Live imaging reveals distinct modes of neutrophil and macrophage migration within interstitial tissues

CCSer2 gates dynein activity at the cell periphery

Cytoplasmic dynein-1 (dynein) is a microtubule-associated, minus end-directed motor that traffics hundreds of different cargos. Dynein must discriminate between cargos and traffic them at the appropriate time from the correct cellular region. How dynein's trafficking activity is regulated in time or cellular space remains poorly understood. Here, we identify CCSer2 as the first known protein to gate dynein activity in the spatial dimension. CCSer2 promotes the migration of developing zebrafish primordium cells, macrophages, and cultured human cells by facilitating the trafficking of cargos that are acted on by peripherally localized dynein. Our data suggest that CCSer2 disfavors the interaction between dynein and its regulator Ndel1 at the cell edge, resulting in localized dynein activation. These findings support a model where the spatial specificity of dynein is achieved by the localization of proteins that trigger Ndel1's release from dynein. We propose that CCSer2 defines a broader class of proteins that activate dynein in distinct microenvironments via regulating Ndel1-dynein interaction.

Haemocyte motility: A marker of inflammation in Mytilus sp

Bivalve immunity relies exclusively on innate cellular and humoral mechanisms, during which cells named haemocytes maraud across tissues to survey the organism and cope with invaders through migration towards infected site. Immune response is therefore governed by haemocyte motility. This review focuses on the different types of haemocyte movement in Mytilus sp. To address their role in immunity, from random patrolling of organs to directed pathogen elimination. By forming cell clusters or aggregates of different sizes, haemocyte displacements define inflammation per se in mussels. Although described for many years, motility can now be quantified by advanced microscopy techniques that give access to cell velocity values, allowing us to quantify inflammation. As various biotic and abiotic factors have been found to modulate haemocyte velocity, this parameter can be considered a marker to assess the inflammation level, paving the way for future developments in determining the immune status of mussels.

Light-powered phagocytic macrophage microrobot (phagobot): both in vitro and in vivo

Micro/nanorobots based on immune cells show great potential for addressing challenging biological and biomedical conditions. However, their powerful innate immune functions, particularly the phagocytosis capabilities, remain a big challenge to fully leverage with the current designs of immune cell-based microrobots. Herein, we report a light-powered phagocytic macrophage microrobot (phagobot), which is capable of robotic navigation toward specific foreign bio-threats and executing precise phagocytosis of these targeted entities under light control. Without genetic modification or nanoengineering of macrophages, the phagobot's "wake-up" program is achieved through direct activation of a resting-state macrophage by a tightly focused near-infrared (NIR) light beam. The phagobot exhibits robotic steering and directional navigation controlled by optical manipulation of the extended pseudopodia within the activated macrophage. It can further execute targeted phagocytic clearance tasks via engulfing various foreign bio-threats, including nanoplastics, microbials, and cancer cell debris. Notably, the phagobot can be constructed in a living larval zebrafish through optical activation and manipulation of the endogenous macrophage, which also exhibits controllable navigation and targeted phagocytic capabilities in vivo. With the intrinsic immune functions of macrophages, our light-powered phagobot represents a novel form of intelligent immune cell-based microrobots, holding many new possibilities for precise immune regulation and treatment for immune-related diseases.

Live imaging in zebrafish reveals tissue-specific strategies for amoeboid migration

Amoeboid cells such as leukocytes can enter and migrate in diverse tissues, even though tissues vary widely in their chemical and mechanical composition. Here, we imaged motile T cells as they colonized peripheral tissues during zebrafish development to determine whether cells tailor their migration strategy to their local tissue environment. We found that T cells in most sites migrated with F-actin-rich, leading-edge pseudopods, matching how they migrate in vitro. T cells notably deviated from this strategy in the epidermis, where they instead migrated using a rearward concentration of F-actin and stable leading-edge blebs. This mode of migration occurs under planar confinement in vitro, and we found that the stratified keratinocyte layers of the epidermis also impose planar-like confinement on leukocytes in vivo. Collectively, our data indicate that immune cells adapt their migration strategy to navigate different tissue geometries in vivo.

Timing matters in macrophage/CD4+ T cell interactions: an agent-based model comparing Mycobacterium tuberculosis host-pathogen interactions between latently infected and naïve individuals

Tuberculosis (TB), caused by Mycobacterium tuberculosis (Mtb), remains a significant health challenge. Clinical manifestations of TB exist across a spectrum with a majority of infected individuals remaining asymptomatic, commonly referred to as latent TB infection (LTBI). In vitro models have demonstrated that cells from individuals with LTBI can better control Mtb growth and form granuloma-like structures more quickly, compared to cells from uninfected (Mtb-naïve) individuals. These in vitro results agree with animal and clinical evidence that LTBI protects, to some degree, against reinfection. However, the mechanisms by which LTBI might offer protection against reinfection remain unclear, and quantifying the relative contributions of multiple control mechanisms is challenging using experimental methods alone. To complement in vitro models, we have developed an in silico agent-based model to help elucidate host responses that might contribute to protection against reinfection. Our simulations indicate that earlier contact between macrophages and CD4+ T cells leads to LTBI simulations having more activated CD4+ T cells and, in turn, more activated infected macrophages, all of which contribute to a decreased bacterial load early on. Our simulations also demonstrate that granuloma-like structures support this early macrophage activation in LTBI simulations. We find that differences between LTBI and Mtb-naïve simulations are driven by TNFα and IFNγ-associated mechanisms as well as macrophage phagocytosis and killing mechanisms. Together, our simulations show how important the timing of the first interactions between innate and adaptive immune cells is, how this impacts infection progression, and why cells from LTBI individuals might be faster to respond to reinfection.IMPORTANCETuberculosis (TB) remains a significant global health challenge, with millions of new infections and deaths annually. Despite extensive research, the mechanisms by which latent TB infection (LTBI) confers protection against reinfection remain unclear. In this study, we developed an in silico agent-based model to simulate early immune responses to Mycobacterium tuberculosis infection based on experimental in vitro infection of human donor cells. Our simulations reveal that early interactions between macrophages and CD4+ T cells, driven by TNFα and IFNγ, are critical for bacterial control and granuloma formation in LTBI. These findings offer new insights into the immune processes involved in TB, which could inform the development of targeted vaccines and host-directed therapies. By integrating experimental data with computational predictions, our research provides a robust framework for understanding TB immunity and guiding future interventions to mitigate the global TB burden.

Microtubule-dependent cell polarity regulates skin-resident macrophage phagocytosis and directed cell migration

Immune cells rapidly respond to tissue damage through dynamic properties of the cytoskeleton. How microtubules control immune cell functions during injury responses remains poorly understood. Within skin, tissue-resident macrophages known as Langerhans cells use dynamic dendrites to surveil the epidermis for damage and migrate through a densely packed epithelium to wounds. Here, we use Langerhans cells within the adult zebrafish epidermis as a model to investigate roles for microtubules in immune cell tissue surveillance, phagocytosis, and directed migration. We describe microtubule organization within Langerhans cells, and show that depolymerizing the microtubule cytoskeleton alters dendrite morphology, debris engulfment, and migration efficiency. We find that the microtubule organizing center positions adjacent to engulfed debris and that its position correlates with navigational pathfinding during directed cell migration. Stabilizing microtubules prevents Langerhans cell motility during directed cell migration by impairing navigation around cellular obstacles. Collectively, our work demonstrates requirements for microtubules in the dynamic actions of tissue-resident macrophages during epithelial surveillance and wound repair.

