Immunological consequences of Epstein-Barr virus replication

Abstract

Epstein-Barr virus (EBV) is a ubiquitous human herpesvirus involved in the pathogenesis of a wide spectrum of malignant and non-malignant diseases. In healthy EBV carriers, the virus is believed to infect two major cellular targets - B lymphocytes and epithelial cells. While EBV latency is established predominantly, if not exclusively, in B-lymphocytes, virus replication can take place both in B cells and epithelial cells. Lytic replication ensures virus transmission to new carriers and replenishes the cellular reservoirs of virus persistence. The generally asymptomatic and harmless persistence of EBV relies on a tightly controlled immune response and distinct modes of virus/cell interactions observed at different stages of EBV life cycle. The aim of this thesis was to characterize the mutual influence of the host immune system and EBV at the replicative stage of virus infection.

We showed that EBV enters monocytes and inhibits their differentiation into dendritic cells (DCs) without the need of viral gene expression. The sensitivity of the cells to virus-induced apoptosis progressively decreases along the process of DC maturation and is strongly dependent on the cell type in which the virus replicated before infecting DC precursors, since epithelial-cells derived viruses exhibited a significantly stronger pro-apoptotic activity than their B cell-derived counterparts. The capacity of the virus to suppress DC development might help in delaying the establishment of EBV specific immunity before the pool of infected B cells reaches the size sufficient for long-term virus persistence.

During virus replication, both B cells and epithelial cells may escape recognition by cytotoxic CD8+ T cells through downregulation of MHC class I molecules. Our work demonstrated that MHC class I heavy-chain and beta2m mRNA and protein synthesis are inhibited during EBV replication. Several other characteristic changes observed in the MHC class I processing and presentation pathway during the lytic cycle were recapitulated by chemical inhibition of protein synthesis. These results were recently confirmed by others and the viral protein responsible for host-cell global protein synthesis shutdown was shown to be encoded by the BGLF5 open reading frame of the EBV genome.

Triggering of receptors of the tumor necrosis factor (TNF) superfamily participates both in determining the fate of B-lymphocytes during the process of their differentiation and in immunologic clearance of virus infected targets. Initiation of EBV lytic cycle counteracted sensitization to death induced by TNF-related apoptosis-inducing ligand (TRAIL) that resulted from B-cell receptor (BCR) triggering in Burkitt s lymphoma cells. Differential modulation of death-transmitting and decoy TRAIL receptors was associated with sensitization to TRAIL in response to BCR-triggering or protection from TRAIL by EBV lytic cycle. Interference with TRAIL-mediated checkpoints in B-cell differentiation may account for the involvement of EBV in autoimmune diseases. Decreased sensitivity to TRAIL may also protect EBV infected cells from recognition by CTL and NK-cells.

B-cell homeostasis is severely perturbed during malaria infection. We showed that the CIDR1á domain of P. falciparum erythrocyte membrane protein 1 (PfEMP1), a multiadhesive protein expressed during the erythrocytic phase of the parasite life cycle, binds to B cells and induces EBV replication. This might partly explain the increased EBV viral load during malaria infection and the increased risk of B cell immortalization in the ontogenesis of endemic Burkitt s lymphoma.

Results presented in this thesis strengthen the notion that EBV replication actively modulates the functioning of the immune system at different levels through complex interactions of viral products with several types of cells and contributes to immune suppression, autoimmunity and tumorogenesis through a number of mechanisms whose details require further characterization. Research lines defined by this work may lead to new approaches towards management of EBV associated diseases.