Influence of proinflammatory cytokines and chemokines on the neuropathogenesis of oncornavirus and immunosuppressive lentivirus infections

Neuropeptide Y Negatively Influences Monocyte Recruitment to the Central Nervous System during Retrovirus Infection

Monocyte infiltration into the CNS is a hallmark of several viral infections of the central nervous system (CNS), including retrovirus infection. Understanding the factors that mediate monocyte migration in the CNS is essential for the development of therapeutics that can alter the disease process. In the current study, we found that neuropeptide Y (NPY) suppressed monocyte recruitment to the CNS in a mouse model of polytropic retrovirus infection. NPY(-/-) mice had increased incidence and kinetics of retrovirus-induced neurological disease, which correlated with a significant increase in monocytes in the CNS compared to wild-type mice. Both Ly6C(hi) inflammatory and Ly6C(lo) alternatively activated monocytes were increased in the CNS of NPY(-/-) mice following virus infection, suggesting that NPY suppresses the infiltration of both cell types. Ex vivo analysis of myeloid cells from brain tissue demonstrated that infiltrating monocytes expressed high levels of the NPY receptor Y2R. Correlating with the expression of Y2R on monocytes, treatment of NPY(-/-) mice with a truncated, Y2R-specific NPY peptide suppressed the incidence of retrovirus-induced neurological disease. These data demonstrate a clear role for NPY as a negative regulator of monocyte recruitment into the CNS and provide a new mechanism for suppression of retrovirus-induced neurological disease.

IMPORTANCE

Monocyte recruitment to the brain is associated with multiple neurological diseases. However, the factors that influence the recruitment of these cells to the brain are still not well understood. In the current study, we found that neuropeptide Y, a protein produced by neurons, affected monocyte recruitment to the brain during retrovirus infection. We show that mice deficient in NPY have increased influx of monocytes into the brain and that this increase in monocytes correlates with neurological-disease development. These studies provide a mechanism by which the nervous system, through the production of NPY, can suppress monocyte trafficking to the brain and reduce retrovirus-induced neurological disease.

Immunochemistry of adenoviruses: limitations and new horizons of gene therapy

Adenoviruses have increasingly been recognized as significant viral pathogens causing high morbidity and mortality especially among immunocompromised individuals such as transplant recipients and AIDS patients. Through the infection process, after the adenovirus fiber and penton are bonded to cell surface receptors through special amino acid moieties, secondary messengers activate protein kinases, pro-inflammatory cytokines and chemokines. Serotype and species specific antibodies also are induced. Recombinant human adenoviruses have been pivotal in the development of gene therapy strategies and have shown a great promise for the treatment of genetic disorders and malignancies. Recent studies have enlightened their harmful immunological effects dependent on fiber and hexon polypeptide structure and receptor binding. Pre-existing antibodies or those elicited by vectors neutralize input recombinant adenovirus particles rendering them ineffective. Mediators induce serious even lethal side effects and cytotoxic reactions which extinguish transgene expression. To overcome these difficulties new strategies are required in the application of recombinant adenoviruses to redirect vector entry from the natural receptors to alternative binding sites or using rare human or animal adenovirus fiber molecules to modify the native fiber structure by altering amino acid structure and creating chimeric fibers. This requires searching for, isolating and characterizing new serotypes, mutants or variants for new generation vectors. Human adenovirus 1 feline isolate (feline adenovirus) might fulfil these criteria.

Inflammation in neuroviral diseases

During any viral infection of the central nervous system (CNS), the extent and nature of neural cell alterations are dictated by the localization of virus replication and, possibly, persistence. However, one additional source of CNS damage comes from the immune response that develops following CNS viral infection. Indeed, despite of its major role in controlling virus spread in the infected CNS, the immune system is equipped with numerous molecular effectors shared with the nervous system that may greatly alter the homeostasis and function of neural cells. Proinflammatory cytokines and metalloproteases belong to this inflammatory cascade. Besides neurovirulence, the crosstalk engaged between neural and immune cells is a major factor determining the outcome of neuroviral infections.

