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In this review, we discuss intravital microscopy of immune cells, starting from its historic origins to current applications in diverse organs. It is clear from a quantitative review of the literature that intravital microscopy is a key tool in both historic and contemporary immunological research, providing unique advances in our understanding of immune responses. We have chosen to focus this review on how intravital microscopy methodologies are used to image specific organs or systems and we present recent descriptions of fundamental immunological processes that could not have been achieved by other methods. The following target organs/systems are discussed in more detail: cremaster muscle, skin (ear and dorsal skin fold chamber), lymph node, liver, lung, mesenteric vessels, carotid artery, bone marrow, brain, spleen, foetus and lastly vessels of the knee joint. Since microscopes were first invented, scientists have constantly been drawn to these marvellous machines. This is because microscopes provide their user with the ability to view first hand minuscule objects and processes they have often dedicated their entire careers to studying. It is this ability to view biology at work in both space and time, from a single molecule to an entire organism that makes imaging such a powerful tool. In this Special Feature, we have compiled a series of articles that discuss the history of microscopes and imaging modalities. We look at how current platforms have influenced basic research of immunology and cell biology as well as their use in the clinic to diagnose and treat disease. We also discuss how future developments in technology will open avenues for an even deeper understanding of fundamental principles in biology and the challenges associated with handling vast amounts of data generated by technology that gives such a high level of deta iled information. Immunology & Cell Biology thanks the coordinator of this Special Feature ‐ Edwin Hawkins ‐ for his planning and input.

Mutant p53s (mutp53) increase cancer invasiveness by upregulating Rab-coupling protein (RCP) and diacylglycerol kinase-α (DGKα)-dependent endosomal recycling. Here we report that mutp53-expressing tumour cells produce exosomes that mediate intercellular transfer of mutp53’s invasive/migratory gain-of-function by increasing RCP-dependent integrin recycling in other tumour cells. This process depends on mutp53’s ability to control production of the sialomucin, podocalyxin, and activity of the Rab35 GTPase which interacts with podocalyxin to influence its sorting to exosomes. Exosomes from mutp53-expressing tumour cells also influence integrin trafficking in normal fibroblasts to promote deposition of a highly pro-invasive extracellular matrix (ECM), and quantitative second harmonic generation microscopy indicates that this ECM displays a characteristic orthogonal morphology. The lung ECM of mice possessing mutp53-driven pancreatic adenocarcinomas also displays increased orthogonal characteristics which precedes metastasis, indicating that mutp53 can influence the microenvironment in distant organs in a way that can support invasive growth. Some p53 mutants promote invasive migration of cancer cells and metastasis of tumours in vivo. However the key mechanistic details behind these phenomena remain unclear. Here the authors propose a non-cell autonomous mechanism involving fibroblasts, whereby mutant p53-expressing cancer cells activate an exosome-mediated mechanism that influences integrin recycling in fibroblasts, thus influencing extracellular matrix remodelling to favour cancer cell invasion and migration.

The rapid response of neutrophils throughout the body to a systemic challenge is a critical first step in resolution of bacterial infection such as Escherichia coli ( E. coli ). Here we delineated the dynamics of this response, revealing novel insights into the molecular mechanisms using lung and spleen intravital microscopy and 3D ex vivo culture of living precision cut splenic slices in combination with fluorescent labelling of endogenous leukocytes. Within seconds after challenge, intravascular marginated neutrophils and lung endothelial cells (ECs) work cooperatively to capture pathogens. Neutrophils retained on lung ECs slow their velocity and aggregate in clusters that enlarge as circulating neutrophils carrying E. coli stop within the microvasculature. The absolute number of splenic neutrophils does not change following challenge; however, neutrophils increase their velocity, migrate to the marginal zone (MZ) and form clusters. Irrespective of their location all neutrophils capturing heat-inactivated E. coli take on an activated phenotype showing increasing surface CD11b. At a molecular level we show that neutralization of ICAM-1 results in splenic neutrophil redistribution to the MZ under homeostasis. Following challenge, splenic levels of CXCL12 and ICAM-1 are reduced allowing neutrophils to migrate to the MZ in a CD29-integrin dependent manner, where the enlargement of splenic neutrophil clusters is CXCR2-CXCL2 dependent. We show directly molecular mechanisms that allow tissue resident neutrophils to provide the first lines of antimicrobial defense by capturing circulating E. coli and forming clusters both in the microvessels of the lung and in the parenchyma of the spleen.

