Bovine Respiratory Syncytial Virus and Histophilus somni Interaction at the Alveolar Barrier

Abstract

ABSTRACTOur previous studies showed thatHistophilus somniand bovine respiratory syncytial virus (BRSV) act synergisticallyin vivoto cause more severe bovine respiratory disease than either agent alone causes. SinceH. somnisurface and secreted immunoglobulin binding protein A (IbpA) causes retraction of bovine alveolar type 2 (BAT2) cells and invasion between BAT2 cellsin vitro, we investigated mechanisms of BRSV-plus-H. somniinfection at the alveolar barrier. BRSV treatment of BAT2 cells prior to treatment with IbpA-richH. somniconcentrated culture supernatant (CCS) resulted in increased BAT2 cell rounding and retraction compared to those with either treatment alone. This mimicked the increased alveolar cell thickening in calves experimentally infected with BRSV followed byH. somnicompared to that in calves infected with BRSV orH. somnialone. BRSV-plus-H. somniCCS treatment of BAT2 cells also enhanced paracellular migration. The effect of matrix metalloproteinases (MMPs) was investigated as well because microarray analysis revealed that treatment with BRSV plusH. somnisynergistically upregulated BAT2 cell expression ofmmp1andmmp3compared to that in cells treated with either agent alone. Enzyme-linked immunosorbent assay (ELISA) confirmed that MMP1 and MMP3 protein levels were similarly upregulated. In collagen I and collagen IV (targets for MMP1 and MMP3, respectively) substrate zymography, digestion was increased with supernatants from dually treated BAT2 cells compared with those from singly treated cells. Enhanced breakdown of collagen IV in the basal lamina and of fibrillar collagen I in the adjacent interstitium in the dual infection may facilitate dissemination ofH. somniinfection.