Herpesvirus-induced expression of sLe(x) and related O-linked glycans in the infected cell

Abstract

Lewis antigens constitute a family of fucosylated carbohydrate antigens (glycotopes), involved in leukocyte homing and related immunological phenomena. These glycotopes are only expressed restrictedly in normal cells, but are induced at appropriate occasions. It is well established that many tumors “hijack” Lewis glycotopes for e.g. extravasation and metastasis, and recent data indicate that also human retroviruses may use a similar strategy for colonization of distal tissues. The overall goal of the present thesis was to explore the prerequisite for this phenomenon to occur in cells infected with herpesviruses, a virus family where persistent infections and immune evasion are important hallmarks. Using confocal immunofluorescence, neo-expression of Lewis antigens was found on cells infected with herpes simplex virus type-1 (HSV-1), varicella-zoster virus (VZV), and cytomegalovirus (CMV). However, whereas the neurotropic viruses VZV and HSV-1 induced sialyl Lewis x (sLex), CMV induced Lewis y (Ley) at the surface of the infected cells. Real time RT-PCR methods for transcriptional analysis of all known human fucosyltransferase genes (FUT) were developed to determine the mechanism behind virus-specific induction of different glycotopes. The herpesviruses investigated were all able to induce transcription of FUT3, FUT5 and FUT6 relevant for sLex and Ley synthesis whereas only CMV induced FUT1, necessary for Ley expression. In most cases the transcriptional activity of these genes was several orders of magnitude larger in virus-infected cells compared to uninfected cells. The viral factors causing neo-expression of glycotopes were explored using FUT5 and the HSV-1 infected cell as a model system. It was found that the transcripts of the immediate early viral gene, designated ICP0, was able to induce FUT5 transcription without assistance of the translated gene product. This finding explained the extremely early occurrence of host FUT5 RNA, detectable as early as one hour post infection. However, several other viral factors were engaged in regulation of the FUT5 transcription downstream the ICP0 induction. The viral glycoprotein gC-1 was identified as a probable candidate as a carrier of O-linked glycans and important regulatory elements of the O-glycosylation sequon of gC-1 were characterized. These regulatory elements were decisive for the social behavior of virus-infected cells in culture. The conclusion of the present work is that herpesviruses possess powerful mechanisms for viral control of the expression of selectin ligands and similar glycotopes, of relevance for tumor metastasizing and tissue invasion of human transforming retroviruses. sLex and Ley constitute targets for development of cancer chemotherapy, but further investigation is necessary to determine whether this approach is applicable also for treatment of herpesvirus-infections.