T-cell responses to the M3 immune evasion protein of murid gammaherpesvirus 68 are partially protective and induced with lytic antigen kinetics

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.

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.

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.

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.

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.

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

Human gammaherpesviruses, Epstein-Barr virus, and human herpesvirus 8/Kaposi's sarcoma-associated herpesvirus are important pathogens associated with diseases, including lymphomas and other malignancies. Murine gammaherpesvirus 68 (MHV-68) is used as an experimental model system to study the host immune control of infection and explore novel vaccine strategies based on latency-deficient live viruses. We studied the properties and the potential of a recombinant MHV-68 (AC-RTA) in which the genes required for persistent infection were replaced by a constitutively expressed viral transcription activator, RTA, which dictates the virus to lytic replication. After intranasal infection of mice, replication of AC-RTA in the lung was attenuated, and no AC-RTA virus or viral DNA was detected in the isolated splenocytes, indicating a lack of latency in the spleen. Infection of the AC-RTA virus elicited both cellular immune responses and virus-specific IgG at a level comparable to that elicited by infection of the wild-type virus. Importantly, vaccination of AC-RTA was able to protect mice against subsequent challenge by the wild-type MHV-68. AC-RTA provides a vaccine strategy for preventing infection of human gammaherpesviruses. Furthermore, our results suggest that immunity to the major latent antigens is not required for protection.

Cell-mediated immunity to Toxoplasma gondii develops primarily by local Th1 host immune responses in the absence of parasite replication

A single inoculation of mice with the live, attenuated Toxoplasma gondii uracil auxotroph strain cps1-1 induces long-lasting immunity against lethal challenge with hypervirulent strain RH. The mechanism for this robust immunity in the absence of parasite replication has not been addressed. The mechanism of long-lasting immunity, the importance of route of immunization, cellular recruitment to the site of infection, and local and systemic inflammation were evaluated. Our results show that infection with cps1-1 elicits long-lasting CD8+ T cell- mediated immunity. We show that immunization with cps1-1-infected dendritic cells elicits long-lasting immunity. Intraperitoneal infection with cps1-1 induced a rapid influx of GR1+ neutrophils and two stages of GR1+CD68+ inflammatory monocyte infiltration into the site of inoculation. CD19+ B cells and CD3+ T cells steadily increase for 8 days after infection. CD8+ T cells were rapidly recruited to the site of infection and increased faster than CD4+ T cells. Surprisingly, cps1-1 infection induced high systemic levels of bioactive IL-12p70 and a very low level and transient systemic IFN-gamma. Furthermore, we show significant levels of these inflammatory cytokines were locally produced at the site of cps1-1 inoculation. These findings offer new insight into immunological mechanisms and local host responses to a non-replicating type I parasite infection associated with development of long-lasting immunity to Toxoplasma gondii.

Comparison of pathogenic properties of the murid gammaherpesvirus (MuHV 4) strains: a role for immunomodulatory proteins encoded by the left (5′-)end of the genome

The murid herpesvirus 4 (MuHV 4) species encompasses 7 isolates, out of which at least two (MHV-68, MHV-72) became in vitro propagated laboratory strains. Following intranasal inoculation, MuHV 4 induces an acute infectious mononucleosis-like syndrome with elevated levels of peripheral blood leukocytes, shifts in the relative proportion of lymphocytes along with the appearance of atypical mononuclear cells. At least two isolates exhibited spontaneous deletions at the left hand (5′-end) of their genome, resulting in the absence of M1, M2, M3 genes (strain MHV-72) and also of the M4 gene (strain MHV-76). Based on DNA sequence amplifications only, another two isolates (MHV-Šum and MHV-60) were shown to possess similar deletions of varying length. During latency (until 24 months post-infection), the mice infected with any MuHV 4 isolate (except MHV-76) developed lymphoproliferative disorders. The lack of tumor formation in MHV-76 infected mice was associated with persistent virus production at late post-infection intervals. In addition to careful analysis of spontaneously occurring 5′-end genome defects, our knowledge of the function of 5′-end genes relies on the behaviour of mutants with corresponding deletions and/or insertions. While M2 and M3 genes encode immune evasion proteins, M4 codes for a soluble glycopeptide acting as immunomodulator and/or immunostimulator.