Autoreactive T cells can infiltrate the CNS to cause disorders such as multiple sclerosis. In order to visualize T cell activation in the CNS, we introduced a truncated fluorescent derivative of nuclear factor of activated T cells (NFAT) as a real-time T cell activation indicator. In experimental autoimmune encephalomyelitis, a rat model of multiple sclerosis, we tracked T cells interacting with structures of the vascular blood-brain barrier (BBB). 2-photon imaging documented the cytoplasmic-nuclear translocation of fluorescent NFAT, indicative of calcium-dependent activation of the T cells in the perivascular space, but not within the vascular lumen. The activation was related to contacts with the local antigen-presenting phagocytes and was noted only in T cells with a high pathogenic potential. T cell activation implied the presentation of an autoantigen, as the weakly pathogenic T cells, which remained silent in the untreated hosts, were activated upon instillation of exogenous autoantigen. Activation did not cogently signal long-lasting arrest, as individual T cells were able to sequentially contact fresh APCs. We propose that the presentation of local autoantigen by BBB-associated APCs provides stimuli that guide autoimmune T cells to the CNS destination, enabling them to attack the target tissue.

In experimental autoimmune encephalitis (EAE), autoimmune T cells are activated in the periphery before they home to the CNS. On their way, the T cells pass through a series of different cellular milieus where they receive signals that instruct them to invade their target tissues. These signals involve interaction with the surrounding stroma cells, in the presence or absence of autoantigens. To portray the serial signaling events, we studied a T-cell-mediated model of EAE combining in vivo two-photon microscopy with two different activation reporters, the FRET-based calcium biosensor Twitch1 and fluorescent NFAT. In vitro activated T cells first settle in secondary (2°) lymphatic tissues (e.g., the spleen) where, in the absence of autoantigen, they establish transient contacts with stroma cells as indicated by sporadic short-lived calcium spikes. The T cells then exit the spleen for the CNS where they first roll and crawl along the luminal surface of leptomeningeal vessels without showing calcium activity. Having crossed the blood-brain barrier, the T cells scan the leptomeningeal space for autoantigen-presenting cells (APCs). Sustained contacts result in long-lasting calcium activity and NFAT translocation, a measure of full T-cell activation. This process is sensitive to anti-MHC class II antibodies. Importantly, the capacity to activate T cells is not a general property of all leptomeningeal phagocytes, but varies between individual APCs. Our results identify distinct checkpoints of T-cell activation, controlling the capacity of myelin-specific T cells to invade and attack the CNS. These processes may be valuable therapeutic targets.autoimmunity | intracellular calcium | T-cell activation | central nervous system | two-photon imaging

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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 Ca2+ sensor for the quantification of the intracellular Ca2+ 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