Type I interferons (IFNs), IFN-α and IFN-β, represent the major effector
cytokines of the host immune response against viruses and other intracellular pathogens. These
cytokines are produced via activation of numerous
pattern recognition receptors, including the
Toll-like receptor signaling network,
retinoic acid-inducible gene-1 (RIG-1),
melanoma differentiation-associated protein-5 (MDA-5) and
interferon gamma-inducible protein-16 (IFI-16). Whilst the contribution of type I IFNs to peripheral immunity is well documented, they can also be produced by almost every cell in the central nervous system (CNS). Furthermore, IFNs can reach the CNS from the periphery to modulate the function of not only microglia and astrocytes, but also neurons and oligodendrocytes, with major consequences for cognition and behavior. Given the pleiotropic nature of type I IFNs, it is critical to determine their exact cellular impact. Inappropriate upregulation of type I IFN signaling and
interferon-stimulated gene expression have been linked to several
CNS diseases termed "interferonopathies" including
Aicardi-Goutieres syndrome and
ubiquitin specific peptidase 18 (USP18)-deficiency. In contrast, in the CNS of mice with virus-induced
neuroinflammation, type I IFNs can limit production of other
cytokines to prevent potential damage associated with chronic
cytokine expression. This capacity of type I IFNs could also explain the therapeutic benefits of exogenous type I IFN in chronic CNS
autoimmune diseases such as
multiple sclerosis. In this review we will highlight the importance of a well-balanced level of type I IFNs for healthy brain physiology, and to what extent dysregulation of this
cytokine system can result in brain 'interferonopathies'.