The main finding of this study was that the administration of doxycycline resulted in a significant reduction in the total number of BAL PMNs in mice challenged with LPS. Interestingly, the measurements of lung permeability in LPS-treated mice were not affected by doxycycline. Thus, doxycycline specifically inhibited PMN migration into the airspaces of the lung without affecting other parameters of lung injury.
Tetracyclines are known to be broad inhibitors of the synthesis and activity of metalloproteinases, including MMP7 (matrilysin) [9, 10]. In the lungs, syndecan-1 shedding is MMP-dependent and primarily mediated by MMP7 [11, 12]. Therefore, we used syndecan-1 shedding as a marker of MMP activity in the lungs in the presence or absence of doxycycline, and found that doxycycline reduced the shedding of syndecan-1 into the BAL fluid.
PMNs incubated in the presence of doxycycline exhibited similar chemotaxis as compared with PMNs incubated without doxycycline. Because KC, the murine ortholog of human IL-8 (CXCL8), is one of the most important PMN chemoattractants in mice, the in vitro results suggest that a defect in PMN migration or its ability to sense a chemokine gradient is an unlikely explanation for the decrease in intra-alveolar PMNs seen in the mice that received doxycycline.
An increasing number of data suggests that tetracyclines in general, and doxycycline in particular have a powerful anti-inflammatory effect. In mice, the administration of doxycycline in the water at a dose of 1.5 mg/kg is protective in a model of LPS-induced sepsis . Furthermore, doxycyline at 2.0 mg/kg attenuates lung injury induced by LPS, doxycycline-resistant S. pneumoniae and bleomycin; and reduces inflammation in a model of asthma [4, 5, 14]. In humans, doxycycline has been used in the treatment of lymphangioleiomyomatosis (LAM) [15, 16] and decreases proteinuria in diabetic nephropathy , although it was found to have no effect in osteoarthritis . Clearly, doxycycline has anti-inflammatory properties that are independent of its antimicrobial activity.
Our study has relevance for scientists investigating the role of specific genes using the rtTA/tetO system. If the proper controls are in place, the rtTA/tetO system is still a powerful way to design in vivo experiments to test research hypotheses; however, comparing mice treated with and without doxycycline can be misleading. Instead, comparisons should be performed between double transgenic mice carrying the entire rtTA/tetO system and single transgenic mice carrying only the tetO operator, all in the presence of doxycycline.