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The epithelial-mesenchymal transition (EMT) of renal epithelial cells (RTECs) has pivotal roles in the development of renal fibrosis. Although the interaction of lymphocyte function-associated antigen 1 (LFA-1) on leukocytes and its ligand, intracellular adhesion molecule 1 (ICAM-1), plays essential roles in most inflammatory reactions, its pathogenetic role in the EMT of RTECs remains to be clarified. In the present study, we investigated the effect of the interaction of LFA-1 on peripheral blood mononuclear cells (PBMCs) and ICAM-1 on HK-2 cells after stimulation with TGF-&bgr;1 on the EMT of RTECs. ICAM-1 was highly expressed in HK-2 cells. After TGF-&bgr;1 stimulation, the chemokines CCL3 and CXCL12 increased on HK-2 cells. After co-culture of PBMCs and HK-2 cells pre-stimulated with TGF-&bgr;1 (0.1 ng/ml) (HK-2-TGF-&bgr;1 (0.1)), the expression of the active form of LFA-1 increased on PBMCs; however, total LFA-1 expression did not change. The expression of the active form of LFA-1 on PBMCs did not increase after co-culture with not CCL3 but CXCL12 knockdown HK-2-TGF-&bgr;1 (0.1). The expression of epithelial cell junction markers (E-cadherin and occludin) further decreased and that of mesenchymal markers (vimentin and fibronectin) further increased in HK-2-TGF-&bgr;1 (0.1) after co-culture with PBMCs for 24 hrs (HK-2-TGF-&bgr;1 (0.1)- PBMCs). The phosphorylation of ERK 1/2 but not smad2 and smad3 increased in HK-2-TGF-&bgr;1 (0.1)-PBMCs. The snail and slug signaling did not increase HK-2-TGF-&bgr;1 (0.1)-PBMCs. Although the migration and invasion of HK-2 cells induced full EMT by a high dose (10.0 ng/ml) and long-term (72-96 hrs) TGF-&bgr;1 stimulation increased, that of HK-2-TGF-&bgr;1 (0.1)-PBMCs did not increase. These results suggested that HK-2 cells stimulated with TGF-&bgr;1 induced conformational activation of LFA-1 on PBMCs by increased CXCL12. Then, the direct interaction of LFA-1 on PBMCs and ICAM-1 on HK-2 cells activated ERK1/2 signaling to accelerate the part of EMT of HK-2 cells induced by TGF-&bgr;1. [ABSTRACT FROM AUTHOR]