Real-time imaging reveals a role for macrophage protrusive motility in melanoma invasion

Macrophages are primary cells of the innate immune system that mediate tumor progression. However, the motile behavior of macrophages and interactions with tumor cells are not well understood. Here, we exploit the optical transparency of larval zebrafish and perform real-time imaging of macrophage-melanoma interactions. We found that macrophages are highly motile in the tumor microenvironment. Macrophages extend dynamic projections between tumor cells that precede invasive melanoma migration. Modulating macrophage motility with a dominant inhibitory mutation in Rac2 inhibits recruitment to the tumor and impairs tumor invasion. However, a hyperactivating mutation in Rac2 does not affect macrophage recruitment but limits macrophage projections into the melanoma mass and reduces invasive melanoma cell migration. Taken together, these findings reveal a role for Rac2-mediated macrophage protrusive motility in melanoma invasion.

The Molecular Mechanism of Macrophages in Response to Mechanical Stress

Macrophages, a type of functionally diversified immune cell involved in the progression of many physiologies and pathologies, could be mechanically activated. The physical properties of biomaterials including stiffness and topography have been recognized as exerting a considerable influence on macrophage behaviors, such as adhesion, migration, proliferation, and polarization. Recent articles and reviews on the physical and mechanical cues that regulate the macrophage's behavior are available; however, the underlying mechanism still deserves further investigation. Here, we summarized the molecular mechanism of macrophage behavior through three parts, as follows: (1) mechanosensing on the cell membrane, (2) mechanotransmission by the cytoskeleton, (3) mechanotransduction in the nucleus. Finally, the present challenges in understanding the mechanism were also noted. In this review, we clarified the associated mechanism of the macrophage mechanotransduction pathway which could provide mechanistic insights into the development of treatment for diseases like bone-related diseases as molecular targets become possible.

Temperature-Related Effects on Disease Susceptibility and Immune Response in Redband Trout (Oncorhynchus mykiss gairdneri) Following Challenge With Flavobacterium columnare

Heat stress can increase disease risk in fishes by reducing immune function. Interactions between redband trout (Oncorhynchus mykiss gairdneri) and Flavobacterium columnare, a causative agent of columnaris disease, provide an opportunity to investigate the effects of temperature on immune function and disease resistance during periods of thermal stress. We conducted three trials to characterise differences in immune function and mortality between redband trout held at 18°C and 21°C following challenge with F. columnare. In trial 1, cumulative per cent mortality (CPM) was low and not statistically different between 18°C and 21°C. In trials 2 and 2, we administered higher challenge doses and observed increased CPM overall and significantly greater CPM at 21°C than 18°C. Redband trout upregulated il-8, tnf-α, igm and igt following infection by F. columnare, suggesting that all of these genes may be involved in immune responses to F. columnare infection. We found no differences in the strength of the immune responses between fish held at 21°C versus 18°C. This indicated that 21°C did not elicit sufficient thermal stress to impair immune function and that increased CPM at 21°C versus 18°C was due to enhanced F. columnare virulence.

The IQGAP-related RasGAP IqgC regulates cell-substratum adhesion in Dictyostelium discoideum

Proper adhesion of cells to their environment is essential for the normal functioning of single cells and multicellular organisms. To attach to the extracellular matrix (ECM), mammalian cells form integrin adhesion complexes consisting of many proteins that together link the ECM and the actin cytoskeleton. Similar to mammalian cells, the amoeboid cells of the protist Dictyostelium discoideum also use multiprotein adhesion complexes to control their attachment to the underlying surface. However, the exact composition of the multiprotein complexes and the signaling pathways involved in the regulation of adhesion in D. discoideum have not yet been elucidated. Here, we show that the IQGAP-related protein IqgC is important for normal attachment of D. discoideum cells to the substratum. Mutant iqgC-null cells have impaired adhesion, whereas overexpression of IqgC promotes directional migration. A RasGAP C-terminal (RGCt) domain of IqgC is sufficient for its localization in the ventral adhesion focal complexes, while RasGAP activity of a GAP-related domain (GRD) is additionally required for the proper function of IqgC in adhesion. We identify the small GTPase RapA as a novel direct IqgC interactor and show that IqgC participates in a RapA-regulated signaling pathway targeting the adhesion complexes that include talin A, myosin VII, and paxillin B. On the basis of our results, we propose that IqgC is a positive regulator of adhesion, responsible for the strengthening of ventral adhesion structures and for the temporal control of their subsequent degradation.

Microfluidic device reveals new insights into impairment of neutrophil transmigration in patients with sepsis

Neutrophils need to migrate through tight tissue spaces to eliminate pathogens, but their movement is often hindered by their large and stiff nuclei. Neutrophil migration is impaired in sepsis patients, but it is unclear whether this defect is related to the deformability of their nuclei. Herein, we designed microfluidic devices with micron-scale narrow slits to simulate biological barriers. This setup allowed us to observe and record neutrophil movement and nuclear deformation in real-time. We also developed a method for morphological analysis to quantify nucleus deformation in numerous individual cells. Our studies showed that neutrophils from healthy individuals could adjust their nuclear shape to squeeze through these constrictions, whereas those from sepsis patients demonstrated less flexibility. Neutrophils with rigid nuclei struggled to pass through narrow gaps and were more likely to rupture under pressure. These findings suggest that the migration defects of neutrophils observed in sepsis may be attributed to the inability of neutrophils to deform their nuclei, highlighting the crucial role of microfluidic technologies in offering new insights into migration defects under pathological conditions.

Integrating mechanism-based T cell phenotypes into a model of tumor-immune cell interactions

Interactions between cancer cells and immune cells in the tumor microenvironment influence tumor growth and can contribute to the response to cancer immunotherapies. It is difficult to gain mechanistic insights into the effects of cell-cell interactions in tumors using a purely experimental approach. However, computational modeling enables quantitative investigation of the tumor microenvironment, and agent-based modeling, in particular, provides relevant biological insights into the spatial and temporal evolution of tumors. Here, we develop a novel agent-based model (ABM) to predict the consequences of intercellular interactions. Furthermore, we leverage our prior work that predicts the transitions of CD8+ T cells from a naïve state to a terminally differentiated state using Boolean modeling. Given the details incorporated to predict T cell state, we apply the integrated Boolean-ABM framework to study how the properties of CD8+ T cells influence the composition and spatial organization of tumors and the efficacy of an immune checkpoint blockade. Overall, we present a mechanistic understanding of tumor evolution that can be leveraged to study targeted immunotherapeutic strategies.

Staphylococcus aureus lipid factors modulate melanoma cell clustering and invasion

The microbiome can influence cancer development and progression. However, less is known about the role of the skin microbiota in melanoma. Here, we took advantage of a zebrafish melanoma model to probe the effects of Staphylococcus aureus on melanoma invasion. We found that S. aureus produces factors that enhance melanoma invasion and dissemination in zebrafish larvae. We used a published in vitro 3D cluster formation assay that correlates increased clustering with tumor invasion. S. aureus supernatant increased clustering of melanoma cells and was abrogated by a Rho-Kinase inhibitor, implicating a role for Rho-GTPases. The melanoma clustering response was specific to S. aureus but not to other staphylococcal species, including S. epidermidis. Our findings suggest that S. aureus promotes melanoma clustering and invasion via lipids generated by the lipase Sal2 (officially known as GehB). Taken together, these findings suggest that specific bacterial products mediate melanoma invasive migration in zebrafish.

