Tunneling nanotubes (TNT) are dynamic connections between cells, which represent a novel route for cell-to-cell communication. A growing body of evidence points TNT towards a role for intercellular exchanges of signals, molecules, organelles, and pathogens, involving them in a diverse array of functions. TNT form among several cell types, including neuronal cells, epithelial cells, and almost all immune cells. In myeloid cells (e.g., macrophages, dendritic cells, and osteoclasts), intercellular communication via TNT contributes to their differentiation and immune functions. Importantly, TNT enable myeloid cells to communicate with a targeted neighboring or distant cell, as well as with other cell types, therefore creating a complex variety of cellular exchanges. TNT also contribute to pathogen spread as they serve as “corridors” from a cell to another. Herein, we addressed the complexity of the definition and in vitro characterization of TNT in innate immune cells, the different processes involved in their formation, and their relevance in vivo. We also assess our current understanding of how TNT participate in immune surveillance and the spread of pathogens, with a particular interest for HIV-1. Overall, despite recent progress in this growing research field, we highlight that further investigation is needed to better unveil the role of TNT in both physiological and pathological conditions.

In metazoans, γ-tubulin acts within two main complexes, γ-tubulin small complexes (γ-TuSCs) and γ-tubulin ring complexes (γ-TuRCs). In higher eukaryotes, it is assumed that microtubule nucleation at the centrosome depends on γ-TuRCs, but the role of γ-TuRC components remains undefined. For the first time, we analyzed the function of all four γ-TuRC–specific subunits in Drosophila melanogaster: Dgrip75, Dgrip128, Dgrip163, and Dgp71WD. Grip-motif proteins, but not Dgp71WD, appear to be required for γ-TuRC assembly. Individual depletion of γ-TuRC components, in cultured cells and in vivo, induces mitotic delay and abnormal spindles. Surprisingly, γ-TuSCs are recruited to the centrosomes. These defects are less severe than those resulting from the inhibition of γ-TuSC components and do not appear critical for viability. Simultaneous cosilencing of all γ-TuRC proteins leads to stronger phenotypes and partial recruitment of γ-TuSC. In conclusion, γ-TuRCs are required for assembly of fully functional spindles, but we suggest that γ-TuSC could be targeted to the centrosomes, which is where basic microtubule assembly activities are maintained.

In the arms race of host–microbe co-evolution, macrophages (Mɸs) have been endowed with strategies to neutralize pathogenic challenge while preserving host integrity. During steady-states conditions, Mɸs perform multiple house-keeping functions governed by their differentiation state, tissue distribution, and signals from the microenvironment. In response to pathogenic challenge and host mediators, however, Mɸs undergo different programs of activation rendering them either pro-inflammatory and microbicidal (M1), or immunosuppressants and tissue repairers (M2). An excessive or prolonged polarization of either program may be detrimental to the host due to potential tissue injury or contribution to pathogenesis. Conversely, intracellular microbes that cause chronic diseases such as tuberculosis and acquired immunodeficiency syndrome exemplify strategies for survival in the host. Indeed, both Mycobacterium tuberculosis (Mtb) and human immunodeficiency virus (HIV-1) are successful intracellular microbes that thrive in Mɸs. Given these microbes not only co-circulate throughout the developing world but each has contributed to prevalence and mortality caused by the other, substantial insights into microbe physiology and host defenses then rest in the attempt to fully understand their influence on Mɸ polarization. This review addresses the role of Mɸ polarization in the immune response to, and pathogenesis of, Mtb and HIV.

The tuberculosis (TB) bacillus, Mycobacterium tuberculosis (Mtb), and HIV-1 act synergistically; however, the mechanisms by which Mtb exacerbates HIV-1 pathogenesis are not well known. Using in vitro and ex vivo cell culture systems, we show that human M(IL-10) anti-inflammatory macrophages, present in TB-associated microenvironment, produce high levels of HIV-1. In vivo, M(IL-10) macrophages are expanded in lungs of co-infected non-human primates, which correlates with disease severity. Furthermore, HIV-1/Mtb co-infected patients display an accumulation of M(IL-10) macrophage markers (soluble CD163 and MerTK). These M(IL-10) macrophages form direct cell-to-cell bridges, which we identified as tunneling nanotubes (TNTs) involved in viral transfer. TNT formation requires the IL-10/STAT3 signaling pathway, and targeted inhibition of TNTs substantially reduces the enhancement of HIV-1 cell-to-cell transfer and overproduction in M(IL-10) macrophages. Our study reveals that TNTs facilitate viral transfer and amplification, thereby promoting TNT formation as a mechanism to be explored in TB/AIDS potential therapeutics.

Key Points: HIV-1 Nef reprograms human macrophage migration favoring the mesenchymal mode, which translates in vivo to macrophage tissue accumulation. Nef enhances mesenchymal migration by influencing podosome organization and function via the phagocyte-specific kinase Hck and WASP.

Significance: Bone deficits are frequent complications observed in HIV-1–infected patients. Our study demonstrates that HIV-1 infects osteoclasts, the cells specialized in bone degradation, using different models including HIV-1–infected humanized mice. We decipher the cellular mechanisms by which HIV-1 contributes to enhanced bone degradation in human osteoclasts, showing that the virus modifies the structure and function of the sealing zone, the bone resorption machinery of osteoclasts. We identify the viral protein Nef as the key factor responsible for such effects. As a proof-of-concept, we correlate bone deficit in transgenic Nef-expressing mice with enhanced osteoclast activity. Therefore, our findings provide formal evidence that osteoclasts constitute HIV-1 host target cells, contributing to bone deficits in vivo.