If you think that Toll-like receptors (TLRs) are all you need to recognize the multitude of bacterial and viral insults, then think again. Shizuo Akira and colleagues now offer an alternative. They show that, instead of TLRs, a recently identified cytoplasmic protein — retinoic-acid-inducible gene I (RIG-I) — is crucial for the detection of RNA viruses by fibroblasts and conventional dendritic cells (DCs).

The first hint that there might be a TLR-independent pathway came from the observation that cells lacking key TLR-signalling molecules — MyD88 (myeloid differentiation primary-response protein 88) and TRIF (Toll/interleukin-1 receptor (TIR)-domain-containing adaptor protein inducing interferon-b) — could still induce expression of type I interferons (IFNs) in response to viral infection. In vitro studies led the authors to consider that RIG-I was a good candidate for mediating this response. So, they knocked out the gene encoding RIG-I in mice and measured the ability of cells from these mice to respond to viruses. Because most RIG-I-deficient mice died in utero, they first studied mouse embryonic fibroblasts (MEFs). The induction of expression of type I IFNs and IFN-inducible genes in response to RNA viruses (such as Newcastle disease virus and vesicular stomatitis virus) was abrogated in RIG-I-deficient MEFs. Moreover, the absence of the IFN response impaired the ability of these cells to clear the virus.

The transcription factors that are required for induction of expression of type I IFNs and pro-inflammatory cytokines are IFN-regulatory factor 3 (IRF3) and nuclear factor-κB, respectively. The activation of both of these transcription factors was also abrogated in infected, RIG-I-deficient MEFs.

Given that DCs are the main producers of type I IFNs, the authors' attention next turned to DCs. Similar to fibroblasts, RIG-I-deficient conventional DCs showed severely impaired production of type I IFNs after viral infection. But deletion of components of the TLR-signalling pathway did not affect the IFN response of wild-type DCs, confirming the crucial role of the RIG-I pathway in this antiviral response.

In contrast to conventional DCs and fibroblasts, RIG-I-deficient plasmacytoid DCs (pDCs; which were derived in vitro from mouse bone-marrow cells, using FLT3 ligand) produced type I IFNs in amounts similar to those produced by wild-type pDCs, after viral infection. Instead of the RIG-I system, pDCs used the TLR system (in particular, TLR7 and TLR9), because induction of expression of antiviral IFNs was severely impaired when pDCs were deficient in these TLRs or their signalling molecules.

So, RIG-I and TLRs can both detect viral components and trigger antiviral responses depending on the cell type.