T-CELL VACCINATION ALTERS THE COURSE OF MURINE HERPESVIRUS 68 INFECTION AND THE ESTABLISHMENT OF VIRAL LATENCY IN MICE

Gammaherpesviruses (γHV) cause lifelong infection and diseases in many species. The human γHV, Epstein-Barr virus (EBV) and Kaposi’s sarcoma-associated herpesvirus, are associated with several malignant human diseases including Burkitt’s lymphoma, nasopharyngeal carcinoma, and Kaposi’s sarcoma (3, 29). Vaccines against these widely disseminated viruses are keenly sought to prevent the associated diseases. Most EBV vaccine studies have been focused on gp350/220, which elicits virus-neutralizing antibody (10, 11, 24). However, this vaccination approach does not induce a cytotoxic T-lymphocyte (CTL) response against infected cells, which is crucial for the control of the virus (30). Vaccines against latency-associated proteins containing CTL epitopes are currently being tested. But the fact that more than 60% of the epitopes recognized by EBV-specific CTL clones are located in regions outside the latent EBNA and LMP-1 proteins suggests that any EBV vaccine based on CTL epitopes needs to include other regions of the viral genome such as the lytic cycle genes (20, 32). EBV studies have been limited to clinically apparent EBV infection in humans, and studies of acute infection are limited to infectious mononucleosis (IM) patients due to the lack of a suitable animal model (26, 36). However, the recently identified murine herpesvirus 68 (MHV-68), a type 2 γHV, shows pathobiological features similar to those of EBV and Kaposi’s sarcoma-associated herpesvirus and represents a useful small-animal model of γHV infection (26, 39, 45). Intranasal administration of MHV-68 to mice results in an acute infection in lung epithelial cells followed by latent infection in B cells, macrophages, and lung epithelium (37, 40, 46). In addition, there is splenomegaly and an expansion of activated CD8+ T cells in blood, characteristics in common with EBV-induced IM (38, 41). Many of these activated cells have been shown to be of a T-cell receptor Vβ4+/CD8+ phenotype irrespective of the major histocompatibility complex (MHC) haplotype (8, 41). CD8+ T cells have been shown to be important in the control of both acute and latent infections, and several lytic cycle CTL epitopes including the dominant epitopes ORF6487–495/Db and ORF61524–531/Kb (2, 9, 33), have been defined recently. Interestingly, these epitopes are expressed not only during the acute lung infection, but also after the establishment of latency in the spleen due to continual low level of viral reactivation (22). Studies by Stewart et al. showed that vaccination against the major membrane antigen gp150, the MHV-68 homologue of EBV gp350/220, reduced the peak numbers of latently infected cells (36). However, latency was still established in this system. Since lytic-phase CD8+ T-cell epitopes are expressed during the acute infection and also after latency has been established (22), we hypothesized that vaccination against lytic cycle CD8+ epitopes might result in better control of both acute and latent infection. In this study, we vaccinated mice with bone marrow-derived dendritic cells pulsed with defined lytic cycle CTL epitopes, including a subdominant CTL epitope from glycoprotein B (gB), the most conserved protein in the herpesvirus family (29, 35). This vaccination approach has been shown to elicit strong CTL responses and also limits the generation of antipeptide antibody (16, 17, 23). The data show that dendritic cell immunization against lytic cycle MHV-68 T-cell epitopes not only leads to partial protection against acute infection in the lungs but also alters the early events in the establishment of latency in the spleen.