Exposure to nanoparticles (NPs) is frequently associated with adverse cardiovascular effects. In contrast, NPs in nanomedicine hold great promise for precise lung‐specific drug delivery, especially considering the extensive pulmonary capillary network that facilitates interactions with bloodstream‐suspended particles. Therefore, exact knowledge about effects of engineered NPs within the pulmonary microcirculation are instrumental for future application of this technology in patients. To unravel the real‐time dynamics of intravenously delivered NPs and their effects in the pulmonary microvasculature, we employed intravital microscopy of the mouse lung. Only PEG‐amine‐QDs, but not carboxyl‐QDs triggered rapid neutrophil recruitment in microvessels and their subsequent recruitment to the alveolar space and was linked to cellular degranulation, TNF‐α, and DAMP release into the circulation, particularly eATP. Stimulation of the ATP‐gated receptor P2X7R induced expression of E‐selectin on microvascular endothelium thereby mediating the neutrophilic immune response. Leukocyte integrins LFA‐1 and MAC‐1 facilitated adhesion and decelerated neutrophil crawling on the vascular surface. In summary, this study unravels the complex cascade of neutrophil recruitment during NP‐induced sterile inflammation. Thereby we demonstrate novel adverse effects for NPs in the pulmonary microcirculation and provide critical insights for optimizing NP‐based drug delivery and therapeutic intervention strategies, to ensure their efficacy and safety in clinical applications. Exact knowledge about interactions and effects of engineered nanoparticles with the pulmonary microcirculation are instrumental for future application of this technology in patients. Using intravital microscopy, this study unravels the complex cascade of neutrophil recruitment during QD nanoparticle‐induced sterile inflammation in the lung microcirculation, mediated by the eATP/P2X7R axis.

Exposure to air pollution, including nanoparticles (NPs), is a major health concern associated with various diseases, triggered by subtle inflammatory responses in the lung. To investigate the dynamic immune response in vivo, lung intravital microscopy (L-IVM), was used to analyze the behavior of alveolar macrophages (AMs) and neutrophils, combined with ventilator-assisted inhalation of nebulized NPs in mice. Inhalation of fluorescent quantum dot NPs (cQDs) and soot-like carbon black NPs (CNPs, ambient pollutants), led to rapid spatially focused recruitment of neutrophils near alveolar deposited NPs. Neutrophil recruitment was initiated by NPs uptake by AMs, dependent on AM motility and AM NP surface recognition. Prior airway application of neutralizing antibodies against alveolar ICAM-1 and LFA-1, leading to reduced AM motility, inhibition of C5aR1 and FcγRI receptor mediated NPs uptake by AMs, as well as neutralizing of TNFα and application of a cellular degranulation inhibitor, abolished the early immune response induced by NPs. Overall, our data demonstrates the crucial role of AM activity (migration, phagocytosis, cytokine release) in the rapid and site-specific recruitment of neutrophils during the early phase of particle inhalation, suggesting these processes to be key events in mounting the immune response upon NP inhalation in the lung.Competing Interest StatementThe authors have declared no competing interest.

Exposure to nanoparticles (NPs) is frequently associated with adverse cardiovascular effects. In contrast, NPs in nanomedicine hold great promise for precise lung-specific drug delivery, especially considering the extensive pulmonary capillary network that facilitates interactions with bloodstream-suspended particles. Therefore, exact knowledge about interactions and effects of engineered NPs with the pulmonary microcirculation are instrumental for future application of this technology in patients. To unravel the real-time dynamics of intravenously delivered NPs and their effects in the pulmonary microvasculature, we employed intravital microscopy of the mouse lung. PEG amine-modified quantum dots (aQDs) with a low potential for biomolecule and cell interactions and carboxyl-modified quantum dots (cQDs) with a high interaction potential were used, representing two different NP subtypes. Only aQDs triggered rapid neutrophil recruitment in microvessels and their subsequent recruitment to the alveolar space. Application of specific inhibitors revealed that the aQDs induced neutrophil recruitment was linked to cellular degranulation, TNF-α, and DAMP release into the circulation, particularly extracellular ATP (eATP). Stimulation of the ATP-gated P2X7R induced the expression of E-selectin on microvascular endothelium with the subsequent E-selectin depended neutrophilic immune response. Leukocyte integrins (LFA-1 and MAC-1) mediated adhesion and reduction in neutrophil crawling velocity on the vascular surface. In summary, this study unravels the complex cascade of neutrophil recruitment during NP-induced sterile inflammation. Thereby we demonstrate novel adverse effects for NPs in the pulmonary microcirculation and provide critical insights for optimizing NP-based drug delivery and therapeutic intervention strategies, to ensure their efficacy and safety in clinical applications.

