Localizing central nervous system immune surveillance: meningeal antigen-presenting cells activate T cells during experimental autoimmune encephalomyelitis.

The onset of neurological signs in experimental autoimmune encephalomyelitis is tightly associated with infiltration and reactivation of T cells in the central nervous system. The anatomic localization of the initial T cell-antigen-presenting cell (APC) interactions leading to reactivation of T cells in the central nervous system is, however, still unclear. We hypothesized that activated CD4(+) T cells gain direct access to the subarachnoid space and become reactivated on encounter with cognate antigen in this compartment. C57Bl/6 mice were immunized with MOG35-55, and interactions between CD4(+) T cells and major histocompatibility class II+ APCs in the subarachnoid space were investigated using flow cytometry, confocal microscopy of leptomeningeal whole-mount preparations, time-lapse microscopy of leptomeningeal explants, and in vitro proliferation assays. CD4(+) T cells, polarized to produce Th1/Th17 cytokines, accumulated in the subarachnoid space early during the course of experimental autoimmune encephalomyelitis, before CD4(+) T cells were detected in the spinal cord parenchyma. At this time point, leptomeningeal but not parenchymal CD4(+) T cells incorporated bromodeoxyuridine, indicating local proliferation of CD4(+) T cells in the subarachnoid space. Time-lapse microscopy indicated that these CD4(+) T cells actively scanned the tissue and interacted with local major histocompatibility class II+ APCs, resulting in long-lasting interactions between CD4(+) T cells and major histocompatibility class II+ APCs, suggestive of immunological synapses. These results support the concept that immune surveillance of the central nervous system involves the subarachnoid space and indicate that the leptomeninges play an important role in experimental autoimmune encephalomyelitis initiation.