Targeted blockade in lethal West Nile virus encephalitis indicates a crucial role for very late antigen (VLA)-4-dependent recruitment of nitric oxide-producing macrophages

Abstract

Infiltration of Ly6Chi monocytes from the blood is a hallmark of viral encephalitis. In mice with lethal encephalitis caused by West Nile virus (WNV), an emerging neurotropic flavivirus, inhibition of Ly6Chi monocyte trafficking into the brain by anti-very late antigen (VLA)-4 integrin antibody blockade at the time of first weight loss and leukocyte influx resulted in long-term survival of up to 60% of infected mice, with subsequent sterilizing immunity. This treatment had no effect on viral titers but appeared to be due to inhibition of Ly6Chi macrophage immigration. Although macrophages isolated from the infected brain induced WNV-specific CD4+ T-cell proliferation, T cells did not directly contribute to pathology, but are likely to be important in viral control, as antibody-mediated T-cell depletion could not reproduce the therapeutic benefit of anti-VLA-4. Instead, 70% of infiltrating inflammatory monocyte-derived macrophages were found to be making nitric oxide (NO). Furthermore, aminoguanidine-mediated inhibition of induced NO synthase activity in infiltrating macrophages significantly prolonged survival, indicating involvement of NO in the immunopathology. These data show for the first time the therapeutic effects of temporally targeting pathogenic NO-producing macrophages during neurotropic viral encephalitis.

Regional astrocyte IFN signaling restricts pathogenesis during neurotropic viral infection

“…This role is supported by studies demonstrating that pharmacological blockade of VLA-4 in the context of neurotropic viral infection can ameliorate disease, largely via suppression of CNS immunopathology (38,53). We note, however, that the experimental consequences of VLA-4 blockade are likely context dependent, varying by disease model and timing of treatment after infection, and that this blockade may act through additional VLA-4 ligands beyond VCAM-1, including JAM-B and fibronectin (56).…”

“…For example, several recent studies have described a role for type I IFN in suppressing the expression of IL-1β (4,49,50), which is known to induce VCAM-1 expression in astrocytes (51,52). VCAM-1-VLA-4 interactions facilitate immune trafficking during CNS infection (38,53). Although CNS immune infiltration is required for viral clearance following WNV infection (9,54), high levels of inflammatory cell infiltration are associated with pathology and bystander injury (11,45,55 (4).…”

“…While VCAM-1 expression on vascular endothelium is a well-known regulator of neuroinflammation (35), VCAM-1 expression on astrocytes has also been shown to promote neuroinflammation by enhancing the penetration of infiltrating leukocytes out of perivascular spaces and into the CNS parenchyma (36,37). Moreover, pharmacological blockade of VCAM-1 binding to its ligand VLA-4 has previously been shown to decrease pathogenic neuroinflammation and improve survival following s.c. WNV infection (38). Given that IFNAR signaling in astrocytes downregulated VCAM-1 expression in the cerebellum following WNV infection, we questioned whether VCAM-1 expression contributed to the enhanced immune infiltration and immunopathol- Relative transcript levels in tissue homogenates were measured by SYBR qRT-PCR for the indicated cytokines and chemokines.…”

Harnessing Nanoparticles for Immune Modulation

“…Once in the target tissue, these cells may differentiate into different effector cells, including DCs, macrophages, and even cells with a microglial phenotype [4,76], depending on the prevailing soluble factor milieu. In some cases these cells have important housekeeping functions (i.e., to digest and clear tissue debris before tissue remodelling); however, in many cases they express high levels of NO via induced NOS2 expression, as well as increased levels of NADPH oxidase, cathepsins, and myeloperoxidase, all of which may demonstrably contribute to further tissue damage in numerous conditions, including experimental autoimmune encephalomyelitis, myocardial infarction, and viral encephalitis [8,77,78]. Such monocyte management has recently emerged as a unique potential treatment modality to limit damage associated with the acute phase of inflammation in numerous disease indications.…”

The West Nile Virus-Like Flavivirus Koutango Is Highly Virulent in Mice due to Delayed Viral Clearance and the Induction of a Poor Neutralizing Antibody Response

“…The majority of murine studies have focused on the virulent lineage I North American strain of WNV, namely, NY99 (WNV NY99 ), or the lineage II Sarafend strain (30)(31)(32)(33), and little is known about the pathogenesis of other strains of WNV, especially those viruses that branch outside lineages I and II.…”