Live imaging in zebrafish reveals tissue-specific strategies for amoeboid migration

Amoeboid cells like leukocytes can enter and migrate within virtually every tissue of the body, even though tissues vary widely in their chemical and mechanical composition. Here, we imaged motile T cells as they colonized peripheral tissues during zebrafish development to ask if cells tailor their migration strategy to their local tissue environment. We found that T cells in most sites migrated with f-actin-rich leading-edge pseudopods, matching how they migrate in vitro . T cells notably deviated from this strategy in the epidermis, where they instead migrated using a rearward concentration of f-actin and stable leading-edge blebs. This mode of migration occurs under planar confinement in vitro , and we correspondingly found the stratified keratinocyte layers of the epidermis impose planar-like confinement on leukocytes in vivo . By imaging the same cell type across the body, our data collectively indicates that cells adapt their migration strategy to navigate different tissue geometries in vivo .

THP-1 Macrophages Limit Neutrophil Transendothelial Migration in a Model Infection

Introduction

Dysregulated neutrophil function plays a significant role in the pathology of infections, cancer, cardiovascular diseases, and autoimmune disorders. Neutrophil activity is influenced by various cell populations, including macrophages, which are crucial regulators. However, the exact role of human macrophages in controlling neutrophil function remains unclear due to a scarcity of studies utilizing human cells in physiologically relevant models.

Methods

We adapted our "Infection-on-a-Chip" microfluidic device to incorporate macrophages within the collagen extracellular matrix, allowing for the study of interactions between human neutrophils and macrophages in a context that mimics in vivo conditions. The integration of THP-1 macrophages was optimized and their effect on the endothelial lumen was characterized, focusing on permeability and structural integrity. The device was then employed to examine the influence of macrophages on neutrophil response to infection with the bacterial pathogen Pseudomonas aeruginosa.

Results

Integration of THP-1 macrophages into the microfluidic device was successfully optimized, showing no increase in endothelial permeability or structural damage. The presence of macrophages was found to significantly reduce neutrophil transendothelial migration in response to Pseudomonas aeruginosa infection.

Conclusions

Our findings highlight the regulatory role of macrophages in modulating neutrophil responses, suggesting potential therapeutic targets to control neutrophil function in various diseases. The modified microfluidic platform offers a valuable tool for mechanistic studies into macrophage-neutrophil interactions in disease contexts.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12195-024-00813-2.

Agent-based model predicts that layered structure and 3D movement work synergistically to reduce bacterial load in 3D in vitro models of tuberculosis granuloma

Tuberculosis (TB) remains a global public health threat. Understanding the dynamics of host-pathogen interactions within TB granulomas will assist in identifying what leads to the successful elimination of infection. In vitro TB models provide a controllable environment to study these granuloma dynamics. Previously we developed a biomimetic 3D spheroid granuloma model that controls bacteria better than a traditional monolayer culture counterpart. We used agent-based simulations to predict the mechanistic reason for this difference. Our calibrated simulations were able to predict heterogeneous bacterial dynamics that are consistent with experimental data. In one group of simulations, spheroids are found to have higher macrophage activation than their traditional counterparts, leading to better bacterial control. This higher macrophage activation in the spheroids was not due to higher counts of activated T cells, instead fewer activated T cells were able to activate more macrophages due to the proximity of these cells to each other within the spheroid. In a second group of simulations, spheroids again have more macrophage activation but also more T cell activation, specifically CD8+ T cells. This higher level of CD8+ T cell activation is predicted to be due to the proximity of these cells to the cells that activate them. Multiple mechanisms of control were predicted. Simulations removing individual mechanisms show that one group of simulations has a CD4+ T cell dominant response, while the other has a mixed/CD8+ T cell dominant response. Lastly, we demonstrated that in spheroids the initial structure and movement rules work synergistically to reduce bacterial load. These findings provide valuable insights into how the structural complexity of in vitro models impacts immune responses. Moreover, our study has implications for engineering more physiologically relevant in vitro models and advancing our understanding of TB pathogenesis and potential therapeutic interventions.

Behavioral dynamics of neuroprotective macrophage polarization in neuropathic pain observed by GHz femtosecond laser two-photon excitation microscopy

Macrophage polarization in neurotoxic (M1) or neuroprotective (M2) phenotypes is known to play a significant role in neuropathic pain, but its behavioral dynamics and underlying mechanism remain largely unknown. Two-photon excitation microscopy (2PEM) is a promising functional imaging tool for investigating the mechanism of cellular behavior, as using near-infrared excitation wavelengths is less subjected to light scattering. However, the higher-order photobleaching effect in 2PEM can seriously hamper its applications to long-term live-cell studies. Here, we demonstrate a GHz femtosecond (fs) 2PEM that enables hours-long live-cell imaging of macrophage behavior with reduced higher-order photobleaching effect-by leveraging the repetition rate of fs pulses according to the fluorescence lifetime of fluorophores. Using this new functional 2PEM platform, we measure the polarization characteristics of macrophages, especially the long-term cellular behavior in efferocytosis, unveiling the dynamic mechanism of neuroprotective macrophage polarization in neuropathic pain. These efforts can create new opportunities for understanding long-term cellular dynamic behavior in neuropathic pain, as well as other neurobiological problems, and thus dissecting the underlying complex pathogenesis.

Cytoneme-mediated intercellular signaling in keratinocytes essential for epidermal remodeling

The skin, the largest organ, functions as a primary defense mechanism. Epidermal stem cells supply undifferentiated keratinocytes that differentiate as they migrate toward the outermost skin layer. Although such a replenishment process is disrupted in various human skin diseases, its underlying mechanisms remain elusive. With high-resolution live imaging and in vivo manipulations, we revealed that Notch signaling between keratinocytes is mediated by signaling filopodia called cytonemes and is essential for proper keratinocyte differentiation and proliferation. Inhibiting keratinocyte cytonemes reduced Notch expression within undifferentiated keratinocytes, leading to abnormal differentiation and hyperproliferation, resembling human skin disease phenotypes. Overproduction of Interleukin (IL)-17 signal, associated with skin diseases like psoriasis, induces psoriatic phenotypes via cytonemes in zebrafish. Our study suggests that intercellular signaling between keratinocytes through cytonemes is critical for epidermal maintenance, and its misregulation could be an origin of human skin diseases.

Comparative transcriptomics coupled to developmental grading via transgenic zebrafish reporter strains identifies conserved features in neutrophil maturation

Neutrophils are evolutionarily conserved innate immune cells playing pivotal roles in host defense. Zebrafish models have contributed substantially to our understanding of neutrophil functions but similarities to human neutrophil maturation have not been systematically characterized, which limits their applicability to studying human disease. Here we show, by generating and analysing transgenic zebrafish strains representing distinct neutrophil differentiation stages, a high-resolution transcriptional profile of neutrophil maturation. We link gene expression at each stage to characteristic transcription factors, including C/ebp-β, which is important for late neutrophil maturation. Cross-species comparison of zebrafish, mouse, and human samples confirms high molecular similarity of immature stages and discriminates zebrafish-specific from pan-species gene signatures. Applying the pan-species neutrophil maturation signature to RNA-sequencing data from human neuroblastoma patients reveals association between metastatic tumor cell infiltration in the bone marrow and an overall increase in mature neutrophils. Our detailed neutrophil maturation atlas thus provides a valuable resource for studying neutrophil function at different stages across species in health and disease.

Cdc42 activity in the trailing edge is required for persistent directional migration of keratinocytes

Fibroblasts migrate discontinuously by generating transient leading-edge protrusions and irregular, abrupt retractions of a narrow trailing edge. In contrast, keratinocytes migrate persistently and directionally via a single, stable, broad protrusion paired with a stable trailing-edge. The Rho GTPases Rac1, Cdc42 and RhoA are key regulators of cell protrusions and retractions. However, how these molecules mediate cell-type specific migration modes is still poorly understood. In fibroblasts, all three Rho proteins are active at the leading edge, suggesting short-range coordination of protrusive Rac1 and Cdc42 signals with RhoA retraction signals. Here, we show that Cdc42 was surprisingly active in the trailing-edge of migrating keratinocytes. Elevated Cdc42 activity colocalized with the effectors MRCK and N-WASP suggesting that Cdc42 controls both myosin activation and actin polymerization in the back. Indeed, Cdc42 was required to maintain the highly dynamic contractile acto-myosin retrograde flow at the trailing edge of keratinocytes, and its depletion induced ectopic protrusions in the back, leading to decreased migration directionality. These findings suggest that Cdc42 is required to stabilize the dynamic cytoskeletal polarization in keratinocytes, to enable persistent, directional migration.

