Platelet activation by Histophilus somni and its lipooligosaccharide induces endothelial cell proinflammatory responses and platelet internalization.

Histophilus somni is a gram-negative coccobacillus that causes respiratory and reproductive disease in cattle. The hallmark of systemic H. somni infection is diffuse vascular inflammation that can lead to an acute central nervous system disease known as thrombotic meningoencephalitis. Previously, we demonstrated that H. somni and its lipooligosaccharide (LOS) activate bovine platelets, leading to expression of P selectin, CD40L, and FasL. Because activated platelets have been reported to induce endothelial cell cytokine production and adhesion molecule expression, we sought to determine if bovine platelets induce proinflammatory and procoagulative changes in bovine pulmonary artery endothelial cells. Endothelial cells were incubated with platelets activated with adenosine diphosphate, H. somni, or H. somni LOS. Incubation with activated bovine platelets significantly increased expression of in adhesion molecules (intercellular adhesion molecule 1, E selectin) and tissue factor, as measured by flow cytometry, real-time polymerase chain reaction, and Western blot analysis. Activated platelets also up-regulated expression of endothelial cell IL-1beta, monocyte chemoattractant protein 1, and macrophage inflammatory protein 1alpha as determined by real-time polymerase chain reaction and an IL-1beta enzyme-linked immunosorbent assay. An interesting and surprising finding was that bovine platelets activated by H. somni or its LOS were internalized by bovine endothelial cells as visualized by transmission electron microscopy. This internalization seemed to correlate with endothelial cell activation and morphological changes indicative of cell stress. These findings suggest that activated platelets might play a role in promoting vascular inflammation during H. somni infection.