Dissecting the host response to a gamma-herpesvirus

Viral interference with antigen presentation

CD8+ T cells play an important role in immunity to viruses. Just how important these cells are is demonstrated by the evolution of viral strategies for blocking the generation or display of peptide-major histocompatibility complex class I complexes on the surfaces of virus-infected cells. Here, we focus on viral interference with antigen presentation; in particular we consider the importance (and difficulty) of establishing the evolutionary significance (that is, the ability to enhance viral transmission) of viral gene products that interfere with antigen presentation in vitro.

A battle for survival: immune control and immune evasion in murine gamma-herpesvirus-68 infection

CD8(+) T cells are generally considered a key defence against herpesviruses. The murine gamma-herpesvirus-68 encodes two proteins that limit their efficacy. M3 neutralizes chemokines, while K3 downregulates MHC class I glycoproteins. The consequence of this evasion is that CD4(+) T cells are essential to the control of persistent infection.

Factors controlling levels of CD8+ T-cell lymphocytosis associated with murine gamma-herpesvirus infection

Intranasal infection of mice with murine gamma-herpesvirus 68 (MHV-68) elicits a striking CD8+ T-cell lymphocytosis following the establishment of latency, which includes a marked increased frequency of Vbeta4+ CD8+ T cells. The Vbeta4+ CD8+ T cells do not recognize a conventional viral peptide, but are stimulated by an uncharacterized ligand expressed on latently infected, activated B cells. The selective expansion of Vbeta4+ CD8+ T cells after MHV-68 infection is observed in all mouse strains examined, although the fold-increase varies widely, ranging from less than twofold to greater than 10-fold. The factors controlling the variation are currently undefined. In the current study, CD8+ T cell activation and Vbeta4+ CD8+ T-cell frequencies were analyzed in 18 inbred strains of mice. The data show that the magnitude of the Vbeta4+ CD8+ T-cell response correlates with the degree of CD8+ T cell-activation, and that both major histocompatibility complex (MHC) and non-MHC genes contribute to the magnitude of the activation. Furthermore, the magnitude of the response does not reflect major differences in susceptibility to viral infection and/or corresponding differences in the acute response. Rather the degree of Vbeta4+ CD8+ T cell activation may be determined by differences in levels of expression of the stimulatory ligand at the peak of latency.

K3-mediated evasion of CD8(+) T cells aids amplification of a latent gamma-herpesvirus

The murine gamma-herpesvirus-68 (MHV-68) K3 protein, like that of the Kaposi's sarcoma associated herpesvirus, down-regulates major histocompatibility complex (MHC) class I expression. However, how this contributes to viral replication in vivo is unclear. After intranasal MHV-68 infection, K3 was transcribed both during acute lytic infection in the lung and during latency establishment in lymphoid tissue. K3-deficient viruses were not cleared more rapidly from the lung, but the number of latently infected spleen cells was reduced and the frequency of virus-specific CD8(+) cytotoxic T lymphocytes (CTLs) was increased. CTL depletion reversed the viral latency deficit. Thus, a major function of K3 appears to be CTL evasion during viral latency expansion.

Characterization of a spontaneous 9.5-kilobase-deletion mutant of murine gammaherpesvirus 68 reveals tissue-specific genetic requirements for latency

Murine gammaherpesvirus 68 (gammaHV68 [also known as MHV-68]) establishes a latent infection in mice, providing a small-animal model with which to identify host and viral factors that regulate gammaherpesvirus latency. While gammaHV68 establishes a latent infection in multiple tissues, including splenocytes and peritoneal cells, the requirements for latent infection within these tissues are poorly defined. Here we report the characterization of a spontaneous 9.5-kb-deletion mutant of gammaHV68 that lacks the M1, M2, M3, and M4 genes and eight viral tRNA-like genes. Previously, this locus has been shown to contain the latency-associated M2, M3, and viral tRNA-like genes. Through characterization of this mutant, we found that the M1, M2, M3, M4 genes and the viral tRNA-like genes are dispensable for (i) in vitro replication and (ii) the establishment and maintenance of latency in vivo and reactivation from latency following intraperitoneal infection. In contrast, following intranasal infection with this mutant, there was a defect in splenic latency at both early and late times, a phenotype not observed in peritoneal cells. These results indicate (i) that there are different genetic requirements for the establishment of latency in different latent reservoirs and (ii) that the genetic requirements for latency depend on the route of infection. While some of these phenotypes have been observed with specific mutations in the M1 and M2 genes, other phenotypes have never been observed with the available gammaHV68 mutants. These studies highlight the importance of loss-of-function mutations in defining the genetic requirements for the establishment and maintenance of herpesvirus latency.

