AbstractsBiology & Animal Science

In the process of co-evolution with their hosts, herpesviruses have developed advanced mechanisms to counteract and evade the innate and adaptive responses of their hosts. Herpesviruses boast an impressive number of immunomodulatory proteins, commonly referred to as immune evasins, and their functions range from decoy receptors and virokines to modulators of the cytotoxic T cell response. Marek’s disease virus (MDV), an alphaherpesvirus, is the causative agent of a lethal disease in chickens characterized by generalized nerve inflammation and rapid lymphoma development. During lytic replication, MDV induces a drastic reduction of major histocompatibility complex (MHC) class I expression on the surface of infected cells, which allows the virus to shield itself from destruction by the cytotoxic T cell response. Currently, it remains unclear a) which proteins are responsible for MDV MHC class I downregulation and b) to what extent this and other immune evasion strategies influence the severity of disease, in particular tumorigenesis. The MDV homologue of the conserved herpesviral UL49.5 gene encodes a small endoplasmic reticulum (ER) transmembrane protein which has been postulated as a likely MHC class I modulator due to its supposed interference with the transporter associated with antigen processing (TAP), a function which has been demonstrated for members of the genus Varicellovirus. Through the generation of a mouse anti-UL49.5 antibody as well as a replication-competent UL49.5 knock-out virus in the course of my thesis project, novel tools for investigation of the pUL49.5 function are now available. However, the presented results within this thesis indicate that MDV pUL49.5 is not responsible for downregulation of MHC class I molecules on the surface of infected primary chicken embryo cells. Investigations with ectopically expressed UL49.5 confirmed those findings and additionally indicated that pUL49.5 does not lead to proteasome-mediated TAP degradation, a function which has been proposed in the past as its likely mode of action. Further investigations of pUL49.5 were obstructed by severe protein stability issues of unknown origin, which could not be solved by inhibiting cellular pathways of protein degradation. These enigmatic observations together with an obvious context- dependence of the protein’s expression (e.g., cell type), make some of my results, as well as previous studies, regarding the function of MDV pUL49.5 difficult to interpret. In a second part of my project, the previously unidentified MDV ORF012 gene was characterized in detail and first evidence for its involvement in immune evasion was obtained. The extensive colinearity of the MDV genome with related herpesviruses has eased functional characterization of many MDV genes. However, MDV contains a number of unique open reading frames (ORFs) that have not yet been characterized regarding their full coding potential and the functions of their products. Among these unique ORFs are two putative ORFs, ORF011* and ORF012*, which are found at the…