The growth determinants and transport properties of tunneling nanotube networks between B lymphocytes

Anikó Osteikoetxea-Molnár, Edina Szabó-Meleg, Eszter Angéla Tóth, Ádám Oszvald, Emese Izsépi, Mariann Kremlitzka, Beáta Biri, László Nyitray, Tamás Bozó, Péter Németh, Miklós Kellermayer, Miklós Nyitrai, Janos Matko

Tunneling nanotubes (TNTs) are long intercellular connecting structures providing a special transport route between two neighboring cells. To date TNTs have been reported in different cell types including immune cells such as T-, NK, dendritic cells, or macrophages. Here we report that mature, but not immature, B cells spontaneously form extensive TNT networks under conditions resembling the physiological environment. Live-cell fluorescence, structured illumination, and atomic force microscopic imaging provide new insights into the structure and dynamics of B cell TNTs. Importantly, the selective interaction of cell surface integrins with fibronectin or laminin extracellular matrix proteins proved to be essential for initiating TNT growth in B cells. These TNTs display diversity in length and thickness and contain not only F-actin, but their majority also contain microtubules, which were found, however, not essential for TNT formation. Furthermore, we demonstrate that Ca(2+)-dependent cortical actin dynamics exert a fundamental control over TNT growth-retraction equilibrium, suggesting that actin filaments form the TNT skeleton. Non-muscle myosin 2 motor activity was shown to provide a negative control limiting the uncontrolled outgrowth of membranous protrusions. Moreover, we also show that spontaneous growth of TNTs is either reduced or increased by B cell receptor- or LPS-mediated activation signals, respectively, thus supporting the critical role of cytoplasmic Ca(2+) in regulation of TNT formation. Finally, we observed transport of various GM1/GM3 (+) vesicles, lysosomes, and mitochondria inside TNTs, as well as intercellular exchange of MHC-II and B7-2 (CD86) molecules which may represent novel pathways of intercellular communication and immunoregulation.