Longitudinal intravital imaging of the femoral bone marrow reveals plasticity within marrow vasculature

Optimized intravital three-photon imaging of intact mouse tibia links plasma cell motility to functional states

Intravital deep bone marrow imaging is crucial to studying cellular dynamics and functions but remains challenging, and minimally invasive methods are needed. We employed a high pulse-energy 1650 nm laser to perform three-photon microscopy in vivo, reaching ≈400 μm depth in intact mouse tibia. Repetition rates of 3 and 4 MHz allowed us to analyze motility patterns of fast and rare cells within unperturbed marrow and to identify a bi-modal migratory behavior for plasma cells. Third harmonic generation (THG) was identified as a label-free marker for cellular organelles, particularly endoplasmic reticulum, indicating protein synthesis capacity. We found a strong THG signal, suggesting high antibody secretion, in one-third of plasma cells while the rest showed low signals. We discovered an inverse relationship between migratory behavior and THG signal, linking motility to functional plasma cell states. This method may enhance our understanding of marrow microenvironment effects on cellular functions.

A semi-automated cell tracking protocol for quantitative analyses of neutrophil swarming to sterile and S. aureus contaminated bone implants in a mouse femur model

Implant-associated osteomyelitis remains a major orthopaedic problem. As neutrophil swarming to the surgical site is a critical host response to prevent infection, visualization and quantification of this dynamic behavior at the native microenvironment of infection will elucidate previously unrecognized mechanisms central to understanding the host response. We recently developed longitudinal intravital imaging of the bone marrow (LIMB) to visualize host cells and fluorescent S. aureus on a contaminated transfemoral implant in live mice, which allows for direct visualization of bacteria colonization of the implant and host cellular responses using two-photon laser scanning microscopy. To the end of rigorous and reproducible quantitative outcomes of neutrophil swarming kinetics in this model, we developed a protocol for robust segmentation, tracking, and quantifications of neutrophil dynamics adapted from Trainable Weka Segmentation and TrackMate, two readily available Fiji/ImageJ plugins. In this work, Catchup mice with tdTomato expressing neutrophils received a transfemoral pin with or without ECFP/EGFP-expressing USA300 methicillin-resistant Staphylococcus aureus (MRSA) to obtain 30-minute LIMB videos at 2-, 4-, and 6-hours post-implantation. The developed semi-automated neutrophil tracking protocol was executed independently by two users to quantify the distance, displacement, speed, velocity, and directionality of the target cells. The results revealed high inter-user reliability for all outcomes (ICC > 0.96; p > 0.05). Consistent with the established paradigm on increased neutrophil swarming during active infection, the results also demonstrated increased neutrophil speed and velocity at all measured time points, and increased displacement at later time points (6 hours) in infected versus uninfected mice (p < 0.05). Neutrophils and bacteria also exhibit directionality during migration in the infected mice. The semi-automated cell tracking protocol provides a streamlined approach to robustly identify and track individual cells across diverse experimental settings and eliminates inter-observer variability.

Angiogenesis is uncoupled from osteogenesis during calvarial bone regeneration

Bone regeneration requires a well-orchestrated cellular and molecular response including robust vascularization and recruitment of mesenchymal and osteogenic cells. In femoral fractures, angiogenesis and osteogenesis are closely coupled during the complex healing process. Here, we show with advanced longitudinal intravital multiphoton microscopy that early vascular sprouting is not directly coupled to osteoprogenitor invasion during calvarial bone regeneration. Early osteoprogenitors emerging from the periosteum give rise to bone-forming osteoblasts at the injured calvarial bone edge. Microvessels growing inside the lesions are not associated with osteoprogenitors. Subsequently, osteogenic cells collectively invade the vascularized and perfused lesion as a multicellular layer, thereby advancing regenerative ossification. Vascular sprouting and remodeling result in dynamic blood flow alterations to accommodate the growing bone. Single cell profiling of injured calvarial bones demonstrates mesenchymal stromal cell heterogeneity comparable to femoral fractures with increase in cell types promoting bone regeneration. Expression of angiogenesis and hypoxia-related genes are slightly elevated reflecting ossification of a vascularized lesion site. Endothelial Notch and VEGF signaling alter vascular growth in calvarial bone repair without affecting the ossification progress. Our findings may have clinical implications for bone regeneration and bioengineering approaches.

Endothelial SMAD1/5 signaling couples angiogenesis to osteogenesis in juvenile bone

Skeletal development depends on coordinated angiogenesis and osteogenesis. Bone morphogenetic proteins direct bone formation in part by activating SMAD1/5 signaling in osteoblasts. However, the role of SMAD1/5 in skeletal endothelium is unknown. Here, we found that endothelial cell-conditional SMAD1/5 depletion in juvenile mice caused metaphyseal and diaphyseal hypervascularity, resulting in altered trabecular and cortical bone formation. SMAD1/5 depletion induced excessive sprouting and disrupting the morphology of the metaphyseal vessels, with impaired anastomotic loop formation at the chondro-osseous junction. Endothelial SMAD1/5 depletion impaired growth plate resorption and, upon long-term depletion, abrogated osteoprogenitor recruitment to the primary spongiosa. Finally, in the diaphysis, endothelial SMAD1/5 activity was necessary to maintain the sinusoidal phenotype, with SMAD1/5 depletion inducing formation of large vascular loops and elevated vascular permeability. Together, endothelial SMAD1/5 activity sustains skeletal vascular morphogenesis and function and coordinates growth plate remodeling and osteoprogenitor recruitment dynamics in juvenile mouse bone.

Longitudinal intravital imaging of the bone marrow for analysis of the race for the surface in a murine osteomyelitis model

Critical knowledge gaps of orthopedic infections pertain to bacterial colonization. The established dogma termed the Race for the Surface posits that contaminating bacteria compete with host cells for the implant post-op, which remains unproven without real-time in vivo evidence. Thus, we modified the murine longitudinal intravital imaging of the bone marrow (LIMB) system to allow real-time quantification of green fluorescent protein (GFP+) host cells and enhanced cyan fluorescent protein (ECFP+) or red fluorescent protein (RFP+) methicillin-resistant Staphylococcus aureus (MRSA) proximal to a transfemoral implant. Following inoculation with ~105 CFU, an L-shaped metal implant was press-fit through the lateral cortex at a 90° angle ~0.150 mm below a gradient refractive index (GRIN) lens. We empirically derived a volume of interest (VOI) = 0.0161 ± 0.000675 mm3 during each imaging session by aggregating the Z-stacks between the first (superior) and last (inferior) in-focus LIMB slice. LIMB postimplantation revealed very limited bacteria detection at 1 h, but by 3 h, 56.8% of the implant surface was covered by ECFP+ bacteria, and the rest were covered by GFP+ host cells. 3D volumetric rendering of the GFP+ and ECFP+ or RFP+ voxels demonstrated exponential MRSA growth between 3 and 6 h in the Z-plane, which was validated with cross-sectional ex vivo bacterial burden analyses demonstrating significant growth by ~2 × 104 CFU/h on the implant from 2 to 12 h post-op (p < 0.05; r2  > 0.98). Collectively, these results show the competition at the surface is completed by 3 h in this model and demonstrate the potential of LIMB to elucidate mechanisms of bacterial colonization, the host immune response, and the efficacy of antimicrobials.

