Two separate mechanisms of enforced viral replication balance innate and adaptive immune activation

The induction of innate and adaptive immunity is essential for controlling viral infections. Limited or overwhelming innate immunity can negatively impair the adaptive immune response. Therefore, balancing innate immunity separately from activating the adaptive immune response would result in a better antiviral immune response. Recently, we demonstrated that Usp18-dependent replication of virus in secondary lymphatic organs contributes to activation of the innate and adaptive immune responses. Whether specific mechanisms can balance innate and adaptive immunity separately remains unknown. In this study, using lymphocytic choriomeningitis virus (LCMV) and replication-deficient single-cycle LCMV vectors, we found that viral replication of the initial inoculum is essential for activating virus-specific CD8⁺ T cells. In contrast, extracellular distribution of virus along the splenic conduits is necessary for inducing systemic levels of type I interferon (IFN-I). Although enforced virus replication is driven primarily by Usp18, B cell–derived lymphotoxin beta contributes to the extracellular distribution of virus along the splenic conduits. Therefore, lymphotoxin beta regulates IFN-I induction independently of CD8⁺ T-cell activity. We found that two separate mechanisms act together in the spleen to guarantee amplification of virus during infection, thereby balancing the activation of the innate and adaptive immune system.     

Production and biomedical applications of virus-like particles derived from polyomaviruses

Virus-like particles (VLPs), aggregates of capsid proteins devoid of viral genetic material, show great promise in the fields of vaccine development and gene therapy. These particles spontaneously self-assemble after heterologous expression of viral structural proteins. This review will focus on the use of virus-like particles derived from polyomavirus capsid proteins. Since their first recombinant production 27 years ago these particles have been investigated for a myriad of biomedical applications. These virus-like particles are safe, easy to produce, can be loaded with a broad range of diverse cargos and can be tailored for specific delivery or epitope presentation. We will highlight the structural characteristics of polyomavirus-derived VLPs and give an overview of their applications in diagnostics, vaccine development and gene delivery.     

Self-Renewal and Toll-like Receptor Signaling Sustain Exhausted Plasmacytoid Dendritic Cells during Chronic Viral Infection

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Residual LCMV antigen in transiently CD4+ T cell‐depleted mice induces high levels of virus‐specific antibodies but only limited B‐cell memory

Infection of C57BL/6 mice with lymphocytic choriomeningitis virus (LCMV) strain Armstrong (Arm) induces an acute infection with rapid virus clearance by CD8⁺ T cells independently of CD4⁺ T cell help. Residual viral antigen may, however, persist for a prolonged time. Here, we demonstrate that mice that had been transiently depleted of CD4⁺ T cells during acute LCMV Arm infection generated high levels of virus‐specific IgG antibodies (Ab) after viral clearance. Robust induction of LCMV‐specific IgG after transient CD4⁺ T cell depletion was dependent on Fcγ receptors but not on the complement receptors CD21/CD35. In contrast to the potent production of LCMV‐specific IgG, the generation of LCMV‐specific isotype‐switched memory B cells after transient CD4⁺ T cell depletion was considerably reduced. Moreover, mice depleted of CD4⁺ T cells during acute infection were strongly impaired in generating a secondary LCMV‐specific B cell response upon LCMV rechallenge. In conclusion, our data indicate that LCMV antigen depots after viral clearance were capable of inducing high levels of virus‐specific IgG. They failed, however, to induce robust virus‐specific B cell memory revealing a previously unappreciated dichotomy of specific Ab production and memory cell formation after priming with residual antigen.     

IL‐33 promotes innate IFN‐γ production and modulates dendritic cell response in LCMV‐induced hepatitis in mice

Recent studies have revealed IL‐33 as a key factor in promoting antiviral T‐cell responses. However, it is less clear as to how IL‐33 regulates innate immunity. In this study, we infected wild‐type (WT) and IL‐33^?/? mice with lymphocytic choriomeningitis virus and demonstrated an essential role of infection‐induced IL‐33 expression for robust innate IFN‐γ production in the liver. We first show that IL‐33 deficiency resulted in a marked reduction in the number of IFN‐γ⁺ γδ T and NK cells, but an increase in that of IL‐17⁺ γδ T cells at 16 h postinfection. Recombinant IL‐33 (rIL‐33) treatment could reverse such deficiency via increasing IFN‐γ‐producing γδ T and NK cells, and inhibiting IL‐17⁺ γδ T cells. We also found that rIL‐33‐induced type 2 innate lymphoid cells were not involved in T‐cell responses and liver injury, since the adoptive transfer of type 2 innate lymphoid cells neither affected the IFN‐γ and TNF‐α production in T cells, nor liver transferase levels in lymphocytic choriomeningitis virus infected mice. Interestingly, we found that while IL‐33 was not required for costimulatory molecule expression, it was critical for DC proliferation and cytokine production. Together, this study highlights an essential role of IL‐33 in regulating innate IFN‐γ‐production and DC function during viral hepatitis.     

