Induction of protective immunity against murine gammaherpesvirus 68 infection in the absence of viral latency

A replication-deficient gammaherpesvirus vaccine protects mice from lytic disease and reduces latency establishment

Gammaherpesviruses are oncogenic viruses that establish lifelong infections and are significant causes of morbidity and mortality. Vaccine strategies to limit gammaherpesvirus infection and disease are in development, but there are no FDA-approved vaccines for Epstein-Barr or Kaposi sarcoma herpesvirus. As a new approach to gammaherpesvirus vaccination, we developed and tested a replication-deficient virus (RDV) platform, using murine gammaherpesvirus 68 (MHV68), a well-established mouse model for gammaherpesvirus pathogenesis studies and preclinical therapeutic evaluations. We employed codon-shuffling-based complementation to generate revertant-free RDV lacking expression of the essential replication and transactivator protein encoded by ORF50 to arrest viral gene expression early after de novo infection. Inoculation with RDV-50.stop exposes the host to intact virion particles and leads to limited lytic gene expression in infected cells yet does not produce additional infectious particles. Prime-boost vaccination of mice with RDV-50.stop elicited virus-specific neutralizing antibody and effector T cell responses in the lung and spleen. In contrast to vaccination with heat-inactivated WT MHV68, vaccination with RDV-50.stop resulted in a near complete abolishment of virus replication in the lung 7 days post-challenge and reduction of latency establishment in the spleen 16 days post-challenge with WT MHV68. Ifnar1-/- mice, which lack the type I interferon receptor, exhibit severe disease and high mortality upon infection with WT MHV68. RDV-50.stop vaccination of Ifnar1-/- mice prevented wasting and mortality upon challenge with WT MHV68. These results demonstrate that prime-boost vaccination with a gammaherpesvirus that is unable to undergo lytic replication offers protection against acute replication, impairs the establishment of latency, and prevents severe disease upon the WT virus challenge. Our study also reveals that the ability of a gammaherpesvirus to persist in vivo despite potent pre-existing immunity is an obstacle to obtaining sterilizing immunity.

Replication-dead gammaherpesvirus vaccine protects against acute replication, reactivation from latency, and lethal challenge in mice

Gammaherpesviruses (GHVs) are oncogenic viruses that establish lifelong infections and are significant causes of human morbidity and mortality. While several vaccine strategies to limit GHV infection and disease are in development, there are no FDA-approved vaccines for human GHVs. As a new approach to gammaherpesvirus vaccination, we developed and tested a replication-dead virus (RDV) platform, using murine gammaherpesvirus 68 (MHV68), a well-established mouse model for gammaherpesvirus pathogenesis studies and preclinical therapeutic evaluations. We employed codon-shuffling-based complementation to generate revertant-free RDV lacking expression of the essential replication and transactivator protein (RTA) encoded by ORF50 to arrest viral gene expression early after de novo infection. Inoculation with RDV-50.stop exposes the host to intact virion particles and leads to limited lytic gene expression in infected cells. Prime-boost vaccination of mice with RDV-50.stop elicited virus-specific neutralizing antibody and effector T cell responses in the lung and spleen. Vaccination with RDV-50.stop resulted in a near complete abolishment of virus replication in the lung 7 days post-challenge and virus reactivation from spleen 16 days post-challenge with WT MHV68. Ifnar1-/- mice, which lack the type I interferon receptor, exhibit severe disease upon infection with WT MHV68. RDV-50.stop vaccination of Ifnar1-/- mice prevented wasting and mortality upon challenge with WT MHV68. These results demonstrate that prime-boost vaccination with a GHV that is unable to undergo lytic replication offers protection against acute replication, reactivation, and severe disease upon WT virus challenge.

Age-associated B cells are long-lasting effectors that impede latent γHV68 reactivation

Age-associated B cells (ABCs; CD19⁺CD11c⁺T-bet⁺) are a unique population that are increased in an array of viral infections, though their role during latent infection is largely unexplored. Here, we use murine gammaherpesvirus 68 (γHV68) to demonstrate that ABCs remain elevated long-term during latent infection and express IFNγ and TNF. Using a recombinant γHV68 that is cleared following acute infection, we show that ABCs persist in the absence of latent virus, though their expression of IFNγ and TNF is decreased. With a fluorescent reporter gene-expressing γHV68 we demonstrate that ABCs are infected with γHV68 at similar rates to other previously activated B cells. We find that mice without ABCs display defects in anti-viral IgG2a/c antibodies and are more susceptible to reactivation of γHV68 following virus challenges that typically do not break latency. Together, these results indicate that ABCs are a persistent effector subset during latent viral infection that impedes γHV68 reactivation.

