Catchup: a mouse model for imaging-based tracking and modulation of neutrophil granulocytes.

Neutrophil granulocyte biology is a central issue of immunological research, but the lack of animal models that allow for neutrophil-selective genetic manipulation has delayed progress. By modulating the neutrophil-specific locus Ly6G with a knock-in allele expressing Cre recombinase and the fluorescent protein tdTomato, we generated a mouse model termed Catchup that exhibits strong neutrophil specificity. Transgene activity was found only in very few eosinophils and basophils and was undetectable in bone marrow precursors, including granulomonocytic progenitors (GMPs). Cre-mediated reporter-gene activation allowed for intravital two-photon microscopy of neutrophils without adoptive transfer. Homozygous animals were Ly6G deficient but showed normal leukocyte cellularity in all measured organs. Ly6G-deficient neutrophils were functionally normal in vitro and in multiple models of sterile or infectious inflammation in vivo. However, Cre-mediated deletion of FcγRIV in neutrophils reduced the cells' recruitment to immune-complex-mediated peritonitis, suggesting a cell-intrinsic role for activating Fc receptors in neutrophil trafficking.

Fully Automated Evaluation of Total Glomerular Number and Capillary Tuft Size in Nephritic Kidneys Using Lightsheet Microscopy.

The total number of glomeruli is a fundamental parameter of kidney function but very difficult to determine using standard methodology. Here, we counted all individual glomeruli in murine kidneys and sized the capillary tufts by combining in vivo fluorescence labeling of endothelial cells, a novel tissue-clearing technique, lightsheet microscopy, and automated registration by image analysis. Total hands-on time per organ was <1 hour, and automated counting/sizing was finished in <3 hours. We also investigated the novel use of ethyl-3-phenylprop-2-enoate (ethyl cinnamate) as a nontoxic solvent-based clearing reagent that can be handled without specific safety measures. Ethyl cinnamate rapidly cleared all tested organs, including calcified bone, but the fluorescence of proteins and immunohistochemical labels was maintained over weeks. Using ethyl cinnamate-cleared kidneys, we also quantified the average creatinine clearance rate per glomerulus. This parameter decreased in the first week of experimental nephrotoxic nephritis, whereas reduction in glomerular numbers occurred much later. Our approach delivers fundamental parameters of renal function, and because of its ease of use and speed, it is suitable for high-throughput analysis and could greatly facilitate studies of the effect of kidney diseases on whole-organ physiology.

CD11c.DTR mice develop a fatal fulminant myocarditis after local or systemic treatment with diphtheria toxin.

To assess the role of alveolar macrophages (AMs) during a pulmonary Aspergillus fumigatus infection AMs were depleted by intratracheal application of diphtheria toxin (DTX) to transgenic CD11c.DTR mice prior to fungal infection. Unexpectedly, all CD11c.DTR mice treated with DTX died within 4-5 days, whether being infected with A. fumigatus or not. Despite measurable impact of DTX on lung functional parameters, these constrictions could not explain the high mortality rate. Instead, DTX-treated CD11c.DTR animals developed fulminant myocarditis (FM) characterized by massive leukocyte infiltration and myocardial cell destruction, including central parts of the heart's stimulus transmission system. In fact, standard limb lead ECG recordings of diseased but not healthy mice showed a "Brugada"-like pattern with an abnormally high ST segment pointing to enhanced susceptibility for potential lethal arrhythmias. While CD11c.DTR mice are extensively used for the characterization of CD11c(+) cells, including dendritic cells, several studies have already mentioned adverse side effects following DTX treatment. Our results demonstrate that this limitation is based on severe myocarditis but not on the expected lung constrictions, and has to be taken into consideration if this animal model is used. Based on these properties, however, the CD11c.DTR mouse might serve as useful animal model for FM.

Very-late-antigen-4 (VLA-4)-mediated brain invasion by neutrophils leads to interactions with microglia, increased ischemic injury and impaired behavior in experimental stroke.

