M. hyopneumoniae field strain XLW-1 was isolated from diseased pigs in Jiangsu Province, China. Porcine alveolar macrophages (PAMs) obtained from three 4–5-week-old M. hyopneumoniae-negative piglets, and serologically negative for PRRSV and porcine circovirus type 2 (PCV2), were prepared as described previously [14] (link), [15] (link). Prior to infection, PAMs were mixed and confirmed negative for M. hyopneumoniae, PRRSV, PCV2, pseudorabies virus, and classical swine fever virus by PCR and RT-PCR. RPMI 1640 medium and fetal bovine serum (FBS) were obtained from GIBCO (Invitrogen). The isolated cells were grown and maintained in RPMI 1640 medium containing 10% (v/v) FBS at 37°C with 5% CO2.
In order to simulate natural conditions of M. hyopneumoniae infection, the tracheal ring were infected first. Tracheas were collected as previously described [40] (link). Briefly, the tracheas were excised aseptically from pigs and submerged in chilled PBS. Tracheas were washed with PBS, and transverse sections (approximately 0.5 cm thick) were prepared by making an incision between the tracheal rings. Each tracheal ring was placed in a 30-mm culture plate insert (Millipore, Bedford, Mass.) containing 3 ml of complete medium.