TLR4 and pulmonary emphysema: Since emphysema is caused by Tlr4 gene disruption in mice [3], [23], and emphysema in smokers correlates with impaired TLR4 expression [7], [8], we initially investigated the specificity of the requirement for TLR4 to protect against emphysema by comparing the lung phenotypes of mice deficient in Tlr4 and the closely-related Tlr2 gene.