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

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 Ca2+-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 Ca2+ 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.

Membrane microdomain organization, calcium signal, and NFAT activation as an important axis in polarized Th cell function.

T helper lymphocytes become polarized upon antigen and cytokine stimuli received after their maturation in the thymus. Since the balance of Th1 and Th2 responses is critical in healthy and pathological immune responses, understanding the molecular base of T cell polarization still remained an important question. Using our Th0/Th1/Th2 hybridoma model system, we performed a comparative study on polarized Th1 and Th2 cells in terms of their membrane raft expression/composition, their TCR mediated activation signaling, and sensitivity to activation-induced cell death (AICD) using flow and image cytometric methods. We show here that the TCR stimulation induced more intense and sustained Ca(2+) -response in Th1 cells compared to Th2 ones correlates well with a shorter nuclear residence time of the Ca(2+) -dependent NFAT transcription factor in Th2 cells. In addition, NFAT translocation directly depended on lipid raft integrity/membrane cholesterol level. Expression pattern of raftophilic accessory proteins (CD4, CD59, and CD48) and lipids (GM1, cholesterol) were also different in the Th1 and Th2 hybridomas, similarly to differentiated spleen Th cells. The activation-induced, remarkably clustered and polarized membrane distribution of TCR/CD3 complex in Th1, but not in Th2 cells, together with an increased raft localization of Kv1.3 ion channels regulating the Ca(2+) -response, are consistent with the above properties of NFAT. Finally, the polarized Th cells, especially Th1, were more sensitive to AICD than their unpolarized Th0 precursor. These results suggest that the membrane microdomain organization-Ca(2+) -signaling-NFAT activation axis is an important determinant of polarized Th cell effector function and fate.

The AC8 IgG3 monoclonal anti-cholesterol antibody modulates uptake and presentation of antigens for T cell activation.

Natural anti-cholesterol antibodies (ACHAs) exist in mammalian species, moreover their level sensitively changes in pathological situations, such as atherosclerosis or HIV infection. The conditions of their production and functional role, however, still remained ill defined. Recently we developed IgG3 type monoclonal ACHAs that selectively react with 'clustered cholesterol' of live immune cells, such as membrane microdomains (lipid rafts and caveolas). These antibodies inhibited HIV-1 infection of Th cells and macrophages by remodeling the HIV-1 receptor/coreceptor distribution in the plasma membrane of target cells. As a novel modulatory effect, here we show that the AC8 IgG3 monoclonal anti-cholesterol antibody (mACHA), but not the AC9 IgM mACHA, spontaneously bind to all professional APCs, such as murine macrophages (Mfs) or bone marrow derived dendritic cells (DCs) and B lymphocytes. Upon binding, AC8 mAb remarkably enhanced the efficiency of yeast uptake by macrophages, but not the uptake of OVA-Ig immune complexes by DCs. Binding to B lymphoma APCs, AC8 mAb remodeled their surface membrane by microclustering rafts and recruiting MHC-II and the CD80 costimulators to common microdomains. The modulated APCs induced an enhanced activation signaling (higher Ca(2+)-signals and NFAT1 activation) in Th cells conjugated with them, relative to untreated APCs. The results presented herein highlight the modulatory potential of the IgG3 type AC8 mAb on both innate and adaptive effector cell functions.

New cholesterol-specific antibodies remodel HIV-1 target cells' surface and inhibit their in vitro virus production.

The importance of membrane rafts in HIV-1 infection is still in the focus of interest. Here, we report that new monoclonal anticholesterol IgG antibodies (ACHAs), recognizing clustered membrane cholesterol (e.g., in lipid rafts), rearrange the lateral molecular organization of HIV-1 receptors and coreceptors in the plasma membrane of HIV-1 permissive human T-cells and macrophages. This remodeling is accompanied with a substantial inhibition of their infection and HIV-1 production in vitro. ACHAs promote the association of CXCR4 with both CD4 and lipid rafts, consistent with the decreased lateral mobility of CXCR4, while Fab fragments of ACHAs do not show these effects. ACHAs do not directly mask the extracellular domains of either CD4 or CXCR4 nor do they affect CXCR4 internalization. No significant inhibition of HIV production is seen when the virus is preincubated with the antibodies prior to infection. Thus, we propose that the observed inhibition is mainly due to the membrane remodeling induced by cholesterol-specific antibodies on the target cells. This, in turn, may prevent the proper spatio-temporal juxtaposition of HIV-1 glycoproteins with CD4 and chemokine receptors, thus negatively interfering with virus attachment/entry.