MicroRNAs (miRNAs) are noncoding RNAs that modulate cellular gene expression, primarily at the post-transcriptional level. We sought to examine the functional role of miR-155 in a model of viral-induced neuroinflammation.

Acute encephalomyelitis and immune-mediated demyelination were induced by intracranial injection with the neurotropic JHM strain of mouse hepatitis virus (JHMV) into C57BL/6 miR-155 ^+/+ wildtype (WT) mice or miR-155 ^−/− mice. Morbidity and mortality, viral load and immune cell accumulation in the CNS, and spinal cord demyelination were assessed at defined points post-infection. T cells harvested from infected mice were used to examine cytolytic activity, cytokine activity, and expression of certain chemokine receptors. To determine the impact of miR-155 on trafficking, T cells from infected WT or miR-155 ^−/− mice were adoptively transferred into RAG1 ^−/− mice, and T cell accumulation into the CNS was assessed using flow cytometry. Statistical significance was determined using the Mantel-Cox log-rank test or Student’s T tests.

Compared to WT mice, JHMV-infected miR-155 ^−/− mice developed exacerbated disease concomitant with increased morbidity/mortality and an inability to control viral replication within the CNS. In corroboration with increased susceptibility to disease, miR-155 ^−/− mice had diminished CD8⁺ T cell responses in terms of numbers, cytolytic activity, IFN-γ secretion, and homing to the CNS that corresponded with reduced expression of the chemokine receptor CXCR3. Both IFN-γ secretion and trafficking were impaired in miR-155 ^−/− , virus-specific CD4⁺ T cells; however, expression of the chemokine homing receptors analyzed on CD4⁺ cells was not affected. Except for very early during infection, there were not significant differences in macrophage infiltration into the CNS between WT and miR-155 ^−/− JHMV-infected mice, and the severity of demyelination was similar at 14 days p.i. between WT and miR-155 ^−/− JHMV-infected mice.

These findings support a novel role for miR-155 in host defense in a model of viral-induced encephalomyelitis. Specifically, miR-155 enhances antiviral T cell responses including cytokine secretion, cytolytic activity, and homing to the CNS in response to viral infection. Further, miR-155 can play either a host-protective or host-damaging role during neuroinflammation depending on the disease trigger.