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Background: Cyclophilin A (CypA) is a ubiquitously distributed intracellular protein as well as a secreted protein and has recently been reported to be an immunomodulatory molecule. The objective of this study was to determine the effect of CypA on dendritic cell (DC) differentiation, activation, and functional maturation. The role of p38 MAP kinase in DC functions was also investigated., Materials and Methods: KG1 cells (CD34+ human myeloblastic cell line) were treated with cytokines (GM-CSF+IL-4) and/or CypA and expression of cell surface markers was analyzed by FACS analysis. The antigen-uptake capacity of different DCs was determined by FITC-dextran uptake assay. Antigen-presentation capacity of DCs was determined by allogeneic mixed lymphocyte reaction (MLR) by [3H] thymidine incorporation assay. To assess the T cell polarization stimulated by KG1 derived DCs, various Th1 and Th2 cytokines secreted by allostimulated CD4+ and CD8+ T cells were determined by Bioplex cytokine assay. Total and phosphorylated p38 MAPK activity in CypA treated DCs was detected by Bioplex p38 total and phosphoprotein assay., Results: During the differentiation of KG1 cells to immature DCs, cell surface expression of CD11b was increased by 30.6% for CypA alone, 55% for CypA plus cytokines, and 44% for cytokines alone. Similarly, CypA alone increased the cell surface expression of CD11c by 59% as compared to CypA plus cytokines (68%) and cytokines alone (50%). CypA up-regulated the antigen uptake capacity of the immature DCs to a greater extent (5 times) as compared to cytokines alone (2.5 times). Moreover, CypA augmented the capacity of DCs to present antigens to allogenic CD8+ T cells, and also increased the secretion of Th1 type cytokines TNF-alpha and IFN-gamma from the allogenic CD4+ T cells. Furthermore, CypA induced the phosphorylation and hence activation of MAP kinase p38. Pre-treatment with SB-203580, a p38 inhibitor, significantly reduced MLR stimulatory capacity of CypA-induced DCs in both CD8+ and CD4+ T cells (P < 0.05)., Conclusions: CypA enhances DC differentiation and maturation by up-regulating CD11b and CD11c expression. CypA can also augment DC antigen uptake and antigen presentation, which may be mediated by the p38 signaling pathway.