Microvilli structures on B lymphocytes: inducible functional domains?

Abstract

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.

Myosin 1c Participates in B Cell Cytoskeleton Rearrangements, Is Recruited to the Immunologic Synapse, and Contributes to Antigen Presentation

“…These structures were previously identified in the surface of activated B cells and have been suggested to be inducible domains that concentrate certain important molecules such as ICAM-1 or MHC-II (27). Microvilli likely are the initial contact sites for cell-cell interactions.…”

“…Interestingly, MHC-II molecules are present and enriched in B cell microvilli structures formed upon activation (Fig. 5A) (27). It is known that these actin-dependent structures can concentrate some transmembrane proteins but exclude others.…”

“…Microvilli are plasma membrane protrusions present over the entire surface of resting lymphocytes (26) that differ in structure and function from lamellipodia-filopodia protrusions. It was previously demonstrated that microvilli are enhanced in number and length after stimulation of naive B cells (27); indeed, these structures may represent functional lymphocyte domains that allow molecule segregation. Analysis of human T cells and a mouse pre-B cell line demonstrated Myo1g enrichment in lymphocyte microvilli (18).…”

Myosin 1g regulates cytoskeleton plasticity, cell migration, exocytosis, and endocytosis in B lymphocytes

“…Microvilli are formed by central actin filament bundles that are connected to the overlying plasma membrane by several linking proteins, including class I myosin. These structures represent functional lymphocyte domains that allow molecule segregation, probably to mediate different cell-cell interactions [36]. Interestingly, microvilli of mature B cells are inducible.…”

“…Interestingly, microvilli of mature B cells are inducible. They are increased in number and length after stimulation with different stimuli such as IL-4, LPS, or anti-CD40 antibodies [36], thus relating the marked presence of microvilli with an activated phenotype of B lymphocytes.…”