Age-associated B cells are long-lasting effectors that impede latent γHV68 reactivation

Age-associated B cells (ABCs; CD19⁺CD11c⁺T-bet⁺) are a unique population that are increased in an array of viral infections, though their role during latent infection is largely unexplored. Here, we use murine gammaherpesvirus 68 (γHV68) to demonstrate that ABCs remain elevated long-term during latent infection and express IFNγ and TNF. Using a recombinant γHV68 that is cleared following acute infection, we show that ABCs persist in the absence of latent virus, though their expression of IFNγ and TNF is decreased. With a fluorescent reporter gene-expressing γHV68 we demonstrate that ABCs are infected with γHV68 at similar rates to other previously activated B cells. We find that mice without ABCs display defects in anti-viral IgG2a/c antibodies and are more susceptible to reactivation of γHV68 following virus challenges that typically do not break latency. Together, these results indicate that ABCs are a persistent effector subset during latent viral infection that impedes γHV68 reactivation.

Latent gammaherpesvirus exacerbates arthritis through modification of age-associated B cells

Epstein-Barr virus (EBV) infection is associated with rheumatoid arthritis (RA) in adults, though the nature of the relationship remains unknown. Herein, we have examined the contribution of viral infection to the severity of arthritis in mice. We have provided the first evidence that latent gammaherpesvirus infection enhances clinical arthritis, modeling EBV's role in RA. Mice latently infected with a murine analog of EBV, gammaherpesvirus 68 (γHV68), develop more severe collagen-induced arthritis and a Th1-skewed immune profile reminiscent of human disease. We demonstrate that disease enhancement requires viral latency and is not due to active virus stimulation of the immune response. Age-associated B cells (ABCs) are associated with several human autoimmune diseases, including arthritis, though their contribution to disease is not well understood. Using ABC knockout mice, we have provided the first evidence that ABCs are mechanistically required for viral enhancement of disease, thereby establishing that ABCs are impacted by latent gammaherpesvirus infection and provoke arthritis.

Deletion of immune evasion genes provides an effective vaccine design for tumor-associated herpesviruses

Vaccines based on live attenuated viruses often induce broad, multifaceted immune responses. However, they also usually sacrifice immunogenicity for attenuation. It is particularly difficult to elicit an effective vaccine for herpesviruses due to an armament of immune evasion genes and a latent phase. Here, to overcome the limitation of attenuation, we developed a rational herpesvirus vaccine in which viral immune evasion genes were deleted to enhance immunogenicity while also attaining safety. To test this vaccine strategy, we utilized murine gammaherpesvirus-68 (MHV-68) as a proof-of-concept model for the cancer-associated human γ-herpesviruses, Epstein-Barr virus and Kaposi sarcoma-associated herpesvirus. We engineered a recombinant MHV-68 virus by targeted inactivation of viral antagonists of type I interferon (IFN-I) pathway and deletion of the latency locus responsible for persistent infection. This recombinant virus is highly attenuated with no measurable capacity for replication, latency, or persistence in immunocompetent hosts. It stimulates robust innate immunity, differentiates virus-specific memory T cells, and elicits neutralizing antibodies. A single vaccination affords durable protection that blocks the establishment of latency following challenge with the wild type MHV-68 for at least six months post-vaccination. These results provide a framework for effective vaccination against cancer-associated herpesviruses through the elimination of latency and key immune evasion mechanisms from the pathogen.

From Superantigens to "Real" Viral Antigens

Studies inspired by Dr. Peter Doherty led to over 16 years of research into the mouse gamma-herpesvirus, γHV68, in the Blackman laboratory. Progress on our understanding of γHV68 biology include insight into the establishment of latency, immune control of the acute and latent stages of infection and experimental vaccines, is described here.

