The role of CD40/CD154 ligation in the upregulation of genes of the proinflammatory NFB signal transduction pathway was explored in primary cultures of human renal proximal tubule epithelial cells. Using a cDNA gene array specific for human NFB signal pathway genes, 38 genes were upregulated at 1 hr and 7 of these genes remained upregulated at 3 hrs. Of these genes ICAM-1 was explored in further detail. Quantitative real time PCR for ICAM-1 mRNA expression confirmed the gene array findings. Western blot analysis and quantitative sandwich-enzyme ELISA confirmed this observation at the protein level. A cell surface ELISA assay showed that ICAM-1 expression doubled by 48 hrs of CD154 exposure, and FACS analysis suggested that both the number of cells expressing ICAM-1 and the expression of ICAM-1 on these cells had increased. A cell adhesion assay using fluorescein-labeled human peripheral mononuclear cells showed that ICAM-1 upregulation resulted in increased mononuclear cell adhesion to the monolayer which was abrogated by pre-treatment of the monolayer with a neutralizing ICAM-1 antibody. The p38 MAPK inhibitor, SB203580, but not the ERK 1/2 inhibitor (PD98059) nor the protein kinase C inhibitor (calphostin) blunted ICAM-1 expression and monocyte adhesion to the monolayer. We conclude that in human renal proximal tubule epithelial cell, CD40 activation upregulates ICAM-1 (and other NFB pathway genes) expression with concomitant enhanced adhesion of mononuclear cells, which is mediated via the p38 MAPK signal transduction pathway.