Neuropeptide Y has a protective role during murine retrovirus-induced neurological disease

Viral infections in the central nervous system (CNS) can lead to neurological disease either directly by infection of neurons or indirectly through activation of glial cells and production of neurotoxic molecules. Understanding the effects of virus-mediated insults on neuronal responses and neurotrophic support is important in elucidating the underlying mechanisms of viral diseases of the CNS. In the current study, we examined the expression of neurotrophin- and neurotransmitter-related genes during infection of mice with neurovirulent polytropic retrovirus. In this model, virus-induced neuropathogenesis is indirect, as the virus predominantly infects macrophages and microglia and does not productively infect neurons or astrocytes. Virus infection is associated with glial cell activation and the production of proinflammatory cytokines in the CNS. In the current study, we identified increased expression of neuropeptide Y (NPY), a pleiotropic growth factor which can regulate both immune cells and neuronal cells, as a correlate with neurovirulent virus infection. Increased levels of Npy mRNA were consistently associated with neurological disease in multiple strains of mice and were induced only by neurovirulent, not avirulent, virus infection. NPY protein expression was primarily detected in neurons near areas of virus-infected cells. Interestingly, mice deficient in NPY developed neurological disease at a faster rate than wild-type mice, indicating a protective role for NPY. Analysis of NPY-deficient mice indicated that NPY may have multiple mechanisms by which it influences virus-induced neurological disease, including regulating the entry of virus-infected cells into the CNS.

Detection of an infectious retrovirus, XMRV, in blood cells of patients with chronic fatigue syndrome

Chronic fatigue syndrome (CFS) is a debilitating disease of unknown etiology that is estimated to affect 17 million people worldwide. Studying peripheral blood mononuclear cells (PBMCs) from CFS patients, we identified DNA from a human gammaretrovirus, xenotropic murine leukemia virus-related virus (XMRV), in 68 of 101 patients (67%) as compared to 8 of 218 (3.7%) healthy controls. Cell culture experiments revealed that patient-derived XMRV is infectious and that both cell-associated and cell-free transmission of the virus are possible. Secondary viral infections were established in uninfected primary lymphocytes and indicator cell lines after their exposure to activated PBMCs, B cells, T cells, or plasma derived from CFS patients. These findings raise the possibility that XMRV may be a contributing factor in the pathogenesis of CFS.

Innate immunity in the pathogenesis of polytropic retrovirus infection in the central nervous system

Neuroinflammation, including astrogliosis, microgliosis, and the production of proinflammatory cytokines and chemokines is a common response in the central nervous system (CNS) to virus infection, including retrovirus infection. However, the contribution of this innate immune response in disease pathogenesis remains unresolved. Analysis of the neuroinflammatory response to polytropic retrovirus infection in the mouse has provided insight into the potential contribution of the innate immune response to retrovirus-induced neurologic disease. In this model, retroviral pathogenesis correlates with the induction of neuroinflammatory responses including the activation of astrocytes and microglia, as well as the production of proinflammatory cytokines and chemokines. Studies of the neurovirulent determinants of the polytropic envelope protein as well as studies with knockout mice suggest that retroviral pathogenesis in the brain is multifaceted and that cytokine and chemokine production may be only one mechanism of disease pathogenesis. Analysis of the activation of the innate immune response to retrovirus infection in the CNS indicates that toll-like receptor 7 (TLR7) is a contributing factor to retrovirus-induced neuroinflammation, but that other factors can compensate for the lack of TLR7 in inducing both neuroinflammation and neurologic disease.

