Microvilli structures on B lymphocytes: inducible functional domains?

• 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 beta-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 GM(1), a marker for lipid rafts. However, although both surface and total levels of GM(1) were similar in B lymphocytes stimulated with either LPS or LPS plus IL-4, GM(1) 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.

Nyckelord

Animals

Antigens; CD/metabolism

Antigens; CD40/metabolism

B-Lymphocytes/immunology/*ultrastructure

Cell Communication/drug effects/physiology

Cholera Toxin/pharmacology

Cyclodextrins/pharmacology

Genes; MHC Class II/*physiology

Intercellular Adhesion Molecule-1/*metabolism

Interleukin-4/pharmacology

Lipopolysaccharides/pharmacology

Lymphocyte Activation/drug effects

Lymphocyte Function-Associated Antigen-1/metabolism

Membrane Glycoproteins/metabolism

Membrane Microdomains/metabolism/*ultrastructure

Mice

Microvilli/immunology/*ultrastructure

Research Support; Non-U.S. Gov't

T-Lymphocytes/immunology

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