MarShie: a clearing protocol for 3D analysis of single cells throughout the bone marrow at subcellular resolution

Analyzing immune cell interactions in the bone marrow is vital for understanding hematopoiesis and bone homeostasis. Three-dimensional analysis of the complete, intact bone marrow within the cortex of whole long bones remains a challenge, especially at subcellular resolution. We present a method that stabilizes the marrow and provides subcellular resolution of fluorescent signals throughout the murine femur, enabling identification and spatial characterization of hematopoietic and stromal cell subsets. By combining a pre-processing algorithm for stripe artifact removal with a machine-learning approach, we demonstrate reliable cell segmentation down to the deepest bone marrow regions. This reveals age-related changes in the marrow. It highlights the interaction between CX3CR1+ cells and the vascular system in homeostasis, in contrast to other myeloid cell types, and reveals their spatial characteristics after injury. The broad applicability of this method will contribute to a better understanding of bone marrow biology.

Minimally invasive longitudinal intravital imaging of cellular dynamics in intact long bone

Intravital two-photon microscopy enables deep-tissue imaging at high temporospatial resolution in live animals. However, the endosteal bone compartment and underlying bone marrow pose unique challenges to optical imaging as light is absorbed, scattered and dispersed by thick mineralized bone matrix and the adipose-rich bone marrow. Early bone intravital imaging methods exploited gaps in the cranial sutures to bypass the need to penetrate through cortical bone. More recently, investigators have developed invasive methods to thin the cortical bone or implant imaging windows to image cellular dynamics in weight-bearing long bones. Here, we provide a step-by-step procedure for the preparation of animals for minimally invasive, nondestructive, longitudinal intravital imaging of the murine tibia. This method involves the use of mixed bone marrow radiation chimeras to unambiguously double-label osteoclasts and osteomorphs. The tibia is exposed by a simple skin incision and an imaging chamber constructed using thermoconductive T-putty. Imaging sessions up to 12 h long can be repeated over multiple timepoints to provide a longitudinal time window into the endosteal and marrow niches. The approach can be used to investigate cellular dynamics in bone remodeling, cancer cell life cycle and hematopoiesis, as well as long-lived humoral and cellular immunity. The procedure requires an hour to complete and is suitable for users with minimal prior expertise in small animal surgery.

Visualizing vasculature and its response to therapy in the tumor microenvironment

Tumor vasculature plays a critical role in the progression and metastasis of tumors, antitumor immunity, drug delivery, and resistance to therapies. The morphological and functional changes of tumor vasculature in response to therapy take place in a spatiotemporal-dependent manner, which can be predictive of treatment outcomes. Dynamic monitoring of intratumor vasculature contributes to an improved understanding of the mechanisms of action of specific therapies or reasons for treatment failure, leading to therapy optimization. There is a rich history of methods used to image the vasculature. This review describes recent advances in imaging technologies to visualize the tumor vasculature, with a focus on enhanced intravital imaging techniques and tumor window models. We summarize new insights on spatial-temporal vascular responses to various therapies, including changes in vascular perfusion and permeability and immune-vascular crosstalk, obtained from intravital imaging. Finally, we briefly discuss the clinical applications of intravital imaging techniques.

Endothelial SMAD1/5 signaling couples angiogenesis to osteogenesis during long bone growth

Skeletal development depends on coordinated angiogenesis and osteogenesis. Bone morphogenetic proteins direct bone development by activating SMAD1/5 signaling in osteoblasts. However, the role of SMAD1/5 in skeletal endothelium is unknown. Here, we found that endothelial cell-conditional SMAD1/5 depletion in juvenile mice caused metaphyseal and diaphyseal hypervascularity, resulting in altered cancellous and cortical bone formation. SMAD1/5 depletion induced excessive sprouting, disrupting the columnar structure of the metaphyseal vessels and impaired anastomotic loop morphogenesis at the chondro-osseous junction. Endothelial SMAD1/5 depletion impaired growth plate resorption and, upon long term depletion, abrogated osteoprogenitor recruitment to the primary spongiosa. Finally, in the diaphysis, endothelial SMAD1/5 activity was necessary to maintain the sinusoidal phenotype, with SMAD1/5 depletion inducing formation of large vascular loops, featuring elevated endomucin expression, ectopic tip cell formation, and hyperpermeability. Together, endothelial SMAD1/5 activity sustains skeletal vascular morphogenesis and function and coordinates growth plate remodeling and osteoprogenitor recruitment dynamics during bone growth.

Quantitative analyses of matrices, osteoblasts, and osteoclasts during bone remodeling using an in vitro system

INTRODUCTION

Bone remodeling plays a central role in the maintenance of bone homeostasis. Our group has established an in vitro system by which the cellular events during bone remodeling can be observed longitudinally. This study used this system to quantitatively analyze osteoblasts, osteoclasts, and matrices to elucidate their temporal changes and correlations.

MATERIALS AND METHODS

Osteoblasts from EGFP mice were cultured to form calcified nodules, followed by co-culture with bone marrow macrophages from Tnfrsf11a^Cre/+ x Ai14 mice for 3 weeks (resorption phase). Then cells were cultured with osteoblast differentiation medium for 3 weeks (formation phase). The same sites were observed weekly using 2-photon microscopy. Matrices were detected using second harmonic generation. Parameters related to matrices, osteoblasts, and osteoclasts were quantified and statistically analyzed.

RESULTS

Resorption and replenishment of the matrix were observed at the same sites by 2 photon microscopy. Gross quantification revealed that matrix and osteoblast parameters decreased in the resorption phase and increased in the formation phase, while osteoclast parameters showed the opposite pattern. When one field of view was divided into 16 regions of interest (ROIs) and correlations between parameters were analyzed in each ROI, decreased and increased matrix volumes were moderately correlated. Parameters of matrices and osteoblasts, and those of matrices and osteoclasts exhibited moderate correlations, while those of osteoblasts and osteoclasts were only weakly correlated.

CONCLUSION

Several correlations between cells and matrix during remodeling were demonstrated quantitatively. This system may be a powerful tool for the research of bone remodeling.

