Visualizing leukocyte trafficking in the living brain with 2-photon intravital microscopy

Antibody-based in vivo leukocyte label for two-photon brain imaging in mice

Significance: To study leukocyte-endothelial interactions in a living system, robust and specific leukocyte labeling techniques are needed for in vivo two-photon microscopy of the cerebral microvasculature. Aim: We tested fluorophore-conjugated anti-CD45.2 monoclonal antibodies (mAb) to optimize dosing and two-photon imaging parameters for leukocyte labeling in healthy mice and a venous microstroke model. Approach: We retro-orbitally injected anti-CD45.2 mAb at 0.04, 0.4, and 2    mg / kg into BALB/c mice and used flow cytometry to analyze antibody saturation. Leukocyte labeling in the cortical microvasculature was examined by two-photon imaging. We also tested the application of CD45.2 mAb in a pathological leukocyte-endothelial adhesion model by photothrombotically occluding cortical penetrating venules. Results: We found that 0.4    mg / kg of anti-CD45.2 antibody intravenously was sufficient to label 95% of circulating leukocytes. There was no depletion of circulating leukocytes after 24 h at the dosages tested. Labeled leukocytes could be observed as deep as 550    μ m from the cortical surface. The antibody reliably labeled rolling, crawling, and adherent leukocytes in venules around the stroke-affected tissues. Conclusion: We show that the anti-CD45.2 mAb is a robust reagent for acute labeling of leukocytes during in vivo two-photon microscopy of the cortical microvasculature.

Multispectral intravital microscopy for simultaneous bright-field and fluorescence imaging of the microvasculature

Intravital video microscopy permits the observation of microcirculatory blood flow. This often requires fluorescent probes to visualize structures and dynamic processes that cannot be observed with conventional bright-field microscopy. Conventional light microscopes do not allow for simultaneous bright-field and fluorescent imaging. Moreover, in conventional microscopes, only one type of fluorescent label can be observed. This study introduces multispectral intravital video microscopy, which combines bright-field and fluorescence microscopy in a standard light microscope. The technique enables simultaneous real-time observation of fluorescently-labeled structures in relation to their direct physical surroundings. The advancement provides context for the orientation, movement, and function of labeled structures in the microcirculation.

Simple and Robust Intravital Microscopy Procedures in Hybrid TIE2GFP-BALB/c Transgenic Mice

PURPOSE

The endeavor of deciphering intricate phenomena within the field of molecular medicine dictates the necessity to investigate tumor/disease microenvironment real-time on cellular level. We, hereby, design simple and robust intravital microscopy strategies, which can be used to elucidate cellular or molecular interactions in a fluorescent mouse model.

PROCEDURES

We crossbred transgenic TIE2GFP mice with nude BALB/c mice, allowing the breeding of immunocompetent and immunodeficient mouse models expressing green fluorescent protein (GFP) in vascular endothelium. Then, we surgically exposed various tissues of interest to perform intravital microscopy.

RESULTS

By utilizing simple tissue preparation procedures and confocal or two-photon microscopy, we produced high-resolution static snapshots, dynamic sequences, and 3D reconstructions of orthotopically grown mammary tumor, skin inflammation, brain, and muscle. The homogenous detection of GFP expressed by endothelial cells and a combination of fluorescence agents enabled landmarking of tumor microenvironment and precise molecular tagging.

CONCLUSION

Simple intravital microscopy procedures on TIE2GFP mice allowed a real-time multi-color visualization of tissue microenvironment, underlining that robust microscopy strategies are relatively simple and can be readily available for many tissues of interest.

Perivascular macrophages create an intravascular niche for CD8+ T cell localisation prior to the onset of fatal experimental cerebral malaria

Objectives

The immunologic events that build up to the fatal neurological stage of experimental cerebral malaria (ECM) are incompletely understood. Here, we dissect immune cell behaviour occurring in the central nervous system (CNS) when Plasmodium berghei ANKA (PbA)‐infected mice show only minor clinical signs.

