CD8+ CD122+ regulatory T cells contain clonally expanded cells with identical CDR3 sequences of the T-cell receptor β-chain

An angel or a devil? Current view on the role of CD8+ T cells in the pathogenesis of myasthenia gravis

BACKGROUND

Myasthenia gravis (MG) and the experimental autoimmune MG (EAMG) animal model are characterized by T-cell-induced and B-cell-dominated autoimmune diseases that affect the neuromuscular junction. Several subtypes of CD4+ T cells, including T helper (Th) 17 cells, follicular Th cells, and regulatory T cells (Tregs), contribute to the pathogenesis of MG. However, increasing evidence suggests that CD8+ T cells also play a critical role in the pathogenesis and treatment of MG.

MAIN BODY

Herein, we review the literature on CD8+ T cells in MG, focusing on their potential effector and regulatory roles, as well as on relevant evidence (peripheral, in situ, cerebrospinal fluid, and under different treatments), T-cell receptor usage, cytokine and chemokine expression, cell marker expression, and Treg, Tc17, CD3+CD8+CD20+ T, and CXCR5+ CD8+ T cells.

CONCLUSIONS

Further studies on CD8+ T cells in MG are necessary to determine, among others, the real pattern of the Vβ gene usage of autoantigen-specific CD8+ cells in patients with MG, real images of the physiology and function of autoantigen-specific CD8+ cells from MG/EAMG, and the subset of autoantigen-specific CD8+ cells (Tc1, Tc17, and IL-17+IFN-γ+CD8+ T cells). There are many reports of CD20-expressing T (or CD20 + T) and CXCR5+ CD8 T cells on autoimmune diseases, especially on multiple sclerosis and rheumatoid arthritis. Unfortunately, up to now, there has been no report on these T cells on MG, which might be a good direction for future studies.

Cellular and Molecular Phenotypes of pConsensus Peptide (pCons) Induced CD8+ and CD4+ Regulatory T Cells in Lupus

Systemic lupus erythematosus (SLE) is a chronic autoimmune disease with widespread inflammation, immune dysregulation, and is associated with the generation of destructive anti-DNA autoantibodies. We have shown previously the immune modulatory properties of pCons peptide in the induction of both CD4⁺ and CD8⁺ regulatory T cells which can in turn suppress development of the autoimmune disease in (NZB/NZW) F1 (BWF1) mice, an established model of lupus. In the present study, we add novel protein information and further demonstrate the molecular and cellular phenotypes of pCons-induced CD4⁺ and CD8⁺ T_reg subsets. Flow cytometry analyses revealed that pCons induced CD8⁺ T_reg cells with the following cell surface molecules: CD25^highCD28^{high and low} subsets (shown earlier), CD62L^high, CD122^low, PD1^low, CTLA4^low, CCR7^low and 41BB^high. Quantitative real-time PCR (qRT-PCR) gene expression analyses revealed that pCons-induced CD8⁺ T_reg cells downregulated the following several genes: Regulator of G protein signaling (RGS2), RGS16, RGS17, BAX, GPT2, PDE3b, GADD45β and programmed cell death 1 (PD1). Further, we confirmed the down regulation of these genes by Western blot analyses at the protein level. To our translational significance, we showed herein that pCons significantly increased the percentage of CD8⁺FoxP3⁺ T cells and further increased the mean fluorescence intensity (MFI) of FoxP3 when healthy peripheral blood mononuclear cells (PBMCs) are treated with pCons (10 μg/ml, for 24-48 hours). In addition, we found that pCons reduced apoptosis in CD4⁺ and CD8⁺ T cells and B220⁺ B cells of BWF1 lupus mice. These data suggest that pCons stimulates cellular, immunological, and molecular changes in regulatory T cells which in turn protect against SLE autoimmunity.

CD8+ Regulatory T Cell - A Mystery to Be Revealed

Regulatory T cells (Treg) are essential to maintain immune homeostasis and prevent autoimmune disorders. While the function and molecular regulation of Foxp3+CD4+ Tregs are well established, much of CD8+ Treg biology remains to be revealed. Here, we will review the heterogenous subsets of CD8+ T cells have been named "CD8+ Treg" and mainly focus on CD122hiLy49+CD8+ Tregs present in naïve mice. CD122hiLy49+CD8+ Tregs, which depends on transcription factor Helios and homeostatic cytokine IL-15, have been established as a non-redundant regulator of germinal center (GC) reaction. Recently, we have demonstrated that TGF-β (Transforming growth factor-β) and transcription factor Eomes (Eomesodermin) are essential for the function and homeostasis of CD8+ Tregs. In addition, we will discuss several open questions regarding the differentiation, function and true identity of CD8+ Tregs as well as a brief comparison between two regulatory T cell subsets critical to control GC reaction, namely CD4+ TFR (follicular regulatory T cells) and CD8+ Tregs.

