SINGLE PRE-EXPOSURE TO A HIGH DOSE OF ZYMOSAN ENHANCES LUNG DEFENSE MECHANISMS AND ACCELERATES THE PULMONARY CLEARANCE OF A BACTERIAL PATHOGEN IN RATS

The present study examines the effects of pre-exposure to zymosan (a 1 --3-beta-glucan from baker yeast) on lung defense against bacterial infection. Rats received a single dose of zymosan A (0.6, 1.2, or 2.5 mg/kg body weight [bw]) or vehicle control (saline) via intratracheal instillation 3 days prior to intratracheal inoculation with 5 x 10(5) Listeria monocytogenes. Left lungs were homogenized and cultured to assess bacterial clearance, and bronchoalveolar lavage was performed on the right lungs to monitor lung inflammation and injury. Prior to bacterial infection, zymosan exposure resulted in elevated inflammation and oxidant production in the lungs. Zymosan treatment followed by infection led to an accelerated pulmonary clearance of bacteria when compared to the saline control group in a dose-dependent fashion. In addition, lower levels of injury and inflammation were associated with the enhanced bacteria clearance observed in zymosan-infected rats. Our findings suggest that zymosan exposure may enhance the lung immune response by activating alveolar macrophages prior to infection, and stimulating T cells involved in the adaptive immune response early after infection, thus resulting in a heightened pulmonary immune response.