Gammaherpesvirus lytic gene expression as characterized by DNA array

Gammaherpesviruses are associated with a number of diseases including lymphomas and other malignancies. Murine gammaherpesvirus 68 (MHV-68) constitutes the most amenable animal model for this family of pathogens. However experimental characterization of gammaherpesvirus gene expression, at either the protein or RNA level, lags behind that of other, better-studied alpha- and beta-herpesviruses. We have developed a cDNA array to globally characterize MHV-68 gene expression profiles, thus providing an experimental supplement to a genome that is chiefly annotated by homology. Viral genes started to be transcribed as early as 3 h postinfection (p.i.), and this was followed by a rapid escalation of gene expression that could be seen at 5 h p.i. Individual genes showed their own transcription profiles, and most genes were still being expressed at 18 h p.i. Open reading frames (ORFs) M3 (chemokine-binding protein), 52, and M9 (capsid protein) were particularly noticeable due to their very high levels of expression. Hierarchical cluster analysis of transcription profiles revealed four main groups of genes and allowed functional predictions to be made by comparing expression profiles of uncharacterized genes to those of genes of known function. Each gene was also categorized according to kinetic class by blocking de novo protein synthesis and viral DNA replication in vitro. One gene, ORF 73, was found to be expressed with alpha-kinetics, 30 genes were found to be expressed with beta-kinetics, and 42 genes were found to be expressed with gamma-kinetics. This fundamental characterization furthers the development of this model and provides an experimental basis for continued investigation of gammaherpesvirus pathology.

Antibody-mediated control of persistent gamma-herpesvirus infection

The human gamma-herpesviruses, EBV and Kaposi's sarcoma-associated herpesvirus, establish life-long latency and can reactivate in immunocompromised individuals. T cells play an important role in controlling persistent EBV infection, whereas a role for humoral immunity is less clear. The murine gamma-herpesvirus-68 has biological and structural similarities to the human gamma-herpesviruses, and provides an important in vivo experimental model for dissecting mechanisms of immune control. In the current studies, CD28(-/-) mice were used to address the role of Abs in control of persistent murine gamma-herpesvirus-68 infection. Lytic infection was controlled in the lungs of CD28(-/-) mice, and latency was maintained in B cells at normal frequencies. Although class-switched virus-specific Abs were initially generated in the absence of germinal centers, titers and viral neutralizing activity rapidly waned. T cell depletion in CD28(-/-) mice with compromised Ab responses, but not in control mice with intact Ab responses, resulted in significant recrudescence from latency, both in the spleen and the lung. Recrudescence could be prevented by passive transfer of immune serum. These data directly demonstrate an important contribution of humoral immunity to control of gamma-herpesvirus latency, and have significant implications for clinical intervention.

Reduced functional capacity of CD8+ T cells expanded by post-exposure vaccination of gamma-herpesvirus-infected CD4-deficient mice

Mice (I-A(b-/-)) that lack CD4(+) T cells remain healthy for at least three months after respiratory exposure to the murine gamma-herpesvirus 68 (gammaHV68), then succumb with symptoms of chronic wasting disease. Postexposure challenge of gammaHV68-infected I-A(b+/+) and I-A(b-/-) mice with a recombinant vaccinia virus (Vacc-p56) expressing an antigenic gammaHV68 peptide caused a massive increase in the numbers of D(b)p56-specific CD8(+) T cells. Previous experiments showed that, despite the large numbers of potential CTL effectors, there was little effect on the long-term survival of the CD4-deficient group and no diminution in the level of persistent virus shedding and latency. Comparison of the expanded CD8(+)D(b)p56(+) sets in the I-A(b+/+) and I-A(b-/-) mice indicated that these two T cell populations were not identical. More CD69(high)CD8(+) D(b)p56(+) T cells were found in the CD4-deficient mice, an effect that might be thought to reflect higher Ag load. By contrast, the mean fluorescence intensity of staining for the CD44 glycoprotein was diminished on CD8(+)D(b)p56(+) T cells from the I-A(b-/-) group, the level of CTL activity was lower on a per cell basis, and the relative prevalence of IFN-gamma(+)TNF-alpha(+) T cells detected after in vitro stimulation with the p56 peptide was decreased. Given that this experimental system provides an accessible model for evaluating postexposure vaccination protocols that might be used in diseases like HIV/AIDS, the further need is to clarify the underlying molecular mechanisms and the relative significance of lack of CD4(+) T help vs higher Ag load for these expanded CD8(+) effector populations.

Tracking T cells with tetramers: new tales from new tools

To understand the success or failure of immune responses against pathogens or tumours requires the direct measurement of specific lymphocytes. Recently, there has been an explosion of data in this field through the use of several new tools for measuring the number and function of T cells. This has allowed immunologists who study human disease and mouse models of infection and cancer to readily track specific T cells--in both time and space. Although there are common patterns, over time, each host-pathogen relationship seems to develop unique characteristics, as reflected in the quality of the T-cell response.