Expression of β-Defensin-3 in Lungs of Immunocompetent Rats with Methicillin-Resistant Staphylococcus aureus Ventilator-Associated Pneumonia

Background. Despite the rising incidence and high rate of treatment failure of ventilator-associated pneumonia (VAP) due to methicillin-resistant Staphylococcus aureus (MRSA), to date there has been no rat model specifically designed for antimicrobial evaluation. beta-Defensin-3 Acronym for beta-defensin-3 is correct as (BD-3) is an antimicrobial peptide and mainly expresses in the gastrointestinal and respiratory tract. It demonstrates a broad spectrum of potent antimicrobial activity against many potentially pathogenic microbes, including multi-resistant Staphylococcus aureus and vancomycin-resistant Enterococcus faecium. Therefore, the authors hypothesized that the expression of BD-3 might change in lungs of rats with MRSA VAP, and this change might play an important role in the pathogenesis of VAP. Materials and Methods. Eighty specific pathogen-free male Sprague-Dawley rats were randomly assigned to the following experimental groups: (1) N (nonventilated pneumonia) group (n = 34): rats were infected intrabronchially with 0.2 mL of 10(10) cfu/mL MRSA inoculum; (2) V (ventilator-associated pneumonia) group (n = 34): rats were ventilated for 4 h using the protective ventilation settings: 6 mL/kg tidal volume, 5 cmH(2)O of positive end-expiratory pressure (PEEP), 88 breaths/min, and FiO2 = 0.21. After ventilation, rats were inoculated intrabronchially with the same amount of MRSA as in N group; (3) P (protective mechanical ventilation) group (n = 6): 0.2 mL of nor-mal saline was instilled into lungs of rats after 4 h of ventilation as in V group; (4) C (control) group (n = 6): only 0.2 mL of normal saline were instilled into lungs of rats. Rats from both P and C groups were killed 48 h after instillation of normal saline. Rats from other two groups were killed 3, 6, 12, 24, 48 h after inoculation. Pneumonia evaluation was performed by macroscopic, histopathologic, and microbiologic criteria. The expression of BD-3 in lungs was determined by immunohistochemistry staining and Western blot analysis. Results. Rats inoculated after 4 h of protective mechanical ventilation rapidly developed progressive pneumonia with heterogeneous distribution of lesions and different degrees of histologic evolution predominating in lower lobes. Lung bacterial concentrations in V group at each time point were significantly higher than those of N group. It was 12.2 +/- 0.9 log(10) cfu/g of tissue at h 48 in V group, and 8.7 +/- 0.4 in N group. Bacteremia occurred in nine of 10 rats at h 48 in V group, while two of 10 rats in N group. Both C and P groups showed a very low level of BD-3. Compared with C group, the expression of BD-3 at h 48 in both N and V groups significantly increased. However, the latter was significantly lower than the former. Conclusions. Rat model of MRSA VAP was obtained by intrabronchially inoculating rats with 2 x 10(9) cfu MRSA after 4 h of protective mechanical ventilation. VAP was more severe than nonventilated pneumonia in terms of histopathologic and microbiologic criteria, especially systemic spread. This may be associated with the inhibited up-regulation of BD-3 expression by mechanical ventilation. (C) 2011 Elsevier Inc. All rights reserved.