Search

The colon is responsible for absorbing fluids. It contains many microorganisms including fungi, which are enriched in its distal segment. The colonic mucosa must thus tightly regulate fluid influx to avoid absorption of fungal metabolites, which can be toxic to epithelial cells and reach the blood circulation, leading to barrier dysfunction and sepsis. How this is achieved remains unknown. Here, we describe a mechanism by which macrophages rapidly check the fluids absorbed through epithelial cells to avoid intoxication of colonocytes. This mechanism relies on a population of distal colon macrophages equipped with "balloon-like" protrusions (BLPs) inserted at the base of epithelial cells. BLPs sample absorbed fluids and stop absorption when fluids contain fungal toxins. Without macrophages or BLPs, epithelial cells keep absorbing fluids containing fungal products, leading to apoptosis and loss of epithelium integrity. These results reveal an unexpected role of macrophages in the maintenance of colon-microbiota interactions in homeostasis.

Sprouting angiogenesis is fundamental for development and contributes to multiple diseases, including cancer, diabetic retinopathy and cardiovascular diseases. Sprouting angiogenesis depends on the invasive properties of endothelial tip cells. However, the current concept assumes that sprouting angiogenesis is based on a universal endothelial tip cell invasive profile. Here, we propose the existence of two modes of sprouting angiogenesis in vivo, filopodia-sensitive and filopodia-insensitive. We disclose that endothelial tip cells use long lamellipodia projections (LLPs) as the main mechanism for invasion, whilst the presence of filopodia is necessary in tissue specific contexts. We further show that LLPs and filopodia protrusions are balanced by myosin-IIA (MIIA) and actin-related protein 2/3 (Arp2/3) activity. Endothelial cell autonomous ablation of MIIA promotes excessive LLPs formation in detriment of filopodia. Conversely, endothelial cell-autonomous ablation of Arp2/3 prevents LLPs development and leads to excessive filopodia formation. We further show that MIIA inhibits integrin-dependent activation of Arp2/3 by regulating Rac1 activity. Our discoveries demonstrate how endothelial tip cells regulate its protrusive activity and will pave the way towards new strategies to block invasive tip cells during sprouting angiogenesis.

International audience; The function of most immune cells depends on their ability to migrate through complex microenvironments, either randomly to patrol for the presence of antigens or directionally to reach their next site of action. The actin cytoskeleton and its partners are key conductors of immune cell migration as they control the intrinsic migratory properties of leukocytes as well as their capacity to respond to cues present in their environment. In this review we focus on the latest discoveries regarding the role of the actomyosin cytoskeleton in optimizing immune cell migration in complex environments, with a special focus on recent insights provided by physical modeling.

Summary Inflammation triggers the differentiation of Ly6Chi monocytes into microbicidal macrophages or monocyte-derived dendritic cells (moDCs). Yet, it is unclear whether environmental inflammatory cues control the polarization of monocytes toward each of these fates or whether specialized monocyte progenitor subsets exist before inflammation. Here, we have shown that naive monocytes are phenotypically heterogeneous and contain an NR4A1- and Flt3L-independent, CCR2-dependent, Flt3+CD11c−MHCII+PU.1hi subset. This subset acted as a precursor for FcγRIII+PD-L2+CD209a+, GM-CSF-dependent moDCs but was distal from the DC lineage, as shown by fate-mapping experiments using Zbtb46. By contrast, Flt3−CD11c−MHCII−PU.1lo monocytes differentiated into FcγRIII+PD-L2−CD209a−iNOS+ macrophages upon microbial stimulation. Importantly, Sfpi1 haploinsufficiency genetically distinguished the precursor activities of monocytes toward moDCs or microbicidal macrophages. Indeed, Sfpi1+/− mice had reduced Flt3+CD11c−MHCII+ monocytes and GM-CSF-dependent FcγRIII+PD-L2+CD209a+ moDCs but generated iNOS+ macrophages more efficiently. Therefore, intercellular disparities of PU.1 expression within naive monocytes segregate progenitor activity for inflammatory iNOS+ macrophages or moDCs., Graphical Abstract, Highlights • Murine Ly6ChiCD115+ monocytes are heterogeneous • DC-related genes (Cd209a and MHCII) are expressed in a subset of FcγRIII+ monocytes • GM-CSF-dependent CD209a+ moDCs are generated by FcγRIII+CD209a+MHCII+ monocytes • iNOS+ macrophages are generated by FcγRIII+CD209a−MHCII− monocytes, Monocytes can differentiate into multiple progenies during inflammation. Here, Menezes et al. show that monocytes from naive mice are heterogeneous and contain distinct precursor subsets giving rise to iNOS+ inflammatory macrophages or GM-CSF-induced CD209a+ monocyte-derived dendritic cells.