Rho-associated kinase regulates Langerhans cell morphology and responsiveness to tissue damage

Skin is often the first physical barrier to encounter invading pathogens and physical damage. Damage to the skin must be resolved quickly and efficiently to maintain organ homeostasis. Epidermal-resident immune cells known as Langerhans cells use dendritic protrusions to dynamically surveil the skin microenvironment, which contains epithelial keratinocytes and somatosensory peripheral axons. The mechanisms governing Langerhans cell dendrite dynamics and responses to tissue damage are not well understood. Using skin explants from adult zebrafish, we show that Langerhans cells maintain normal surveillance activity following axonal degeneration and use their dynamic dendrites to engulf small axonal debris. By contrast, a ramified-to-rounded shape transition accommodates the engulfment of larger keratinocyte debris. We find that Langerhans cell dendrites are richly populated with actin and sensitive to a broad spectrum actin inhibitor. We further show that Rho-associated kinase (ROCK) inhibition leads to elongated dendrites, perturbed clearance of large debris, and reduced Langerhans cell migration to tissue-scale wounds. Altogether, our work describes the unique dynamics of Langerhans cells and involvement of the ROCK pathway in immune cell responses to damage of varying magnitude.

Light-sheet autofluorescence lifetime imaging with a single-photon avalanche diode array

Significance

Fluorescence lifetime imaging microscopy (FLIM) of the metabolic co-enzyme nicotinamide adenine dinucleotide (phosphate) [NAD(P)H] is a popular method to monitor single-cell metabolism within unperturbed, living 3D systems. However, FLIM of NAD(P)H has not been performed in a light-sheet geometry, which is advantageous for rapid imaging of cells within live 3D samples.

Aim

We aim to design, validate, and demonstrate a proof-of-concept light-sheet system for NAD(P)H FLIM.

Approach

A single-photon avalanche diode camera was integrated into a light-sheet microscope to achieve optical sectioning and limit out-of-focus contributions for NAD(P)H FLIM of single cells.

Results

An NAD(P)H light-sheet FLIM system was built and validated with fluorescence lifetime standards and with time-course imaging of metabolic perturbations in pancreas cancer cells with 10 s integration times. NAD(P)H light-sheet FLIM in vivo was demonstrated with live neutrophil imaging in a larval zebrafish tail wound also with 10 s integration times. Finally, the theoretical and practical imaging speeds for NAD(P)H FLIM were compared across laser scanning and light-sheet geometries, indicating a 30 × to 6 × acquisition speed advantage for the light sheet compared to the laser scanning geometry.

Conclusions

FLIM of NAD(P)H is feasible in a light-sheet geometry and is attractive for 3D live cell imaging applications, such as monitoring immune cell metabolism and migration within an organism.

Semi-supervised analysis of myeloid and T cell behavior in ex vivo ovarian tumor slices reveals changes in cell motility after treatments

Studies of the high-grade serous ovarian cancer (HGSOC) tumor microenvironment, the most lethal gynecological cancer, aim to enhance the efficiency of established therapies. Cell motility is an important process of anti-tumor response. Using ex vivo human and mouse HGSOC tumor slices combined with time-lapse imaging, we assessed the motility of CD8+ T and myeloid cells. We developed a semi-supervised analysis of cell movements, identifying four cell behaviors: migrating, long migrating, static, and wobbling. Tumor slices were maintained 24h ex vivo, retaining viability and cell movements. Ex vivo treatments with lipopolysaccharide altered CD8+ T and myeloid cell behavior. In vivo chemotherapy reduced ex vivo cell movements in human and mouse tumors and differentially affected CD8+ T and myeloid cells in chemo-sensitive and chemo-resistant mouse models. Ex vivo tumor slices can extend in vivo mouse studies to human, providing a stepping stone to translate mouse cancer studies to clinical trials.

The role of G protein-coupled receptor in neutrophil dysfunction during sepsis-induced acute respiratory distress syndrome

Sepsis is defined as a life-threatening dysfunction due to a dysregulated host response to infection. It is a common and complex syndrome and is the leading cause of death in intensive care units. The lungs are most vulnerable to the challenge of sepsis, and the incidence of respiratory dysfunction has been reported to be up to 70%, in which neutrophils play a major role. Neutrophils are the first line of defense against infection, and they are regarded as the most responsive cells in sepsis. Normally, neutrophils recognize chemokines including the bacterial product N-formyl-methionyl-leucyl-phenylalanine (fMLP), complement 5a (C5a), and lipid molecules Leukotriene B4 (LTB4) and C-X-C motif chemokine ligand 8 (CXCL8), and enter the site of infection through mobilization, rolling, adhesion, migration, and chemotaxis. However, numerous studies have confirmed that despite the high levels of chemokines in septic patients and mice at the site of infection, the neutrophils cannot migrate to the proper target location, but instead they accumulate in the lungs, releasing histones, DNA, and proteases that mediate tissue damage and induce acute respiratory distress syndrome (ARDS). This is closely related to impaired neutrophil migration in sepsis, but the mechanism involved is still unclear. Many studies have shown that chemokine receptor dysregulation is an important cause of impaired neutrophil migration, and the vast majority of these chemokine receptors belong to the G protein-coupled receptors (GPCRs). In this review, we summarize the signaling pathways by which neutrophil GPCR regulates chemotaxis and the mechanisms by which abnormal GPCR function in sepsis leads to impaired neutrophil chemotaxis, which can further cause ARDS. Several potential targets for intervention are proposed to improve neutrophil chemotaxis, and we hope that this review may provide insights for clinical practitioners.

Light sheet autofluorescence lifetime imaging with a single photon avalanche diode array

Single photon avalanche diode (SPAD) array sensors can increase the imaging speed for fluorescence lifetime imaging microscopy (FLIM) by transitioning from laser scanning to widefield geometries. While a SPAD camera in epi-fluorescence geometry enables widefield FLIM of fluorescently labeled samples, label-free imaging of single-cell autofluorescence is not feasible in an epi-fluorescence geometry because background fluorescence from out-of-focus features masks weak cell autofluorescence and biases lifetime measurements. Here, we address this problem by integrating the SPAD camera in a light sheet illumination geometry to achieve optical sectioning and limit out-of-focus contributions, enabling fast label-free FLIM of single-cell NAD(P)H autofluorescence. The feasibility of this NAD(P)H light sheet FLIM system was confirmed with time-course imaging of metabolic perturbations in pancreas cancer cells with 10 s integration times, and in vivo NAD(P)H light sheet FLIM was demonstrated with live neutrophil imaging in a zebrafish tail wound, also with 10 s integration times. Finally, the theoretical and practical imaging speeds for NAD(P)H FLIM were compared across laser scanning and light sheet geometries, indicating a 30X to 6X frame rate advantage for the light sheet compared to the laser scanning geometry. This light sheet system provides faster frame rates for 3D NAD(P)H FLIM for live cell imaging applications such as monitoring single cell metabolism and immune cell migration throughout an entire living organism.

