Submitted to: Virus Research
Publication Type: Peer reviewed journal
Publication Acceptance Date: 11/18/2005
Publication Date: 6/1/2006
Citation: Ridpath, J.F., Bendfeldt, S., Neill, J.D., Liebler-Tenorio, E. 2006. Lyphocytopathogenic activity in vitro correlates with high virulence in vivo for BVDV type 2 strains: criteria for a third biotype of BVDV. Virus Research. 118(1-2):62-69. Interpretive Summary: Bovine viral diarrhea virus (BVDV) is an important pathogen of cattle. Different strains of BVDV cause different disease syndromes. Some cause very severe clinical disease and some cause very mild clinical disease. To date, the only way to determine if a virus caused severe disease (i.e. was a virulent virus) or caused mild disease (i.e. was a low virulence virus) was to infect animals with the virus and observe the results. This limited the research that could be done to determine why some viruses were highly virulent and others less so. This research describes a model that uses cultured cells rather that live animals to differentiate between virulence levels of BVD. It is important in that use of this model will reduce the need to use live animals and will significantly cut the cost and difficulty of studying virulence in BVDV strains. Preliminary results using this model indicate that virulence is associated with the death of cells associated with the immune system.
Technical Abstract: Two biotypes of bovine viral diarrhea viruses (BVDV), cytopathic (cp) and noncytopathic (ncp), are recognized based on their activity in cultured epithelial cells. Biotype does not correlate to virulence in acute infections as BVDV strains associated with severe acute BVD outbreaks are all noncytopathic based on their growth characteristics in cultured epithelial cells. Previous studies have shown that acute infections with highly virulent BVDV result in depletion of cells in lymphoid tissues. In this study, flow cytometry demonstrated that infection with highly virulent BVDV strains was associated with a pronounced reduction in circulating lymphocyte numbers and increased numbers of apoptotic and necrotic circulating lymphocytes in vivo. Infection with low virulence BVDV did not result in a significant increase in death of circulating lymphocytes. Thus there appeared to be a correlation between lymphocytopenia and virulence. To study the interaction of BVDV strains with lymphoid cells in the laboratory, we developed an in vitro model that used a bovine lymphoid cell line (BL-3 cells). Using this model it was found that while BVDV strains are segregated into two biotypes based on their activity in cultured epithelial cell, they may be segregated into three biotypes based on their activity in cultured lymphoid cells. These three biotypes are noncytopathogenic (no obvious effects on the viability of either cultured epithelial or lymphoid cells), cytopathogenic (cytopathic effect and cell death in both cultured epithelial and lymphoid cells within 48 hours of infection) and lymphocytopathogenic (no effect on cultured epithelial cells, however, cell death in cultured lymphoid cells is observed within 5 days of infection). The proposed lymphocytopathic biotype correlates with virulence in acute infections in vivo. Cell death caused by the lymphocytopathogenic biotype was not associated with changes in the processing of the NS2/3 viral protein. Further, the cytopathic effect induced in cultured lymphoid cells by ncp highly virulent BVDV strains occurs by a different mechanism than the cytopathic effect induced by cp BVDV strains.