Intravital imaging to study cancer progression and metastasis

Navigation through the bulk tumour, entry into the blood vasculature, survival in the circulation, exit at distant sites and resumption of proliferation are all steps necessary for tumour cells to successfully metastasize. The ability of tumour cells to complete these steps is highly dependent on the timing and sequence of the interactions that these cells have with the tumour microenvironment (TME), including stromal cells, the extracellular matrix and soluble factors. The TME thus plays a major role in determining the overall metastatic phenotype of tumours. The complexity and cause-and-effect dynamics of the TME cannot currently be recapitulated in vitro or inferred from studies of fixed tissue, and are best studied in vivo, in real time and at single-cell resolution. Intravital imaging (IVI) offers these capabilities, and recent years have been a time of immense growth and innovation in the field. Here we review some of the recent advances in IVI of mammalian models of cancer and describe how IVI is being used to understand cancer progression and metastasis, and to develop novel treatments and therapies. We describe new techniques that allow access to a range of tissue and cancer types, novel fluorescent reporters and biosensors that allow fate mapping and the probing of functional and phenotypic states, and the clinical applications that have arisen from applying these techniques, reporters and biosensors to study cancer. We finish by presenting some of the challenges that remain in the field, how to address them and future perspectives.

Two-Photon Excitation Spectra of Various Fluorescent Proteins within a Broad Excitation Range

Two-photon excitation fluorescence laser-scanning microscopy is the preferred method for studying dynamic processes in living organ models or even in living organisms. Thanks to near-infrared and infrared excitation, it is possible to penetrate deep into the tissue, reaching areas of interest relevant to life sciences and biomedicine. In those imaging experiments, two-photon excitation spectra are needed to select the optimal laser wavelength to excite as many fluorophores as possible simultaneously in the sample under consideration. The more fluorophores that can be excited, and the more cell populations that can be studied, the better access to their arrangement and interaction can be reached in complex systems such as immunological organs. However, for many fluorophores, the two-photon excitation properties are poorly predicted from the single-photon spectra and are not yet available, in the literature or databases. Here, we present the broad excitation range (760 nm to 1300 nm) of photon-flux-normalized two-photon spectra of several fluorescent proteins in their cellular environment. This includes the following fluorescent proteins spanning from the cyan to the infrared part of the spectrum: mCerulean3, mTurquoise2, mT-Sapphire, Clover, mKusabiraOrange2, mOrange2, LSS-mOrange, mRuby2, mBeRFP, mCardinal, iRFP670, NirFP, and iRFP720.

Advances and challenges in intravital imaging of craniofacial and dental progenitor cells

Craniofacial and appendicular bone homeostasis is dynamically regulated by a balance between bone formation and resorption by osteoblasts and osteoclasts, respectively. Despite the developments in multiple imaging techniques in bone biology, there are still technical challenges and limitations in the investigation of spatial/anatomical location of rare stem/progenitor cells and their molecular regulation in tooth and craniofacial bones of living animals. Recent advances in live animal imaging techniques for the craniofacial and dental apparatus can provide new insights in real time into bone stem/progenitor cell dynamics and function in vivo. Here, we review the current inventions and applications of the noninvasive intravital imaging technique and its practical uses and limitations in the analysis of stem/progenitor cells in craniofacial and dental apparatus in vivo. Furthermore, we also explore the potential applications of intravital microscopy in the dental field.

Comparison of the 3D-Microstructure Between Alveolar and Iliac Bone for Enhanced Bioinspired Bone Graft Substitutes

In oral- and maxillofacial bone augmentation surgery, non-vascularized grafts from the iliac crest demonstrate better clinical performance than alveolar bone grafts. The underlying mechanisms are not fully understood but are essential for the enhancement of bone regeneration scaffolds. Synchrotron Radiation µ-CT at a pixel size of 2.3 μm was used to characterize the gross morphology and the vascular and osteocyte lacuna porosity of patient-matched iliac crest/alveolar bone samples. The results suggest a difference in the spatial distribution of the vascular pore system. Fluid simulations reveal the permeability tensor to be more homogeneous in the iliac crest, indicating a more unidirectional fluid flow in alveolar bone. The average distance between bone mineral and the closest vessel pore boundary was found to be higher in alveolar bone. At the same time, osteocyte lacunae density is higher in alveolar bone, potentially compensating for the longer average distance between the bone mineral and vessel pores. The present study comprehensively quantified and compared the 3D microarchitecture of intraindividual human alveolar and iliac bone. The identified difference in pore network architecture may allow a bone graft from the iliac crest to exhibit higher regeneration potential due to an increased capacity to connect with the surrounding pore network of the residual bone. The results may contribute to understanding the difference in clinical performance when used as bone grafts and are essential for optimization of future scaffold materials.

NAD(P)H fluorescence lifetime imaging of live intestinal nematodes reveals metabolic crosstalk between parasite and host

Infections with intestinal nematodes have an equivocal impact: they represent a burden for human health and animal husbandry, but, at the same time, may ameliorate auto-immune diseases due to the immunomodulatory effect of the parasites. Thus, it is key to understand how intestinal nematodes arrive and persist in their luminal niche and interact with the host over long periods of time. One basic mechanism governing parasite and host cellular and tissue functions, metabolism, has largely been neglected in the study of intestinal nematode infections. Here we use NADH (nicotinamide adenine dinucleotide) and NADPH (nicotinamide adenine dinucleotide phosphate) fluorescence lifetime imaging of explanted murine duodenum infected with the natural nematode Heligmosomoides polygyrus and define the link between general metabolic activity and possible metabolic pathways in parasite and host tissue, during acute infection. In both healthy and infected host intestine, energy is effectively produced, mainly via metabolic pathways resembling oxidative phosphorylation/aerobic glycolysis features. In contrast, the nematodes shift their energy production from balanced fast anaerobic glycolysis-like and effective oxidative phosphorylation-like metabolic pathways, towards mainly anaerobic glycolysis-like pathways, back to oxidative phosphorylation/aerobic glycolysis-like pathways during their different life cycle phases in the submucosa versus the intestinal lumen. Additionally, we found an increased NADPH oxidase (NOX) enzymes-dependent oxidative burst in infected intestinal host tissue as compared to healthy tissue, which was mirrored by a similar defense reaction in the parasites. We expect that, the here presented application of NAD(P)H-FLIM in live tissues constitutes a unique tool to study possible shifts between metabolic pathways in host-parasite crosstalk, in various parasitic intestinal infections.