ORF6 and ORF61 Expressing MVA Vaccines Impair Early but Not Late Latency in Murine Gammaherpesvirus MHV-68 Infection

Gammaherpesviruses (γHV) are important pathogens causing persistent infections which lead to several malignancies in immunocompromised patients. Murine γHV 68 (MHV-68), a homolog to human EBV and KSHV, has been employed as a classical pathogen to investigate the molecular pathogenicity of γHV infections. γHV express distinct antigens during lytic or latent infection and antigen-specific T cells have a significant role in controlling the acute and latent viral infection, although the quality of anti-viral T cell responses required for protective immunity is not well-understood. We have generated recombinant modified vaccinia virus Ankara (recMVA) vaccines via MVA-BAC homologous recombination technology expressing MHV-68 ORF6 and ORF61 antigens encoding both MHC class I and II-restricted epitopes. After vaccination, we examined T cell responses before and after MHV-68 infection to determine their involvement in latent virus control. We show recognition of recMVA- and MHV-68-infected APC by ORF6 and ORF61 epitope-specific T cell lines in vitro. The recMVA vaccines efficiently induced MHV-68-specific CD8+ and CD4+ T cell responses after a single immunization and more pronounced after homologous prime/boost vaccination in mice. Moreover, we exhibit protective capacity of prophylactic recMVA vaccination during early latency at day 17 after intranasal challenge with MHV-68, but failed to protect from latency at day 45. Further T cell analysis indicated that T cell exhaustion was not responsible for the lack of protection by recMVA vaccination in long-term latency at day 45. The data support further efforts aiming at improved vaccine development against γHV infections with special focus on targeting protective CD4+ T cell responses.

Gammaherpesvirus Colonization of the Spleen Requires Lytic Replication in B Cells

Gammaherpesviruses infect lymphocytes and cause lymphocytic cancers. Murid herpesvirus-4 (MuHV-4), Epstein-Barr virus, and Kaposi's sarcoma-associated herpesvirus all infect B cells. Latent infection can spread by B cell recirculation and proliferation, but whether this alone achieves systemic infection is unclear. To test the need of MuHV-4 for lytic infection in B cells, we flanked its essential ORF50 lytic transactivator with loxP sites and then infected mice expressing B cell-specific Cre (CD19-Cre). The floxed virus replicated normally in Cre^- mice. In CD19-Cre mice, nasal and lymph node infections were maintained; but there was little splenomegaly, and splenic virus loads remained low. Cre-mediated removal of other essential lytic genes gave a similar phenotype. CD19-Cre spleen infection by intraperitoneal virus was also impaired. Therefore, MuHV-4 had to emerge lytically from B cells to colonize the spleen. An important role for B cell lytic infection in host colonization is consistent with the large CD8⁺ T cell responses made to gammaherpesvirus lytic antigens during infectious mononucleosis and suggests that vaccine-induced immunity capable of suppressing B cell lytic infection might reduce long-term virus loads.IMPORTANCE Gammaherpesviruses cause B cell cancers. Most models of host colonization derive from cell cultures with continuous, virus-driven B cell proliferation. However, vaccines based on these models have worked poorly. To test whether proliferating B cells suffice for host colonization, we inactivated the capacity of MuHV-4, a gammaherpesvirus of mice, to reemerge from B cells. The modified virus was able to colonize a first wave of B cells in lymph nodes but spread poorly to B cells in secondary sites such as the spleen. Consequently, viral loads remained low. These results were consistent with virus-driven B cell proliferation exploiting normal host pathways and thus having to transfer lytically to new B cells for new proliferation. We conclude that viral lytic infection is a potential target to reduce B cell proliferation.

Latent virus infection upregulates CD40 expression facilitating enhanced autoimmunity in a model of multiple sclerosis

Epstein-Barr virus (EBV) has been identified as a putative environmental trigger of multiple sclerosis (MS) by multiple groups working worldwide. Previously, we reported that when experimental autoimmune encephalomyelitis (EAE) was induced in mice latently infected with murine γ-herpesvirus 68 (γHV-68), the murine homolog to EBV, a disease more reminiscent of MS developed. Specifically, MS-like lesions developed in the brain that included equal numbers of IFN-γ producing CD4⁺ and CD8⁺ T cells and demyelination, none of which is observed in MOG induced EAE. Herein, we demonstrate that this enhanced disease was dependent on the γHV-68 latent life cycle and was associated with STAT1 and CD40 upregulation on uninfected dendritic cells. Importantly, we also show that, during viral latency, the frequency of regulatory T cells is reduced via a CD40 dependent mechanism and this contributes towards a strong T helper 1 response that resolves in severe EAE disease pathology. Latent γ-herpesvirus infection established a long-lasting impact that enhances subsequent adaptive autoimmune responses.

