Persistent viral infections are a major health concern. One obstacle inhibiting the clearance of persistent infections is functional inactivation of antiviral T cells. Although such immunosuppression occurs rapidly after infection, the mechanisms that induce the loss of T-cell activity and promote viral persistence are unknown. Herein we document that persistent viral infection in mice results in a significant upregulation of interleukin (IL)-10 by antigen-presenting cells, leading to impaired T-cell responses. Genetic removal of Il10 resulted in the maintenance of robust effector T-cell responses, the rapid elimination of virus and the development of antiviral memory T-cell responses. Therapeutic administration of an antibody that blocks the IL-10 receptor restored T-cell function and eliminated viral infection. Thus, we identify a single molecule that directly induces immunosuppression leading to viral persistence and demonstrate that a therapy to neutralize IL-10 results in T-cell recovery and the prevention of viral persistence.

In prion and Alzheimer's diseases, the roles played by amyloid versus nonamyloid deposits in brain damage remain unresolved. In scrapie-infected transgenic mice expressing prion protein (PrP) lacking the glycosylphosphatidylinositol (GPI) membrane anchor, abnormal protease-resistant PrPres was deposited as amyloid plaques, rather than the usual nonamyloid form of PrPres. Although PrPres amyloid plaques induced brain damage reminiscent of Alzheimer's disease, clinical manifestations were minimal. In contrast, combined expression of anchorless and wild-type PrP produced accelerated clinical scrapie. Thus, the PrP GPI anchor may play a role in the pathogenesis of prion diseases.

Understanding, treating, and preventing diseases caused by immunosuppression and/or persistent infections remain both a major challenge in biomedical research and an important health goal. For a virus or any infectious agent to persist, it must utilize strategies to suppress or evade the host's immune response. Here, we report that two dissimilar viruses employ a common maneuver to cause a profound immunosuppression. Measles virus (MV) and lymphocytic choriomeningitis virus (LCMV) interfere with dendritic cell (DC) development and expansion in vivo and in vitro. The underlying mechanism for this is through the generation of type I interferon (IFN) that acts via a signal transducer and activator of a transcription (STAT)2-dependent, but STAT1-independent, pathway. Thus, viruses subvert the known antiviral effect of type I IFN through STAT2-specific signaling to benefit their survival. These observations have implications for understanding and developing therapies to treat diseases caused by immunosuppression and/or persistent infections.

The CXC chemokine ligand 10 (CXCL10) is a non-ELR CXC chemokine that exerts a potent chemotactic effect on activated T cells through binding the receptor CXCR3 (5). CXCL10 is expressed within tissues following viral infection, suggesting an important role for this chemokine in host defense by contributing to lymphocyte activation, extravasation, and accumulation of virus-specific T cells within sites of infection. Indeed, recent studies with antibody-mediated targeting of CXCL10 and CXCL10 Ϫ/Ϫ mice demonstrated that the absence of CXCL10 function results in increased mortality accompanied by increased viral titers and reduced T-cell infiltration within the brains of mice infected intracerebrally with a murine coronavirus (mouse hepatitis virus [MHV]) (8,20). In addition, CXCL10 expression modulates the pathogenesis of liver disease in adenovirus-infected mice and transgenic mice capable of replicating hepatitis B virus by attracting CD8 ϩ T lymphocytes into the liver (2, 11).These studies indicate that CXCL10 functions as a sentinel molecule in host defense and is important in the development of a protective T-cell response following viral infection. Recent findings have also illustrated an important role for chemokines in innate defense following viral infection. For example, in addition to its chemotactic effect on T cells, CXCL10 has also been shown to induce natural killer (NK) cell migration following viral infection (2, 11, 22, 23). Expression of both CXCL10 and CXC chemokine ligand 9 (CXCL9) has been found to contribute to antiviral immune responses in the absence of T and B cells (24). Although these chemokines have a demonstrated direct antimicrobial effect (6), their protective effect following vaccinia virus infection was a result of enhanced NK cell trafficking and activation (24).Our laboratory is interested in the functional contributions of chemokines and chemokine receptors in both host defense and disease progression within the context of coronavirus infection of the central nervous system (CNS). Intracerebral infection of susceptible strains of mice with MHV results in an acute encephalomyelitis followed by a chronic immune-mediated demyelinating disease that is similar in pathology to the human demyelinating disease multiple sclerosis (16). A robust expression of chemokine genes occurs within the CNS following MHV infection that precedes and accompanies leukocyte entry (18).Although NK cells can be readily detected within the CNS early following MHV infection, their precise contributions to antiviral immune responses within the CNS have not been well established. Furthermore, CXCL10 is prominently expressed as early as day 1 postinfection, suggesting that this molecule may function in enhancing innate immune responses by attracting NK cells into the CNS. Therefore, to further understand the relationship between CXCL10 and the innate immune response to viral infection of the CNS, we constructed a recombinant MHV capable of expressing mouse CXCL10. Intracerebral infection of RAG1 Ϫ/Ϫ mice with t...

