Regulatory T cells suppress Th17 cell Ca2+ signaling in the spinal cord during murine autoimmune neuroinflammation

T lymphocyte motility and interaction dynamics with other immune cells are vital determinants of immune responses. Regu-latory T (Treg) cells prevent autoimmune disorders by suppressing excessive lymphocyte activity, but how interstitial motility pat-terns of Treg cells limit neuroinflammation is not well understood. We used two-photon microscopy to elucidate the spatial organi-zation, motility characteristics, and interactions of endogenous Treg and Th17 cells together with antigen-presenting cells (APCs) within the spinal cord leptomeninges in experimental autoim-mune encephalomyelitis (EAE), an animal model of multiple sclerosis. Th17 cells arrive before the onset of clinical symptoms, distribute uniformly during the peak, and decline in numbers during later stages of EAE. In contrast, Treg cells arrive after Th17 cells and persist during the chronic phase. Th17 cells meander widely, interact with APCs, and exhibit cytosolic Ca2+ transients and elevated basal Ca2+ levels before the arrival of Treg cells. In contrast, Treg cells adopt a confined, repetitive-scanning motility while contacting APCs. These locally confined but highly motile Treg cells limit Th17 cells from accessing APCs and suppress Th17 cell Ca2+ signaling by a mechanism that is upstream of store-operated Ca2+ entry. Finally, Treg cell depletion increases APC numbers in the spinal cord and exaggerates ongoing neuroinflam-mation. Our results point to fundamental differences in motility characteristics between Th17 and Treg cells in the inflamed spinal cord and reveal three potential cellular mechanisms by which Treg cells regulate Th17 cell effector functions: reduction of APC den-sity, limiting access of Th17 cells to APCs, and suppression of Th17 Ca2+ signaling.