Lack of Paxillin phosphorylation promotes single-cell migration in vivo

Focal adhesions are structures that physically link the cell to the extracellular matrix for cell migration. Although cell culture studies have provided a wealth of information regarding focal adhesion biology, it is critical to understand how focal adhesions are dynamically regulated in their native environment. We developed a zebrafish system to visualize focal adhesion structures during single-cell migration in vivo. We find that a key site of phosphoregulation (Y118) on Paxillin exhibits reduced phosphorylation in migrating cells in vivo compared to in vitro. Furthermore, expression of a non-phosphorylatable version of Y118-Paxillin increases focal adhesion disassembly and promotes cell migration in vivo, despite inhibiting cell migration in vitro. Using a mouse model, we further find that the upstream kinase, focal adhesion kinase, is downregulated in cells in vivo, and cells expressing non-phosphorylatable Y118-Paxillin exhibit increased activation of the CRKII-DOCK180/RacGEF pathway. Our findings provide significant new insight into the intrinsic regulation of focal adhesions in cells migrating in their native environment.

Macrophages undergo a behavioural switch during wound healing in zebrafish

In response to wound signals, macrophages are immediately recruited to the injury where they acquire distinct phenotypes and functions, playing crucial roles both in host defense and healing process. Although macrophage phenotypes have been intensively studied during wound healing, mostly using markers and expression profiles, the impact of the wound environment on macrophage shape and behaviour, and the underlying mechanisms deserve more in-depth investigation. Here, we sought to characterize the dynamics of macrophage recruitment and behaviour during aseptic wounding of the caudal fin fold of the zebrafish larva. Using a photo-conversion approach, we demonstrated that macrophages are recruited to the wounded fin fold as a single wave where they switch their phenotype. Intravital imaging of macrophage shape and trajectories revealed that wound-macrophages display a highly stereotypical set of behaviours and change their shape from amoeboid to elongated shape as wound healing proceeds. Using a pharmacological inhibitor of 15-lipoxygenase and protectin D1, a specialized pro-resolving lipid, we investigated the role of polyunsaturated fatty acid metabolism in macrophage behaviour. While inhibition of 15-lipoxygenase using PD146176 or Nordihydroguaiaretic acid (NDGA) decreases the switch from amoeboid to elongated shape, protectin D1 accelerates macrophage reverse migration and favours elongated morphologies. Altogether, our findings suggest that individual macrophages at the wound switch their phenotype leading to important changes in behaviour and shape to adapt to changing environment, and highlight the crucial role of lipid metabolism in the control of macrophage behaviour plasticity during inflammation in vivo.

An evolutionary and physiological perspective on cell-substrate adhesion machinery for cell migration

Cell-substrate adhesion is a critical aspect of many forms of cell migration. Cell adhesion to an extracellular matrix (ECM) generates traction forces necessary for efficient migration. One of the most well-studied structures cells use to adhere to the ECM is focal adhesions, which are composed of a multilayered protein complex physically linking the ECM to the intracellular actin cytoskeleton. Much of our understanding of focal adhesions, however, is primarily derived from in vitro studies in Metazoan systems. Though these studies provide a valuable foundation to the cell-substrate adhesion field, the evolution of cell-substrate adhesion machinery across evolutionary space and the role of focal adhesions in vivo are largely understudied within the field. Furthering investigation in these areas is necessary to bolster our understanding of the role cell-substrate adhesion machinery across Eukaryotes plays during cell migration in physiological contexts such as cancer and pathogenesis. In this review, we review studies of cell-substrate adhesion machinery in organisms evolutionary distant from Metazoa and cover the current understanding and ongoing work on how focal adhesions function in single and collective cell migration in an in vivo environment, with an emphasis on work that directly visualizes cell-substrate adhesions. Finally, we discuss nuances that ought to be considered moving forward and the importance of future investigation in these emerging fields for application in other fields pertinent to adhesion-based processes.

A two-step search and run response to gradients shapes leukocyte navigation in vivo

Migrating cells must interpret chemical gradients to guide themselves within tissues. A long-held principle is that gradients guide cells via reorientation of leading-edge protrusions. However, recent evidence indicates that protrusions can be dispensable for locomotion in some contexts, raising questions about how cells interpret endogenous gradients in vivo and whether other mechanisms are involved. Using laser wound assays in zebrafish to elicit acute endogenous gradients and quantitative analyses, we demonstrate a two-stage process for leukocyte chemotaxis in vivo: first a “search” phase, with stimulation of actin networks at the leading edge, cell deceleration, and turning. This is followed by a “run” phase, with fast actin flows, cell acceleration, and persistence. When actin dynamics are perturbed, cells fail to resolve the gradient, suggesting that pure spatial sensing of the gradient is insufficient for navigation. Our data suggest that cell contractility and actin flows provide memory for temporal sensing, while expansion of the leading edge serves to enhance gradient sampling.

Amoeboid-like migration ensures correct horizontal cell layer formation in the developing vertebrate retina

Migration of cells in the developing brain is integral for the establishment of neural circuits and function of the central nervous system. While migration modes during which neurons employ predetermined directional guidance of either preexisting neuronal processes or underlying cells have been well explored, less is known about how cells featuring multipolar morphology migrate in the dense environment of the developing brain. To address this, we here investigated multipolar migration of horizontal cells in the zebrafish retina. We found that these cells feature several hallmarks of amoeboid-like migration that enable them to tailor their movements to the spatial constraints of the crowded retina. These hallmarks include cell and nuclear shape changes, as well as persistent rearward polarization of stable F-actin. Interference with the organization of the developing retina by changing nuclear properties or overall tissue architecture hampers efficient horizontal cell migration and layer formation showing that cell-tissue interplay is crucial for this process. In view of the high proportion of multipolar migration phenomena observed in brain development, the here uncovered amoeboid-like migration mode might be conserved in other areas of the developing nervous system.

Centrosome Positioning in Migrating Dictyostelium Cells

Directional cell migration and the establishment of polarity play an important role in development, wound healing, and host cell defense. While actin polymerization provides the driving force at the cell front, the microtubule network assumes a regulatory function, in coordinating front protrusion and rear retraction. By using Dictyostelium discoideum cells as a model for amoeboid movement in different 2D and 3D environments, the position of the centrosome relative to the nucleus was analyzed using live-cell microscopy. Our results showed that the centrosome was preferentially located rearward of the nucleus under all conditions tested for directed migration, while the nucleus was oriented toward the expanding front. When cells are hindered from straight movement by obstacles, the centrosome is displaced temporarily from its rearward location to the side of the nucleus, but is reoriented within seconds. This relocalization is supported by the presence of intact microtubules and their contact with the cortex. The data suggest that the centrosome is responsible for coordinating microtubules with respect to the nucleus. In summary, we have analyzed the orientation of the centrosome during different modes of migration in an amoeboid model and present evidence that the basic principles of centrosome positioning and movement are conserved between Dictyostelium and human leukocytes.

Laser-mediated osteoblast ablation triggers a pro-osteogenic inflammatory response regulated by reactive oxygen species and glucocorticoid signaling in zebrafish

In zebrafish, transgenic labeling approaches, robust regenerative responses and excellent in vivo imaging conditions enable precise characterization of immune cell behavior in response to injury. Here, we monitored osteoblast-immune cell interactions in bone, a tissue which is particularly difficult to in vivo image in tetrapod species. Ablation of individual osteoblasts leads to recruitment of neutrophils and macrophages in varying numbers, depending on the extent of the initial insult, and initiates generation of cathepsin K+ osteoclasts from macrophages. Osteoblast ablation triggers the production of pro-inflammatory cytokines and reactive oxygen species, which are needed for successful macrophage recruitment. Excess glucocorticoid signaling as it occurs during the stress response inhibits macrophage recruitment, maximum speed and changes the macrophage phenotype. Although osteoblast loss is compensated for within a day by contribution of committed osteoblasts, macrophages continue to populate the region. Their presence is required for osteoblasts to fill the lesion site. Our model enables visualization of bone repair after microlesions at single-cell resolution and demonstrates a pro-osteogenic function of tissue-resident macrophages in non-mammalian vertebrates.

