AbstractDendritic cells (DCs) develop from bone marrow (BM) progenitor cells and mature in response to external signals to elicit functions important for innate and adaptive immunity. Interferon consensus sequence binding protein (ICSBP; also called interferon regulatory factor 8 [IRF-8]) is a hematopoietic cell–specific transcription factor expressed in BM progenitor cells that contributes to myeloid cell development. In light of our earlier observation that ICSBP−/− mice lack CD8α+DCs, we investigated the role of ICSBP in DC development in vitro in the presence of Flt3 ligand. Immature ICSBP−/− DCs developed from BM progenitor cells showed assorted defects, did not mature in response to activation signals, and failed to express CD8α and interleukin 12 (IL-12) p40, a feature consistent with ICSBP−/− DCs in vivo. We show that retroviral introduction of ICSBP restores the development of immature DCs that can fully mature on activation signals. All the defects seen with ICSBP−/− DCs were corrected after ICSBP transduction, including the expression of CD8α and IL-12 p40 as well as major histocompatability complex class II and other costimulatory molecules. ICSBP is known to regulate gene expression by interacting with partner proteins PU.1 and IRFs, thereby binding to target elements ISRE and EICE. Analysis of a series of ICSBP mutants showed that the intact DNA-binding activity as well as the ability to interact with partner proteins are required for the restoration of DC development/maturation, pointing to the transcriptional function of ICSBP as a basis of restoration. Taken together, this study identifies ICSBP as a factor critical for both early differentiation and final maturation of DCs.