The Use Of Two Photon Microscopy To Image Vaso-Occulsion In Pulmonary Microvessels Of Living Mice With Sickle Cell Disease

Although sickle cell anemia is initiated by red cell pathology, it is accompanied by an inflammatory immune response involving platelets and white blood cells that contribute to vaso-occlusvie episodes including painful vaso-occlusive crises (pVOC) and acute chest syndrome (ACS). In order to better understand the cellular and molecular bases of vaso-occlusion we are in the process of developing procedures to image microvessels in the lung, liver and spleen of living mice by 2-photon microscopy, a procedure that is based on excitation of a fluorophore by two photons simultaneously. The two-photon technique utilizes infrared light that efficiently penetrates tissues up to 200 microns with low phototoxicity allowing time-lapse imaging. Two-photon intravital microscopy can be used to study the behavior of intravascular cells during vaso-occlusive events.

Mice are prepared for lung intravital microscopy by the intraperitoneal injection of a mixture of ketamine and xylazine. Additional anesthesia is added during experimentation. The trachea is opened and the mouse intubated. The chest is opened to allow access to the left lobe of the lung through a window that is a few millimeters in diameter. PBS is applied to keep the lung moist. A custom built suction device is placed on the lung and covered with a cover glass at the same time pressure is exerted to seal the organ and the glass cover together. Throughout the procedure, the mouse is held at a temperature of 37°C. Once surgery is completed, a mixture of antibodies coupled to fluorophores is given by retro-orbital injection. In order to minimize photobleaching we used antibodies conjugated to Alexa Fluor 488, Alexa Fluor 555 or Alexa Fluor 647.

We are able to visualize and quantify interactions between red blood cells, white blood cells, and endothelial cells as well as the expression of adhesion molecules on endothelial cells in real time. During pVOC triggered by hypoxia, cell adhesion of neutrophils, lymphocytes and monocytes to the endothelium is observed that is associated with an increase in endothelial expression of ICAM-1 and V-CAM. We label endothelial cells with anti-CD31, lymphocytes with anti-CD45, monocytes with anti-Ly6C and neutrophils with anti-Ly6G. Platelets are labeled with anti-CD41 or anti-CD62P, NK cells with anti-NKp46, and macrophages with anti-F4/80 and anti-CD1d. We are able to quantify cell shape, rolling, adhesion and movement.

Our preliminary results demonstrate that it is possible in real time to image the sequence of events occurring during pulmonary vasoocclusion in sickle cell disease. In conclusion, intravital 2-photon microscopy holds great potential for enabling us to better understand inflammatory responses within the blood vessels of living SCD mice.