OBJECTIVE: CD27 is a member of the tumor necrosis factor receptor (TNFR) superfamily and is expressed on T, B, and NK cells. The signaling via CD27 plays pivotal roles in T-T and T-B interaction. CD27 is a useful marker in assessing the number of circulation B cells and B cell subsets because it permits one step identification of the major B cell compartments, CD27- naive and CD27+ memory B cells as well as CD27high plasma cells. We have analyzed the mechanisms underlying the regulation of CD27 expression. METHODS: Isolation B cells and Raji cells were cultured with PMA. The levels of cell surface CD27 and CD 27 mRNA were analyzed by FACs staining and RT-PCR. Raji cells were cultured with phorbol 12-myristate 13-acetate (PMA), with or without pretreated shedding inhibitor BB3103 and TAPI-1. sCD27 was measured in culture supernatant by ELISA. Cell lysates were analyzed for PKC isotype activation by Western blot. We used PKC inhibitor Ly379196 and rottlen. RESULTS: Membrane expression of CD27 was down-regulated after PMA stimulation without cytotoxic effect in B cells. Furthermore, PMA treatment could directly reduce CD27 mRNA without intermediate protein synthesis in B cells. In contrast, PMA treatment induced soluble form of CD27 (sCD27), which was shed from the cell surface and was found in PMA treatment B cell culture supernatant. PMA-induced sCD27 proteins were decreased with shedding inhibitor BB3103 and TAPI-1. PMA-induced down regulation of CD27 expressions were quenched with protein kinase C (PKC) inhibitor Staurosporin, PKC-beta inhibitor rottlerin and PKC-delta inhibitor Ly379196. CONCLUSION: These data suggest that PMA-induced activation of PKC plays a crucial role in down-regulation of the expression of the CD27 and up-regulation of the shedding of the CD27 in human B cells.