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PU.1 is a hematopoietic cell–specific transcription factor belonging to the Ets family. We hypothesized that PU.1 is involved in MHC class II expression in dendritic cells (DCs).The role of PU.1 in MHC class II expression in DCs was analyzed.Transcriptional regulation of the DC-specific pI promoter of the class II transactivator (CIITA) gene and subsequent MHC class II expression was investigated by using PU.1 small interfering RNA (siRNA) and reporter, chromatin immunoprecipitation, and electrophoretic mobility shift assays.PU.1 siRNA introduction suppressed MHC class II expression, allogeneic and syngeneic T-cell activation activities of bone marrow–derived DCs (BMDCs) with reduction of CIITA mRNA driven by the DC-specific promoter pI, and MHC class II mRNA. The chromatin immunoprecipitation assay showed constitutive binding of PU.1 to the pI region in BMDCs, whereas acetylation of histone H3 on pI was suppressed by LPS stimulation in parallel with shutdown of CIITA transcription. PU.1 transactivated the pI promoter throughcis-elements at −47/−44 and −30/−27 in a reporter assay and to which PU.1 directly bound in an electrophoretic mobility shift assay. Acetylation of histones H3 and H4 on pI was reduced in PU.1 siRNA–introduced BMDCs. Knockdown of interferon regulatory factor 4 or 8, which is a heterodimer partner of PU.1, by siRNA did not affect pI-driven CIITA transcription or MHC class II expression.PU.1 basally transactivates the CIITA pI promoter in DCs by functioning as a monomeric transcription factor and by affecting histone modification, resulting in the subsequent expression and function of MHC class II.