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Simple Summary: Breast cancer is one of the most common cancers in women and despite improvements in treatments, patients suffering from metastatic disease still have a very low survival rate. A rate-limiting step is the seeding of tumor cells from the blood into the metastatic site. Macrophages are a key cell type in the metastatic niche, which are important for tumor cell seeding despite being separated from the tumor cells in the vasculature by an endothelial barrier. We found that macrophages and tumor cells make thin membranous connections through the endothelial barrier to assist in tumor cell crossing. These structures are similar to tunneling nanotubes that we previously showed were important for tumor cell invasion. We found that these macrophage-mediated connections were important for tumor crossing the endothelial barrier and for metastatic seeding. This new finding suggests that targeting tunneling nanotubes may limit tumor spread. Macrophages are important players involved in the progression of breast cancer, including in seeding the metastatic niche. However, the mechanism by which macrophages in the lung parenchyma interact with tumor cells in the vasculature to promote tumor cell extravasation at metastatic sites is not clear. To mimic macrophage-driven tumor cell extravasation, we used an in vitro assay (eTEM) in which an endothelial monolayer and a matrigel-coated filter separated tumor cells and macrophages from each other. The presence of macrophages promoted tumor cell extravasation, while macrophage conditioned media was insufficient to stimulate tumor cell extravasation in vitro. This finding is consistent with a requirement for direct contact between macrophages and tumor cells. We observed the presence of Thin Membranous Connections (TMCs) resembling similar structures formed between macrophages and tumor cells called tunneling nanotubes, which we previously demonstrated to be important in tumor cell invasion in vitro and in vivo. To determine if TMCs are important for tumor cell extravasation, we used macrophages with reduced levels of endogenous M-Sec (TNFAIP2), which causes a defect in tunneling nanotube formation. As predicted, these macrophages showed reduced macrophage-tumor cell TMCs. In both, human and murine breast cancer cell lines, there was also a concomitant reduction in tumor cell extravasation in vitro when co-cultured with M-Sec deficient macrophages compared to control macrophages. We also detected TMCs formed between macrophages and tumor cells through the endothelial layer in the eTEM assay. Furthermore, tumor cells were more frequently found in pores under the endothelium that contain macrophage protrusions. To determine the role of macrophage-tumor cell TMCs in vivo, we generated an M-Sec deficient mouse. Using an in vivo model of experimental metastasis, we detected a significant reduction in the number of metastatic lesions in M-Sec deficient mice compared to wild type mice. There was no difference in the size of the metastases, consistent with a defect specific to tumor cell extravasation and not metastatic outgrowth. Additionally, with an examination of time-lapse intravital-imaging (IVI) data sets of breast cancer cell extravasation in the lungs, we could detect the presence of TMCs between extravascular macrophages and vascular tumor cells. Overall, our data indicate that macrophage TMCs play an important role in promoting the extravasation of circulating tumor cells in the lungs. [ABSTRACT FROM AUTHOR]

Abstract Background HIV-1 promotes the formation of tunneling nanotubes (TNTs) that connect distant cells, aiding cell-to-cell viral transmission between macrophages. Our recent study suggests that the cellular protein M-Sec plays a role in these processes. However, the timing, mechanism, and to what extent M-Sec contributes to HIV-1 transmission is not fully understood, and the lack of a cell line model that mimics macrophages has hindered in-depth analysis. Results We found that HIV-1 increased the number, length and thickness of TNTs in a manner dependent on its pathogenic protein Nef and M-Sec in U87 cells, as observed in macrophages. In addition, we found that M-Sec was required not only for TNT formation but also motility of U87 cells, both of which are beneficial for viral transmission. In fact, M-Sec knockdown in U87 cells led to a significantly delayed viral production in both cellular and extracellular fractions. This inhibition was observed for wild-type virus, but not for a mutant virus lacking Nef, which is known to promote not only TNT formation but also migration of infected macrophages. Conclusions By taking advantage of useful features of U87 cells, we provided evidence that M-Sec mediates a rapid and efficient cell–cell transmission of HIV-1 at an early phase of infection by enhancing both TNT formation and cell motility.

Background: HIV-1 promotes the formation of tunneling nanotubes (TNTs) that connect distant cells, aiding cell-to-cell viral transmission between macrophages. Our recent study suggests that the cellular protein M-Sec plays a role in these processes. However, the timing, mechanism, and to what extent M-Sec contributes to HIV-1 transmission is not fully understood, and the lack of a cell line model that mimics macrophages has hindered in-depth analysis. Results: We found that HIV-1 increased the number, length and thickness of TNTs in a manner dependent on its pathogenic protein Nef and M-Sec in U87 cells, as observed in macrophages. In addition, we found that M-Sec was required not only for TNT formation but also motility of U87 cells, both of which are beneficial for viral transmission. In fact, M-Sec knockdown in U87 cells led to a significantly delayed viral production in both cellular and extracellular fractions. This inhibition was observed for wild-type virus, but not for a mutant virus lacking Nef, which is known to promote not only TNT formation but also migration of infected macrophages. Conclusions: By taking advantage of useful features of U87 cells, we provided evidence that M-Sec mediates a rapid and efficient cell–cell transmission of HIV-1 at an early phase of infection by enhancing both TNT formation and cell motility. [ABSTRACT FROM AUTHOR]

Abstract: The application of polymeric nanoparticles (NPs) has a promising future for targeting and delivering drugs into the central nervous system (CNS). However, the fate of NPs once entered in the brain after crossing the blood–brain barrier (BBB) and taken up into neuronal cells is a neglected area of study. Thus, here, we investigate the possible mechanisms of a cell-to-cell transport of poly-lactide-co-glycolide (PLGA) NPs modified with a glycopeptide (g7-NPs), already demonstrated to be able to cross the BBB after in vivo administration in rodents. We also tested antibody (Ab) -modified g7-NPs both in vitro and in vivo to investigate the possibility of specific targeting. Our results show that g7-NPs can be transported intra- and inter-cellularly within vesicles after vesicular internalization. Moreover, cell-to-cell transport is mediated by tunneling-nanotube (TNT)-like structures in cell lines and most interestingly in glial as well as neuronal cells in vitro. The transport is dependent on F-actin and can be increased by induction of TNT-like structures overexpressing M-Sec, a central factor and inducer of TNT formation. Moreover, cell-to-cell transport occurs independently from NP surface modification with antibodies. These in vitro findings were in part confirmed by in vivo evidence after i.p. administration of NPs in mice. [Copyright &y& Elsevier]