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

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