Document Type

Dissertation - Open Access

Award Date

2021

Degree Name

Doctor of Philosophy (PhD)

Department / School

Veterinary and Biomedical Sciences

First Advisor

Radhey Kaushik

Second Advisor

Christopher Chase

Abstract

BVDV is a major pathogen of ruminants that causes immune dysfunction and lead to secondary bacterial infection. The underlying mechanism of this dysfunction is not well understood. Based on previous results, we believe that macrophages play a central role in BVDV-associated immune dysfunction. Previously, we have shown that the infected monocyte-derived macrophage (MDM) supernatant of the high virulent strains of BVDV significantly induced lymphocyte apoptosis. In the current study, we performed experiments to identify the apoptotic factors in the infected MDM supernatants in vitro. Also, we have extended our investigation to include testing the impact of infected macrophages on neutrophil viability, surface marker expression, and functional activity. The infected-MDM supernatants did not impact neutrophil viability. The supernatants did down-regulate CD18 and significantly down regulated L-selectin expression. Interestingly the supernatants had no significant effect on neutrophil phagocytic activity nor oxidative burst. Only cytopathic BVDV-infected supernatant significantly down-regulated nitric oxide production and neutrophil extracellular traps (NET) induction in pre-stimulated cells. Viral-protein-depleted-MDM supernatants were still capable of inducing lymphocyte apoptosis. Tumor necrosis factor-alpha and interleukin-1-beta, two apoptosis-related cytokines, were not detected in the virulent supernatant. Our data suggested an important role of macrophages in the mechanism of immune dysfunction that affects neutrophil function and induced lymphocyte apoptosis. Unlike lymphocyte apoptosis that depends on the highly virulent BVDV, the negative impact on neutrophil functional activity seems to be biotype specific, especially the BVDV cytopathic (Cp) biotype that is the biotype in most of the commercially available vaccine in the US. Our data also suggested that neither viral proteins nor two proinflammatory cytokines are involved in inducing lymphocyte apoptosis. However further testing is required to identify these immune-dysfunction-related factors. The second part of the dissertation included both an in vivo investigation of bison viral infections and an in vitro study on susceptibility of a primary cell line to 4 viruses associated with bovine respiratory disease. The in vivo study observed the tropism and co-infection of BVDV and Bosavirus, a novel Copiparvo virus, in different tissues of the American bison. The in vitro study focused on comparing the viral susceptibility and immune response of a primary bovine tracheal cells and the established BT cell line to BRD-related viral pathogen. We found that the co-infection of both viruses negatively impacts the replicative ability of each other in different tissues on the viral nucleic acid level. The viral load of BVDV in the skin of persistently infected (PI) cattle is higher than that of the American bison. Skin ear notches are the gold standard used for detecting BVDV PI cattle, however this finding may indicate for bison other samples besides ear notch-skin samples may be better for PI diagnostics depending the viral load in the tissues. The in vitro cell culture study found that we can rely on biologically relevant primary bovine tracheal cells in studying BVDV and BHV-1 the primary cells had similar susceptibility, growth kinetics and IFN-I response to that of the established bovine turbinate (BT) cell line.

Publisher

South Dakota State University

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Rights Statement

In Copyright