Highly resolved intravital striped-illumination microscopy of germinal centers

Monitoring cellular communication by intravital deep-tissue multi-photon microscopy is the key for understanding the fate of immune cells within thick tissue samples and organs in health and disease. By controlling the scanning pattern in multi-photon microscopy and applying appropriate numerical algorithms, we developed a striped-illumination approach, which enabled us to achieve 3-fold better axial resolution and improved signal-to-noise ratio, i.e. contrast, in more than 100 µm tissue depth within highly scattering tissue of lymphoid organs as compared to standard multi-photon microscopy. The acquisition speed as well as photobleaching and photodamage effects were similar to standard photo-multiplier-based technique, whereas the imaging depth was slightly lower due to the use of field detectors. By using the striped-illumination approach, we are able to observe the dynamics of immune complex deposits on secondary follicular dendritic cells - on the level of a few protein molecules in germinal centers

High-resolution intravital microscopy

Cellular communication constitutes a fundamental mechanism of life, for instance by permitting transfer of information through synapses in the nervous system and by leading to activation of cells during the course of immune responses. Monitoring cell-cell interactions within living adult organisms is crucial in order to draw conclusions on their behavior with respect to the fate of cells, tissues and organs. Until now, there is no technology available that enables dynamic imaging deep within the tissue of living adult organisms at sub-cellular resolution, i.e. detection at the level of few protein molecules. Here we present a novel approach called multi-beam striped-illumination which applies for the first time the principle and advantages of structured-illumination, spatial modulation of the excitation pattern, to laser-scanning-microscopy. We use this approach in two-photon-microscopy - the most adequate optical deep-tissue imaging-technique. As compared to standard two-photon-microscopy, it achieves significant contrast enhancement and up to 3-fold improved axial resolution (optical sectioning) while photobleaching, photodamage and acquisition speed are similar. Its imaging depth is comparable to multifocal two-photon-microscopy and only slightly less than in standard single-beam two-photon-microscopy. Precisely, our studies within mouse lymph nodes demonstrated 216% improved axial and 23% improved lateral resolutions at a depth of 80 µm below the surface. Thus, we are for the first time able to visualize the dynamic interactions between B cells and immune complex deposits on follicular dendritic cells within germinal centers (GCs) of live mice. These interactions play a decisive role in the process of clonal selection, leading to affinity maturation of the humoral immune response. This novel high-resolution intravital microscopy method has a huge potential for numerous applications in neurosciences, immunology, cancer research and developmental biology. Moreover, our striped-illumination approach is able to improve the resolution of any laser-scanning-microscope, including confocal microscopes, by simply choosing an appropriate detector

T7 RNA Polymerase Functions In Vitro without Clustering

Many nucleic acid polymerases function in clusters known as factories. We investigate whether the RNA polymerase (RNAP) of phage T7 also clusters when active. Using ‘pulldowns’ and fluorescence correlation spectroscopy we find that elongation complexes do not interact in vitro with a Kd<1 µM. Chromosome conformation capture also reveals that genes located 100 kb apart on the E. coli chromosome do not associate more frequently when transcribed by T7 RNAP. We conclude that if clustering does occur in vivo, it must be driven by weak interactions, or mediated by a phage-encoded protein

Automated quantification of hematopoietic cell - stromal cell interactions in histological images of undecalcified bone

Confocal microscopy is the method of choice for the analysis of localization of multiple cell types within complex tissues such as the bone marrow. However, the analysis and quantification of cellular localization is difficult, as in many cases it relies on manual counting, thus bearing the risk of introducing a rater-dependent bias and reducing interrater reliability. Moreover, it is often difficult to judge whether the co-localization between two cells results from random positioning, especially when cell types differ strongly in the frequency of their occurrence. Here, a method for unbiased quantification of cellular co-localization in the bone marrow is introduced. The protocol describes the sample preparation used to obtain histological sections of whole murine long bones including the bone marrow, as well as the staining protocol and the acquisition of high-resolution images. An analysis workflow spanning from the recognition of hematopoietic and non-hematopoietic cell types in 2-dimensional (2D) bone marrow images to the quantification of the direct contacts between those cells is presented. This also includes a neighborhood analysis, to obtain information about the cellular microenvironment surrounding a certain cell type. In order to evaluate whether co-localization of two cell types is the mere result of random cell positioning or reflects preferential associations between the cells, a simulation tool which is suitable for testing this hypothesis in the case of hematopoietic as well as stromal cells, is used. This approach is not limited to the bone marrow, and can be extended to other tissues to permit reproducible, quantitative analysis of histological data

Unilateral dampening of Bmp activity by Nodal generates cardiac left-right asymmetry

Signaling by Nodal and Bmp is essential for cardiac laterality. How activities of these pathways translate into left-right asymmetric organ morphogenesis is largely unknown. We show that, in zebrafish, Nodal locally reduces Bmp activity on the left side of the cardiac field. This effect is mediated by the extracellular matrix enzyme Hyaluronan synthase 2, expression of which is induced by Nodal. Unilateral reduction of Bmp signaling results in lower expression of nonmuscle myosin II and higher cell motility on the left, driving asymmetric displacement of the entire cardiac field. In silico modeling shows that left-right differences in cell motility are sufficient to induce a robust, directional migration of cardiac tissue. Thus, the mechanism underlying the formation of cardiac left-right asymmetry involves Nodal modulating an antimotogenic Bmp activity

Microanatomical Analysis And Quantification Of Plasma Cell Niche Interactions In The Bone Marrow

INTRODUCTION: Long-lived plasma cells (PCs), responsible for the production of long-term antibody titers, have been shown to survive in the bone marrow for months to years in the absence of antigen. They are supported by a special microenvironment, the PC survival niche. Various cell types have been reported to contribute to this niche by providing survival factors, e.g. CXCL12-producing reticular stromal cells. Additionally, hematopoietic cells have been shown to mediate PC survival in vivo, amongst them megakaryocytes and eosinophils, but the spatiotemporal dynamics of the various niche components in the tissue remain elusive. AIMS: The aim of our work is to analyze the cellular and molecular composition of plasma cell survival niches in the bone marrow in situ. METHODS: In order to unambiguously quantify the localization of PCs, we are analyzing bone marrow cryosections and whole mounts for PCs, stromal cells, vasculature and accessory niche cells. Additionally, we have developed a computer modeling approach which allows us to distinguish random co-localization from non-random cell positioning. Using these approaches, we have previously shown that PCs directly contact reticular stromal cells in a non-random fashion, while 30% of PCs are found in 10 µm vicinity to eosinophils, which represent only transient contributors to the niche. We have now analyzed PC localization in relation to mineralized bone, bone marrow vasculature and hematopoietic cell types in 3 dimensions and found that PC niches are situated at large distance to sinusoids. RESULTS: Semi-automated 3D analyses of whole mounts allow for a comprehensive and unbiased quantification of PC localization and their possible interactions with accessory niche cells in the bone marrow. We show that the survival niche for long-lived PCs is located distant from sinusoidal blood vessels, in contrast to what has been reported for the hematopoietic stem cell niche. We are now testing ways to mobilize PCs from their niches, which should result in shifted PC localization - from vessel-distant to peri-sinusoidal spaces. We are further exploring ways to perform multiplexed histological analysis using multi-epitope ligand cartography (MELC) in the bone marrow in order to further characterize the plasma cell niche