Toll-like receptor 4 in normal and inflamed lungs and other organs of pig, dog and cattle

Bacterial diseases, especially those of the lung caused by Gram-negative bacteria, inflict significant economic loss associated with mortality and morbidity in domestic animals. Toll-like receptor 4 (TLR4) has recently been recognized as a major receptor for cellular interactions with lipopolysaccharides derived from Gram-negative bacteria. However, there are no data on the expression of TLR4 in various organs of domestic animals. We performed immunohistochemistry and immuno-gold electron microscopy to localize TLR4 in lung and seven other organs from normal pig, dog and calf (n=2 each) and in inflamed lungs from calves (n=4) challenged with Mannheimia hemolytica. The data show TLR4 in macrophages in lung, small intestine, liver and spleen in all the species and pulmonary intravascular macrophages in calves and pigs. Epithelium in lung, small intestine, cornea and convoluted and straight renal tubules was stained for TLR4. Vascular endothelium of large blood vessels only in lungs and skin was positive, and skeletal muscles were negative for TLR4. In inflamed lungs, airway epithelium showed reduced staining for TLR4 while staining in macrophages remained unaltered. These are the first immunocytochemical data on TLR4 expression in domestic animal species and show similarity in TLR4 staining in macrophages, epithelium and vascular endothelium among dog, pig and cattle.