[The DRFZ-a pioneer in research on the interaction between immune and stromal cells during de- and regeneration of the musculoskeletal system]

Rheumatoid arthritis and osteoarthritis are two related chronic diseases of the musculoskeletal system which are particularly pronounced in the region of joints and bones. Their pathogeneses are associated with chronic inflammation, which can disrupt homeostasis in bones and articular cartilage. Degradation products deriving from articular cartilage can contribute to the exacerbation of inflammation in the joint region. Mechanical stimuli and blood vessels also play a central role in both the regulation of bone growth as well as in the regeneration of bone tissue. Not only chronic inflammatory processes but also hormonal changes after menopause or undesired effects of glucocorticoid therapy have an influence on the balance between bone resorption and deposition, by promoting the former and reducing the latter. This results in decreased bone quality and, in some cases, considerable loss of bone or osteoporosis. An in-depth understanding of these processes at the molecular, cellular, and tissue level, as well as of the changes present in chronic inflammatory diseases, has been the focus of research at the German Rheumatism Research Center (Deutsches Rheuma-Forschungszentrum, DRFZ) since its foundation. Based on an improved understanding of these mechanisms, the DRFZ aims to develop improved prevention and treatment strategies with effects even in early disease stages.

[B lymphocytes and plasma cells as drivers of rheumatic diseases]

Verschiedene Arbeitsgruppen am Deutschen Rheuma-Forschungszentrum Berlin haben in enger Zusammenarbeit mit der Medizinischen Klinik mit Schwerpunkt Rheumatologie und Klinische Immunologie an der Charité wichtige Beiträge zur Bedeutung der B‑Zellen und Plasmazellen bei rheumatischen Erkrankungen geleistet, die nicht nur für die Rheumatologie, sondern für alle klinischen Fachgebiete, in denen antikörpervermittelte Erkrankungen eine Rolle spielen, relevant sind. Insbesondere wird auf die gestörte B‑Zell-Homöostase, die Bedeutung des Immunglobulin M(IgM)-Fc-Rezeptors für die Regulation der Autoimmunität, die Rolle der langlebigen Gedächtnis-Plasmazelle bei der Aufrechterhaltung der Autoimmunität sowie die Sicherung ihres Überlebens in speziellen, von Stromazellen organisierten Nischen im Knochenmark und in entzündeten Geweben eingegangen. Die Forschungsergebnisse haben zu einem besseren Verständnis der immunologischen und molekularen Mechanismen bei rheumatischen Erkrankungen und ihrer Therapie beigetragen. Die Identifizierung der langlebigen Gedächtnis-Plasmazelle hat zu vielversprechenden therapeutischen Ansätzen mit kurativem Potenzial bei Autoimmunerkrankungen geführt.

Immunotoxicity mechanisms of perfluorinated compounds PFOA and PFOS

Perfluorinated and polyfluorinated compounds (PFASs) are a class of synthetic chemical substances that are widely used in human production and life, such as fire-fighting foams, textiles and clothing, surfactants, and surface protective agents. Perfluorooctane sulfonic acid (PFOS) and perfluorooctanoic acid (PFOA) are the most abundant and common perfluorinated compounds in biota and humans. Currently, PFOA and PFOS have been listed in the Stockholm Convention on Persistent Organic Pollutants, and their production has been halted in many countries. However, because the high-energy carbon-fluorine bond can make it resistant to hydrolysis, photolysis, microbial degradation, and vertebrate metabolism, PFOA and PFOS show environmental persistence and bioaccumulation and hence, are of great concern to humans and wildlife. PFOA and PFOS have toxic effects on the immune system of the body. This article reviewed the effects of PFOA and PFOS on immune organs such as the spleen, bone marrow, and thymus of mice and zebrafish, and the effects on non-specific immune functions such as the skin barrier, intestinal mucosal barrier, and humoral immunity. We also reviewed the influence of specific immune functions based on cellular immunity, and further summarized the possible immune toxicity mechanisms such as AIM2 inflammasome activation, gene dysregulation, and signal pathway disorders caused by PFOA and PFOS. The aim of this review was to provide a reference for further understanding of the immunotoxicity and the responsible mechanism of PFOA and PFOS.

Quantification of bone marrow interstitial pH and calcium concentration by intravital ratiometric imaging

The fate of hematopoietic stem cells (HSCs) can be directed by microenvironmental factors including extracellular calcium ion concentration ([Ca2+]e), but the local [Ca2+]e around individual HSCs in vivo remains unknown. Here we develop intravital ratiometric analyses to quantify the absolute pH and [Ca2+]e in the mouse calvarial bone marrow, taking into account the pH sensitivity of the calcium probe and the wavelength-dependent optical loss through bone. Unexpectedly, the mean [Ca2+]e in the bone marrow (1.0 ± 0.54 mM) is not significantly different from the blood serum, but the HSCs are found in locations with elevated local [Ca2+]e (1.5 ± 0.57 mM). With aging, a significant increase in [Ca2+]e is found in M-type cavities that exclusively support clonal expansion of activated HSCs. This work thus establishes a tool to investigate [Ca2+]e and pH in the HSC niche with high spatial resolution and can be broadly applied to other tissue types.

Vascular Regulation of Hematopoietic Stem Cell Homeostasis, Regeneration, and Aging

Purpose of Review

Hematopoietic stem cells (HSCs) sit at the top of the hierarchy that meets the daily burden of blood production. HSC maintenance relies on extrinsic cues from the bone marrow (BM) microenvironment to balance stem cell self-renewal and cell fate decisions. In this brief review, we will highlight the studies and model systems that define the centralized role of BM vascular endothelium in modulating HSC activity in health and stress.

Recent Findings

The BM microenvironment is composed of a diverse array of intimately associated vascular and perivascular cell types. Recent dynamic imaging studies, coupled with single-cell RNA sequencing (scRNA-seq) and functional readouts, have advanced our understanding of the HSC-supportive cell types and their cooperative mechanisms that govern stem cell fate during homeostasis, regeneration, and aging. These findings have established complex and discrete vascular microenvironments within the BM that express overlapping and unique paracrine signals that modulate HSC fate.

Summary

Understanding the spatial and reciprocal HSC-niche interactions and the molecular mechanisms that govern HSC activity in the BM vascular microenvironment will be integral in developing therapies aimed at ameliorating hematological disease and supporting healthy hematopoietic output.

Anatomy of Hematopoiesis and Local Microenvironments in the Bone Marrow. Where to?

The shape and spatial organization -the anatomy- of a tissue profoundly influences its function. Knowledge of the anatomical relationships between parent and daughter cells is necessary to understand differentiation and how the crosstalk between the different cells in the tissue leads to physiological maintenance and pathological perturbations. Blood cell production takes place in the bone marrow through the progressive differentiation of stem cells and progenitors. These are maintained and regulated by a heterogeneous microenvironment composed of stromal and hematopoietic cells. While hematopoiesis has been studied in extraordinary detail through functional and multiomics approaches, much less is known about the spatial organization of blood production and how local cues from the microenvironment influence this anatomy. Here, we discuss some of the studies that revealed a complex anatomy of hematopoiesis where discrete local microenvironments spatially organize and regulate specific subsets of hematopoietic stem cells and/or progenitors. We focus on the open questions in the field and discuss how new tools and technological advances are poised to transform our understanding of the anatomy of hematopoiesis.

