Human monocytes represent a competitive source of interferon-α in peripheral blood

Interferon-alpha (IFN-α) has a critical role in antiviral immunity and plasmacytoid dendritic cells (pDCs) have been demonstrated as the principal IFN-α source after Toll-like receptor (TLR) 7 and 9 stimulation. Little is known about the contribution of pDC-independent IFN-α sources to total IFN-α production capacity of human peripheral blood. Using an array of pathogen associated molecular patterns (PAMPs), Poly(I:C)/Dotap represented the second strongest IFN-α stimulus in total PBMC. Poly(I:C)/Dotap induced three times more IFN-α, when compared to TLR7-stimulation (R848) and four times less, when compared to TLR9-stimulation. Dotap (mediator of cellular uptake) dramatically increased Poly(I:C)-induced IFN-α production. Sorting experiments and ELISpot assays revealed that monocytes and not myeloid DCs are the main IFN-α source after Poly(I:C)/Dotap stimulation. ELISpot analyses demonstrated the highest IFN-α spot numbers after Poly(I:C)/Dotap stimulation. Although pDCs produced highest IFN-α levels per cell, monocytes represent a competing IFN-α source in total PBMC due to their high frequency.