Image enhancement of the in vivo leukocyte-endothelium contact zone using optical sectioning microscopy

Recoil and stiffening by adherent leukocytes in response to fluid shear

Prolonged exposure to fluid shear stress alters leukocyte functions associated with the immune response. We examined the initial response of freshly isolated human leukocytes to fluid shear stress under high magnification. Adherent leukocytes exhibit a rapid biomechanical response to physiological levels of fluid shear stress. After passive displacement in the direction of a constant fluid shear stress, adherent leukocytes actively recoil back in the opposite direction of the fluid flow. Recoil is observed within seconds of the applied fluid shear stress. Simultaneously, fluid shear stress induces a stiffening of the cell. The immediate cell displacement in response to a step increase in fluid shear stress is greatly attenuated in subsequent steps compared to the initial fluid shear stress step. Recoil is not mediated by actin polymerization-dependent mechanisms, as cytochalasin D had no effect on this early response. However, stiffening was determined in part by an intact actin cytoskeleton. Inhibiting myosin force generation with ML-7 abolished the recoil and stiffening responses, implicating force generation by myosin as an important contributor to the early leukocyte response to fluid shear stress. This initial shear stress response may be particularly important in facilitating leukocyte attachment under sustained fluid shear stress by the flowing blood in the microcirculation.

Enumeration of leukocyte infiltration in solid tumors by confocal laser scanning microscopy

Background

Leukocytes commonly infiltrate solid tumors, and have been implicated in the mechanism of spontaneous regression in some cancers. Conventional techniques for the quantitative estimation of leukocyte infiltrates in tumors rely on light microscopy of immunostained thin tissue sections, in which an arbitrary assessment (based on low, medium or high levels of infiltration) of antigen density is made by the pathologist. These estimates are relatively subjective and often require the opinion of a second pathologist. In addition, since thin tissue sections are cut, no data regarding the three-dimensional distribution of antigen can be obtained.

Results

To overcome these problems, we have designed a method to enumerate leukocyte infiltration into tumors, using confocal laser scanning microscopy of fluorescently immunostained leukocytes in thick tissue sections. Using image analysis software, a threshold was applied to eliminate unstained tissue and residual noise. The total antigen volume in the scanned tissue was calculated and divided by the mean cell volume (calculated by "seeding" ten individual cells) to obtain the cell count. Using this method, we compared the calculated leukocyte counts with those obtained manually by ten laboratory personnel. There was no significant difference (P > 0.05) between the cell counts obtained by either method.

We then compared leukocyte infiltration into seven tumors and matched non-malignant tissue obtained from the periphery of the resected tissue. There was a significant increase in the infiltration of all leukocyte subsets into the tumors compared to minimal numbers in the non-malignant tissue.

Conclusion

From these results we conclude that this method may be of considerable use for the enumeration of cells in tissues. Furthermore, since it can be performed by laboratory technical staff, less time input is required by the pathologist in assessing the degree of leukocyte infiltration into tumors.

Probing enhanced cytochrome P450 2B1/2 activity in rat hepatocyte spheroids through confocal laser scanning microscopy

Cytochrome P450 (CYP450) enzymes are essential for xenobiotic metabolism. Although CYP450s are found in many tissues, CYP2B1/2 are primarily expressed in the rat liver. The constitutive expression in vivo of CYP2B1/2 is low but it is induced in the presence of various drugs such as phenobarbital (PB). In this study, CYP2B1/2 activity in cultured hepatocytes was assessed in situ with the introduction of a fluorogenic substrate, pentoxyresorufin. The product of 7-pentoxyresorufin-O-dealkylation (PROD), which is catalyzed specifically by CYP2B1/2, was detected using confocal laser scanning microscopy (CLSM). Primary hepatocytes cultured as monolayers on collagen-coated surfaces exhibited background PROD activity and minimal PB inducibility after 4 days in culture. In contrast, rat hepatocytes organized in compacted aggregates, or spheroids, exhibited higher levels of PROD activity and retained their ability for PB induction. The results from the CLSM analysis were verified by RT-PCR and Western immunoblotting analysis. Furthermore, CLSM in conjunction with image processing techniques and three-dimensional reconstruction revealed the localization of enhanced PROD activity in the center of spheroids. The results support the use of CLSM as a powerful tool for investigating CYP2B1/2 activity in cultured rat hepatocytes.

Confocal imaging of flows in artificial venular bifurcations

We describe a new experimental methodology for visualizing three-dimensional structures in microscopic tubes under flow conditions. Through the use of microfabrication techniques, artificial venular bifurcations are constructed from glass tubes with semicircular cross sections (radius = 50 mu). Aqueous fluorescent solutions are infused into the tubes at flow rates of about 1 microliter/min, a value comparable to blood flow in the microcirculation. The flow is imaged using a combination of confocal microscopy and three-dimensional image reconstruction software techniques. The quantitative accuracy of the experimental method is evaluated by measuring the "separation surface," a formation resulting from converging flows at a bifurcation. Details of the fabrication process, fluidics, confocal microscopy, image reconstructions, optical effects, and computations are described. We show the first three-dimensional visualization of a microscopic flow structure using confocal microscopy, and within certain limitations, quantitative agreement between the measured and computed positions of the separation surface.

Flow resistance and drag forces due to multiple adherent leukocytes in postcapillary vessels

Computational fluid dynamics was used to model flow past multiple adherent leukocytes in postcapillary size vessels. A finite-element package was used to solve the Navier-Stokes equations for low Reynolds number flow of a Newtonian fluid past spheres adhering to the wall of a cylindrical vessel. We determined the effects of sphere number, relative geometry, and spacing on the flow resistance in the vessel and the fluid flow drag force acting to sweep the sphere off the vessel wall. The computations show that when adherent leukocytes are aligned on the same side of the vessel, the drag force on each of the interacting leukocytes is less than the drag force on an isolated adherent leukocyte and can decrease by up to 50%. The magnitude of the reduction depends on the ratio of leukocyte to blood vessel diameter and distance between adherent leukocytes. However, there is an increase in the drag force when leukocytes adhere to opposite sides of the vessel wall. The increase in resistance generated by adherent leukocytes in vessels of various sizes is calculated from the computational results. The resistance increases with decreasing vessel size and is most pronounced when leukocytes adhere to opposite sides of the vessel.

Development of a side-view chamber for studying cell-surface adhesion under flow conditions

Observing microscopic specimens is often useful in studies of cellular interaction with a vascular wall. We have developed an in vitro side-view flow chamber that permits observations from the side of the cell's contact with various adhesive surfaces under dynamic flow conditions. This side-view flow chamber consists of two precision rectangular glass tubes called microslides. A smaller microslide is inserted into a larger one to create a flow channel with a flat surface on which either cultured vascular endothelium can be grown or purified adhesion molecules can be coated. Two optical prisms with a 45 degrees chromium-coated surface are used along the flow channel to generate light illumination and observation pathways. The side-view images of cell-substrate contact can be obtained using a light microscope. This design allows us not only to measure the effects of flow on cell-surface adhesion strength, but also to have close observation of cell deformation and adhesive contact to various surfaces in shear flow. In addition, this chamber can readily serve for a conventional top-view flow channel, similar to the parallel-plate flow chambers used in many areas. The development of such a side-view flow chamber can be beneficial to various in vitro applications in cellular studies that require an edge view, especially for various cell interactions with cultured vascular endothelium or surfaces containing single-type adhesive molecules under flow conditions.