Plasma cell survival: The intrinsic drivers, migratory signals, and extrinsic regulators

Antibody-secreting cells (ASC) are the effectors of protective humoral immunity and the only cell type that produces antibodies or immunoglobulins in mammals. In addition to their formidable capacity to secrete massive quantities of proteins, ASC are terminally differentiated and have unique features to become long-lived plasma cells (LLPC). Upon antigen encounter, B cells are activated through a complex multistep process to undergo fundamental morphological, subcellular, and molecular transformation to become an efficient protein factory with lifelong potential. The ASC survival potential is determined by factors at the time of induction, capacity to migration from induction to survival sites, and ability to mature in the specialized bone marrow microenvironments. In the past decade, considerable progress has been made in identifying factors regulating ASC longevity. Here, we review the intrinsic drivers, trafficking signals, and extrinsic regulators with particular focus on how they impact the survival potential to become a LLPC.

Imaging of Bone Marrow Plasma Cells and of Their Niches

Decade-long survival of plasma cells in the bone marrow has long been a puzzling matter. To understand how plasma cells are maintained and supported by survival-niches to account for lifelong antibody production demands new intravital imaging techniques that are able to follow up a single cell and their interaction with other cell types in situ. We achieved to successfully establish longitudinal imaging of the bone marrow (LIMB) that is based on an implantable endoscopic device. In this chapter, basic approaches on how to investigate plasma cell-stroma interaction and surgical implantation procedures are introduced.

The Bone Marrow as Sanctuary for Plasma Cells and Memory T-Cells: Implications for Adaptive Immunity and Vaccinology

The bone marrow (BM) is key to protective immunological memory because it harbors a major fraction of the body’s plasma cells, memory CD4+ and memory CD8+ T-cells. Despite its paramount significance for the human immune system, many aspects of how the BM enables decade-long immunity against pathogens are still poorly understood. In this review, we discuss the relationship between BM survival niches and long-lasting humoral immunity, how intrinsic and extrinsic factors define memory cell longevity and show that the BM is also capable of adopting many responsibilities of a secondary lymphoid organ. Additionally, with more and more data on the differentiation and maintenance of memory T-cells and plasma cells upon vaccination in humans being reported, we discuss what factors determine the establishment of long-lasting immunological memory in the BM and what we can learn for vaccination technologies and antigen design. Finally, using these insights, we touch on how this holistic understanding of the BM is necessary for the development of modern and efficient vaccines against the pandemic SARS-CoV-2.

Maintenance of quiescent immune memory in the bone marrow

The adaptive immune system has the important ability to generate and maintain a memory for antigens once encountered. Recent progress in understanding the organization of immunological memory has challenged the established paradigm of maintenance of memory by restless, circulating, and "homeostatically" proliferating lymphocytes. Among other tissues, the bone marrow has emerged as a preferred resting place for memory lymphocytes providing both local and systemic long-term protection. Why the bone marrow? There, mesenchymal stromal cells provide a privileged environment for quiescent memory B and T lymphocytes, the protagonists of secondary immune reactions, and for memory plasma cells providing persistent humoral immunity. In this review, we discuss the dedicated role of the bone marrow for the maintenance of memory lymphocytes and its implications for immunological memory.

Membrane budding is a major mechanism of in vivo platelet biogenesis

How platelets are produced by megakaryocytes in vivo remains controversial despite more than a century of investigation. Megakaryocytes readily produce proplatelet structures in vitro; however, visualization of platelet release from proplatelets in vivo has remained elusive. We show that within the native prenatal and adult environments, the frequency and rate of proplatelet formation is incompatible with the physiological demands of platelet replacement. We resolve this inconsistency by performing in-depth analysis of plasma membrane budding, a cellular process that has previously been dismissed as a source of platelet production. Our studies demonstrate that membrane budding results in the sustained release of platelets directly into the peripheral circulation during both fetal and adult life without induction of cell death or proplatelet formation. In support of this model, we demonstrate that in mice deficient for NF-E2 (the thrombopoietic master regulator), the absence of membrane budding correlates with failure of in vivo platelet production. Accordingly, we propose that membrane budding, rather than proplatelet formation, supplies the majority of the platelet biomass.

Plasma cell dynamics in the bone marrow niche

Using intravital imaging, we report that bone marrow (BM) plasma cells (PCs) are motile. BM PCs exhibit a unique migration pattern, characterized by intermittent periods of high motility and longer stretches of confined migration or arrest. BM PCs accumulate into clusters, which have reduced cell motility. APRIL promotes cluster formation and overall PC motility in the BM. Although CXCL12 and its receptor, CXCR4, promote PC motility in the BM, VLA4 activity promotes arrest. However, blocking either pathway promotes PC egress from the BM. Under steady-state conditions, BM PCs recirculate to other bones and spleen. In older mice, overall PC motility and recirculation increase, and this is correlated with increased CXCR4 expression, which depends on PC age or maturation rather than mouse age. Altogether, these results suggest that changes in PC motility and CXCR4 expression are linked with survival of long-lived PCs in the BM.

Benet et al. report that bone marrow (BM) plasma cells (PCs) are motile and found in clusters organized by the survival factor APRIL. They identify CXCR4 and VLA4 as key factors regulating PC motility and retention in the BM. PCs recirculate to other bones. PC motility increases with age.

Intravital imaging of orthotopic and ectopic bone

Bone homeostasis is dynamically regulated by a balance between bone resorption by osteoclasts and bone formation by osteoblasts. Visualizing and evaluating the dynamics of bone cells in vivo remain difficult using conventional technologies, including histomorphometry and imaging analysis. Over the past two decades, multiphoton microscopy, which can penetrate thick specimens, has been utilized in the field of biological imaging. Using this innovative technique, the in vivo dynamic motion of bone metabolism-related cells and their interactions has been revealed. In this review, we summarize previous approaches used for bone imaging and provide an overview of current bone tissue imaging methods using multiphoton excitation microscopy.

