Haemophilus somnus induces apoptosis in bovine endothelial cells in vitro

Haemophilus somnus causes pneumonia, reproductive failure, infectious myoca rditis, thrombotic meningoencephalitis, and other diseases in cattle. Altho ugh vasculitis is commonly seen as a result of systemic H. somnus infection s, the pathogenesis of vascular damage is poorly characterized. In this stu dy, we demonstrated that H. somnus (pathogenic isolates 649, 2336, and 8025 and asymptomatic carrier isolates 127P and 129Pt) induce apoptosis of bovi ne endothelial cells in a time- and dose-dependent manner, as determined by Hoechst 33342 staining, terminal deoxynucleotidyl transferase-mediated dUT P-FITC nick end labeling, DNA fragmentation, and transmission electron micr oscopy, H. somnus induced endothelial cell apoptosis in as little as Ih of incubation and did not require extracellular growth of the bacteria, Viable H. somnus organisms induced greater endothelial cell apoptosis than heat-k illed organisms. Since viable H. somnus cells release membrane fibrils and blebs, which contain lipooligosaccharide (LOS) and immunoglobulin binding p roteins, we examined culture filtrates for their ability to induce endothel ial cell apoptosis, Culture filtrates induced similar levels of endothelial cell apoptosis, as did viable H. somnus organisms. Heat inactivation of H. somnus culture filtrates partially reduced the apoptotic effect on endothe lial cells, which suggested the presence of both heat-labile and heat-stabl e factors. We found that H. somnus LOS, which is heat stable, induced endot helial cell apoptosis in a time- and dose-dependent manner and was inhibite d by the addition of polymyxin B. These data demonstrate that H. somnus and its LOS induce endothelial cell apoptosis, which may play a role in produc ing vasculitis in vivo.