CD4 T cells specific for a latency-associated γ-herpesvirus epitope are polyfunctional and cytotoxic

The oncogenic γ-herpesviruses EBV and Kaposi sarcoma-associated herpesvirus are ubiquitous human pathogens that establish lifelong latent infections maintained by intermittent viral reactivation and reinfection. Effector CD4 T cells are critical for control of viral latency and in immune therapies for virus-associated tumors. In this study, we exploited γHV68 infection of mice to enhance our understanding of the CD4 T cell response during γ-herpesvirus infection. Using a consensus prediction approach, we identified 16 new CD4 epitope-specific responses that arise during lytic infection. An additional epitope encoded by the M2 protein induced uniquely latency-associated CD4 T cells, which were not detected at the peak of lytic infection but only during latency and were not induced postinfection with a latency-deficient virus. M2-specific CD4 T cells were selectively cytotoxic, produced multiple antiviral cytokines, and sustained IL-2 production. Identification of latency-associated cytolytic CD4 T cells will aid in dissecting mechanisms of CD4 immune control of γ-herpesvirus latency and the development of therapeutic approaches to control viral reactivation and pathology.

KSHV targeted therapy: an update on inhibitors of viral lytic replication

Kaposi’s sarcoma-associated herpesvirus (KSHV) is the causative agent of Kaposi’s sarcoma, primary effusion lymphoma and multicentric Castleman’s disease. Since the discovery of KSHV 20 years ago, there is still no standard treatment and the management of virus-associated malignancies remains toxic and incompletely efficacious. As the majority of tumor cells are latently infected with KSHV, currently marketed antivirals that target the virus lytic cycle have shown inconsistent results in clinic. Nevertheless, lytic replication plays a major role in disease progression and virus dissemination. Case reports and retrospective studies have pointed out the benefit of antiviral therapy in the treatment and prevention of KSHV-associated diseases. As a consequence, potent and selective antivirals are needed. This review focuses on the anti-KSHV activity, mode of action and current status of antiviral drugs targeting KSHV lytic cycle. Among these drugs, different subclasses of viral DNA polymerase inhibitors and compounds that do not target the viral DNA polymerase are being discussed. We also cover molecules that target cellular kinases, as well as the potential of new drug targets and animal models for antiviral testing.

An essential role for γ-herpesvirus latency-associated nuclear antigen homolog in an acute lymphoproliferative disease of cattle

Wildebeests carry asymptomatically alcelaphine herpesvirus 1 (AlHV-1), a γ-herpesvirus inducing malignant catarrhal fever (MCF) to several ruminant species (including cattle). This acute and lethal lymphoproliferative disease occurs after a prolonged asymptomatic incubation period after transmission. Our recent findings with the rabbit model indicated that AlHV-1 infection is not productive during MCF. Here, we investigated whether latency establishment could explain this apparent absence of productive infection and sought to determine its role in MCF pathogenesis. First, whole-genome cellular and viral gene expression analyses were performed in lymph nodes of MCF-developing calves. Whereas a severe disruption in cellular genes was observed, only 10% of the entire AlHV-1 genome was expressed, contrasting with the 45% observed during productive infection in vitro. In vivo, the expressed viral genes included the latency-associated nuclear antigen homolog ORF73 but none of the regions known to be essential for productive infection. Next, genomic conformation analyses revealed that AlHV-1 was essentially episomal, further suggesting that MCF might be the consequence of a latent infection rather than abortive lytic infection. This hypothesis was further supported by the high frequencies of infected CD8(+) T cells during MCF using immunodetection of ORF73 protein and single-cell RT-PCR approaches. Finally, the role of latency-associated ORF73 was addressed. A lack of ORF73 did not impair initial virus replication in vivo, but it rendered AlHV-1 unable to induce MCF and persist in vivo and conferred protection against a lethal challenge with a WT virus. Together, these findings suggest that a latent infection is essential for MCF induction.