Prevention of CD8 T Cell Deletion during Chronic Viral Infection

During chronic viral infections, CD8 T cells rapidly lose antiviral and immune-stimulatory functions in a sustained program termed exhaustion. In addition to this loss of function, CD8 T cells with the highest affinity for viral antigen can be physically deleted. Consequently, treatments designed to restore function to exhausted cells and control chronic viral replication are limited from the onset by the decreased breadth of the antiviral T cell response. Yet, it remains unclear why certain populations of CD8 T cells are deleted while others are preserved in an exhausted state. We report that CD8 T cell deletion during chronic viral infection can be prevented by therapeutically lowering viral replication early after infection. The initial resistance to deletion enabled long-term maintenance of antiviral cytolytic activity of the otherwise deleted high-affinity CD8 T cells. In combination with decreased virus titers, CD4 T cell help and prolonged interactions with costimulatory molecules B7-1/B7-2 were required to prevent CD8 T cell deletion. Thus, therapeutic strategies to decrease early virus replication could enhance virus-specific CD8 T cell diversity and function during chronic infection.     

CD8 T cell immunodominance shifts during the early stages of acute LCMV infection independently from functional avidity maturation

Virus-specific T cell responses are often directed to a small subset of possible epitopes and their relative magnitude defines their hierarchy. We determined the size and functional avidity of 4 representative peptide-specific CD8⁺ T cell populations in C57BL/6 mice at different time points after lymphocytic choriomeningitis virus (LCMV) infection. We found that the frequency of different peptide-specific T cell populations in the spleen changed independently over the first 8 days after infection. These changes were not associated with a larger or more rapid increase in functional avidity and yet still resulted in a shift in the final immunodominance hierarchy. Thus, the immunodominance observed at the peak of an antiviral T cell response is not necessarily determined by the initial size or rate of functional avidity maturation of peptide-specific T cell populations.     

Serological study of the lymphochoriomeningitis virus (LCMV) in an inner city of Argentina

Lymphocytic choriomeningitis virus (LCMV) is the prototype of the family Arenaviridae and is associated with the natural reservoir, Mus domesticus (Md). It causes meningitis and a flu-like illness characterized by malaise, myalgia, retrorbital headache, and photophobia. This study presents the data obtained in a rodent and human serological study during 6 years (1998-2003) in the city of Rio Cuarto, Argentina. Antibodies anti-LCMV were sought by ELISA in rodents and humans. LCMV was found only in Md species in 9.4% of animals. The results also show some seasonal, no significant variations in the prevalence of the infection. Distribution of positive mice was not modified significantly by trapping sites, sex, or age of the animals. The prevalence of LCMV positive urban residents was found to be consistently low (1-3.6%) along the study period, with overage prevalence of 3.3% and values in males (4.6%) significantly higher than in females (2.6%) (P < 0.05). Seven of 432 pregnant women were found to be LCMV positive, but the absence of LCMV antibodies in the newborns demonstrated that the mothers were infected before pregnancy. This study is the first evidence on endemic LCMV in an Argentine city located outside the endemic area of Argentine hemorrhagic fever (AHF) and described the need to study other areas and increase awareness of this viral infection.     

Commentary: T cells get by with a little help from their friends

In animal models, lymphocytic choriomeningitis virus (LCMV) may be controlled after acute infection or may establish various levels of persistence. Cytotoxic responses mediated by CD8(+) T cells are responsible for both initial control of LCMV and for immunopathology. As discussed in this article, there is emerging evidence that the levels of antigen to which the immune system is exposed over time are important in controlling CD8(+) T cell activation, memory responses and exhaustion, and that these levels are affected by the efficiency of T cell help and the presence of antibody. To enable lasting control of LCMV infection, CD8(+) T cells, CD4(+) T cell help and B cells are all required. These findings have important implications for the prevention and treatment of infection by viruses such as hepatitis B and C viruses, cytomegalovirus and HIV. See accompanying article http://dx.doi.org/10.1002/eji.200324717