Tracheal whole mounts or cross-sections were processed for immunoperoxidase histochemistry using techniques described previously [26, 27] . Briefly, specimens were incubated for 2 h in PBS containing 5% normal goat serum (Jackson ImmunoResearch Laboratories, West Grove, PA) and then for 36 h in PBS containing 1% normal goat serum and MRC OX6 anti-MHC class II monoclonal antibody (PharMingen, San Diego, CA) at a 1:1000 dilution [3, 4] . The tissues were washed again for 6 h in PBS followed by a 24-h incubation in peroxidase-conjugated goat anti-mouse IgG (Jackson) at a 1:200 dilution.