Patients With Severe Multiple Sclerosis Exhibit Functionally Altered CD8(+) Regulatory T Cells

BACKGROUND AND OBJECTIVES: Multiple sclerosis (MS) is a chronic inflammatory and demyelinating disease of the CNS. Studies of immune dysfunction in MS have mostly focused on CD4(+) Tregs, but the role of CD8(+) Tregs remains largely unexplored. We previously evidenced the suppressive properties of rat and human CD8(+)CD45RC(low/neg) Tregs from healthy individuals, expressing Forkhead box P3 (FOXP3) and acting through interferon-gamma (IFN-γ), transforming growth factor beta (TGFβ), and interleukin-34 (IL-34). secretions to regulate immune responses and control diseases such as transplant rejection. To better understand CD8(+)CD45RC(low/neg) Tregs contribution to MS pathology, we further investigated their phenotype, function, and transcriptome in patients with MS. METHODS: We enrolled adults with relapsing-remitting MS and age-matched and sex-matched healthy volunteers (HVs). CD8(+) T cells were segregated based on low or lack of expression of CD45RC. First, the frequency in CSF and blood, phenotype, transcriptome, and function of CD8(+)CD45RC(low) and (neg) were investigated according to exacerbation status and secondarily, according to clinical severity based on the MS severity score (MSSS) in patients with nonexacerbating MS. We then induced active MOG(35-55) EAE in C57Bl/6 mice and performed adoptive transfer of fresh and expanded CD8(+)CD45RC(neg) Tregs to assess their ability to mitigate neuroinflammation in vivo. RESULTS: Thirty-one untreated patients with relapsing-remitting MS were compared with 40 age-matched and sex-matched HVs. We demonstrated no difference of CSF CD8(+)CD45RC(low) and CD8(+)CD45RC(neg) proportions, but blood CD8(+)CD45RC(low) frequency was lower in patients with MS exacerbation when compared with that in HVs. CD8(+)CD45RC(neg) Tregs but not CD8(+)CD45RC(low) showed higher suppressive capacities in vitro in MS patients with exacerbation than in patients without acute inflammatory attack. In vitro functional assays showed a compromised suppression capacity of CD8(+)CD45RC(low) Tregs in patients with nonexacerbating severe MS, defined by the MSSS. We then characterized murine CD8(+)CD45RC(neg) Tregs and demonstrated the potential of CD45RC(neg) cells to migrate to the CNS and mitigate experimental autoimmune encephalomyelitis in vivo. DISCUSSION: Altogether, these results suggest a defect in the number and function of CD8(+)CD45RC(low) Tregs during MS relapse and an association of CD8(+)CD45RC(low) Tregs dysfunction with MS severity. Thus, CD8(+)CD45RC(low/neg) T cells might bring new insights into the pathophysiology and new therapeutic approaches of MS.