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Equine herpesvirus type 1 (EHV-1) remains a severe threat to horse industry despite widespread vaccination. The control of EHV-1 infection is mainly dependent on cellular immunity that is mediated by CD8+ cytotoxic T lymphocytes (CTLs). EHV-1, as its relatives in the large Herpesviridae family, has evolved strategies to evade CTL immunity by interfering with the major histocompatibility complex class I (MHC-I) antigen presentation pathway. In the first part of this thesis, we identified a novel immunomodulatory protein involved in the downregulation of MHC-I from the cell surface. The responsible viral protein, pUL56, which is encoded by EHV-1 open reading frame 1 (ORF1), is a phosphorylated early protein, which is expressed as different forms and predominantly localizes to Golgi-derived membranes. In addition, the transmembrane (TM) domain of pUL56 was shown to be indispensable for correct subcellular localization and proper function. The function of pUL56 was independent of that mediated by pUL49.5, a viral protein known to inhibit the transporter associated with antigen processing (TAP) and encoded by EHV-1 and related viruses. Surprisingly, pUL56 by itself was not capable of downregulating MHC-I and likely needs (an)other unidentified viral protein(s) to perform this action.
EHV-1 has recently been demonstrated to be a promising viral vehicle for delivery of foreign antigens. In the second part of the thesis, we utilized ORF1 as the locus for the insertion of foreign genes, more specifically the VP2 and/or VP5 genes of bluetongue virus serotype 8 (BTV-8). BTV-8 can infect most domestic and wild ruminants species and was responsible for an epizootic in northern Europe in 2006. The EHV-1 recombinant viruses generated stably expressed the transgenes and grew with kinetics that were identical to those of parental virus in vitro. After immunization of mice, a BTV-8-specific neutralizing antibody response was elicited. In a challenge experiment using a lethal dose of BTV-8, 100% of interferonreceptor- deficient (IFNAR-/-) mice vaccinated with the recombinant EHV-1 carrying both VP2 and VP5, but not VP2 alone, survived. VP7 was not included in the vectored vaccines and was successfully used as a marker for differentiating infected from vaccinated animals (DIVA).
In conclusion, EHV-1 ORF1-encoded pUL56 is a novel immune evasion protein involved in the interference of MHC-I surface expression, but is unable to perform its function outside of the context of viral infection. An EHV-I recombinant carrying VP2 and VP5 of BTV-8 in the ORF1 locus is capable of eliciting protective immunity in a murine infection model and as such may be an alternative for BTV vaccination strategies.