Gammaherpesvirus latency induces antibody-associated thrombocytopenia in mice

Human herpesviruses establish lifelong latency. Viral recrudescence can lead to the development of cancers, immunoproliferative disorders, transplantation complications, and thrombocytopenia. Although platelet-specific autoantibodies have been reported in patients infected with the Epstein-Barr virus (EBV), the mechanisms by which thrombocytopenia is induced remain unclear, as do the relative contributions of lytic viral replication and latent viral gene expression. The human gammaherpesviruses are tightly restricted in their ability to infect other mammals, so they are difficult to study in live animal models. Here we show that infection of mice with murine gammaherpesvirus-68 (γHV68), a rodent-specific pathogen closely related to EBV, induces the production of platelet-binding antibodies and causes thrombocytopenia. Infection of antibody-deficient mice does not lead to thrombocytopenia, indicating the platelet decrease is mediated by antibody. Additionally, infection with a latency-null recombinant γHV68 does not induce thrombocytopenia, suggesting factors associated with viral latency drive the infection-induced antibody-mediated thrombocytopenia. These studies describe an important animal model of gammaherpesvirus-induced autoimmune thrombocytopenia and demonstrate that this pathology is mediated by antibody and dependent on viral latency. This model will allow studies of the underlying mechanisms of disease progression and the testing of therapeutic strategies for the alleviation of virus-induced thrombocytopenia.

Vaccine prospect of Kaposi sarcoma-associated herpesvirus

Infection of Kaposi sarcoma-associated herpesvirus (KSHV) or human herpesvirus-8 (HHV-8) is estimated to account for 34,000 new cancer cases globally. Unlike other herpesviruses, KSHV is not ubiquitous but is highly prevalent in some areas, such as sub-Saharan Africa where Kaposi sarcoma is the leading cancer among adults. While latent infection of KSHV plays a major and direct role in tumorigenesis, viral lytic replication also makes significant contributions to this process. Efforts to develop a KSHV vaccine are limited, but studies with EBV have provided important lessons. Informative vaccine research has been conducted in the mouse infection model of a closely related rodent virus, murine gammaherpesvirus-68 (MHV-68 or γHV-68). This mouse model has generated fundamental principles for an effective vaccination strategy. KSHV vaccines designed to prevent a naïve host from infection and to boost the immune control of KSHV in persistently infected people will have major impact on individuals who are at a high risk of developing KSHV-associated diseases.

The latency-associated nuclear antigen, a multifunctional protein central to Kaposi's sarcoma-associated herpesvirus latency

Latency-associated nuclear antigen (LANA) is encoded by the Kaposi's sarcoma (KS)-associated herpesvirus (KSHV) open reading frame 73. LANA is expressed during latent KSHV infection of cells, including tumor cells, such as primary effusion lymphoma, KS and multicentric Castleman's disease. Latently infected cells have multiple extrachromosomal copies of covalently closed circular KSHV genomes (episomes) that are stably maintained in proliferating cells. LANA's best characterized function is that of mediating episome persistence. It does so by binding terminal repeat sequences to the chromosomal matrix, thus ensuring episome replication with each cell division and efficient DNA segregation to daughter nuclei after mitosis. To achieve these functions, LANA associates with different host cell proteins, including chromatin-associated proteins and proteins involved in DNA replication. In addition to episome maintenance, LANA has transcriptional regulatory effects and affects cell growth. LANA exerts these functions through interactions with different cell proteins.

γ-Herpesvirus reactivation differentially stimulates epitope-specific CD8 T cell responses

The γ-herpesviruses are characterized by their ability to establish lifelong latency. Subsequent immune suppression leads to viral reactivation from latency and the onset of a variety of pathologies, including lymphoproliferative disease and cancers. CD8 T cells play a key role in preventing reactivation of latent virus. Therefore, to develop effective therapeutic immune strategies, it is essential to understand the maintenance of CD8 T cell responses during latency. Because the γ-herpesviruses are highly species-specific and mice cannot be infected with the human pathogens, EBV or Kaposi's sarcoma-associated herpesvirus, we have used a natural rodent γ-herpesvirus experimental infection model, γ-herpesvirus-68. In this report, we show that during long-term latent infection, naive CD8 T cells are recruited into the ongoing immune response in an epitope-specific manner. When virus reactivation is induced in vivo, the recruitment of CD8 T cells for some, but not all, epitopes is enhanced. The variation in recruitment is not due to differences in epitope presentation. We also show that CD8 T cells that are newly stimulated during reactivation are functionally impaired compared with acutely stimulated cells in terms of cytokine production. Thus, our results demonstrate unexpected complexity in the response of CD8 T cells specific for different viral epitopes that were stimulated during acute infection, quiescent latency, and reactivation.