Methods

A 2‐photon intravital microscopy (2P‐IVM) brain imaging model was used to study the spatiotemporal context of early immunological events in situ during ECM.

Results

Early in the disease course, antigen‐specific CD8⁺ T cells came in contact and arrested on the endothelium of post‐capillary venules. CD8⁺ T cells typically adhered adjacent to, or were in the near vicinity of, perivascular macrophages (PVMs) that line post‐capillary venules. Closer examination revealed that CD8⁺ T cells crawled along the inner vessel wall towards PVMs that lay on the abluminal side of large post‐capillary venules. ‘Activity hotspots’ in large post‐capillary venules were characterised by T‐cell localisation, activated morphology and clustering of PVM, increased abutting of post‐capillary venules by PVM and augmented monocyte accumulation. In the later stages of infection, when mice exhibited neurological signs, intravascular CD8⁺ T cells increased in number and changed their behaviour, actively crawling along the endothelium and displaying frequent, short‐term interactions with the inner vessel wall at hotspots.

Conclusion

Our study suggests an active interaction between PVM and CD8⁺ T cells occurs across the blood–brain barrier (BBB) in early ECM, which may be the initiating event in the inflammatory cascade leading to BBB alteration and neuropathology.

Biology of the human blood-nerve barrier in health and disease

A highly regulated endoneurial microenvironment is required for normal axonal function in peripheral nerves and nerve roots, which structurally consist of an outer collagenous epineurium, inner perineurium consisting of multiple concentric layers of specialized epithelioid myofibroblasts that surround the innermost endoneurium, which consists of myelinated and unmyelinated axons embedded in a looser mesh of collagen fibers. Endoneurial homeostasis is achieved by tight junction-forming endoneurial microvessels that control ion, solute, water, nutrient, macromolecule and leukocyte influx and efflux between the bloodstream and endoneurium, and the innermost layers of the perineurium that control interstitial fluid component flux between the freely permeable epineurium and endoneurium. Strictly speaking, endoneurial microvascular endothelium should be considered the blood-nerve barrier (BNB) due to direct communication with circulating blood. The mammalian BNB is considered the second most restrictive vascular system after the blood-brain barrier (BBB) based on classic in situ permeability studies. Structural alterations in endoneurial microvessels or interactions with hematogenous leukocytes have been described in several human peripheral neuropathies; however major advances in BNB biology in health and disease have been limited over the past 50 years. Guided by transcriptome and proteome studies of normal and pathologic human peripheral nerves, purified primary and immortalized human endoneurial endothelial cells that form the BNB and leukocytes from patients with well-characterized peripheral neuropathies, validated by in situ or ex vivo protein expression studies, data are emerging on the molecular and functional characteristics of the human BNB in health and in specific peripheral neuropathies, as well as chronic neuropathic pain. These early advancements have the potential to not only increase our understanding of how the BNB works and adapts or fails to adapt to varying insult, but provide insights relevant to pathogenic leukocyte trafficking, with translational potential and specific therapeutic application for chronic peripheral neuropathies and neuropathic pain.

Neutrophil Recruitment: From Model Systems to Tissue-Specific Patterns

Neutrophil recruitment is not only vital for host defense, but also relevant in pathological inflammatory reactions, such as sepsis. Model systems have been established to examine different steps of the leukocyte recruitment cascade in vivo and in vitro under inflammatory conditions. Recently, tissue-specific recruitment patterns have come into focus, requiring modification of formerly generalized assumptions. Here, we summarize existing models of neutrophil recruitment and highlight recent discoveries in organ-specific recruitment patterns. New techniques show that previously stated assumptions of integrin activation and tissue invasion may need revision. Similarly, neutrophil recruitment to specific organs can rely on different organ properties, adhesion molecules, and chemokines. To advance our understanding of neutrophil recruitment, organ-specific intravital microscopy methods are needed.