Protection against retrovirus pathogenesis by SR protein inhibitors

Indole derivatives compounds (IDC) are a new class of splicing inhibitors that have a selective action on exonic splicing enhancers (ESE)-dependent activity of individual serine-arginine-rich (SR) proteins. Some of these molecules have been shown to compromise assembly of HIV infectious particles in cell cultures by interfering with the activity of the SR protein SF2/ASF and by subsequently suppressing production of splicing-dependent retroviral accessory proteins. For all replication-competent retroviruses, a limiting requirement for infection and pathogenesis is the expression of the envelope glycoprotein which strictly depends on the host splicing machinery. Here, we have evaluated the efficiency of IDC on an animal model of retroviral pathogenesis using a fully replication-competent retrovirus. In this model, all newborn mice infected with a fully replicative murine leukemia virus (MLV) develop erythroleukemia within 6 to 8 weeks of age. We tested several IDC for their ability to interfere ex vivo with MLV splicing and virus spreading as well as for their protective effect in vivo. We show here that two of these IDC, IDC13 and IDC78, selectively altered splicing-dependent production of the retroviral envelope gene, thus inhibiting early viral replication in vivo, sufficiently to protect mice from MLV-induced pathogenesis. The apparent specificity and clinical safety observed here for both IDC13 and IDC78 strongly support further assessment of inhibitors of SR protein splicing factors as a new class of antiretroviral therapeutic agents.

Ligand up-regulation does not correlate with a role for CCR1 in pathogenesis in a mouse model of non-lymphocyte-mediated neurological disease

CCR1 ligands, including CCL3, CCL5, and CCL7, are up-regulated in a number of neurological disorders in humans and animal models. CCR1 is expressed by multiple cell types in the central nervous system (CNS), suggesting that receptor signaling by neuronal cell types may influence pathogenesis. In the current study, the authors used a mouse model of retrovirus infection to study the contribution of CCR1 to neuropathogenesis in the absence of lymphocyte recruitment to the CNS. In this model, infection of neonatal mice with the neurovirulent retrovirus Fr98 results in increased expression of proinflammatory chemokines in the CNS, activation of glial cells, and development of severe neurological disease. Surprisingly, no difference in neuropathogenesis was observed between CCR1-sufficient and CCR1-deficient mice following infection with the neuropathogenic virus Fr98. CCR1 was also not necessary for control of virus replication in the brain or virus-induced activation of astroglia. Additionally, CCR1 deficiency did not affect the up-regulation of its ligands, CCL3, CCL5, or CCL7. Thus, CCR1 did not appear to have a notable role in Fr98-induced pathogenesis, despite the correlation between ligand expression and disease development. This suggests that in the absence of inflammation, CCR1 may have a very limited role in neuropathogenesis.

Severe acute respiratory syndrome coronavirus infection of mice transgenic for the human Angiotensin-converting enzyme 2 virus receptor

Animal models for severe acute respiratory syndrome (SARS) coronavirus infection of humans are needed to elucidate SARS pathogenesis and develop vaccines and antivirals. We developed transgenic mice expressing human angiotensin-converting enzyme 2, a functional receptor for the virus, under the regulation of a global promoter. A transgenic lineage, designated AC70, was among the best characterized against SARS coronavirus infection, showing weight loss and other clinical manifestations before reaching 100% mortality within 8 days after intranasal infection. High virus titers were detected in the lungs and brains of transgene-positive (Tg+) mice on days 1 and 3 after infection. Inflammatory mediators were also detected in these tissues, coinciding with high levels of virus replication. Lower virus titers were also detected in other tissues, including blood. In contrast, infected transgene-negative (Tg-) mice survived without showing any clinical illness. Pathologic examination suggests that the extensive involvement of the central nervous system likely contributed to the death of Tg+ mice, even though viral pneumonia was present. Preliminary studies with mice of a second lineage, AC63, in which the transgene expression was considerably less abundant than that in the AC70 line, revealed that virus replication was largely restricted to the lungs but not the brain. Importantly, despite significant weight loss, infected Tg+ AC63 mice eventually recovered from the illness without any mortality. The severity of the disease that developed in these transgenic mice--AC70 in particular--makes these mouse models valuable not only for evaluating the efficacy of antivirals and vaccines, but also for studying SARS coronavirus pathogenesis.