Importance of antibody in virus infection and vaccine-mediated protection by a latency-deficient recombinant murine γ-herpesvirus-68

The human γ-herpesviruses EBV and Kaposi's sarcoma-associated herpesvirus establish lifelong latent infections, can reactivate in immunocompromised individuals, and are associated with the development of malignancies. Murine γ-herpesvirus-68 (γHV68), a rodent pathogen related to EBV and Kaposi's sarcoma-associated herpesvirus, provides an important model to dissect mechanisms of immune control and investigate vaccine strategies. Infection of mice with γHV68 elicits robust antiviral immunity, and long-term protection from γHV68 reactivation requires both cellular and humoral immune responses. Vaccination of mice with AC-replication and transcription activator (RTA), a highly lytic latency-null recombinant γHV68, results in complete protection from wild-type γHV68 infection that lasts for at least 10 mo. In this report, we examine the immune correlates of AC-RTA-mediated protection and show that sterilizing immunity requires both T cells and Ab. Importantly, Ab was also critical for mitigating viral infection in the brain, and in the absence of Ab-mediated control, amplification of the AC-RTA virus in the brain resulted in fatality. Our results highlight important considerations in the development of vaccination strategies based on live-attenuated viruses.

Persistent infection of a gammaherpesvirus in the central nervous system

Human gammaherpesvirus infections of the central nervous system (CNS) have been linked to various neurological diseases. Murine gammaherpesvirus 68 (MHV-68), genetically related and biologically similar to human gammaherpesviruses, infects the CNS in laboratory mice. However, viral persistency of MHV-68 has not been studied following CNS infection. In this study, we undertook the noninvasive bioluminescence imaging of a recombinant MHV-68 expressing the firefly luciferase (M3FL) to monitor virus progression after CNS infection. The M3FL virus inoculated in the brain systemically spread to the abdominal area in bioluminescence imaging, which was further confirmed by detection of viral genome and transcripts. The disseminated wild-type virus established latency in the spleen. Moreover, the treatment of the infected mice with CsA induced reactivation of latent MHV-68 from the brain and the spleen. Our results suggest that MHV-68 may persist both inside and outside the CNS once it gains access to the CNS.

Noninvasive biophotonic imaging for studies of infectious disease

According to World Health Organization estimates, infectious organisms are responsible for approximately one in four deaths worldwide. Animal models play an essential role in the development of vaccines and therapeutic agents but large numbers of animals are required to obtain quantitative microbiological data by tissue sampling. Biophotonic imaging (BPI) is a highly sensitive, nontoxic technique based on the detection of visible light, produced by luciferase-catalysed reactions (bioluminescence) or by excitation of fluorescent molecules, using sensitive photon detectors. The development of bioluminescent/fluorescent microorganisms therefore allows the real-time noninvasive detection of microorganisms within intact living animals. Multiple imaging of the same animal throughout an experiment allows disease progression to be followed with extreme accuracy, reducing the number of animals required to yield statistically meaningful data. In the study of infectious disease, the use of BPI is becoming widespread due to the novel insights it can provide into established models, as well as the impact of the technique on two of the guiding principles of using animals in research, namely reduction and refinement. Here, we review the technology of BPI, from the instrumentation through to the generation of a photonic signal, and illustrate how the technique is shedding light on infection dynamics in vivo.

Prospects of a novel vaccination strategy for human gamma-herpesviruses

Due to the oncogenic potential associated with persistent infection of human gamma-herpesviruses, including Epstein-Barr virus (EBV or HHV-4) and Kaposi's sarcoma-associated herpesvirus (KSHV or HHV-8), vaccine development has focused on subunit vaccines. However, the results using an animal model of mouse infection with a related rodent virus, murine gamma-herpesvirus 68 (MHV-68, γHV-68, or MuHV-4), have shown that the only effective vaccination strategy is based on live attenuated viruses, including viruses engineered to be incapable of establishing persistence. Vaccination with a virus lacking persistence would eliminate many potential complications. Progress in understanding persistent infections of EBV and KSHV raises the possibility of engineering a live attenuated virus without persistence. Therefore, we should keep the option open for developing a live EBV or KSHV vaccine.