A Novel Cervical Spinal Cord Window Preparation Allows for Two-Photon Imaging of T-Cell Interactions with the Cervical Spinal Cord Microvasculature during Experimental Autoimmune Encephalomyelitis

T-cell migration across the blood-brain barrier (BBB) is a crucial step in the pathogenesis of experimental autoimmune encephalomyelitis (EAE), an animal model of multiple sclerosis (MS). Two-photon intravital microscopy (2P-IVM) has been established as a powerful tool to study cell-cell interactions in inflammatory EAE lesions in living animals. In EAE, central nervous system inflammation is strongly pronounced in the spinal cord, an organ in which 2P-IVM imaging is technically very challenging and has been limited to the lumbar spinal cord. Here, we describe a novel spinal cord window preparation allowing to use 2P-IVM to image immune cell interactions with the cervical spinal cord microvascular endothelium during EAE. We describe differences in the angioarchitecture of the cervical spinal cord versus the lumbar spinal cord, which will entail different hemodynamic parameters in these different vascular beds. Using T cells as an example, we demonstrate the suitability of this novel methodology in imaging the post-arrest multistep T-cell extravasation across the cervical spinal cord microvessels. The novel methodology includes an outlook to the analysis of the cellular pathway of T-cell diapedesis across the BBB by establishing visualization of endothelial junctions in this vascular bed.

Imaging Nanotherapeutics in Inflamed Vasculature by Intravital Microscopy

Intravital microscopy (IVM) is the application of light microscopy to real time study biology of live animal tissues in intact and physiological conditions with the high spatial and temporal resolution. Advances in imaging systems, genetic animal models and imaging probes, IVM has offered quantitative and dynamic insight into cell biology, immunology, neurobiology and cancer. In this review, we will focus on the targeting of nanotherapeutics to inflamed vasculature. We will introduce the basic concept and principle of IVM and demonstrate that IVM is a powerful tool used to quantitatively determine the molecular mechanisms of interactions between nanotherapeutics and neutrophils or endothelium in living mice. In the future, it is needed to develop new imaging systems and novel imaging contrast agents to better understand molecular mechanisms of tissue processing of nanotherapeutics in vivo.

Current Evidence for a Role of the Kynurenine Pathway of Tryptophan Metabolism in Multiple Sclerosis

The kynurenine pathway (KP) is the major metabolic pathway of the essential amino acid tryptophan (TRP). Stimulation by inflammatory molecules, such as interferon-γ (IFN-γ), is the trigger for induction of the KP, driving a complex cascade of production of both neuroprotective and neurotoxic metabolites, and in turn, regulation of the immune response and responses of brain cells to the KP metabolites. Consequently, substantial evidence has accumulated over the past couple of decades that dysregulation of the KP and the production of neurotoxic metabolites are associated with many neuroinflammatory and neurodegenerative diseases, including Parkinson’s disease, AIDS-related dementia, motor neurone disease, schizophrenia, Huntington’s disease, and brain cancers. In the past decade, evidence of the link between the KP and multiple sclerosis (MS) has rapidly grown and has implicated the KP in MS pathogenesis. KP enzymes, indoleamine 2,3-dioxygenase (IDO-1) and tryptophan dioxygenase (highest expression in hepatic cells), are the principal enzymes triggering activation of the KP to produce kynurenine from TRP. This is in preference to other routes such as serotonin and melatonin production. In neurological disease, degradation of the blood–brain barrier, even if transient, allows the entry of blood monocytes into the brain parenchyma. Similar to microglia and macrophages, these cells are highly responsive to IFN-γ, which upregulates the expression of enzymes, including IDO-1, producing neurotoxic KP metabolites such as quinolinic acid. These metabolites circulate systemically or are released locally in the brain and can contribute to the excitotoxic death of oligodendrocytes and neurons in neurological disease principally by virtue of their agonist activity at N-methyl-d-aspartic acid receptors. The latest evidence is presented and discussed. The enzymes that control the checkpoints in the KP represent an attractive therapeutic target, and consequently several KP inhibitors are currently in clinical trials for other neurological diseases, and hence may make suitable candidates for MS patients. Underpinning these drug discovery endeavors, in recent years, several advances have been made in how KP metabolites are assayed in various biological fluids, and tremendous advancements have been made in how specimens are imaged to determine disease progression and involvement of various cell types and molecules in MS.