Neuronal injury from ischemic stroke is aggravated by invading peripheral immune cells. Early infiltrates of neutrophil granulocytes and T-cells influence the outcome of stroke. So far, however, neither the timing nor the cellular dynamics of neutrophil entry, its consequences for the invaded brain area, or the relative importance of T-cells has been extensively studied in an intravital setting. Here, we have used intravital two-photon microscopy to document neutrophils and brain-resident microglia in mice after induction of experimental stroke. We demonstrated that neutrophils immediately rolled, firmly adhered, and transmigrated at sites of endothelial activation in stroke-affected brain areas. The ensuing neutrophil invasion was associated with local blood-brain barrier breakdown and infarct formation. Brain-resident microglia recognized both endothelial damage and neutrophil invasion. In a cooperative manner, they formed cytoplasmic processes to physically shield activated endothelia and trap infiltrating neutrophils. Interestingly, the systemic blockade of very-late-antigen-4 immediately and very effectively inhibited the endothelial interaction and brain entry of neutrophils. This treatment thereby strongly reduced the ischemic tissue injury and effectively protected the mice from stroke-associated behavioral impairment. Behavioral preservation was also equally well achieved with the antibody-mediated depletion of myeloid cells or specifically neutrophils. In contrast, T-cell depletion more effectively reduced the infarct volume without improving the behavioral performance. Thus, neutrophil invasion of the ischemic brain is rapid, massive, and a key mediator of functional impairment, while peripheral T-cells promote brain damage. Acutely depleting T-cells and inhibiting brain infiltration of neutrophils might, therefore, be a powerful early stroke treatment.

Quantitative Visualization of Leukocyte Infiltrate in a Murine Model of Fulminant Myocarditis by Light Sheet Microscopy.

Light-sheet fluorescence microscopy (LSFM), in combination with chemical clearing protocols, has become the gold standard for analyzing fluorescently labelled structures in large biological specimens, and is down to cellular resolution. Meanwhile, the constant refinement of underlying protocols and the enhanced availability of specialized commercial systems enable us to investigate the microstructure of whole mouse organs and even allow for the characterization of cellular behavior in various live-cell imaging approaches. Here, we describe a protocol for the spatial whole-mount visualization and quantification of the CD45+ leukocyte population in inflamed mouse hearts. The method employs a transgenic mouse strain (CD11c.DTR)that has recently been shown to serve as a robust, inducible model for the study of the development of fulminant fatal myocarditis, characterized by lethal cardiac arrhythmias. This protocol includes myocarditis induction, intravital antibody-mediated cell staining, organ preparation, and LSFM with subsequent computer-assisted image post-processing. Although presented as a highly-adapted method for our particular scientific question, the protocol represents the blueprint of an easily adjustable system that can also target completely different fluorescent structures in other organs and even in other species.

Survival of residual neutrophils and accelerated myelopoiesis limit the efficacy of antibody-mediated depletion of Ly-6G+ cells in tumor-bearing mice.

Expansion of Ly-6G(+) myeloid cells has been reported in most murine cancer models. However, divergent findings exist regarding the role and effect of these cells on host immunity and tumor progression. Antibody-mediated depletion of Ly-6G(+) cells is a common technique to assess the in vivo relevance of these cells. Interpretation of results crucially depends on the efficacy and course of depletion. We established murine head and neck cancer models and analyzed the efficacy of antibody-mediated depletion by flow cytometry, conventional histology, and intravital imaging with a novel Ly-6G-transgenic mouse model. The first phase of depletion was characterized by effective elimination of Ly-6G(+) cells from the peripheral blood. Nevertheless, viable, resistant cells were found to reside in the tumor tissue and spleen. This peripheral depletion phase was associated with high systemic levels of granulocyte colony-stimulating factor and KC and enhanced splenic production of Ly-6G(+) cells. Even under sustained treatment with either αGr-1 or αLy-6G antibodies, peripheral blood depletion ended after approximately 1 wk and was followed by reappearance of immature Ly-6G(+) cells with an immunoregulatory phenotype. Reappearance of these depletion-resistant immature cells was enhanced in tumor-bearing, compared with naïve, control mice. Collectively, our data suggest that depletion of Ly-6G(+) myeloid cells in tumor-bearing mice is counteracted by the persistence of intratumoral cells, enhanced extramedullary granulopoiesis, and accelerated reappearance of immature cells. Hence, extensive monitoring of in vivo kinetics and tissue distribution of Ly-6G(+) cells is required in depletion studies.