Dendritic cells loaded with tumor B cells elicit broad immunity against murine gammaherpesvirus 68 but fail to prevent long-term latency

It is still unknown whether a noninfectious gammaherpesvirus vaccine is able to prevent or reduce virus persistence. This led us to use dendritic cells loaded with tumor B cells as a vaccine approach for the murine gammaherpesvirus 68 (gammaHV68) model of infection. Dendritic cells loaded with UV-irradiated latently infected tumor B cells induce broad, strong, and long-lasting immunity against gammaHV68. Dendritic cell vaccination prevents the enlargement of lymph nodes and severely limits acute infection and early latency but does not prevent gammaHV68 from establishing long-term latency. Our findings support the concept that attenuated viruses may be the best vaccine option for preventing gammaherpesvirus persistence.

Two kinetic patterns of epitope-specific CD8 T-cell responses following murine gammaherpesvirus 68 infection

Murine gammaherpesvirus 68 (gammaHV68) provides an important experimental model for understanding mechanisms of immune control of the latent human gammaherpesviruses. Antiviral CD8 T cells play a key role throughout three separate phases of the infection: clearance of lytic virus, control of the latency amplification stage, and prevention of reactivation of latently infected cells. Previous analyses have shown that T-cell responses to two well-characterized epitopes derived from ORF6 and ORF61 progress with distinct kinetics. ORF6(487)-specific cells predominate early in infection and then decline rapidly, whereas ORF61(524)-specific cells continue to expand through early latency, due to sustained epitope expression. However, the paucity of identified epitopes to this virus has limited our understanding of the overall complexities of CD8 T-cell immune control throughout infection. Here we screened 1,383 predicted H-2(b)-restricted peptides and identified 33 responses, of which 21 have not previously been reported. Kinetic analysis revealed a spectrum of T-cell responses based on the rapidity of their decline after the peak acute response that generally corresponded to the expression patterns of the two previously characterized epitopes. The slowly declining responses that were maintained during latency amplification proliferated more rapidly and underwent maturation of functional avidity over time. Furthermore, the kinetics of decline was accelerated following infection with a latency-null mutant virus. Overall, the data show that gammaHV68 infection elicits a highly heterogeneous CD8 T-cell response that segregates into two distinctive kinetic patterns controlled by differential epitope expression during the lytic and latency amplification stages of infection.

Kaposi Sarcoma Pathogenesis: A Triad of Viral Infection, Oncogenesis and Chronic Inflammation

Kaposi sarcoma (KS) is a complex cancer that arises from the initial infection of an appropriate endothelial or progenitor cell by Kaposi Sarcoma Herpesvirus/Human Herpesvirus-8 (KSHV/HHV8). However, the majority of KS cases occur when infected patients also suffer from some coincident form of immune deregulation, providing a favorable microenvironment for tumor development. Cellular hallmarks of KS progression include both the hyper-proliferation of KSHV-infected cells and the infiltration of immune modulatory cells into KS lesions, which together result in chronic inflammation, the induction of angiogenesis and tumor growth. This review describes the current understanding of the interactions between KSHV and host responses that result in this unusual cancer, along with existing treatments and prospects for future therapeutic approaches.

Vaccines to prevent infections by oncoviruses

It has been estimated that viruses are etiological agents in approximately 12% of human cancers. Most of these cancers can be attributed to infections by human papillomavirus (HPV), hepatitis B virus (HBV), hepatitis C virus (HCV), Epstein-Barr virus (EBV), and Kaposi's sarcoma-associated herpesvirus (KSHV). Prophylactic vaccines against other pathogenic viruses have an excellent record as public health interventions in terms of safety, effectiveness, and ability to reach economically disadvantaged populations. These considerations should prompt efforts to develop and implement vaccines against oncoviruses. Safe and effective HBV and HPV vaccines, based on virus-like particles, are commercially available, and the major focus is now on vaccine delivery, especially to low-resource settings. HCV and EBV vaccines are under active development, but few clinical trials have been conducted, and none of the candidate vaccines has proven to be sufficiently effective to warrant commercialization. Efforts to develop KSHV vaccines have been more limited.