Protecting mucosal barriers, γδ T cells hold promise for the development of new cancer immunotherapies. In mice, γδ T cells can largely be segregated into CD27+ and CD27− cells, and their functions are modulated by interactions with surrounding cells. However, which cells communicate directly with γδ T cells in lung adenocarcinoma remains unknown. To address this, we combined flow cytometry, confocal microscopy, and scRNA-seq, using an autochthonous genetically engineered mouse model and different γδ T cell-deficient settings. We found that γδ T cells were increased in tumour-bearing lungs, with an altered phenotype. CD27− and CD27+ γδ T cells differed in their localisation and interactions including their tropism for macrophages. Overall, we propose a model where CD27+ γδ T cells undermine the differentiation of tumour-associated macrophages into airway macrophages, fostering a negative outcome in lung adenocarcinoma. Determining its translatability to human health may offer new avenues for immunotherapeutic strategies. Guardians of pulmonary homeostasis, γδ T cells remain enigmatic regarding their role in lung adenocarcinoma. Raffo-Iraolagoitia et al. report that a subset of γδ T cells impairs the differentiation of tumour-associated macrophages into airway macrophages, relevant for the outcome of lung adenocarcinoma.

The pulmonary immune system defends a huge surface area directly in contact with the contents of the air we breathe. Neutrophils, the most abundant immune cell in the pulmonary vasculature, are critical to immunity but they are also capable of generating life-threatening pathology. Natural Killer cells are the most highly represented lymphocyte subset in the lung, but relatively little is known about their localization, motility or the specific mechanisms by which they contribute to local homeostasis. Here, we used lung-intravital microscopy to directly visualise and quantify neutrophil and natural killer cell dynamics in the pulmonary vasculature of live mice. This approach revealed unexpected sessile behaviour by intravascular natural killer cells. Interactions with natural killer cells made neutrophils scan the endothelium more slowly over larger distances and reduced the number of neutrophils that accumulated in an LPS-triggered inflammatory challenge. This represents a new paradigm by which natural killer cells contribute to lung physiology by diminishing potentially pathogenic neutrophil accumulation.

Neutrophils have been implicated in poor outcomes in cancer and severe inflammation. We found that neutrophils expressing intermediate levels of Ly6G (Ly6GInt) were present in mouse cancer models and more abundant in those with high rates of spontaneous metastasis. Maturation, age, tissue localization and functional capacity all drive neutrophil heterogeneity. Recent studies have proposed various markers to distinguish between these heterogeneous sub-populations; however, these markers are limited to specific models of inflammation and cancer. Here, we identify and define Ly6G expression level as a robust and reliable marker to distinguish neutrophils at different stages of maturation. Ly6GInt neutrophils were bona fide immature neutrophils with reduced immune regulatory and adhesion capacity. Whereas the bone marrow is a more recognised site of granulopoiesis, the spleen also produces neutrophils in homeostasis and cancer. Strikingly, neutrophils matured faster in the spleen than in the bone marrow with unique transcriptional profiles. We propose that developmental origin is critical in neutrophil identity and postulate that neutrophils that develop in the spleen supplement the bone marrow by providing an intermediate more mature reserve before emergency haematopoiesis. Competing Interest Statement DAM is a paid director and shareholder of Fibrofind limited. DJM receives research funding from Puma Bio-technology and Merck. OJS and TGB receive research funding from AstraZeneca. Footnotes * Accession code and funding information updated