Description of CD8+ Regulatory T Lymphocytes and Their Specific Intervention in Graft-versus-Host and Infectious Diseases, Autoimmunity, and Cancer

Gershon and Kondo described CD8⁺ Treg lymphocytes as the first ones with regulating activity due to their tolerance ability to foreign antigens and their capacity to inhibit the proliferation of other lymphocytes. Regardless, CD8⁺ Treg lymphocytes have not been fully described-unlike CD4⁺ Treg lymphocytes-because of their low numbers in blood and the lack of specific and accurate population markers. Still, these lymphocytes have been studied for the past 30 years, even after finding difficulties during investigations. As a result, studies have identified markers that define their subpopulations. This review is focused on the expression of cell membrane markers as CD25, CD122, CD103, CTLA-4, CD39, CD73, LAG-3, and FasL as well as soluble molecules such as FoxP3, IFN-γ, IL-10, TGF-β, IL-34, and IL-35, in addition to the lack of expression of cell activation markers such as CD28, CD127 CD45RC, and CD49d. This work also underlines the importance of identifying some of these markers in infections with several pathogens, autoimmunity, cancer, and graft-versus-host disease as a strategy in their prevention, monitoring, and cure.

CD8+CD122+CD49dlow regulatory T cells maintain T-cell homeostasis by killing activated T cells via Fas/FasL-mediated cytotoxicity

The Fas/FasL (CD95/CD178) system is required for immune regulation; however, it is unclear in which cells, when, and where Fas/FasL molecules act in the immune system. We found that CD8(+)CD122(+) cells, which are mostly composed of memory T cells in comparison with naïve cells in the CD8(+)CD122(-) population, were previously shown to include cells with regulatory activity and could be separated into CD49d(low) cells and CD49d(high) cells. We established in vitro and in vivo experimental systems to evaluate the regulatory activity of CD122(+) cells. Regulatory activity was observed in CD8(+)CD122(+)CD49d(low) but not in CD8(+)CD122(+)CD49d(high) cells, indicating that the regulatory cells in the CD8(+)CD122(+) population could be narrowed down to CD49d(low) cells. CD8(+)CD122(-) cells taken from lymphoproliferation (lpr) mice were resistant to regulation by normal CD122(+) Tregs. CD122(+) Tregs taken from generalized lymphoproliferative disease (gld) mice did not regulate wild-type CD8(+)CD122(-) cells, indicating that the regulation by CD122(+) Tregs is Fas/FasL-dependent. CD122(+) Tregs taken from IL-10-deficient mice could regulate CD8(+)CD122(-) cells as equally as wild-type CD122(+) Tregs both in vitro and in vivo. MHC class I-missing T cells were not regulated by CD122(+) Tregs in vitro. CD122(+) Tregs also regulated CD4(+) cells in a Fas/FasL-dependent manner in vitro. These results suggest an essential role of Fas/FasL as a terminal effector of the CD122(+) Tregs that kill activated T cells to maintain immune homeostasis.

CD8(+)CD122(+) T-Cells: A Newly Emerging Regulator with Central Memory Cell Phenotypes

CD8(+)CD122(+) T-cells have been traditionally described as antigen-specific memory T-cells that respond to previously encountered antigens more quickly and vigorously than their naïve counterparts. However, mounting evidence has demonstrated that murine CD8(+)CD122(+) T-cells exhibit a central memory phenotype (CD44(high)CD62L(high)), regulate T cell homeostasis, and act as regulatory T-cells (Treg) by suppressing both autoimmune and alloimmune responses. Importantly, naturally occurring murine CD8(+)CD122(+) Tregs are more potent in immunosuppression than their CD4(+)CD25(+) counterparts. They appear to be acting in an antigen-non-specific manner. Human CD8(+)CXCR3(+) T-cells are the equivalent of murine CD8(+)CD122(+) Tregs and also exhibit central memory phenotypes. In this mini-review article, we will summarize recent progresses in their phenotypes, homeostatic expansion, antigen-specificity, roles in the suppression of alloimmune and autoimmune responses, and the mechanisms underlying their inhibitory function.

Regulatory CD8(+)CD122 (+) T-cells predominate in CNS after treatment of experimental stroke in male mice with IL-10-secreting B-cells

