The upregulation of MHC class II determinants by airway epithelial cells has also been observed during Sendai virus infection [8] . Additionally, OX6 immunoreactivity has been reported in other types of epithelial cells after exposure to endotoxin or IFN-c [9, 10, 34] . It is not clear whether the M. pulmonis organisms themselves or perhaps their ability to stimulate IFN-c production is driving the MHC class II expression by epithelial cells.