Visualization of hepatobiliary excretory function by intravital multiphoton microscopy

Intravital imaging of hepatobiliary excretion is vital for elucidating liver metabolism. In this work, we describe a novel method to observe the intravital dynamics of the uptake, processing, and excretion of an organic anion, 6-carboxyfluorescein diacetate (6-CFDA) in the hepatobiliary system. This is achieved by the use of multiphoton microscopy and an intravital hepatic imaging chamber. The high-quality images show sequential uptake and processing of 6-CFDA from the hepatocytes and the subsequent excretion into bile canaliculi within approximately 50 min. This is a promising technique to study intravital hepatic physiology and metabolism.     

Experimental pathology by intravital microscopy and genetically encoded fluorescent biosensors

The invention of two‐photon excitation microscopes widens the potential application of intravital microscopy (IVM) to the broad field of experimental pathology. Moreover, the recent development of fluorescent protein‐based, genetically encoded biosensors provides an ideal tool to visualize the cell function in live animals. We start from a brief review of IVM with two‐photon excitation microscopes and genetically encoded biosensors based on the principle of Förster resonance energy transfer (FRET). Then, we describe how IVM using biosensors has revealed the pathogenesis of several disease models.     

An intravital microscopy model for studies of immune complex induced inflammation at the microvascular level

The hamster cheek pouch model was used for studies of immune complex induced inflammation. Vascular leakage and leukocyte accumulation were observed after topical application of antigen. This occurred at the post-capillary venules, where also antigen localization could be seen, indicating immune complex deposition at these sites.     

Confocal fluorescent intravital microscopy of the murine spleen

Intravital microscopy has provided many insights into cellular interactions in various secondary lymphoid tissues. Because this technique allows for the visualization of cellular movement in real-time, it has been very powerful. However, until now, it has been difficult to apply this technique to the spleen. We report a technique that utilizes the Nikon RCM-8000 scanning laser, confocal microscope that allows for visualization of cellular movement in real-time in the rodent spleen. Using fluorescently labeled high molecular weight dextran or monoclonal antibodies, we are able to visualize fluorescently labeled cells rolling, tethering, and adhering in the spleen. In addition, we show that the majority of blood flow to the spleen remains within the white pulp nodules, as do most transferred erythrocytes at early time points. This is the first report of intravital microscopy of the spleen using a method that allows for easy identification of transferred cells.     

Visualizing recall immune responses with multi‐photon microscopy

Multi‐photon microscopy has increased our understanding of T‐cell behaviour during the initiation of primary immune responses. In this issue of Immunology a study uses real‐time imaging of lymph nodes to observe and compare the early events in naive, tolerized and primed CD4 T‐cell responses.     

A novel approach for studying microcirculation in bone defects by intravital fluorescence microscopy

Angiogenic and inflammatory responses to biodegradable scaffolds were previously studied using the dorsal skinfold chamber for testing different scaffold materials. In this model, the angiogenic response originates from the soft tissue of the skin. Herein, we introduce a new model that allows the study of developing microcirculation of bone defects for testing tissue-engineered constructs. A bone defect was prepared in the femur of Balb/c mice by inserting a pin for intramedullary fixation, and a custom-made observation window fixed over the defect allowed constant observation. This study included three different groups: empty defect (control), defect filled with porous poly(L-lactide-co-glycolide), and beta-tricalcium-phosphate scaffolds. Starting from 6 days after surgery, angiogenesis, neovascularization, leukocyte-endothelial cell interaction, and microvascular permeability were analyzed over 22 days by using intravital fluorescence microscopy. The empty defects showed no signs of angiogenesis during the observation period, but a distinct increase of capillary density was detected in the scaffold-containing defects. Surprisingly, the histological sections of the scaffold-treated defects showed new bone formation 22 days after implantation. We present a new bone chamber model for intravital long-term study of scaffold materials suitable for bone reconstruction in mice by using fluorescence microscopy.     

Immune-complex-induced inflammatory reaction studied by intravital microscopy Role of histamine and arachidonic acid metabolites

An immune-complex-induced inflammatory reaction was elicited in the hamster cheek pouch microvasculature of ovalburnin (OA)-immunized animals by exposure to 1 or 100 g/ml OA. The low antigen dose caused arteriolar constriction, transient platelet aggregation, and a reversible increase in vascular leakage at postcapillary venules. With the high antigen dose, the constriction and platelet aggregation were more pronounced and the vascular leakage was prolonged. This antigen dose also caused a massive PMNL accumulation in small venules, which coincided with the prolonged vascular leakage. Histamine was released in the reaction as pretreatment with mepyramine largely inhibited the leakage response to 1 g/ ml OA. With 100 mg/ml OA, only the initial phase of vascular leakage was inhibited by mepyramine, leaving the prolonged vascular leakage and PMNL accumulation unaltered. Pretreatment with methylprednisolone 16–18 h prior to the experiments reduced both phases of vascular leakage as well as the PMNL accumulation. Pre-treatment with the combined cyclo- and lipoxygenase inhibitors BW755C or nordihydroguaiaretic acid (NDGA) reduced the initial vasoconstriction induced with 100 g/ml OA, thereby augmenting the initial vascular leakage. Despite this, the prolonged phase of vascular leakage was reduced in the NDGA-treated animals. Cyclooxygenase products were not found to play a crucial role in mediating the vascular response; on the contrary, indomethacin pretreatment slightly potentiated the vascular leakage induced by the low antigen dose.     