Effect of Dry Eye Disease on the Kinetics of Lacrimal Gland Dendritic Cells as Visualized by Intravital Multi-Photon Microscopy

The lacrimal gland (LG) is the main source of the tear film aqueous layer and its dysfunction results in dry eye disease (DED), a chronic immune-mediated disorder of the ocular surface. The desiccating stress (DS) murine model that mimics human DED, results in LG dysfunction, immune cell infiltration, and consequently insufficient tear production. To date, the immune cell kinetics in DED are poorly understood. The purpose of this study was to develop a murine model of intravital multi-photon microscopy (IV-MPM) for the LG, and to investigate the migratory kinetics and 3D morphological properties of conventional dendritic cells (cDCs), the professional antigen presenting cells of the ocular surface, in DED. Mice were placed in a controlled environmental chamber with low humidity and increased airflow rate for 2 and 4 weeks to induce DED, while control naïve transgenic mice were housed under standard conditions. DED mice had significantly decreased tear secretion and increased fluorescein staining (p < 0.01) compared to naïve controls. Histological analysis of the LG exhibited infiltrating mononuclear and polymorphonuclear cells (p < 0.05), as well as increased LG swelling (p < 0.001) in DED mice compared to controls. Immunofluorescence staining revealed increased density of cDCs in DED mice (p < 0.001). IV-MPM of the LG demonstrated increased density of cDCs in the LGs of DED mice, compared with controls (p < 0.001). cDCs were more spherical in DED at both time points compared to controls (p < 0.001); however, differences in surface area were found at 2 weeks in DED compared with naïve controls (p < 0.001). Similarly, 3D cell volume was significantly lower at 2 weeks in DED vs. the naïve controls (p < 0.001). 3D instantaneous velocity and mean track speed were significantly higher in DED compared to naïve mice (p < 0.001). Finally, the meandering index, an index for directionality, was significant increased at 4 weeks after DED compared with controls and 2 weeks of DED (p < 0.001). Our IV-MPM study sheds light into the 3D morphological alterations and cDC kinetics in the LG during DED. While in naïve LGs, cDCs exhibit a more dendritic morphology and are less motile, they became more spherical with enhanced motility during DED. This study shows that IV-MPM represents a robust tool to study immune cell trafficking and kinetics in the LG, which might elucidate cellular alterations in immunological diseases, such as DED.

Intravital imaging of megakaryocytes

The dynamics of platelet formation could only be investigated since the development of two-photon microscopy in combination with suitable fluorescent labeling strategies. In this review paper, we give an overview of recent advances in fluorescence imaging of the bone marrow that have contributed to our understanding of platelet biogenesis during the last decade. We make a brief survey through the perspectives and limitations of today's intravital imaging, but also discuss complementary methods that may help to piece together the puzzle of megakaryopoiesis and platelet formation.

Functional characterization of a low-frequency V1937I variant in FASN associated with susceptibility to esophageal squamous cell carcinoma

Metabolic reprogramming has been regarded as one of the core hallmarks of cancer and increased de novo fatty acid synthesis has been documented in multiple tumors including esophageal squamous cell carcinoma (ESCC). Our previous exome-wide analyses found a Val1937Ile variant (rs17848945) in the 34th exon of fatty acid synthase (FASN) that showed a strong association with the risk of ESCC. In this study, we performed a series of functional assays to investigate the biological functions underlying this variant in the development of ESCC. We demonstrated that FASN was upregulated in ESCC and both knockdown and knockout of FASN significantly inhibited ESCC cell proliferation, suggesting a tumor promoter role for this gene in ESCC. Furthermore, the results showed that overexpression of FASN[I] in the ESCC cells substantially enhanced cell proliferation, compared with overexpression of FASN[V], or the control vector. Intriguingly, we found that the FASN[I] variant can enhance the enzyme activity of FASN, and, thus, increase the amount of the FASN end-product, palmitate in the ESCC cells. We also observed elevated palmitate levels in the plasma of the FASN[I] genotype carriers among a total of 632 healthy Chinese adults. In conclusion, our results suggested that the FASN V1937I variant influenced ESCC cell proliferation and susceptibility by altering the catabolic activity of FASN on palmitate. These findings may highlight an important role of palmitate metabolism in the development of ESCC and may contribute to the personalized medicine of this disease.

Intravital Imaging Techniques for Biomedical and Clinical Research

Intravital imaging, the direct visualization of cells and tissues within a living animal, is a technique that has been employed for the better part of a century. The advent of confocal and multiphoton microscopy has dramatically improved the power of intravital imaging, making it possible to obtain optical sections of tissues non-destructively. This review discusses the various techniques used for intravital imaging, describes how intravital imaging provides information about cellular and tissue dynamics not possible to be garnered by other techniques, and details several ways in which intravital imaging is making a direct impact on the clinical care of patients. © 2019 International Society for Advancement of Cytometry.

Limbostomy: Longitudinal Intravital Microendoscopy in Murine Osteotomies

Bone healing involves the interplay of immune cells, mesenchymal cells, and vasculature over the time course of regeneration. Approaches to quantify the spatiotemporal aspects of bone healing at cellular resolution during long bone healing do not yet exist. Here, a novel technique termed Limbostomy is presented, which combines intravital microendoscopy with an osteotomy. This design allows a modular combination of an internal fixator plate with a gradient refractive index (GRIN) lens at various depths in the bone marrow and can be combined with a surgical osteotomy procedure. The field of view (FOV) covers a significant area of the fracture gap and allows monitoring cellular processes in vivo. The GRIN lens causes intrinsic optical aberrations which have to be corrected. The optical system was characterized and a postprocessing algorithm was developed. It corrects for wave front aberration-induced image plane deformation and for background and noise signals, enabling us to observe subcellular processes. Exemplarily, we quantitatively and qualitatively analyze angiogenesis in bone regeneration. We make use of a transgenic reporter mouse strain with nucleargreen fluorescent protein and membrane-bound tdTomato under the Cadherin-5 promoter. We observe two phases of vascularization. First, rapid vessel sprouting pervades the FOV within 3-4 days after osteotomy. Second, the vessel network continues to be dynamically remodeled until the end of our observation time, 14 days after surgery. Limbostomy opens a unique set of opportunities and allows further insight on spatiotemporal aspects of bone marrow biology, for example, hematopoiesis, analysis of cellular niches, immunological memory, and vascularization in the bone marrow during health and disease. © 2020 The Authors. Cytometry Part A published by Wiley Periodicals, Inc. on behalf of International Society for Advancement of Cytometry.

Intravital imaging of eosinophils: Unwrapping the enigma

Eosinophils are traditionally associated with allergic and parasitic inflammation. More recently, eosinophils have also been shown to have roles in diverse processes including development, intestinal health, thymic selection, and B-cell survival with the majority of these insights being derived from murine models and in vitro assays. Despite this, tools to measure the dynamic activity of eosinophils in situ have been lacking. Intravital microscopy is a powerful tool that enables direct visualization of leukocytes and their dynamic behavior in real-time in a wide range of processes in both health and disease. Until recently eosinophil researchers have not been able to take full advantage of this technology due to a lack of tools such as genetically encoded reporter mice. This mini-review examines the history of intravital microscopy with a focus on eosinophils. The development and use of eosinophil-specific Cre (EoCre) mice to create GFP and tdTomato fluorescent reporter animals is also described. Genetically encoded eosinophil reporter mice combined with intravital microscopy provide a powerful tool to add to the toolbox of technologies that will help us unravel the mysteries still surrounding this cell.

