Using adult spleens of naïve mice will not work. You might use the spleens of new-born mice and the GM-CSF driven DC cultures will pretty much look like as with bone marrow. Another option is to inject CpG-ODN (B-type such as 1668 or 1826) 6-7 days before you harvest spleens of adult mice. Those spleen cells (after CpG injection) can be used for GM-CSF driven DC cultures in a similar fashion as using bone marrow. A protocol for this method has been described: J Immunol. 1999 Feb 15;162(4):2368-74.