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International Immunology, Vol. 16, No. 2, pp. 353-364, February 2004
© 2004 Japanese Society for Immunology

Microvilli structures on B lymphocytes: inducible functional domains?

Gediminas Greicius1, Lisa Westerberg1, Edward J. Davey2, Eva Buentke3, Annika Scheynius3, Johan Thyberg1 and Eva Severinson1

1 Department of Cell and Molecular Biology, Karolinska Institutet, 17177 Stockholm, Sweden 2 Department of Genetics and Pathology, Uppsala University, 75185 Uppsala, Sweden 3 Department of Medicine, Unit of Clinical Allergy research, Karolinska Institutet and Hospital, Stockholm, 17176 Stockholm, Sweden

Correspondence to: E. Severinson; E-mail: eva.severinson@ cmb.ki.se
Transmitting editor: P. W. Kincade

Interactive contact between B lymphocytes and T cells is necessary for their expansion during an immune response. It has been shown that B lymphocytes receive signals from T cells, such as IL-4 and cross-linking of CD40, which are crucial for their differentiation. We previously found that these factors induce formation of microvilli on B cells and that this was correlated with increased homotypic adhesion of B lymphocytes. In this study we have investigated if IL-4 induce segregation of proteins to microvilli and lipid rafts. Using immuno-electron microscopy we analyzed cell-surface distribution of molecules involved in B–T cell co-activation. Recruitment to detergent-resistant membrane fractions was analyzed using sucrose gradient centrifugation. We found that microvilli were enriched in ICAM-1 and MHC class II molecules. In contrast, LFA-1 and CD40 were more abundant on the smooth cell surfaces, while B7-2 (CD86) was randomly distributed. We also discovered that depletion of cholesterol, using ß-methyl-cyclodextrin, lowered the number of microvilli, indicating that intact lipid rafts are required for their expression. Moreover, activation of B lymphocytes by lipopolysaccharide (LPS) induced increased expression of GM1, a marker for lipid rafts. However, although both surface and total levels of GM1 were similar in B lymphocytes stimulated with either LPS or LPS plus IL-4, GM1 was mainly expressed on microvilli in LPS plus IL-4-stimulated cells. Taken together, our results indicate that microvilli represent distinct inducible membrane domains that can regulate direct cell–cell interactions via grouping and three-dimensional presentation of cell-surface receptors.

Keywords: adhesion, B cell, IL-4, lipid raft, microvilli


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