Intravital Microscopy of the Beating Murine Heart to Understand Cardiac Leukocyte Dynamics

Cardiovascular disease is the leading cause of worldwide mortality. Intravital microscopy has provided unprecedented insight into leukocyte biology by enabling the visualization of dynamic responses within living organ systems at the cell-scale. The heart presents a uniquely dynamic microenvironment driven by periodic, synchronous electrical conduction leading to rhythmic contractions of cardiomyocytes, and phasic coronary blood flow. In addition to functions shared throughout the body, immune cells have specific functions in the heart including tissue-resident macrophage-facilitated electrical conduction and rapid monocyte infiltration upon injury. Leukocyte responses to cardiac pathologies, including myocardial infarction and heart failure, have been well-studied using standard techniques, however, certain questions related to spatiotemporal relationships remain unanswered. Intravital imaging techniques could greatly benefit our understanding of the complexities of in vivo leukocyte behavior within cardiac tissue, but these techniques have been challenging to apply. Different approaches have been developed including high frame rate imaging of the beating heart, explantation models, micro-endoscopy, and mechanical stabilization coupled with various acquisition schemes to overcome challenges specific to the heart. The field of cardiac science has only begun to benefit from intravital microscopy techniques. The current focused review presents an overview of leukocyte responses in the heart, recent developments in intravital microscopy for the murine heart, and a discussion of future developments and applications for cardiovascular immunology.

Spatial Distribution of Macrophages During Callus Formation and Maturation Reveals Close Crosstalk Between Macrophages and Newly Forming Vessels

Macrophages are essential players in the process of fracture healing, acting by remodeling of the extracellular matrix and enabling vascularization. Whilst activated macrophages of M1-like phenotype are present in the initial pro-inflammatory phase of hours to days of fracture healing, an anti-inflammatory M2-like macrophage phenotype is supposed to be crucial for the induction of downstream cascades of healing, especially the initiation of vascularization. In a mouse-osteotomy model, we provide a comprehensive characterization of vessel (CD31⁺, Emcn⁺) and macrophage phenotypes (F4/80, CD206, CD80, Mac-2) during the process of fracture healing. To this end, we phenotype the phases of vascular regeneration-the expansion phase (d1-d7 after injury) and the remodeling phase of the endothelial network, until tissue integrity is restored (d14-d21 after injury). Vessels which appear during the bone formation process resemble type H endothelium (CD31^hiEmcn^hi), and are closely connected to osteoprogenitors (Runx2⁺, Osx⁺) and F4/80⁺ macrophages. M1-like macrophages are present in the initial phase of vascularization until day 3 post osteotomy, but they are rare during later regeneration phases. M2-like macrophages localize mainly extramedullary, and CD206⁺ macrophages are found to express Mac-2⁺ during the expansion phase. VEGFA expression is initiated by CD80⁺ cells, including F4/80⁺ macrophages, until day 3, while subsequently osteoblasts and chondrocytes are main contributors to VEGFA production at the fracture site. Using Longitudinal Intravital Microendoscopy of the Bone (LIMB) we observe changes in the motility and organization of CX3CR1⁺ cells, which infiltrate the injury site after an osteotomy. A transient accumulation, resulting in spatial polarization of both, endothelial cells and macrophages, in regions distal to the fracture site, is evident. Immunofluorescence histology followed by histocytometric analysis reveals that F4/80⁺CX3CR1⁺ myeloid cells precede vascularization.

Intravital microscopy: Imaging host-parasite interactions in lymphoid organs

Intravital microscopy allows imaging of biological phenomena within living animals, including host-parasite interactions. This has advanced our understanding of both, the function of lymphoid organs during parasitic infections, and the effect of parasites on such organs to allow their survival. In parasitic research, recent developments in this technique have been crucial for the direct study of host-parasite interactions within organs at depths, speeds and resolution previously difficult to achieve. Lymphoid organs have gained more attention as we start to understand their function during parasitic infections and the effect of parasites on them. In this review, we summarise technical and biological findings achieved by intravital microscopy with respect to the interaction of various parasites with host lymphoid organs, namely the bone marrow, thymus, lymph nodes, spleen and the mucosa-associated lymphoid tissue, and present a view into possible future applications.

Three-dimensional intravital imaging in bone research

Intravital imaging has emerged as a novel and efficient tool for visualization of in situ dynamics of cellular behaviors and cell-microenvironment interactions in live animals, based on desirable microscopy techniques featuring high resolutions, deep imaging and low phototoxicity. Intravital imaging, especially based on multi-photon microscopy, has been used in bone research for dynamics visualization of a variety of physiological and pathological events at the cellular level, such as bone remodeling, hematopoiesis, immune responses and cancer development, thus, providing guidance for elucidating novel cellular mechanisms in bone biology as well as guidance for new therapies. This review is aimed at interpreting development and advantages of intravital imaging in bone research, and related representative discoveries concerning bone matrices, vessels, and various cells types involved in bone physiologies and pathologies. Finally, current limitations, further refinement, and extended application of intravital imaging in bone research are concluded.

Complex vectorial optics through gradient index lens cascades

Graded index (GRIN) lenses are commonly used for compact imaging systems. It is not widely appreciated that the ion-exchange process that creates the rotationally symmetric GRIN lens index profile also causes a symmetric birefringence variation. This property is usually considered a nuisance, such that manufacturing processes are optimized to keep it to a minimum. Here, rather than avoiding this birefringence, we understand and harness it by using GRIN lenses in cascade with other optical components to enable extra functionality in commonplace GRIN lens systems. We show how birefringence in the GRIN cascades can generate vector vortex beams and foci, and how it can be used advantageously to improve axial resolution. Through using the birefringence for analysis, we show that the GRIN cascades form the basis of a new single-shot Müller matrix polarimeter with potential for endoscopic label-free cancer diagnostics. The versatility of these cascades opens up new technological directions.

The manufacturing process for GRIN lenses causes a symmetric birefringence variation which is considered a deficiency. Here, the authors show how this birefringence can generate vector vortex beams and form the basis of a Müller matrix polarimeter with potential for endoscopic label-free cancer diagnostics.

