Immunopathogenesis of severe acute respiratory disease in Zaire ebolavirus-infected pigs

Ebola viruses (EBOV) are filamentous single-stranded RNA viruses of the family Filoviridae. Zaire ebolavirus (ZEBOV) causes severe haemorrhagic fever in humans, great apes and non-human primates (NHPs) with high fatality rates. In contrast, Reston ebolavirus (REBOV), the only species found outside Africa, is lethal to some NHPs but has never been linked to clinical disease in humans despite documented exposure. REBOV was isolated from pigs in the Philippines and subsequent experiments confirmed the susceptibility of pigs to both REBOV and ZEBOV with predilection for the lungs. However, only ZEBOV caused severe lung pathology in 5-6 weeks old pigs leading to respiratory distress. To further elucidate the mechanisms for lung pathology, microarray analysis of changes in gene expression was performed on lung tissue from ZEBOV-infected pigs. Furthermore, systemic effects were monitored by looking at changes in peripheral blood leukocyte subsets and systemic cytokine responses. Following oro-nasal challenge, ZEBOV replicated mainly in the respiratory tract, causing severe inflammation of the lungs and consequently rapid and difficult breathing. Neutrophils and macrophages infiltrated the lungs but only the latter were positive for ZEBOV antigen. Genes for proinflammatory cytokines, chemokines and acute phase proteins, known to attract immune cells to sites of infection, were upregulated in the lungs, causing the heavy influx of cells into this site. Systemic effects included a decline in the proportion of monocyte/dendritic and B cells and a mild proinflammatory cytokine response. Serum IgM was detected on day 5 and 6 post infection. In conclusion, a dysregulation/over-activation of the pulmonary proinflammatory response may play a crucial role in the pathogenesis of ZEBOV infection in 5-6 weeks old pigs by attracting inflammatory cells to the lungs.