The
oligoadenylate synthetase (OAS)-
RNase L pathway is a potent
interferon (IFN)-induced
antiviral activity. Upon sensing
double-stranded RNA, OAS produces 2',5'-oligoadenylates (2-5A), which activate
RNase L. Murine coronavirus (mouse hepatitis virus [MHV]) nonstructural
protein 2 (ns2) is a 2',5'-phosphodiesterase (PDE) that cleaves 2-5A, thereby antagonizing
RNase L activation. PDE activity is required for robust replication in myeloid cells, as a mutant of MHV (ns2(H126R)) encoding an inactive PDE fails to antagonize
RNase L activation and replicates poorly in bone marrow-derived macrophages (BMM), while ns2(H126R) replicates to high titer in several types of nonmyeloid cells, as well as in IFN receptor-deficient (Ifnar1(-/-)) BMM. We reported previously that myeloid cells express significantly higher basal levels of OAS transcripts than nonmyeloid cells. Here, we investigated the contributions of Oas gene expression, basal IFN signaling, and virus-induced IFN to
RNase L activation.
Infection with ns2(H126R) activated
RNase L in Ifih1(-/-) BMM to a similar extent as in wild-type (WT) BMM, despite the lack of IFN induction in the absence of MDA5 expression. However, ns2(H126R) failed to induce
RNase L activation in BMM treated with IFNAR1-blocking antibody, as well as in Ifnar1(-/-) BMM, both expressing low basal levels of Oas genes. Thus, activation of
RNase L does not require virus-induced IFN but rather correlates with adequate levels of basal Oas gene expression, maintained by basal IFN signaling. Finally, overexpression of
RNase L is not sufficient to compensate for inadequate basal OAS levels.
IMPORTANCE: The
oligoadenylate synthetase (OAS)-
RNase L pathway is a potent
antiviral activity. Activation of
RNase L during murine coronavirus (mouse hepatitis virus [MHV])
infection of myeloid cells correlates with high basal Oas gene expression and is independent of virus-induced
interferon secretion. Thus, our data suggest that cells with high basal Oas gene expression levels can activate
RNase L and thereby inhibit virus replication early in
infection upon exposure to viral
double-stranded RNA (dsRNA) before the induction of
interferon and prior to transcription of
interferon-stimulated
antiviral genes. These findings challenge the notion that activation of the OAS-
RNase L pathway requires virus to induce type I IFN, which in turn upregulates OAS gene expression, as well as to provide dsRNA to activate OAS. Our data further suggest that myeloid cells may serve as sentinels to restrict viral replication, thus protecting other cell types from
infection.