The influence of cell type on alphavirus type I interferon induction

Important veterinary and human pathogens, Alphaviruses are positive-sense single stranded RNA viruses comprised of two groups: the Old World alphaviruses (e.g. Sindbis virus [SINV] and Chikungunya virus [CHIKV]) and the New World alphaviruses (e.g. Venezuelan [VEEV] and eastern [EEEV] equine encephalitis virus). Because sensitivity of these viruses to the effects of type I interferon (1FN-alpha/beta) correlates with the severity of disease in vivo, IFN-alpha/beta is a critical factor in disease survival. However, little is understood about the host pattern recognition receptors (PRRs), downstream signaling molecules or cell-types involved in induction of IFN-alpha/beta. Infection of murine fibroblasts (MEFs) with wild-type (wt) alphaviruses did not stimulate production of IFN-a/R despite the fact that wt SINV or VEEV infection resulted in activation and translocation of interferon regulatory factor 3 (IRF-3) to the nucleus. In contrast, a non-cytopathic SINV (39nc) defective in shut-off of host macromolecular synthesis induced IFN-alpha/beta production at late times post infection and was dependent upon the cytoplasmic kinase TBK-1. Examination of PRRs revealed that IFN-alpha/beta production by 39nc was dependent on the RNA-binding proteins MDA-5 and PKR, but not RIG-I. We hypothesize that wt alphaviruses do not specifically block IFN-alpha/beta induction pathways but, instead, may inhibit IFN-alpha/beta production through inhibition of host macromolecular synthesis.;Although wt alphavirus infection of MEFs did not result in IFN-alpha/beta production, IFN-alpha/beta could be detected in the serum of infected mice as early as 8-12 hpi suggesting that a different type of cell produced IFN-alpha/beta in vivo. Examination of conventional dendritic cells revealed that these cells were not responsible for the early systemic IFN-alpha/beta but, instead, cells in plasmacytoid DC (pDC) cultures and immature macrophage (IM) cultures produced early IFN-alpha/beta. SINV infection of pDC cultures activated pDCs, but purified pDCs were non-permissive to infection and failed to produce detectable IFN-alpha/beta. Therefore, another cell in the heterogeneous cultures was likely responsible. IM cultures infected with SINV produced IFN-alpha/beta in a toll-like receptor-independent but IRF-dependent manner. The SINV-infected cells within pDC cultures, IMCs, and the popliteal lymph node of infected mice were CD11c-CD11b +, indicating a previously unappreciated role of immature macrophage-like cells as important IFN-alpha/beta producers after alphavirus infection.