EARLY ACTIVATION MARKERS EXPRESSED BY HUMAN PERIPHERAL DENDRITIC CELLS Peter Hellman and Håkan Eriksson University of Malmoe, Department of Biomedical Laboratory Science E-mail address: peter.hellman@hs.mah.se Two major populations of immature dendritic cells, myeloid (M-DCs) and plasmacytoid (P-DCs) can be identified in human peripheral blood. Activation of these subsets through their Toll-like receptors (TLRs) (TLR4 for M-DCs and TLR9 for P-DCs) induced production of the chemokine Il-8, already within two hours of stimulation. The production of IL-8 preceded the expression of the activation marker CD40 in both M-DCs and P-DCs. Although both populations of DCs secreted Il-8 upon activation, the levels of Il-8 produced was several times higher in the M-DCs compared to the P-DCs population. Before activation both subsets of DCs expressed the IL-8 receptor type B (CD128b), however, upon stimulation the Il-8 receptor became undetectable in both M-DCs and P-DCs. Increased expression of MHC class II molecules is regarded as an early activation marker of DCs. However, only the P-DCs showed a significantly increased expression of MHC class II after 4 hours of stimulation through TLR9. Noteworthy, the M-DCs showed an unexpected increase of MHC class II molecules after conditioning in medium for 4 hours, and no further increase in MHC class II expression after stimulation through TLR4 was observed. In conclusion, we propose that during activation of human DCs the production of Il-8 and loss of CD128b are the earliest signs of activation preceding both MHC class II, CD40, CD80 and CD86 expression.