Berlin marathon (n = 18), which is known to be a pro inflammatory event (RUNINFLAME). All subjects showed increased CRP levels 24 after the run as compared with their pre-run levels, but not directly after the run. Notably, Ang-2 levels were highly increased directly after the run and returned to pre-run levels 24 h later, suggesting that Ang-2 is a very rapid marker of inflammation. For localization of Ang-1, Ang-2 and their receptor Tie2 in the lung, immunohistochemistry was performed on human lung tissue of pneumonia patients and healthy controls. Ang-1 was found to be expressed in various cell types, whereas Ang-2 and the receptor Tie2 were exclusively expressed in endothelial cells. In further in vitro studies, we observed that stimulation of endothelial cells (HUVECs) with pneumolysin (PLY), an important pathogenic factor of S. pneumonia, was associated with significantly increased Ang-2 release, while no effect on IL-6 release was documented. In Electrical cell substrate impedance sensing (ECIS) experiments Ang-2 had permeabilizing effects on HUVEC monolayers. Hypothesizing that Ang-2 might be involved in PLY-induced permeability, mice were intravenously treated with specific siRNA against Ang-2 (Atuplex), and their isolated perfused and ventilated lungs (IPML) stimulated with pneumolysin (PLY), an important pathogenic factor of S. pneumoniae. Lungs of mice treated with Ang-2 specific siRNA showed reduced permeability upon PLY stimulation compared to lungs of mice treated with luciferase-specific siRNA as control, suggesting that Ang-2 contributes to PLY-induced permeability in intact lungs. Finally, the role of the Ang/Tie2 system was examined in a mouse model of pneumococcal pneumonia. Pulmonary mRNA expression of Ang-2 increased in the course of pneumonia, while Ang-1 and Tie2 mRNA expressions were reduced. Remarkably, Ang-1 treatment of S. pneumoniae starting 22 h after infection led to reduced cytokine secretion, neutrophil recruitment and lung permeability 48 h after infection.
In summary, the results suggest that the Ang/Tie2 system plays a central role in pneumoniaevoked inflammation and hyperpermeability. As discussed, Ang-2 does not seem to be suitable for risk stratification in severe pneumonia. However, the results from the different experimental approaches suggest that targeting the Ang-1/Ang-2 balance may provide a new therapeutic perspective in severe pneumonia.