Abstract
The balance between factor B, a positive mediator and factor H, a negative regulator of the complement alternative pathway, keeps the activity of this pathway tightly controlled. Severe dengue virus infection has been associated with hyperactivity of the alternative pathway. Here we investigated the molecular events responsible for the hyperactivity of the complement alternative pathway during dengue virus infection. Both factor B and H mRNA levels are significantly increased in dengue virus-infected endothelial cells and macrophages; however extracellular factor B but not factor H protein is induced thus creating an imbalance of the alternative pathway. In contrast, TLR stimulation induced both factor B and factor H mRNA and protein. Quantitation of intracellular factor B and H proteins demonstrated induction only in antigen-positive cells, suggesting that uninfected bystander cells do not contribute to production of factor H and B. Additionally IFN-b, a cytokine with a central role in regulating innate immune responses against dengue virus, mediates induction of factor B and H in endothelial cells but not macrophages. This study demonstrates that the activity of the complement alternative pathway in the microenvironment of endothelial cells and macrophages is altered by dengue infection. Induction of factor B but not factor H protein will result in an imbalance and complement overactivity, as is observed in dengue patients. Additionally, overactivity of complement at the endothelial cell surface would be predicted to induce functional changes, such as increased vascular permeability, a hallmark of dengue disease.