SummaryThe migration of immune cells is guided by specific chemical signals, such as chemokine gradients. Their trajectories can also be diverted by physical cues and obstacles imposed by the cellular environment, such as topography, rigidity, adhesion, or hydraulic resistance. On the example of hydraulic resistance, it was shown that neutrophil preferentially follow paths of least resistance, a phenomenon referred to as barotaxis. We here combined quantitative imaging and physical modeling to show that barotaxis results from a force imbalance at the scale of the cell, which is amplified by the acto-myosin intrinsic polarization capacity. Strikingly, we found that macropinocytosis specifically confers to immature dendritic cells a unique capacity to overcome this physical bias by facilitating external fluid transport across the cell, thereby enhancing their space exploration capacity in vivo and promoting their tissue-patrolling function. Conversely, mature dendritic cells, which down-regulate macropinocytosis, were found to be sensitive to hydraulic resistance. Theoretical modeling suggested that barotaxis, which helps them avoid dead-ends, may accelerate their migration to lymph nodes, where they initiate adaptive immune responses. We conclude that the physical properties of the microenvironment of moving cells can introduce biases in their migratory behaviors but that specific active mechanisms such as macropinocytosis have emerged to diminish the influence of these biases, allowing motile cells to reach their final destination and efficiently fulfill their functions.

SummaryIntracellular Toll-like receptor 3 (TLR3), TLR7, and TLR9 localize in endosomes and recognize single-stranded RNA and nucleotides from viruses and bacteria. This interaction induces their conformational changes resulting in the production of proinflammatory cytokines and upregulation of cell surface molecules. TLR9 requires a proteolytic cleavage for its signaling. Here, we report that myeloid and plasmacytoid dendritic cells (DCs) deficient for the asparagine endopeptidase (AEP), a cysteine lysosomal protease, showed a decrease in the secretion of proinflammatory cytokines in response to TLR9 stimulation in vitro and in vivo. Upon stimulation, full-length TLR9 was cleaved into a 72 kDa fragment and this processing was strongly reduced in DCs lacking AEP. Processed TLR9 coeluted with the adaptor molecule MyD88 and AEP after size exclusion chromatography. When expressed in AEP-deficient DCs, the 72 kDa proteolytic fragment restored TLR9 signaling. Thus, our results identify an endocytic protease playing a critical role in TLR processing and signaling in DCs.

Antigen capture and presentation onto MHC class II molecules by B lymphocytes is mediated by their surface antigen receptor - the B-cell receptor (BCR). The BCR must therefore coordinate the transport of MHC class II- and antigen-containing vesicles for them to converge and ensure efficient processing. Recently, progress has been made in understanding which and how these vesicular transport events are molecularly linked to BCR signaling. In particular, recent studies have emphasized the key roles of membrane microdomains and the actin cytoskeleton in regulation of membrane trafficking upon BCR engagement.

International audience; T cells interact with dendritic cells (DCs) for periods lasting from minutes to hours. However, a causal link between the duration of this interaction and the efficiency of T cell activation has not been established in vivo. Employing intravital two-photon imaging, we manipulated T cell-DC interactions in real time and found that the first T cell-DC encounter often resulted in a long-lived interaction. Moreover, the cessation of T cell receptor-major histocompatibility complex signals promoted cellular dissociation, suggesting that antigen availability on DCs regulates contact duration. Finally, at least 6 hr of in vivo T cell-DC interaction were required for naive CD4 + T cells to undergo clonal expansion. These results establish the importance of prolonged T cell-DC interactions for efficient CD4 + T cell activation in vivo.