To unravel CNS-specific effects of regulatory Foxp3+ T cells (Tregs) in experimental neuroinflammation.

Pathology driving T helper (Th)17 cells can be suppressed in their activity and effector mechanisms. However, it is not known whether regulating mechanisms succeed once pathologic T cells have entered the CNS.

We applied intra-vital two-photon laser scanning microscopy, flow cytometry, histology and RNA-sequencing in Th17 adoptive transfer models.

Strikingly, suppressive effects of Tregs on autoreactive T cells in the CNS are strictly antigen-specific. While transfer of antigen-specific Tregs ameliorated or even prevented experimental autoimmune encephalomyelitis (EAE) symptoms, non-antigen-specific Tregs failed to do so. Using live imaging, functional heterogeneity of Th17 cells and Tregs is characterized by largely distinct motility profiles and interaction parameters, whereby Th17 cells screen and interact with potential targets and Tregs reveal a place-bound behavior. As in other proinflammatory environments, suppressive effects of Tregs are supported by increased motility and reduced overall contacts of Th17 cells while undergoing frequent transient interactions with Tregs. In contrast, Tregs formed preferentially stable and long-lasting contacts with microglia cells. RNA-sequencing of microglia isolated from EAE mice revealed distinct phenotypical changes induced by Treg contacts.

Instead of directly influencing T effector cells as known from the peripheral immune system, antigen-specific Tregs mediate their suppressive effect in the inflamed CNS via modulation of microglia activity.