Neutrophils: Amoeboid Migration and Swarming Dynamics in Tissues

Neutrophils are key cells of our innate immune response with essential roles for eliminating bacteria and fungi from tissues. They are also the prototype of an amoeboid migrating leukocyte. As one of the first blood-recruited immune cell types during inflammation and infection, these cells can invade almost any tissue compartment. Once in the tissue, neutrophils undergo rapid shape changes and migrate at speeds higher than most other immune cells. They move in a substrate-independent manner in interstitial spaces and do not follow predetermined tissue paths. Instead, neutrophil navigation is largely shaped by the chemokine and chemoattractant milieu around them. This highlights the decisive role of attractant-sensing G-protein coupled receptors (GPCRs) and downstream molecular pathways for controlling amoeboid neutrophil movement in tissues. A diverse repertoire of cell-surface expressed GPCRs makes neutrophils the perfect sentinel cell type to sense and detect danger-associated signals released from wounds, inflamed interstitium, dying cells, complement factors or directly from tissue-invading microbes. Moreover, neutrophils release attractants themselves, which allows communication and coordination between individual cells of a neutrophil population. GPCR-mediated positive feedback mechanisms were shown to underlie neutrophil swarming, a population response that amplifies the recruitment of amoeboid migrating neutrophils to sites of tissue injury and infection. Here we discuss recent findings and current concepts that counteract excessive neutrophil accumulation and swarm formation. In particular, we will focus on negative feedback control mechanisms that terminate neutrophil swarming to maintain the delicate balance between tissue surveillance, host protection and tissue destruction.

Neutrophils in acute inflammation: current concepts and translational implications

Modulation of neutrophil recruitment and function is crucial for targeting inflammatory cells to sites of infection to combat invading pathogens while, at the same time, limiting host tissue injury or autoimmunity. The underlying mechanisms regulating recruitment of neutrophils, 1 of the most abundant inflammatory cells, have gained increasing interest over the years. The previously described classical recruitment cascade of leukocytes has been extended to include capturing, rolling, adhesion, crawling, and transmigration, as well as a reverse-transmigration step that is crucial for balancing immune defense and control of remote organ endothelial leakage. Current developments in the field emphasize the importance of cellular interplay, tissue environmental cues, circadian rhythmicity, detection of neutrophil phenotypes, differential chemokine sensing, and contribution of distinct signaling components to receptor activation and integrin conformations. The use of therapeutics modulating neutrophil activation responses, as well as mutations causing dysfunctional neutrophil receptors and impaired signaling cascades, have been defined in translational animal models. Human correlates of such mutations result in increased susceptibility to infections or organ damage. This review focuses on current advances in the understanding of the regulation of neutrophil recruitment and functionality and translational implications of current discoveries in the field with a focus on acute inflammation and sepsis.

Macrophage network dynamics depend on haptokinesis for optimal local surveillance

Macrophages are key immune cells with important roles for tissue surveillance in almost all mammalian organs. Cellular networks made up of many individual macrophages allow for optimal removal of dead cell material and pathogens in tissues. However, the critical determinants that underlie these population responses have not been systematically studied. Here, we investigated how cell shape and the motility of individual cells influences macrophage network responses in 3D culture settings and in mouse tissues. We show that surveying macrophage populations can tolerate lowered actomyosin contractility, but cannot easily compensate for a lack of integrin-mediated adhesion. Although integrins were dispensable for macrophage chemotactic responses, they were crucial to control cell movement and protrusiveness for optimal surveillance by a macrophage population. Our study reveals that β1 integrins are important for maintaining macrophage shape and network sampling efficiency in mammalian tissues, and sets macrophage motility strategies apart from the integrin-independent 3D migration modes of many other immune cell subsets.

Molecular Insights Into Neutrophil Biology From the Zebrafish Perspective: Lessons From CD18 Deficiency

Neutrophils are key players in innate immunity and originate from the bone marrow of the adult mammalian organism. In mammals, mature neutrophils are released from the bone marrow into the peripheral blood where they circulate until their recruitment to sites of inflammation in a multistep adhesion cascade. Here, adhesion molecules of the β₂ integrin family (CD11/CD18) are critically required for the initial neutrophil adhesion to the inflamed endothelium and several post-adhesion steps allowing their extravasation into the inflamed tissue. Within the mammalian tissue, interstitial neutrophil migration can occur widely independent of β₂ integrins. This is in sharp contrast to neutrophil recruitment in zebrafish larvae (Danio rerio) where neutrophils originate from the caudal hematopoietic tissue and mainly migrate interstitially to sites of lesion upon the early onset of inflammation. However, neutrophils extravasate from the circulation to the inflamed tissue in zebrafish larvae at later-time points. Although zebrafish larvae are a widely accepted model system to analyze neutrophil trafficking in vivo, the functional impact of β₂ integrins for neutrophil trafficking during acute inflammation is completely unknown in this model. In this study, we generated zebrafish with a genetic deletion of CD18, the β subunit of β₂ integrins, using CRISPR/Cas9 technology. Sequence alignments demonstrated a high similarity of the amino acid sequences between zebrafish and human CD18 especially in the functionally relevant I-like domain. In addition, the cytoplasmic domain of CD18 harbors two highly conserved NXXF motifs suggesting that zebrafish CD18 may share functional properties of human CD18. Accordingly, CD18 knock-out (KO) zebrafish larvae displayed the key symptoms of patients suffering from leukocyte adhesion deficiency (LAD) type I due to defects in ITGB2, the gene for CD18. Importantly, CD18 KO zebrafish larvae showed reduced neutrophil trafficking to sites of sterile inflammation despite the fact that an increased number of neutrophils was detectable in the circulation. By demonstrating the functional importance of CD18 for neutrophil trafficking in zebrafish larvae, our findings shed new light on neutrophil biology in vertebrates and introduce a new model organism for studying LAD type I.

Glucose inhibits haemostasis and accelerates diet-induced hyperlipidaemia in zebrafish larvae

Hyperglycaemia damages the microvasculature in part through the reduced recruitment of immune cells and interference with platelet signalling, leading to poor wound healing and accelerated lipid deposition in mammals. We investigated the utility of zebrafish larvae to model the effect of exogenous glucose on neutrophil and macrophage recruitment to a tail wound, wound-induced haemostasis, and chicken egg yolk feed challenge-induced hyperlipidaemia by supplementing larvae with exogenous glucose by immersion or injection. Neither method of glucose supplementation affected the recruitment of neutrophils and macrophages following tail transection. Glucose injection reduced thrombocyte retention and fibrin plug formation while only thrombocyte retention was reduced by glucose immersion following tail transection. We observed accelerated lipid accumulation in glucose-injected larvae challenged with high fat chicken egg yolk feeding. Our study identifies conserved and divergent effects of high glucose on inflammation, haemostasis, and hyperlipidaemia in zebrafish larvae compared to mammals.

