TLR4 and pulmonary emphysema: We therefore propose that the reason Tlr4−/− mice develop emphysema while Tlr2−/− mice do not may be attributable, at least in part, to qualitative and quantitative differences in the TLR2 and TLR4 ligands in the lung (currently unknown) which are required to activate TLR4 to protect against the onset of emphysema.