To our knowledge, these studies are the first evidence that human autoreactive CNS-specific CD8+ T-cells play an immune regulatory role, in contrast to foreign-antigen-specific effectors. Moreover, our studies show a clear clinical relevance for this regulatory role, in that suppressive activity is greatly diminished during relapses of MS and recovered as the patients enter remission. Classically, CNS-targeted, MHC Class I-restricted CD8+ T-cells are thought to have a pathogenic role in disease, with reports demonstrating in vitro
cytotoxic killing of oligodendrocytes [8
]. However, our studies identify an unexpected and novel immune regulatory role in both HC and quiescent MS patients, corroborating studies in EAE, where CNS-specific CD8+ T-cells inhibited disease, whereas control antigen-specific ones did not [15
]. Sporadic, acute exacerbations are characteristic of the relapsing-remitting form of MS. While MS suppressor cell dysfunction has been recognized for decades, the role of CNS-specific CD8+ T-cells remains elusive in the context of accumulating disability, axon trans-section, and gliosis which are characteristic of secondary progressive MS [33
]. It appears that CNS-specific regulatory ability is directly or indirectly involved in the mechanism of MS clinical phase changes. It still remains unclear whether underlying pathology of chronic progressive MS exhibit similar deficient suppressor CD8+ T-cell activity [25
We are only beginning to understand the role of autoreactive, regulatory (“autoregulatory”) T-cells in autoimmune disease [35
]. There has been some evidence that autoantigen-specific CD8+ T-cells may have immune regulatory properties in diabetes models [36
]. Thus, chronic stimulation of CD8+ T-cells with low TCR avidity may induce regulatory function [37
], perhaps explaining the therapeutic generation of antigen-specific, cytotoxic immune suppressor CD8+ T-cells following chronic copolymer-based therapy of MS [14
]. This may also explain the difference between the roles of foreign-antigen-specific CD8+ T-cells vs. autoreactive ones that tend to bear lower avidity TCR, presumably following thymic deletion of higher avidity responders. In contrast to Qa1/HLA-E-restricted suppressor CD8+ T-cells that recognize immune cell-derived peptides, autoregulatory CD8+ T-cells are stimulated by the same tissue antigens that are targets of destructive effector cells, thereby creating an autoregulatory tolerance loop.
The assay system utilized in our studies took advantage a proliferation dye (CFSE), a cellular tracking dye (CMTPX) and overlapping antigenic peptide pools to monitor neuroantigen-specific CD8+ T-cell suppressive ability. This assay has excellent sensitivity and specificity for detecting functional antigen-specific suppressive ability, by allowing the exclusion of suppressor populations from the analysis. Moreover, it allows an unbiased characterization of T-cell suppressive ability without limited range of HLA haplotype or epitopes. Finally, the assay also enables separate concurrent characterization of CD4+ and CD8+ responses within the same culture. Thus, this unique approach allowed us to discover and quantify this novel autoregulatory function of CNS-specific CD8+ T-cells.
This novel concept also unveils a potential strategy for immune therapy. While using autoreactive CD8+ T-cells as therapy may seem unorthodox, this is principally similar to generating autoantigen-reactive CD4+CD25+FOXP3+ Treg
s for adoptive immunotherapy. Other forms of autoreactive CD4+ Treg
s (Tr1, Th3) have also shown promise in animal models. CD8+ T-cells, representing an underappreciated arm of peripheral immune tolerance, afford an attractive form of adoptive immunotherapy, especially in the context of clinical relapses. In that regard, we have shown recently that CNS-specific CD8+ T-cells can inhibit ongoing EAE [15
], dependent on cytotoxic and immune modulatory mechanisms. The phenotypic characteristics of regulatory CD8+ T-cells are not definite and, depending on the model, may range from a CD28(-) [38
], γδ+ [45
], CD25+ [46
], CD122+ [16
], CD103+ [47
], PD-1+ [49
] or FOXP3+ [50
], among others [54
]. In which context neuroantigen-specific CD8+ T-cells regulate, and how, is still unclear. Our preliminary studies reveal autoregulatory CNS-specific suppressor activity in multiple such subsets, with the common features being cytokine- and contact-dependent processes (including cytotoxicity) and an absolute requirement for HLA-Class I (data not shown). Detailed dissection of the characteristics and mechanisms of these cells will be an important pursuit to develop a therapeutic approach.
To summarize, our studies demonstrate a novel, clinically relevant role for neuroantigen-specific CD8+ T-cells, revealing a potential pathway of intrinsic immune regulation that may have implications for the therapy of human MS and other immune-mediated disorders.