Candida auris Cell Wall Mannosylation Contributes to Neutrophil Evasion through Pathways Divergent from Candida albicans and Candida glabrata

Candida auris, a recently emergent fungal pathogen, has caused invasive infections in health care settings worldwide. Mortality rates approach 60% and hospital spread poses a public health threat. Compared to other Candida spp., C. auris avoids triggering the antifungal activity of neutrophils, innate immune cells that are critical for responding to many invasive fungal infections, including candidiasis. However, the mechanism underpinning this immune evasion has been largely unknown. Here, we show that C. auris cell wall mannosylation contributes to the evasion of neutrophils ex vivo and in a zebrafish infection model. Genetic disruption of mannosylation pathways (PMR1 and VAN1) diminishes the outer cell wall mannan, unmasks immunostimulatory components, and promotes neutrophil engagement, phagocytosis, and killing. Upon examination of these pathways in other Candida spp. (Candida albicans and Candida glabrata), we did not find an impact on neutrophil interactions. These studies show how C. auris mannosylation contributes to neutrophil evasion though pathways distinct from other common Candida spp. The findings shed light on innate immune evasion for this emerging pathogen.

IMPORTANCE The emerging fungal pathogen Candida auris presents a global public health threat. Therapeutic options are often limited for this frequently drug-resistant pathogen, and mortality rates for invasive disease are high. Previous study has demonstrated that neutrophils, leukocytes critical for the antifungal host defense, do not efficiently recognize and kill C. auris. Here, we show how the outer cell wall of C. auris promotes immune evasion. Disruption of this mannan polysaccharide layer renders C. auris susceptible to neutrophil killing ex vivo and in a zebrafish model of invasive candidiasis. The role of these mannosylation pathways for neutrophil evasion appears divergent from other common Candida species.

Molecular Mechanisms of Borrelia burgdorferi Phagocytosis and Intracellular Processing by Human Macrophages

Lyme disease is the most common vector-borne illness in North America and Europe. Its causative agents are spirochetes of the Borrelia burgdorferi sensu latu complex. Infection with borreliae can manifest in different tissues, most commonly in the skin and joints, but in severe cases also in the nervous systems and the heart. The immune response of the host is a crucial factor for preventing the development or progression of Lyme disease. Macrophages are part of the innate immune system and thus one of the first cells to encounter infecting borreliae. As professional phagocytes, they are capable of recognition, uptake, intracellular processing and final elimination of borreliae. This sequence of events involves the initial capture and internalization by actin-rich cellular protrusions, filopodia and coiling pseudopods. Uptake into phagosomes is followed by compaction of the elongated spirochetes and degradation in mature phagolysosomes. In this review, we discuss the current knowledge about the processes and molecular mechanisms involved in recognition, capturing, uptake and intracellular processing of Borrelia by human macrophages. Moreover, we highlight interactions between macrophages and other cells of the immune system during these processes and point out open questions in the intracellular processing of borreliae, which include potential escape strategies of Borrelia.

Mvda is required for zebrafish early development

BACKGROUND

The MVD gene mutations are identified in porokeratosis, which is considered a skin-specific autoinflammatory keratinization disease. However, the biological function of MVD gene remains largely unknown. Therefore, we analyzed the function of mvda gene, orthologous to the human MVD gene, in developing zebrafish.

METHODS

Morpholino antisense oligonucleotide technique was used to generate mvda loss-of-function phenotypes. Knockdown of mvda was confirmed by RT-PCR and Sanger sequencing. Scanning and transmission electron microscopy were performed to analyze the morphology of the epidermis. Angiogenesis study was presented using the Tg(fli1a:EGFP)^y1 transgenic strain. In addition, acridine orange staining was used to examine the apoptotic cells in vivo.

RESULTS

As expected, the mvda morphants showed abnormal morphology of the epidermis. Moreover, we observed ectopic sprouts in trunk angiogenesis and impaired formation of the caudal vein plexus in the mvda-deficient zebrafish. Besides, increased apoptosis was found throughout the tail, heart, and eyes in mvda zebrafish morphants.

CONCLUSIONS

These findings indicated the essential role of mvda in the early development of zebrafish. This was the first in vivo knockdown study of the zebrafish mvda gene, which might offer insight into the biological function of the human MVD gene.

Ccdc103 promotes myeloid cell proliferation and migration independent of motile cilia

The pathology of primary ciliary dyskinesia (PCD) is predominantly attributed to impairment of motile cilia. However, PCD patients also have perplexing functional defects in myeloid cells, which lack motile cilia. Here, we show that coiled-coil domain-containing protein 103 (CCDC103), one of the genes that, when mutated, is known to cause PCD, is required for the proliferation and directed migration of myeloid cells. CCDC103 is expressed in human myeloid cells, where it colocalizes with cytoplasmic microtubules. Zebrafish ccdc103/schmalhans (smh) mutants have macrophages and neutrophils with reduced proliferation, abnormally rounded cell morphology and an inability to migrate efficiently to the site of sterile wounds, all of which are consistent with a loss of cytoplasmic microtubule stability. Furthermore, we demonstrate that direct interactions between CCDC103 and sperm associated antigen 6 (SPAG6), which also promotes microtubule stability, are abrogated by CCDC103 mutations from PCD patients, and that spag6 zebrafish mutants recapitulate the myeloid defects observed in smh mutants. In summary, we have illuminated a mechanism, independent of motile cilia, to explain functional defects in myeloid cells from PCD patients. This article has an associated First Person interview with the first author of the paper.

Microphysiological Systems for Studying Cellular Crosstalk During the Neutrophil Response to Infection

Neutrophils are the primary responders to infection, rapidly migrating to sites of inflammation and clearing pathogens through a variety of antimicrobial functions. This response is controlled by a complex network of signals produced by vascular cells, tissue resident cells, other immune cells, and the pathogen itself. Despite significant efforts to understand how these signals are integrated into the neutrophil response, we still do not have a complete picture of the mechanisms regulating this process. This is in part due to the inherent disadvantages of the most-used experimental systems: in vitro systems lack the complexity of the tissue microenvironment and animal models do not accurately capture the human immune response. Advanced microfluidic devices incorporating relevant tissue architectures, cell-cell interactions, and live pathogen sources have been developed to overcome these challenges. In this review, we will discuss the in vitro models currently being used to study the neutrophil response to infection, specifically in the context of cell-cell interactions, and provide an overview of their findings. We will also provide recommendations for the future direction of the field and what important aspects of the infectious microenvironment are missing from the current models.

A robust and flexible CRISPR/Cas9-based system for neutrophil-specific gene inactivation in zebrafish

CRISPR/Cas9-based tissue-specific knockout techniques are essential for probing the functions of genes in embryonic development and disease using zebrafish. However, the lack of capacity to perform gene-specific rescue or live imaging in the tissue-specific knockout background has limited the utility of this approach. Here, we report a robust and flexible gateway system for tissue-specific gene inactivation in neutrophils. Using a transgenic fish line with neutrophil-restricted expression of Cas9 and ubiquitous expression of single guide (sg)RNAs targeting rac2, specific disruption of the rac2 gene in neutrophils is achieved. Transient expression of sgRNAs targeting rac2 or cdk2 in the neutrophil-restricted Cas9 line also results in significantly decreased cell motility. Re-expressing sgRNA-resistant rac2 or cdk2 genes restores neutrophil motility in the corresponding knockout background. Moreover, active Rac and force-bearing F-actins localize to both the cell front and the contracting tail during neutrophil interstitial migration in an oscillating fashion that is disrupted when rac2 is knocked out. Together, our work provides a potent tool that can be used to advance the utility of zebrafish in identifying and characterizing gene functions in a tissue-specific manner.

