In the present invention, CD8+ conventional dendritic cells (CD8+ cDCs) and equivalents thereof (eCD8+ cDCs) in mouse and human have been established as major source of IFN-lambda (IFN-A) in response to double-stranded (ds) nucleic acids. The invention relates to therapeutic applications of ds nucleic acids or analogs thereof targeting CD8+ andlor eCD8+ cDCs in the prevention andlor treatment of infectious diseases, preferably viral infections, or cancer. Furthermore, the invention relates to an in vitro method for producing IFN-A andlor generating or obtaining a population of IFN-A producing CD8+ or eCD8+ cDCs as well as in vitro method for detecting or screening for CD8+ and/or eCD8+ cDCs. In addition, the invention relates to a Flt3-ligand or a MCSF receptor ligand for use in increasing the level of CD8+ andlor eCD8+ cDCs in a subject suffering from an infectious disease or cancer.