Metastasis Organotropism: Redefining the Congenial Soil

Metastasis is the most devastating stage of cancer progression and causes the majority of cancer-related deaths. Clinical observations suggest that most cancers metastasize to specific organs, a process known as "organotropism." Elucidating the underlying mechanisms may help identify targets and treatment strategies to benefit patients. This review summarizes recent findings on tumor-intrinsic properties and their interaction with unique features of host organs, which together determine organ-specific metastatic behaviors. Emerging insights related to the roles of metabolic changes, the immune landscapes of target organs, and variation in epithelial-mesenchymal transitions open avenues for future studies of metastasis organotropism.

Mechanical Loading Promotes the Expansion of Primitive Osteoprogenitors and Organizes Matrix and Vascular Morphology in Long Bone Defects

Elucidating the effects of mechanical stimulation on bone repair is crucial for optimization of the healing process. Specifically, the regulatory role that mechanical loading exerts on the osteogenic stem cell pool and vascular morphology during healing is incompletely understood. Because dynamic loading has been shown to enhance osteogenesis and repair, we hypothesized that loading induces the expansion of the osteoprogenitor cell population within a healing bone defect, leading to an increased presence of osteogenic cells. We further hypothesized that loading during the repair process regulates vascular and collagen matrix morphology and spatial interactions between vessels and osteogenic cells. To address these hypotheses, we used a mechanobiological bone repair model, which produces a consistent and reproducible intramembranous repair response confined in time and space. Bilateral tibial defects were created in adult C57BL/6 mice, which were subjected to axial compressive dynamic loading either during the early cellular invasion phase on postsurgical days (PSDs) 2 to 5 or during the matrix deposition phase on PSD 5 to 8. Confocal and two-photon microscopy was used to generate high-resolution three-dimensional (3D) renderings of longitudinal thick sections of the defect on PSD 10. Endomucin (EMCN)-positive vessels, Paired related homeobox 1 (Prrx1+) stem cell antigen-1 positive (Sca-1+) primitive osteoprogenitors (OPCs), and osterix positive (Osx+) preosteoblasts were visualized and quantified using deep tissue immunohistochemistry. New bone matrix was visualized with second harmonic generation autofluorescence of collagen fibers. We found that mechanical loading during the matrix deposition phase (PSD 5 to 8) increased vessel volume and number, and aligned vessels and collagen fibers to the load-bearing direction of bone. Furthermore, loading led to a significant increase in the proliferation and number of Prrx1+ Sca-1+ primitive OPCs, but not Osx+ preosteoblasts within the defect. Together, these data illustrate the adaptation of both collagen matrix and vascular morphology to better withstand mechanical load during bone repair, and that the mechanoresponsive cell population consists of the primitive osteogenic progenitors. © 2019 American Society for Bone and Mineral Research.

In the Right Place, at the Right Time: Spatiotemporal Conditions Determining Plasma Cell Survival and Function

Plasma cells (PCs), the B lineage cells responsible for producing and secreting antibodies (Abs), are critical cellular components of the humoral immune system. While most of the antibody-secreting cells in the body have a rather short lifetime of a few days, some of them can become long-lived and persist in the body over the entire life span of an individual. The majority of these long-lived plasma cells secretes protective antibodies against pathogens, and are thereby crucial for the humoral component of immunological memory. The generation of these protective antibody-secreting cells can be triggered by an exposure to pathogens, and also by vaccination. Although the majority of plasma cells are protective, sometimes long-lived plasma cells produce autoreactive antibodies, which contribute to the pathogenesis and perpetuation of chronic autoimmune diseases, including lupus erythematosus, rheumatoid arthritis, or multiple sclerosis. In order to promote the formation of protective antibody-secreting cells and to target pathogenic plasma cells, it is crucial to understand the signals which promote their longevity and allow them to exert their function. In recent years, it has become clear that plasma cells depend on extrinsic factors for their survival, leading to the concept that certain tissue microenvironments promote plasma cell retention and longevity. However, these niches are not static structures, but also have dynamic features with respect to their cellular composition. Here, we review what is known about the molecular and cellular composition of the niches, and discuss the impact of dynamic changes within these microenvironments on plasma cell function. As plasma cell metabolism is tightly linked to their function, we present new tools, which will allow us to analyze metabolic parameters in the plasma cell niches in vivo over time.

Collagen I-based scaffolds negatively impact fracture healing in a mouse-osteotomy-model although used routinely in research and clinical application

Although several biomaterials for bone regeneration have been developed in the last decades, clinical application of bone morphogenetic protein 2 is clinically only approved when applied on an absorbable bovine collagen I scaffold (ACS) (Helistat; ACS-H). In research, another ACS, namely Lyostypt (ACS-L) is frequently used as a scaffold in bone-linked studies. Nevertheless, until today, the influence of ACS alone on bone healing remains unknown. Unexpectedly, in vitro studies using ASC-H revealed a suppression of osteogenic differentiation and a significant reduction of cell vitality when compared to ASC-L. In mice, we observed a significant delay in bone healing when applying ACS-L in the fracture gap during femoral osteotomy. The results of our study show for the first time a negative influence of both ACS-H and ACS-L on bone formation demonstrating a substantial need for more sophisticated delivery systems for local stimulation of bone healing in both clinical application and research. STATEMENT OF SIGNIFICANCE: Our study provides evidence-based justification to promote the development and approval of more suitable and sophisticated delivery systems in bone healing research. Additionally, we stimulate researchers of the field to consider that the application of those scaffolds as a delivery system for new substances represents a delayed healing approach rather than a normal bone healing which could greatly impact the outcome of those studies and play a pivotal role in the translation to the clinics. Moreover, we provide impulses on underlying mechanism involving the roles of small-leucine rich proteoglycans (SLRP) for further detailed investigations.

CXCR4, but not CXCR3, drives CD8+ T-cell entry into and migration through the murine bone marrow

The BM serves as a blood-forming organ, but also supports the maintenance and immune surveillance function of many T cells. Yet, in contrast to other organs, little is known about the molecular mechanisms that drive T-cell migration to and localization inside the BM. As BM accumulates many CXCR3-expressing memory CD8⁺ T cells, we tested the involvement of this chemokine receptor, but found that CXCR3 is not required for BM entry. In contrast, we could demonstrate that CXCR4, which is highly expressed on both naive and memory CD8⁺ T cells in BM, is critically important for homing of all CD8⁺ T-cell subsets to the BM in mice. Upon entry into the BM parenchyma, both naïve and memory CD8⁺ T cells locate close to sinusoidal vessels. Intravital imaging experiments revealed that CD8 T cells are surprisingly immobile and we found that they interact with ICAM-1+VCAM-1+BP-1+ perivascular stromal cells. These cells are the major source of CXCL12, but also express key survival factors and maintenance cytokines IL-7 and IL-15. We therefore conclude that CXCR4 is not only crucial for entry of CD8⁺ T cells into the BM, but also controls their subsequent localization toward BM niches that support their survival.