The classical MHC II genes in humans, consist of the HLA-DR, HLA-DP and HLA-DQ allotypes, with multiple alleles of each . Their gene products function to present peptides derived from internalized protein antigen to the T cell receptor on CD4+ T cells. Efficient loading of antigen onto MHC II is vitally dependent on the accessory molecule Ii. Ii comes in several isoforms (p33, p35, p41 and p43) due to alternative translation initiation start sites (p35 and p43), and splicing of exon 6 (p41 and p43) , these isoforms have been attributed various functions. The stochiometric ratio of Ii to MHC II determines the efficiency of peptide loading and ultimately the adaptive responses to protein antigen. Expression of MHC II is in general restricted to a set of specialized immune cells collectively termed Professional APCs . The three major APCs in mammals; Mθs, B lymphocytes and DCs are differentially specialized to serve their individual functions in their hosts . DCs are uniquely potent in their ability to induce T cell responses, and MDDCs are currently being examined for use in cell-based immuno-therapies to treat cancer in human patients. How these cells compare to conventional, in vivo generated, DCs largely remains to be determined. Here we investigate the transcriptional responses of the Ii isoforms to the total MHC II following LPS stimulation in human blood MDDCs. We find that the transcription responses differ from what has previously been described for primary DCs. The MHC II and Ii display a prolonged transcriptional response compared to primary DCs, with elevated transcriptional activity for up to 12 hours for MHC II and up to 24 hours for Ii. Elevated amounts of Ii protein is observed in mature MDDCs, with approximately 40% more Ii protein present in 48 hours matured cells as compared to immature. The levels of Ii p33 protein display an inverse regulation to Ii p33/35 mRNA levels, with an immediate decrease, followed by a marked increase in Ii p33 from 12-48 hours after stimulation. This finding provides vital information in order to enhance the efficiency of MDDC-based tumor vaccines with regard to the timing and method of antigen loading.