Chemokines induce the directional migration of targeted populations of leukocytes during periods of inflammation. Moreover, these molecules also regulate T-cell activation and differentiation following antigenic stimulation. In the present study, the contributions of the CC chemokine ligand 3 (CCL3) to the differentiation and migration of effector T cells in response to viral infection of the central nervous system (CNS) were analyzed. CCL3؊/؊ mice infected with mouse hepatitis virus exhibited a significant reduction of virus-specific CD8 ؉ T cells within the CNS, correlating with delayed viral clearance. Decreased infiltration of CD8 ؉ T cells into infected CCL3 ؊/؊ mice was associated with enhanced accumulation of primed CD8 ؉ T cells in cervical lymph nodes. Although virus-specific CD8 ؉ T cells from CCL3 ؊/؊ mice were CD44 high , they remained CD62L high and CD25 low , retained CCR7 expression, and contained limited transcripts of the proinflammatory chemokine receptors CCR5 and CXCR3 compared with virus-specific CD8 ؉ T cells from CCL3 ؉/؉ mice. Furthermore, the absence of CCL3 impaired the cytokine production and cytolytic activity of CD8 ؉ T cells. In addition, macrophage accumulation within the CNS was significantly decreased in infected CCL3 ؊/؊ mice, correlating with reduced demyelination. These results suggest that CCL3 not only mediates macrophage chemotaxis but also significantly enhances differentiation of primed CD8 ؉ T cells into effector cells and their release into circulation, thus potentiating effective migration to the site of infection. Induction of an effective immune response following viral infection depends on the activation of virus-specific CD8ϩ T cells that are able to migrate to infected tissues and eliminate virus. Differentiation of naïve T cells into antigen-specific effector T cells is generally accomplished by antigen stimulation and cytokine signaling within the secondary lymphoid tissues. Effector cells exit these tissues and migrate to the site of infection where they are capable of exerting diverse antiviral responses (2,5,17,37).The distinct migration patterns of naïve and effector T cells depends upon the expression of adhesion molecules called homing receptors on the surface of T cells. Expression of CD62L (L-selectin) and the chemokine receptor CCR7 contribute to the migration of naïve T cells into secondary lymphoid organs (13,33,35). Recruitment of effector T cells to sites of infection is accompanied by downregulation of expression of CD62L and CCR7 followed by the upregulation of CD25 (interleukin 2 receptor [IL-2R] ␣ chain) and CD44 as well as increased expression of proinflammatory chemokine receptors such as CXCR3 and CCR5 (27,34). T-cell differentiation ultimately results in migration to the site of infection and virus elimination via production of cytokines such as gamma interferon (IFN-␥) and/or cell-mediated cytolysis. Although the mechanisms contributing to the generation of effector CD8 ϩ T cells are not completely known, recent evidence indicates that expo...

Prion protein (PrP) is a required factor for susceptibility to transmissible spongiform encephalopathy or prion diseases. In transgenic mice, expression of prion protein (PrP) from another species often confers susceptibility to prion disease from that donor species. For example, expression of deer or elk PrP in transgenic mice has induced susceptibility to chronic wasting disease (CWD), the prion disease of cervids. In the current experiments, transgenic mice expressing two naturally occurring allelic variants of deer PrP with either glycine (G) or serine (S) at residue 96 were found to differ in susceptibility to CWD infection. G96 mice were highly susceptible to infection, and disease appeared starting as early as 160 days postinfection. In contrast, S96 mice showed no evidence of disease or generation of disease-associated protease-resistant PrP (PrPres) over a 600-day period. At the time of clinical disease, G96 mice showed typical vacuolar pathology and deposition of PrPres in many brain regions, and in some individuals, extensive neuronal loss and apoptosis were noted in the hippocampus and cerebellum. Extraneural accumulation of PrPres was also noted in spleen and intestinal tissue of clinically ill G96 mice. These results demonstrate the importance of deer PrP polymorphisms in susceptibility to CWD infection. Furthermore, this deer PrP transgenic model is the first to demonstrate extraneural accumulation of PrPres in spleen and intestinal tissue and thus may prove useful in studies of CWD pathogenesis and transmission by oral or other natural routes of infection.

We investigated extraneural manifestations in scrapie-infected transgenic mice expressing prion protein lacking the glycophosphatydylinositol membrane anchor. In the brain, blood, and heart, both abnormal protease-resistant prion protein (PrPres) and prion infectivity were readily detected by immunoblot and by inoculation into nontransgenic recipients. The titer of infectious scrapie in blood plasma exceeded 10(7) 50% infectious doses per milliliter. The hearts of these transgenic mice contained PrPres-positive amyloid deposits that led to myocardial stiffness and cardiac disease.

Phosphoinositide 3-kinase activation is important for lymphocyte proliferation and survival. Disrupting the gene that encodes the major phosphoinositide 3-kinase regulatory isoform p85α impairs B cell development and proliferation. However, T cell functions are intact in the absence of p85α. In this study, we test the hypothesis that the related isoform p85β is an essential regulatory subunit for T cell signaling. Unexpectedly, T cells lacking p85β showed a marked increase in proliferation and decreased death when stimulated with anti-CD3 plus IL-2. Both CD4+ and CD8+ T cells completed more cell divisions. Transcriptional profiling revealed reduced levels of caspase-6 mRNA in p85β-deficient T cells, which was paralleled by reduced caspase-6 enzyme activity. Increased T cell accumulation was also observed in vivo following infection of p85β-deficient mice with mouse hepatitis virus. Together, these results suggest a unique role for p85β in limiting T cell expansion.