Interaction of Bluetongue virus with cells of the immune system

Lead Research Organisation: The Pirbright Institute
Department Name: UNLISTED

Abstract

Dendritic cells (DC) have a unique capacity to stimulate responses of naive T lymphocytes and are therefore central to the induction of immune responses and determine the outcome of infectious challenge or vaccination. Little is known of the interaction of BTV with DC, although preliminary studies performed in collaboration with Dr I Schwartz-Cornil, INRA, indicate that BTV replicates in ovine DC without inducing cell death. We will focus on how antigen from live and inactivated BTV interacts with bovine DC’s and determine the ability of bovine DC subsets to present antigen to T cells. In addition, we will develop and refine assays to measure BTV-specific T-cell responses in order to analyse the immune response to vaccination and infection. It has been proposed that the capacity for viral production without a cytopathic effect, as observed in BTV-infected ovine DC, involves budding of BTV. Our recent studies have demonstrated that BTV accumulates in uropods in BTV-infected lymphocytes in vitro. Uropods are the trailing edge of migrating T cells which selectively concentrate molecules involved in intercellular adhesion, whereas the leading edge is enriched in receptors involved in recognition of chemokines, antigens and substrate-adhesion molecules. Upon contact of T cells with other T cells or antigen-presenting cells, a characteristic polarized arrangement of molecules at cell-cell junctions, known as the immunological synapse, is induced. Recent studies have demonstrated that this synapse can play a role in the cell-to-cell spread of lymphotropic viruses and is a mechanism by which viruses can spread in the presence of neutralising antibody. In this project the role of virological synapses in the cell-to-cell spread of BTV will be determined.
 
Description The pathogenesis of bluetongue virus (BTV) was studied in sheep. BTV replication was demonstrated in two major cell types: vascular endothelial cells, and agranular leukocytes which morphologically resemble lymphocytes, monocytes/macrophages and/or dendritic cells. Two organs (the skin and tonsils) were shown to support relatively high levels of BTV replication, although they have not previously been proposed as important replication sites during BTV infection. The high level of BTV replication in the skin is thought to be of major significance for the pathogenesis and transmission of BTV (via biting insects).
Ovine conventional dendritic cells (cDCs) were shown to be the primary targets of BTV and contribute to the primary dissemination of BTV from the skin to draining lymph nodes. Lymph cDCs support BTV replication. BTV infection of cDCs enhanced their survival rate and resulted in their activation. BTV-infected cDCs stimulated antigen-specific CD4 and CD8 lymphocyte proliferation and gamma interferon production. BTV initially targets cDCs while preserving their functional properties, reflecting the optimal adaptation of the virus to its host cells for its first spread.
DCs, especially plasmacytoid DCs (pDCs), produce large amounts of type I interferon (IFN) upon infection with viruses, which has impacts on the physiopathology of the viral infections and on the quality of the adaptive immunity. BTV infection of sheep induced IFN in skin, lymph and in blood. Although BTV replicated in a substantial fraction of the cDCs and pDCs in vitro, only pDCs responded to BTV by producing a significant amount of IFN. BTV replication in pDCs was not essential for IFN production since it was still induced by UV-inactivated BTV. Other inflammatory cytokines were also induced by UV-BTV in primary pDCs. The intracellular pathways involved in induction of IFN in pDC were analysed. These findings have implications for the design of efficient vaccines against dsRNA viruses.
BTV is most successfully controlled by vaccination of susceptible ruminant populations. Currently two different types of BTV vaccines are used for this purpose; inactivated, mostly monovalent vaccine formulations and modified live virus vaccines (MLVs). Clinical signs and viraemia in Dorset Poll sheep vaccinated with BTV-4 and BTV-16 MLVs BTV-4 and BTV-16 MLVs or inoculated with homogenates of midges (C. sonorensis and C. nubeculosus) previously infected with BTV-4 MLV were analysed. All sheep vaccinated with the two MLVs mounted an infectious viraemia lasting for a minimum of 9 up to 23 days post vaccination and developed a range of clinical signs associated with BTV infection. High levels of BTV were detected in the blood, indicating a high potential for infection of vector insects and onward transmission. These studies strongly indicate that, although effective in the endemic areas for which they were designed, the use of these MLVs vaccines in northern Europe, especially in sheep breeds in which they have not been tested, would be a matter of concern.
Exploitation Route The interaction of BTV with bovine dendritic cells is under investigation and will be compared with that in ovine dendritic cells. Similar studies are also being undertaken to determine the interaction with Schmallenberg virus with ruminant dendritic cells.
Sectors Agriculture, Food and Drink