Clinical stroke induces inflammatory processes leading to cerebral and splenic injury and profound peripheral immunosuppression. IL-10 expression is elevated during major CNS diseases and limits inflammation in the brain. Recent evidence demonstrated that transfer of IL-10(+) B-cells reduced infarct volume in male C57BL/6J (wild-type, WT) recipient mice when given 24 h prior to or 4 h after middle cerebral artery occlusion (MCAO). The purpose of this study was to determine if passively transferred IL-10(+) B-cells can exert therapeutic and immunoregulatory effects when injected 24 h after MCAO induction in B-cell-sufficient male WT mice. The results demonstrated that IL-10(+) B-cell treated mice had significantly reduced infarct volumes in the ipsilateral cortex and hemisphere and improved neurological deficits vs. Vehicle-treated control mice after 60 min occlusion and 96 h of reperfusion. The MCAO-protected B-cell recipient mice had less splenic atrophy and reduced numbers of activated, inflammatory T-cells, decreased infiltration of T-cells and a less inflammatory milieu in the ischemic hemispheres compared with Vehicle-treated control mice. These immunoregulatory changes occurred in concert with the predominant appearance of IL-10-secreting CD8(+)CD122(+) Treg cells in both the spleen and the MCAO-affected brain hemisphere. This study for the first time demonstrates a major neuroprotective role for IL-10(+) B-cells in treating MCAO in male WT mice at a time point well beyond the ~4 h tPA treatment window, leading to the generation of a dominant IL-10(+)CD8(+)CD122(+) Treg population associated with spleen preservation and reduced CNS inflammation.

Identification of peptide-specific TCR genes by in vitro peptide stimulation and CDR3 length polymorphism analysis

Identification of TCR genes specific for tumor-associated antigens (TAAs) is necessary for TCR gene modification of T cells, which is applied in anti-tumor adoptive T cell therapy (ACT). The usual identification methods are based on isolating single peptide-responding T cells and cloning the TCR gene by in vitro expansion or by single-cell RT-PCR. However, the long and exacting in vitro culture period and demanding operational requirements restrict the application of these methods. Immunoscope is an effective tool that profiles a repertoire of TCRs and identifies significantly expanded clones through CDR3 length analysis. In this study, a survivin-derived mutant peptide optimized for HLA-A2 binding was selected to load DCs and activate T cells. The monoclonal expansion of TCRA and TCRB genes was separately identified by Immunoscope analysis and following sequence identification, the properly paired TCR genes were transferred into T cells. Peptide recognition and cytotoxicity assays indicated that TCR-modified PBMCs could respond to both the mutant and wild type peptides and lyse target cells. These results show that combining Immunoscope with in vitro peptide stimulation provides an alternative and superior method for identifying specific TCR genes, which represents a significant advance for the application of TCR gene-modified T cells.

A naturally occurring CD8(+)CD122(+) T-cell subset as a memory-like Treg family

Despite extensive studies on CD4(+)CD25(+) regulatory T cells (Tregs) during the past decade, the progress on their clinical translation remains stagnant. Mounting evidence suggests that naturally occurring CD8(+)CD122(+) T cells are also Tregs with the capacity to inhibit T-cell responses and suppress autoimmunity as well as alloimmunity. In fact, they are memory-like Tregs that resemble a central memory T cell (TCM) phenotype. The mechanisms underlying their suppression are still not well understood, although they may include IL-10 production. We have recently demonstrated that programmed death-1 (PD-1) expression distinguishes between regulatory and memory CD8(+)CD122(+) T cells and that CD8(+)CD122(+) Tregs undergo faster homeostatic proliferation and are more potent in the suppression of allograft rejection than conventional CD4(+)CD25(+) Tregs. These findings may open a new line of investigation for accelerating effective Treg therapies in the clinic. In this review, we summarize the significant progress in this promising field of CD8(+)CD122(+) Treg research and discuss their phenotypes, suppressive roles in autoimmunity and alloimmunity, functional requirements, mechanisms of action and potential applications in the clinic.

Natural CD8+CD122+ T cells are more potent in suppression of allograft rejection than CD4+CD25+ regulatory T cells

Despite extensive studies on CD4+CD25+ regulatory T cells (Tregs), their application in adoptive transfer therapies is still not optimal in immune-competent wild-type (WT) animal models. Therefore, it is compelling to search for more potent Tregs for potential clinical application. Mounting evidence has shown that naturally occurring CD8+CD122+ T cells are also Tregs. However, their suppression in allograft rejection, efficiency in suppression and underlying mechanisms remain unclear. Using a murine allotransplantation model, we reported here that CD8+CD122+ Tregs were actually more potent in suppression of allograft rejection and underwent more rapid homeostatic proliferation than their CD4+CD25+ counterparts. Moreover, they produced more IL-10 and were more potent in suppressing T cell proliferation in vitro. Deficiency in IL-10 in CD4+CD25+ and CD8+CD122+ Tregs resulted in their reduced but equal suppression in vivo and in vitro, suggesting that IL-10 is responsible for more effective suppression by CD8+CD122+ than CD4+CD25+ Tregs. Importantly, transfer of CD8+CD122+ Tregs together with the administration of recombinant IL-15 significantly prolonged allograft survival in WT mice. Thus, for the first time, we demonstrate that naturally arising CD8+CD122+ Tregs not only inhibit allograft rejection but also exert this suppression more potently than their CD4+CD25+ counterparts. This novel finding may have important implications for tolerance induction.