Developing B cells in the bone marrow express CD25 during the pre-B-cell stage [8, 9] but the function of CD25 on these immature B cells is largely unknown as they do not proliferate in response to IL-2 MK-1775 ic50 [9]. CD25+ B cells in the periphery are today believed to be activated B cells; however, most of these studies are performed in vitro [10] and very little is known about the expression of CD25

on B cells after activation in vivo. The CD25+ B-cell population consists of about 1% of the whole B-cell population in a naïve mouse spleen and previous studies have revealed considerable phenotypical difference between the CD25+ B cells in bone marrow and those present in secondary lymphoid organs [2]. While CD25+ B cells isolated from bone marrow displayed an immature phenotype, CD25+ B cells isolated from secondary lymphoid organs display a more mature and activated phenotype when compared with XL765 cell line CD25− B cells characterized by higher expression

of surface IgA and IgG as well as a higher expression of the costimulatory molecules CD80 and CD86 [2]. In addition, we have shown that human circulating CD25+ B cells display different phenotypic and functional properties when compared with the CD25− B cells. CD25+ B cells performed significantly better as antigen-presenting cells in allogeneic mixed lymphocyte reaction (MLR) and B cell–specific blocking of the CD25 expression led to abrogation of the

MLR. CD25+ B cells also expressed significantly higher levels of surface immunoglobulin but lacked the ability to secrete them [3]. Overall, the human CD25+ B cells display a more mature phenotype and seem belong to the pentoxifylline memory B-cell population [4]. The aim of this study has been to analyse the functional properties of CD25+ B cells in mice with respect to immunoglobulin and cytokine production, antigen presentation, migration and homing. Our results clearly show that CD25+ B cells are highly differentiated and might belong to the memory B-cell subset. Mouse strains.  Naval Medical Research Institute (NMRI) and C57BL/6 female mice were used. C57Bl/6 mice were used only in the mixed lymphocyte reaction experiments. Permission from the local animal research ethics committee, in accordance with national animal welfare legislation, was obtained for all the mice experiments. B-cell isolation.  Spleens were passed through a 70-μm nylon mesh (BD Bioscience, Erembodegem, Belgium) into a Petri dish containing 10 ml phosphate-buffered saline (PBS). Cell suspension was centrifuged; the pellet resuspended in NH4Cl solution (0,83%, pH 7.29) and kept on ice for 7 min to lyse erythrocytes, followed by two washing steps in cold PBS. The cells were counted and incubated with optimal concentration of Fc-block (2.4G2; BD Bioscience) for 8 min at room temperature to avoid unspecific binding via Fc-receptor interaction.