The transcription factor RelB is required for proper development and function of dendritic cells (DCs), and its expression is upregulated early during differentiation from a variety of progenitors. We explored this mechanism of upregulation in the KG1 cell line model of a DC progenitor and in the differentiation-resistant KG1a subline. RelB expression is relatively higher in untreated KG1a cells but is upregulated only during differentiation of KG1 by an early enhancement of transcriptional elongation, followed by an increase in transcription initiation. Restoration of protein kinase CII (PKCII) expression in KG1a cells allows them to differentiate into DCs. We show that PKCII also downregulated constitutive expression of NF-B in KG1a-transfected cells and restores the upregulation of RelB during differentiation by increased transcriptional initiation and elongation. The two mechanisms are independent and sensitive to PKC signaling levels. Conversely, RelB upregulation was inhibited in primary human monocytes where PKCII expression was knocked down by small interfering RNA targeting. Altogether, the data show that RelB expression during DC differentiation is controlled by PKCII-mediated regulation of transcriptional initiation and elongation.Dendritic cells (DCs) constitute a heterogeneous population of professional antigen-presenting cells with a unique role in the activation of naive T lymphocytes and the establishment of immunological tolerance and memory (2, 3). Their central role as regulators of the immune response has led to the examination of the underlying intracellular signaling pathways and gene expression that may regulate the differentiation of precursors into DCs (21, 60), most notably, the Rel/NF-B family of transcription factors (41, 61).NF-B exists in mammals as a homodimer or heterodimer of the Rel proteins NFB1 (p50/p105), NFB2 (p52/p100), c-Rel, RelA (p65), and RelB. The dimer can be sequestered in the cytoplasm by the inhibitory IBs, which in the canonical activation pathway can be phosphorylated, ubiquitinated, and degraded by the proteasome to allow the nuclear translocation of NF-B and activate transcription (63). On the other hand, the NF-B2 precursor form p100 can inhibit directly the nuclear translocation of heterodimers due to their IB-like ankyrin repeats and is activated by an alternative pathway that requires the NF-B-inducing kinase (NIK) and IKK␣ (62). In this pathway, the bound precursor p100 is processed in a regulated fashion to the p52 form to allow translocation to the nucleus. The p105 subunit also contains IB-like ankyrin repeats that could sequester NF-B dimers in the cytoplasm. However, its processing to the p50 form is constitutive (22,30).Overexpression of IB in mature DCs downregulates major histocompatibility complex (MHC) class II; the costimulatory molecules CD80, CD86, and CD40; and the proinflammatory cytokine tumor necrosis factor alpha (TNF-␣), indicating that antigen presentation is dependent on NF-B function (64). Likewise, the inhibitory p100 precursor ...