In order to study mechanisms of induction of IFN-α by Newcastle disease virus (NDV), we used two replicon systems which are based respectively on DNA and RNA of the Semliki forest virus (SFV) and transfected these into baby hamster kidney cells (BHK) which do not produce interferon-α. Co-incubation of BHK cells which were transfected with the two vector systems, with human PBMCs, showed that production of IFN-α takes place by two different ways. When using the DNA-based SFV vector, only transfectants expressing cell surface HN molecules of NDV (and not the mock-transfected cells) elicited such a response via interaction of these HN molecules with viral receptors on PBMCs. In contrast, BHK cells transfected with RNA which had been in vitro transcribed from the RNA-based SFV vector without foreign gene as insert (mock-transfected) elicited a comparable IFN-α response. Transfer by transfection of poly(I:C), an analogue of double stranded RNA (dsRNA), into the BHK cells induced also by itself the production of IFN-α. Therefore induction of ‘danger signals’ (as double-strand RNA replicative intermediates) might be responsible for this discrepancy observed in IFN-α induction in PBMCs between the two studied SFV vector systems based on transfection of DNA and on RNA. These observations highlight two ways of IFN-α induction which additively may explain the high interferonogenic capacity of NDV as virus: i) via cell-surface expressed HN after transfection of the DNA-based SFV replicon and ii) via transfection of self-amplifying RNA.

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