An extended vision for dynamic high-resolution intravital immune imaging

The past few years have seen the application of confocal and especially two-photon microscopy to the dynamic high-resolution imaging of lymphocytes and antigen presenting cells within organs such as lymph nodes and thymus. After summarizing some of the published results obtained to date using these methods, we describe our view of how this technology will develop and be applied in the near future. This includes its extension to a wide variety of non-lymphoid tissues, to the tracking of functional responses in addition to migratory behavior, to the analysis of molecular events previously studied only in vitro, to dissection of the interplay between hematopoietic and stromal elements, to visualization of a wider array of cell types including neutrophils, macrophages, NK cells, NKT cells and others, and to the interaction of the host with infectious agents. Reaching these goals will depend on a combination of new tools for genetic manipulations, novel fluorescent reporters, enhanced instrumentation, and better surgical techniques for the extended imaging of live animals. The end result will be a new level of understanding of how orchestrated cell movement and interaction contribute to the physiological and pathological activities of the immune system.     

Imaging acute thermal burns by photoacoustic microscopy

The clinical significance of a burn depends on the percentage of total body involved and the depth of the burn. Hence a noninvasive method that is able to evaluate burn depth would be of great help in clinical evaluation. To this end, photoacoustic microscopy is used to determine the depth of acute thermal burns by imaging the total hemoglobin concentration in the blood that accumulates along the boundaries of injuries as a result of thermal damage to the vasculature. We induce acute thermal burns in vivo on pig skin with cautery. Photoacoustic images of the burns are acquired after skin excision. In a burn treated at 175 degrees C for 20 s, the maximum imaged burn depth is 1.73+/-0.07 mm. In burns treated at 150 degrees C for 5, 10, 20, and 30 s, respectively, the trend of increasing maximum burn depth with longer thermal exposure is demonstrated.     

Visualization of mucosal homeostasis via single- and multiphoton intravital fluorescence microscopy

FIVM has provided many insights into the regulation of immunity. We report the validation of an approach for visualizing murine small bowel via single- and multiphoton FIVM. Tissue damage is limited to ～200 μm, immediately adjacent to the incision, as confirmed by intravital PI staining. Treatment with 10 KDa dextran-FITC and 70 KDa dextran-TR confirms that perfusion is intact. Selective filtration of 10 KDa but not 70 KDa dextran from the blood indicated that kidney function is also intact. Interestingly, lamina propria vasculature is semipermeable to 10 KDa dextran. Next, reporter mice expressing egfp from the CX3CR1 locus, egfp from the FoxP3 locus, or RFP from the IL-17F locus were used to track DC subsets, FoxP3(+) Tregs, or Th17f cells, respectively. Resident cx3cr1(+/egfp) cells were sessile but actively probed the surrounding microenvironment. Both T cell populations patrol the lamina propria, but the Th17f cells migrate more rapidly than Tregs. Together, these data demonstrate intact vascular perfusion, while intravitally visualizing the mucosal surface of the small bowel. Lastly, the cx3cr1(+) DCs and T cells display activity similar to that found in steady-state, secondary lymphoid organs.     

Dorsal skinfold chamber technique for intravital microscopy in nude mice.

For many years, observation chambers, implanted in various animal species and in man, have been used for intravital microscopy of the microcirculation in granulation tissue, preformed tissue, or of the microvascularization of tissue implants. We describe herein a skinfold chamber model for the intravital microscopic investigation of striated skin muscle in immunoincompetent, nude mice over a minimum period of 2 weeks. Using fluorescent markers for contrast enhancement of plasma (fluorescein isothiocyanate-dextran) and leukocytes (acridine orange), the presented model allows the quantitative analysis of 1) the microhemodynamic parameters microvessel diameter and red blood cell velocity in arterioles (16 to 50 mu diameter), capillaries (4 to 9 mu diameter), and post-capillary venules (19 to 60 mu diameter), 2) leukocyte/endothelium interaction in these vessel segments, 3) functional capillary density and intercapillary distance, and 4) endothelial cell integrity. These parameters can be assessed in the microcirculation of the striated muscle tissue under normal or pathological conditions, as well as in the microcirculation of transplanted xenogeneic (human) neoplastic and non-neoplastic tissue grafts.