Imaging of immune cell behavior and function in multiple sclerosis and experimental autoimmune encephalomyelitis

To visualize the entire process of encephalitogenic T cell infiltration into the target organ, we performed intravital imaging by using two‐photon microscopy in experimental autoimmune encephalomyelitis, the animal model of multiple sclerosis. Intravital imaging documented that T cells first appear in the leptomeningeal blood vessels where they crawl in an integrin‐dependent manner and scan the intraluminal surface for extravasation sites. After diapedesis, the T cells continue to crawl on the abluminal surface, where they meet local antigen presenting cells ( APC ) that can provide stimuli to the T cells for the subsequent infiltration into the central nervous system ( CNS ) parenchyma. Although flow cytometric analysis documented that the infiltrated T cells upregulated their activation markers in the CNS meninges, it was unclear at which scanning step the activation occurred. We recently introduced two genetically encoded fluorescent T cell activation sensors for intravital imaging. The first is a fluorescent resonance energy transfer‐based Ca 2+ sensor for the quantification of the intracellular Ca 2+ concentration, a major step in T cell receptor signaling. The second sensor is a truncated nuclear factor of activated T cells fused to green fluorescent protein, which subcellular localization corresponds to the T cell activation state. Introducing these sensors into the lymphocytes enables the visualization of the interactions of encephalitogenic T cells with different blood–brain barrier structures, and allows us to assess the functional aspect of these interactions directly in vivo . This model system can be further used to evaluate therapeutic compounds and to better understand the activities of T cells in vivo .