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Adhikary, Dinesh (2006): Investigation of the T helper cell response against Epstein-Barr virus. Dissertation, LMU München: Faculty of Medicine
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Abstract

The Epstein-Barr virus (EBV) is associated with a number of human malignancies. Following primary infection, the virus persists lifelong in the infected host by latently infecting B cells and occasional cycles of reactivation, virus production and re-infection. Adoptively transferred EBV-specific T cells, generated by repeated stimulation with autologous lymphoblastoid cell lines (LCL) in vitro, are able to cure post-transplant lymphoproliferative disease (PTLD). However, the generation of these vaccines is labor and cost intensive precluding their general availability for all patients at risk. Novel insights into the mechanisms of protective antiviral immunity is expected to provide a better understanding of the pathogenesis of EBV-associated diseases and to facilitate the development of novel and generally available immunotherapeutic options. The aim of this work was to assess specificity and breadth of the EBV-specific T helper cell response, using two different experimental strategies. To define specificity, LCL-stimulated CD4+ T cell lines were established from 23 EBV-negative and -positive donors. The T cell lines generated from EBV-negative donors responded poorly against LCL and failed to show EBV-specificity. By contrast, all T cell lines established from healthy virus carriers were EBV-specific. Half of the lines from acutely EBV-infected patients with infectious mononucleosis (IM) were also EBV-specific, while the other half recognized EBV-positive and EBV-negative target cells. Unexpectedly, the EBV-specific T cell lines did not recognize latent antigens of EBV expressed in all LCL. Instead, these lines were specific for lytic cycle antigens predominantly derived from virion proteins. Several of the T cell lines recognized BNRF1, a viral tegument protein. Most T cell lines, however, recognized different virion antigens, suggesting that the family of virion proteins forms the immunodominant targets of the EBV-specific T helper cell response. Studies on the breadth of the EBV-specific T helper response demonstrated that all healthy virus carriers maintain CD4+ T cell memory to lytic cycle antigens. T cells specific for virion antigens recognized EBV-positive cells directly and, surprisingly, a much higher percentage of target cells than those expressing lytic cycle proteins. Antigen was efficiently transferred to bystander B cells by receptor-mediated uptake of released virions, resulting in recognition of target cells incubated with less than one virion per cell. T cell recognition did not require productive infection and occurred early after virus entry before latency was established. By secreting perforin and granzyme B upon antigen recognition, virion-specific T helper cells inhibited proliferation of LCLs and suppressed the outgrowth of LCLs following infection of primary B cells with EBV. These results established a novel role for virion-specific T helper cells in the control of EBV infection, and identify virion proteins as important immune targets. The findings have implications for the treatment of diseases associated with EBV and potentially other coated viruses infecting MHC class II-positive cells.