Recent advances in microscopic techniques for visualizing leukocytes in vivo

Leukocytes are inherently motile and interactive cells. Recent advances in intravital microscopy approaches have enabled a new vista of their behavior within intact tissues in real time. This brief review summarizes the developments enabling the tracking of immune responses in vivo.

TRIO Platform: A Novel Low Profile In vivo Imaging Support and Restraint System for Mice

High resolution, in vivo optical imaging of the mouse brain over time often requires anesthesia, which necessitates maintaining the animal's body temperature and level of anesthesia, as well as securing the head in an optimal, stable position. Controlling each parameter usually requires using multiple systems. Assembling multiple components into the small space on a standard microscope stage can be difficult and some commercially available parts simply do not fit. Furthermore, it is time-consuming to position an animal in the identical position over multiple imaging sessions for longitudinal studies. This is especially true when using an implanted gradient index (GRIN) lens for deep brain imaging. The multiphoton laser beam must be parallel with the shaft of the lens because even a slight tilt of the lens can degrade image quality. In response to these challenges, we have designed a compact, integrated in vivo imaging support system to overcome the problems created by using separate systems during optical imaging in mice. It is a single platform that provides (1) sturdy head fixation, (2) an integrated gas anesthesia mask, and (3) safe warm water heating. This THREE-IN-ONE (TRIO) Platform has a small footprint and a low profile that positions a mouse's head only 20 mm above the microscope stage. This height is about one half to one third the height of most commercially available immobilization devices. We have successfully employed this system, using isoflurane in over 40 imaging sessions with an average of 2 h per session with no leaks or other malfunctions. Due to its smaller size, the TRIO Platform can be used with a wider range of upright microscopes and stages. Most of the components were designed in SOLIDWORKS® and fabricated using a 3D printer. This additive manufacturing approach also readily permits size modifications for creating systems for other small animals.

Modeling leukocyte trafficking at the human blood-nerve barrier in vitro and in vivo geared towards targeted molecular therapies for peripheral neuroinflammation

Peripheral neuroinflammation is characterized by hematogenous mononuclear leukocyte infiltration into peripheral nerves. Despite significant clinical knowledge, advancements in molecular biology and progress in developing specific drugs for inflammatory disorders such as rheumatoid arthritis, inflammatory bowel disease, and multiple sclerosis, there are currently no specific therapies that modulate pathogenic peripheral nerve inflammation. Modeling leukocyte trafficking at the blood-nerve barrier using a reliable human in vitro model and potential intravital microscopy techniques in representative animal models guided by human observational data should facilitate the targeted modulation of the complex inflammatory cascade needed to develop safe and efficacious therapeutics for immune-mediated neuropathies and chronic neuropathic pain.

In vivo multi-modal imaging of experimental autoimmune uveoretinitis in transgenic reporter mice reveals the dynamic nature of inflammatory changes during disease progression

Background

Experimental autoimmune uveoretinitis (EAU) is a widely used experimental animal model of human endogenous posterior uveoretinitis. In the present study, we performed in vivo imaging of the retina in transgenic reporter mice to investigate dynamic changes in exogenous inflammatory cells and endogenous immune cells during the disease process.

Methods

Transgenic mice (C57Bl/6 J Cx₃cr1^GFP/+, C57Bl/6 N CD11c-eYFP, and C57Bl/6 J LysM-eGFP) were used to visualize the dynamic changes of myeloid-derived cells, putative dendritic cells and neutrophils during EAU. Transgenic mice were monitored with multi-modal fundus imaging camera over five time points following disease induction with the retinal auto-antigen, interphotoreceptor retinoid binding protein (IRBP_1–20). Disease severity was quantified with both clinical and histopathological grading.

