Antigen receptor-mediated signaling pathways in transitional immature B cells Dorottya Ko ¨vesdi a , Katalin Pa ´szty b ,A ´ gnes Enyedi c , Endre Kiss a , Ja ´nos Matko ´ a , Katalin Luda ´nyi d ,E ´ va Rajnavo ¨lgyi d , Gabriella Sa ´rmay a, * a Department of Immunology, Eo ¨tvo ¨s Lora ´nd University, Pa ´zma ´ny Pe ´ter se ´ta ´ny 1/c, H-1117, Budapest, Hungary b National Medical Center, Dio ´szegi u. 64. H-1113, Budapest, Hungary c Membrane Research Group of the Hungarian Academy of Sciences, Na ´dor u. 7. H-1051, Budapest, Hungary d Institute of Immunology, Faculty of Medicine, University of Debrecen, Nagyerdei Ko ¨ru ´t 98, H-4012, Debrecen, Hungary Received 4 December 2003; received in revised form 8 January 2004; accepted 8 January 2004 Available online 16 March 2004 Abstract Engagement of antigen receptors on immature B cells induces apoptosis, while at the mature stage, it stimulates cell activation and proliferation. The difference in B cell receptor (BCR)-mediated signaling pathways regulating death or survival of B cells is not fully understood. We aimed to characterize the pathway leading to BCR-driven apoptosis. Transitional immature B cells were obtained from the spleen of sublethally irradiated and auto-reconstituted mice. We have detected a short-lived BCR-driven activation of mitogen-activated protein kinases (ERK1/2 and p38 MAPK) and Akt/PKB in transitional immature B cells that correlated with the lack of c-Fos expression, reduced phosphorylation of Akt substrates and a susceptibility for apoptosis. Simultaneous signaling through BCR and CD40 protected immature B cells from apoptosis, however, without inducing Bcl-2 expression. The BCR-induced apoptosis of immature B cells is a result of the collapse of mitochondrial membrane potential and the subsequent activation of caspase-3. D 2004 Elsevier B.V. All rights reserved. Keywords: Apoptosis; B lymphocytes; Development; Signal transduction; Transitional immature cells 1. Introduction B cell maturation proceeds through developmentally determined checkpoints where the surface immunoglobulin controls the ability of the cells to discriminate between self and non-self molecules [1–3]. B cell receptor (BCR) consisting of immunoglobulin heavy and light chains and the heterodimer of signal transducing chains, Iga and Igh, is first expressed on immature B cells in the bone marrow. Engagement of BCR in this environment results in either the rearrangement of the light chain genes to avoid self- reactivity or the elimination of the autoreactive B cells by apoptosis [4–6]. Consequently, the mature B cell popula- tion becomes tolerant for self-structures, while it reacts to foreign molecules with cell activation, proliferation and antibody production. Therefore, the immature stage, when the BCR governs the antigen-dependent negative selection of B cells, is crucial in avoiding self-reactivity. After leaving the bone marrow, the B cells are at the transitional immature stage. These cells colonize the peripheral lym- phoid organs, where they first encounter foreign antigens. Transitional B cells can be distinguished from the mature population by a series of surface markers, such as heat stabile antigen (HSA/CD24) and high surface IgM and low/ medium CD21 expression [7,8]. Only about 10–30% of these cells enter the mature B cell pool and the rest go through negative selection [9,10]. High-affinity interaction of the antigen with the BCR on mature B cells leads to cell activation, and eventually, in response to additional signals such as those mediated by CD40 ligand on T cells and cytokines, to the differentiation into antibody producing plasma cells. Recognition of antigen or self molecules with low affinity and the lack of survival signals drives transi- tional B cells to apoptosis or, less frequently, may induce 0898-6568/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cellsig.2004.01.005 * Corresponding author. Tel.: +36-1-209-0555/8662; fax: +36-1-381- 2176. E-mail address: sarmayg@cerberus.elte.hu (G. Sa ´rmay). www.elsevier.com/locate/cellsig Cellular Signalling 16 (2004) 881 – 889