Bioengineered in vitro models of leukocyte-vascular interactions

Leukocytes continuously circulate our body through the blood and lymphatic vessels. To survey invaders or abnormalities and defend our body against them, blood-circulating leukocytes migrate from the blood vessels into the interstitial tissue space (leukocyte extravasation) and exit the interstitial tissue space through draining lymphatic vessels (leukocyte intravasation). In the process of leukocyte trafficking, leukocytes recognize and respond to multiple biophysical and biochemical cues in these vascular microenvironments to determine adequate migration and adhesion pathways. As leukocyte trafficking is an essential part of the immune system and is involved in numerous immune diseases and related immunotherapies, researchers have attempted to identify the key biophysical and biochemical factors that might be responsible for leukocyte migration, adhesion, and trafficking. Although intravital live imaging of in vivo animal models has been remarkably advanced and utilized, bioengineered in vitro models that recapitulate complicated in vivo vascular structure and microenvironments are needed to better understand leukocyte trafficking since these in vitro models better allow for spatiotemporal analyses of leukocyte behaviors, decoupling of interdependent biological factors, better controlling of experimental parameters, reproducible experiments, and quantitative cellular analyses. This review discusses bioengineered in vitro model systems that are developed to study leukocyte interactions with complex microenvironments of blood and lymphatic vessels. This review focuses on the emerging concepts and methods in generating relevant biophysical and biochemical cues. Finally, the review concludes with expert perspectives on the future research directions for investigating leukocyte and vascular biology using the in vitro models.

Characterization of immune cell migration using microfabrication

The immune system provides our defense against pathogens and aberrant cells, including tumorigenic and infected cells. Motility is one of the fundamental characteristics that enable immune cells to find invading pathogens, control tissue damage, and eliminate primary developing tumors, even in the absence of external treatments. These processes are termed "immune surveillance." Migration disorders of immune cells are related to autoimmune diseases, chronic inflammation, and tumor evasion. It is therefore essential to characterize immune cell motility in different physiologically and pathologically relevant scenarios to understand the regulatory mechanisms of functionality of immune responses. This review is focused on immune cell migration, to define the underlying mechanisms and the corresponding investigative approaches. We highlight the challenges that immune cells encounter in vivo, and the microfabrication methods to mimic particular aspects of their microenvironment. We discuss the advantages and disadvantages of the proposed tools, and provide information on how to access them. Furthermore, we summarize the directional cues that regulate individual immune cell migration, and discuss the behavior of immune cells in a complex environment composed of multiple directional cues.

The cell biology of inflammation: From common traits to remarkable immunological adaptations

Tissue damage triggers a rapid and robust inflammatory response in order to clear and repair a wound. Remarkably, many of the cell biology features that underlie the ability of leukocytes to home in to sites of injury and to fight infection—most of which are topics of intensive current research—were originally observed in various weird and wonderful translucent organisms over a century ago by Elie Metchnikoff, the “father of innate immunity,” who is credited with discovering phagocytes in 1882. In this review, we use Metchnikoff’s seminal lectures as a starting point to discuss the tremendous variety of cell biology features that underpin the function of these multitasking immune cells. Some of these are shared by other cell types (including aspects of motility, membrane trafficking, cell division, and death), but others are more unique features of innate immune cells, enabling them to fulfill their specialized functions, such as encapsulation of invading pathogens, cell–cell fusion in response to foreign bodies, and their self-sacrifice as occurs during NETosis.

Migration of primordial germline cells is negatively regulated by surrounding somatic cells during early embryogenesis in Drosophila melanogaster

Cell migration is an important morphogenetic process necessary at different stages of individual development and body functioning. The initiation and maintenance of the cell movement state requires the activation of many factors involved in the regulation of transcription, signal transduction, adhesive interactions, modulation of membranes and the cytoskeleton. However, cell movement depends on the status of both migrating and surrounding cells, interacting with each other during movement. The surrounding cells or cell matrix not only form a substrate for movement, but can also participate in the spatio-temporal regulation of the migration. At present, there is no exact understanding of the genetic mechanisms of this regulation. To determine the role of the cell environment in the regulation of individual cell migration, we studied the migration of primordial germline cells (PGC) during early embryogenesis in Drosophila melanogaster. Normally, PGC are formed at the 3rd stage of embryogenesis at the posterior pole of the embryo. During gastrulation (stages 6-7), PGC as a consolidated cell group passively transfers into the midgut primordium. Further, PGC are individualized, acquire an amoeboid form, and actively move through the midgut epithelium and migrate to the 5-6 abdominal segment of the embryo, where they form paired embryonic gonads. We screened for genes expressed in the epithelium surrounding PGC during early embryogenesis and affecting their migration. We identified the myc, Hph, stat92E, Tre-1, and hop genes, whose RNA interference leads to premature active PGC migration at stages 4-7 of embryogenesis. These genes can be divided into two groups: 1) modulators of JAK/STAT pathway activity inducing PGC migration (stat92E, Tre-1, hop), and 2) myc and Hph involved in epithelial morphogenesis and polarization, i. e. modifying the permeability of the epithelial barrier. Since a depletion of each of these gene products resulted in premature PGC migration, we can conclude that, normally, the somatic environment negatively regulates PGC migration during early Drosophila embryogenesis.

The Olfactory Organ Is Populated by Neutrophils and Macrophages During Early Development

The immune system of vertebrates is characterized by innate and adaptive immunity that function together to form the natural defense system of the organism. During development innate immunity is the first to become functional and is mediated primarily by phagocytic cells, including macrophages, neutrophils, and dendritic cells. In the olfactory sensory system, the same sensory neurons in contact with the external environment have their first synapse within the central nervous system. This unique architecture presents a potential gateway for the entry of damaging or infectious agents to the nervous system. Here we used zebrafish as a model system to examine the development of the olfactory organ and to determine whether it shares immune characteristics of a host defense niche described in other tissues. During early development, both neutrophils and macrophages appear coincident with the generation of the primitive immune cells. The appearance of neutrophils and macrophages in the olfactory organs occurs as the blood and lymphatic vascular system is forming in the same region. Making use of the neurogenic properties of the olfactory organ we show that damage to the olfactory sensory neurons in larval zebrafish triggers a rapid immune response by local and non-local neutrophils. In contrast, macrophages, although present in greater numbers, mount a slower response to damage. We anticipate our findings will open new avenues of research into the role of the olfactory-immune response during normal neurogenesis and damage-induced regeneration and contribute to our understanding of the formation of a potential host defense immune niche in the peripheral nervous system.

Neutrophils infiltrate into the spiral ligament but not the stria vascularis in the cochlea during lipopolysaccharide-induced inflammation

It has been challenging to apply intravital imaging for monitoring the inner ear, as the anatomical location and intricate structure hamper the access of imaging instruments to the inner ear of live mice. By employing intravital imaging of the cochlea in live mice with two-photon microscopy, we investigated neutrophil infiltration into the cochlea tissue and its characteristics under a lipopolysaccharide (LPS)-induced inflammatory state. Methods: Cochlea inflammation was induced by LPS injection to the middle ear. Using two-photon intravital microscopy with specifically designed surgical exteriorization of the cochlea in live mice, we investigated the dynamic features of neutrophils in the lateral wall of the cochlea. The molecular expression pattern of the cochlea lateral wall was also investigated during the LPS-induce inflammation. Results: Despite the contention of whether neutrophils are recruited to the spiral ligament (SL) during inflammation, we observed that LPS-induced inflammation of the middle ear, which mimics acute otitis media, triggered neutrophil migration to the SL in the lateral wall. Notably, massive neutrophil infiltration to the SL occurred 2 days after LPS inoculation, but there was no neutrophil infiltration into the stria vascularis (SV) region. At 1 day after LPS-induced cochlear inflammation, increased mRNA expression of interleukin-1β, interleukin-6 were identified in both the SL and SV, while the ICAM-1 mRNA expression increased only in the SL. The differential reactivity of ICAM-1 is likely responsible for the different neutrophil recruitment pattern in the cochlea. Conclusion: Intravital imaging of the cochlea revealed that neutrophil recruitment and infiltration during inflammation are spatially controlled and exclusively observed in the SL but not in the SV and organ of Corti.