Oligodendroglia are limited in type I interferon induction and responsiveness in vivo

Abstract

Type I interferons (IFNα/β) provide a primary defense against infection. Nevertheless, the dynamics of IFNα/β induction and responsiveness by central nervous system (CNS) resident cells in vivo in response to viral infections are poorly understood. Mice were infected with a neurotropic coronavirus with tropism for oligodendroglia and microglia to probe innate antiviral responses during acute encephalomyelitis. Expression of genes associated with the IFNα/β pathways was monitored in microglia and oligodendroglia purified from naïve and infected mice by fluorescent activated cell sorting. Compared with microglia, oligodendroglia were characterized by low basal expression of mRNA encoding viral RNA sensing pattern recognition receptors (PRRs), IFNα/β receptor chains, interferon sensitive genes (ISG), as well as kinases and transcription factors critical in IFNα/β signaling. Although PRRs and ISGs were upregulated by infection in both cell types, the repertoire and absolute mRNA levels were more limited in oligodendroglia. Furthermore, although oligodendroglia harbored higher levels of viral RNA compared with microglia, Ifnα/β was only induced in microglia. Stimulation with the double stranded RNA analogue poly I:C also failed to induce Ifnα/β in oligodendroglia, and resulted in reduced and delayed induction of ISGs compared with microglia. The limited antiviral response by oligodendroglia was associated with a high threshold for upregulation of Ikkε and Irf7 transcripts, both central to amplifying IFNα/β responses. Overall, these data reveal that oligodendroglia from the adult CNS are poor sensors of viral infection and suggest they require exogenous IFNα/β to establish an antiviral state.

Figure 1

Oligodendroglia are limited in basal and inducible expression of viral RNA sensing PRRs. Microglia and oligodendroglia purified from spinal cords of naïve (A) or JHMV infected (B) PLP‐GFP mice were assayed for Mda5, Rig‐I, Tlr3, and Tlr7 transcripts at the indicated times p.i. Data represent the average of three separate experiments ± standard error of the mean (SEM). “*” denotes P ≤ 0.05 comparing microglia to oligodendroglia at each timepoint; “#” indicates P ≤ 0.05 compared with basal levels for each cell population.

Figure 2

Kinetics of IFNα/β, IFNλ, and IFNγ expression during viral infection. Wt mice infected with JHMV were analyzed for kinetics of Ifnβ, Ifnα4, IFNλ, and IFNγ mRNA induction in spinal cords. Data represent the mean ± SEM for three mice per group; “#” indicates P ≤ 0.05 compared with basal levels.

Figure 3

MDA5 and RIG‐I upregulation is IFNα/β dependent. Wt and IFNAR^−/− mice infected with JHMV were used to compare induction of Mda5 and Rig‐I mRNA in oligodendroglia and microglia purified from spinal cords. Data are average of three individual experiments ± SEM as described in Fig. 1. “*” denotes P ≤ 0.05 comparing wt to IFNAR^−/− populations at each timepoint.

Figure 4

Oligodendroglia do not induce IFNα/β transcripts during JHMV infection. Microglia and oligodendroglia purified from spinal cords of naïve or JHMV infected PLP‐GFP mice were assessed for JHMV‐N, Ifnβ, and Ifnα4 transcripts. Data are average values ± SEM of three independent experiments. BD indicates below detection. “*” denotes P ≤ 0.05 comparing microglia to oligodendroglia.

Figure 5

Oligodendroglia are limited in basal and inducible expression of PRR associated signaling factors during JHMV infection. Microglia and oligodendroglia from naïve or JHMV‐infected mice were assayed for Ikkε, Irf3, and Irf7 transcripts as indicated in Fig. 1. Data show the average of three separate experiments ± SEM for Ikkε and Irf7 and represent one of three separate experiments with similar results for Irf3. “*” denotes P ≤ 0.05 comparing microglia to oligodendroglia at each timepoint; “#” indicates P ≤ 0.05 compared with basal levels for each cell population.

Figure 6

Oligodendroglia have limited IFNα/β receptor signaling capacity. Microglia and oligodendroglia from naïve or JHMV infected mice were assayed for transcripts encoding the IFNAR1, soluble IFNAR2 (IFNAR2a) and IFNAR2 transmembrane (IFNAR2c) chains (A) or Stat1 and Irf9 transcripts (B). Data in (A) represent the average of two experiments; data in (B) represent the average of three independent experiments ± SEM as indicated in Fig. 1. “*” denotes P ≤ 0.05 comparing microglia to oligodendroglia at each timepoint; “#” indicates P ≤ 0.05 compared with basal levels for each cell population.

Figure 7

Delayed induction of antiviral ISG in oligodendroglia. Microglia and oligodendroglia from naïve or JHMV infected mice were assayed for Pkr, Oas2, Adar1, and Ifit2 transcripts as indicated in Fig. 1. Data represent the average of three separate experiments ± SEM. “*” denotes P ≤ 0.05 comparing microglia to oligodendroglia at each timepoint; “#” indicates P ≤ 0.05 compared with basal levels for each cell population.

Figure 8

Kinetics of CNS IFNα/β responses to poly I:C. Wt mice were injected intracerebrally with 200 μg poly I:C to assess kinetics of Ifnβ, Ifnα4, Ifit1, and Ifit2 mRNA induction in spinal cords. Data represent the mean ± SEM for three mice per group; “#” indicates P ≤ 0.05 compared with basal levels.

Figure 9

Distinct in vivo responses of oligodendroglia and microglia to poly I:C. Microglia and oligodendroglia purified from spinal cords at 4 or 12 h post poly I:C injection were analyzed for expression of Ifnα4, Ifnβ, Mda5, Rig‐I, Ikkε, Irf7, Ifit1, and Ifit2 transcripts. Data are representative of two independent experiments each with 10 pooled mice per experiment.