Results

In the normal C57Bl/6 J Cx₃cr1^GFP/+ mouse Cx₃cr1-expressing microglia were evenly distributed in the retina. In C57Bl/6 N CD11c-eYFP mice clusters of CD11c-expressing cells were noted in the retina and in C57Bl/6 J LysM-eGFP mice very low numbers of LysM-expressing neutrophils were observed in the fundus. Following immunization with IRBP_1–20, fundus examination revealed accumulations of Cx₃cr1-GFP⁺ myeloid cells, CD11c-eYFP⁺ cells and LysM-eGFP⁺ myelomonocytic cells around the optic nerve head and along retinal vessels as early as day 14 post-immunization. CD11c-eYFP⁺ cells appear to resolve marginally earlier (day 21 post-immunization) than Cx₃cr1-GFP⁺ and LysM-eGFP⁺ cells. The clinical grading of EAU in transgenic mice correlated closely with histopathological grading.

Conclusions

These results illustrate that in vivo fundus imaging of transgenic reporter mice allows direct visualization of various exogenously and endogenously derived leukocyte types during EAU progression. This approach acts as a valuable adjunct to other methods of studying the clinical course of EAU.

Electronic supplementary material

The online version of this article (doi:10.1186/s12974-015-0235-6) contains supplementary material, which is available to authorized users.

Real-time imaging reveals the dynamics of leukocyte behaviour during experimental cerebral malaria pathogenesis

During experimental cerebral malaria (ECM) mice develop a lethal neuropathological syndrome associated with microcirculatory dysfunction and intravascular leukocyte sequestration. The precise spatio-temporal context in which the intravascular immune response unfolds is incompletely understood. We developed a 2-photon intravital microscopy (2P-IVM)-based brain-imaging model to monitor the real-time behaviour of leukocytes directly within the brain vasculature during ECM. Ly6C^hi monocytes, but not neutrophils, started to accumulate in the blood vessels of Plasmodium berghei ANKA (PbA)-infected MacGreen mice, in which myeloid cells express GFP, one to two days prior to the onset of the neurological signs (NS). A decrease in the rolling speed of monocytes, a measure of endothelial cell activation, was associated with progressive worsening of clinical symptoms. Adoptive transfer experiments with defined immune cell subsets in recombinase activating gene (RAG)-1-deficient mice showed that these changes were mediated by Plasmodium-specific CD8⁺ T lymphocytes. A critical number of CD8⁺ T effectors was required to induce disease and monocyte adherence to the vasculature. Depletion of monocytes at the onset of disease symptoms resulted in decreased lymphocyte accumulation, suggesting reciprocal effects of monocytes and T cells on their recruitment within the brain. Together, our studies define the real-time kinetics of leukocyte behaviour in the central nervous system during ECM, and reveal a significant role for Plasmodium-specific CD8⁺ T lymphocytes in regulating vascular pathology in this disease.

Cerebral malaria (CM) is a severe complication of Plasmodium falciparum infection that takes a significant toll on human life. Blockage of the brain blood vessels contributes to the clinical signs of CM, however we know little about the precise pathological events that lead to this disease. To this end, studies in Plasmodium-infected mice, that also develop a similar fatal disease, have proven useful. These studies have revealed an important role for leukocytes not so much in protecting but rather promoting pathology in the brain. To better understand leukocyte behaviour during experimental CM, we established a brain-imaging model that allows us to ‘peek’ into the brain of living mice and watch immunological events as they unfold. We found that worsening of disease was accompanied by an accumulation of monocytes in the blood vessels. Monocyte accumulation was regulated by activated CD8⁺ T cells but only when present in critical numbers. Monocyte depletion resulted in reduced T cell trafficking to the brain, but this did not result in improved disease outcome. Our studies reveal the orchestration of leukocyte accumulation in real time during CM, and demonstrate that CD8⁺ T cells play a crucial role in promoting clinical signs in this disease.