Non-Hodgkin’s Lymphoma (NHL) causes many deaths world wide, and is one of the few cancers that have a continual increase in incidence and mortality rates over the last few decades. Diffused large B cell lymphoma (DLBCL) is an aggressive B cell lymphoma that accounts approximately 40% of all NHL-B cells. We have previously shown that dysregulated CD40 ligand (CD40L/CD154) expression in DLBCL maintains lymphoma cell growth and cell survival. CD40 ligand is a member of the TNF superfamily of proteins that has a wide variety effects throughout the immune system and is critical for both cellular and humoral immunity. It was originally thought that CD154 expression was restricted to activated CD4+ T lymphocytes. However, expression of CD154 has also been demonstrated in non-lymphocytic leukocytes such as mast cells, basophils, eosinophils, dendritic cells and macrophages. More importantly, dysregulated CD154 expression has been noted in a number of diseases, including systemic lupus erythematosus (SLE), Alzheimer’s, and in B cell lymphomas. Although the functional analysis of CD154 has been extensive, little is known about the mechanisms controlling CD154 expression in activated T cells, let alone in normal and malignant B cells. In this study, we show that constitutive NF-kB and NFAT activation in aggressive lymphoma B cells directly interact and synergistically regulate CD154 gene expression. Immunoprecipitation and confocal analyses indicate that NFATc1 and NF-kB (p65 and c-rel) complex and colocalize in lymphoma B cells. Chromatin Immunoprecipitation (CHIP) assays analysis demonstrate that NFATc1, p65 and c-rel bind to the CD154 promoter, that is enhanced by HDAC inhibitor (TSA or SAHA) treatments. HDAC inhibitors also enhance CD154 promoter activity, providing evidence that CD154 transcriptional regulation involves histone acetylation. Over-expression studies show a synergistic effect by NFATc1 and NF-kB (c-rel) on the CD154 promoter. Promoter deletion and site direct mutagenesis studies on the CD154 promoter reveal that the transcriptional regulation of CD154 requires two sites, the distal kB site at −1180 and a proximal NFAT site at −250. These sites bind to both, NFAT and NF-kB proteins, as wells as the DNA architectural protein HMG-1, indicating that the CD154 promoter DNA may loop and form an enhanceosome-like complex. Studies using siRNA to p65, c-rel, or NFATc1 repress CD154 promoter activity, indicating that these transcription factors are necessary and sufficient for CD154 gene transcription. These findings strongly suggest a novel control mechanism for CD154 gene transcription, therefore, providing potential treatment modalities for lymphoma B cells as well as other disorders involving dysredulated CD154 expression.