In contrast to conventional Tregs, Tr1 cells do not express the transcription factor Foxp3 and only a transient expression of CD25; however, they co-expressed CD49b, LAG-3, and CD226 cell-surface markers in humans and mice [
22]. C-MAF, AhR, BAFT, and IRF1 are critical transcription factors for Tr1 cell differentiation [
23,
24]. Tr1 cells exert their immunosuppressive effects through IL-10 expression and by killing effector cells via Granzyme-B and Perforin (Fig.
1) [
6]. Several studies have reported the critical involvement of IL-10 cytokine in the suppression of EAE models associated with an increase in Tr1 cells [
25]. In EAE, the transfer of
in vitro generated OVA-specific Tr1 cells prevents the development of neurological symptoms when OVA peptide is injected intracranially [
26]. Moreover,
in vivo induction of Tr1 cells with soluble myelin basic protein (MBP) reverses ongoing disease in rats immunized with MBP [
27]. Regulatory Tr1 cells regulate EAE partially through a mechanism involving IL-10 [
28]. Myelin-specific Tr1 cells injected mice-mediated delay onset of EAE associated with a reduction in the severity of clinical signs [
29]. Another challenge is to successfully induce regulatory T cells directly
in vivo to avoid T cell transfer that is challenging in humans. Interestingly, nasal anti-CD3 administration can induce Tr1-like T cells
in vivo that are further able to constrain inflammation in the progressive animal model of multiple sclerosis in an IL-10-dependent manner by regulating astrocytes and microglia function [
30]. Tr1 cells also display immunosuppressive functions in the human setting. Tr1 cells isolated from MS patients display impaired IL-10 production and altered IL-10-mediated suppressive effects when compared with healthy controls [
31,
32]. In this line, considering that Tr1 function is impaired in MS, some studies investigated the effect of Tr1 differentiation by tolerogenic dendritic cells (tolDCs). A phase 1b clinical trial showed the feasibility and safety of treating a patient with MS with tolDCs loaded with myelin peptides to increase IL-10 levels in PBMCs as well as the frequency of Tr1 cells [
33]. Furthermore, tolDCs exert some of their effects through the cytokine IL-27 that has been identified as a key inducer of Tr1 and inhibitor of Th17 during autoimmunity [
6,
24]. IL-27 plays a suppressive role during EAE as demonstrated by more severe disease in IL-27R-deficient mice [
34]. Furthermore, IL-27 treatment reduces the severity of EAE by a mechanism dependent on IL-10 [
35]. Relevant to the human disease, the beneficial impact of IFNβ, first-line therapy for relapsing-remitting MS, is associated with IL-27 induction, which promotes the production of IL10 by dendritic cells [
36]. Finally, IL-27 is expressed by astrocytes in brain biopsies of human MS lesions suggesting that IL-27 regulates T cell response also locally within the brain of MS patients [
37]. Therefore, IL-27 (acting on Tr1 cells) plays a critical role in controlling autoimmunity, providing a putative target therapy.