BackgroundDouble stranded RNA (dsRNA) is widely accepted as an RNA motif recognized as a danger signal by the cellular sentries. However, the biology of non-segmented negative strand RNA viruses, or Mononegavirales, is hardly compatible with the production of such dsRNA.Methodology and Principal FindingsDuring measles virus infection, the IFN-β gene transcription was found to be paralleled by the virus transcription, but not by the virus replication. Since the expression of every individual viral mRNA failed to activate the IFN-β gene, we postulated the involvement of the leader RNA, which is a small not capped and not polyadenylated RNA firstly transcribed by Mononegavirales. The measles virus leader RNA, synthesized both in vitro and in vivo, was efficient in inducing the IFN-β expression, provided that it was delivered into the cytosol as a 5′-trisphosphate ended RNA. The use of a human cell line expressing a debilitated RIG-I molecule, together with overexpression studies of wild type RIG-I, showed that the IFN-β induction by virus infection or by leader RNA required RIG-I to be functional. RIG-I binds to leader RNA independently from being 5-trisphosphate ended; while a point mutant, Q299A, predicted to establish contacts with the RNA, fails to bind to leader RNA. Since the 5′-triphosphate is required for optimal RIG-I activation but not for leader RNA binding, our data support that RIG-I is activated upon recognition of the 5′-triphosphate RNA end.Conclusions/SignificanceRIG-I is proposed to recognize Mononegavirales transcription, which occurs in the cytosol, while scanning cytosolic RNAs, and to trigger an IFN response when encountering a free 5′-triphosphate RNA resulting from a mislocated transcription activity, which is therefore considered as the hallmark of a foreign invader.

Measles virus (MV) causes profound immunosuppression, resulting in high infant mortality. The mechanisms are poorly understood, largely due to the lack of a suitable animal model. Here, we report that particular MV proteins, in the absence of MV replication, could generate a systemic immunosuppression in mice through two pathways: (1) via MV-nucleoprotein and its receptor FcgammaR on dendritic cells; and (2) via virus envelope glycoproteins and the MV-hemagglutinin cellular receptor, CD46. The effects comprise reduced hypersensitivity responses associated with impaired function of dendritic cells, decreased production of IL-12, and the loss of antigen-specific T cell proliferation. These results introduce a novel model for testing the immunosuppressive potential of anti-measles vaccines and reveal a specific mechanism of MV-induced modulation of inflammatory reactions.

During acute measles virus (MV) infection

Measles virus (MV) nucleoprotein (N) is a cytosolic protein that is released into the extracellular compartment after apoptosis and/or secondary necrosis of MV-infected cells in vitro. Thus, MV-N becomes accessible to inhibitory cell-surface receptors: FcγRIIB and an uncharacterized nucleoprotein receptor (NR). MV-N is composed of two domains: NCORE (aa 1–400) and NTAIL (aa 401–525). To assess the contribution of MV-N domains and of these two receptors in suppression of cell proliferation, a human melanoma HT144 cell line expressing (HT144IIB1) or lacking FcγRIIB1 was used as a model. Specific and exclusive NCORE–FcγRIIB1 and NTAIL–NR interactions were shown. Moreover, NTAIL binding to human NR predominantly led to suppression of cell proliferation by arresting cells in the G0/G1 phases of the cell cycle, rather than to apoptosis. NCORE binding to HT144IIB1 cells primarily triggered caspase-3 activation, in contrast to HT144IIB1/IC− cells lacking the FcγRIIB1 intra-cytoplasmic tail, thus demonstrating the specific inhibitory effect of the NCORE–FcγRIIB1 interaction. MV-N- and NCORE-mediated apoptosis through FcγRIIB1 was inhibited by the pan-caspase inhibitor zVAD-FMK, indicating that apoptosis was dependent on caspase activation. By using NTAIL deletion proteins, it was also shown that the region of NTAIL responsible for binding to human NR and for cell growth arrest maps to one of the three conserved boxes (Box1, aa 401–420) found in N of Morbilliviruses. This work unveils novel mechanisms by which distinct domains of MV-N may display different immunosuppressive activities, thus contributing to our comprehension of the immunosuppressive state associated with MV infection. Finally, MV-N domains may be good tools to target tumour cell proliferation and/or apoptosis.