In order to identify the cell type(s) producing IFN-ab in vivo, we isolated splenocytes from Ad infected animals 8 h after virus treatment and separated them into CD11c + (DC-containing) and CD11c 2 (non-DC) populations. Both CD11c + and CD11c 2 populations contained viral DNA (not shown). This finding is in accordance with the report of Morelli et al describing that both splenic DC and non-DC contain the virus in Ad-infected mice [49] . However, quantitative RT-PCR determination of IFN-a and IFN-b mRNA in both populations revealed the presence of IFNab mRNAs predominantly in the DC-containing CD11c + fraction ( Fig. 2A, B ), but not in the macrophage containing CD11c 2 fraction. In contrast to IFN-a and IFN-b, the mRNA levels of IL-6, another cytokine known to be induced by Ad, were comparably upregulated in both the CD11c + and CD11c 2 population (Fig. 2C ). Since the latter non-DC population comprises more than 95% of all mouse splenocytes [50] , we conclude that most of the IL-6 made in the Ad-infected spleen is of non-DC origin.