Secondary Logo

Share this article on:

Advances on Non-CD4 + Foxp3+ T Regulatory Cells: CD8+, Type 1, and Double Negative T Regulatory Cells in Organ Transplantation

Ligocki, Ann J. PhD1; Niederkorn, Jerry Y. PhD1

doi: 10.1097/TP.0000000000000813
Review

The overwhelming body of research on T regulatory cells (Treg) has focused on CD4 + CD25 + Foxp3+ T cells. However, recent years have witnessed a resurgence in interest in CD4 − CD8+, CD4 − CD8− (double negative [DN]), and CD4 + Foxp3− type 1 Treg (Tr1) Treg and their role in controlling autoimmune diseases and in promoting the survival of organ allografts and xenografts. CD8+ and DN Treg can arise spontaneously (natural Treg) or can be induced in situ. Both CD8+ and DN Treg have been shown to enhance the survival of organ allografts and xenografts. Additionally, both can suppress alloimmune responses by contact-dependent mechanisms by either inducing apoptosis or mediating direct cytolysis of effector T cells. CD8+, DN, and Tr1 Treg can also act in a contact-independent manner by elaborating soluble immunosuppressive factors, such as TGF-β and IL-10. Applying CD8+, DN, and Tr1 Treg for enhancing the survival of organ allografts and xenografts is still in its infancy but holds significant potential. Furthermore, there is a need for a more comprehensive understanding of how current immunosuppressive therapies applied to organ transplantations affect the wide array of Treg populations.

Ligocki and Niederkorn provide a timely review on the role of non-canonical FoxP3-negative regulatory T cells. Defining the role of these cells in transplantation is at its infancy but tantalizing data suggest that these cells may have significant therapeutic potential.

1 Department of Ophthalmology, University of Texas Southwestern Medical Center, Dallas, TX.

Received 2 January 2015. Revision requested 6 April 2015.

Accepted 20 April 2015.

Funding: Supported by NIH grants EY007641, EY005631, and EY020799 and Research to Prevent Blindness.

The authors declare no conflicts of interest.

A.J.L. and J.Y.N. contributed to the writing of the article.

Correspondence: Jerry Y. Niederkorn, Department of Ophthalmology, U.T. Southwestern Medical Center, 5323 Harry Hines Boulevard, Dallas, TX 75390-9057. (jerry.niederkorn@utsouthwestern.edu).

Shortly after the role of T cells in protective immunity was recognized, Gershon and coworkers1 proposed that a subpopulation of T cells might be capable of suppressing the immune response. Accordingly, the term “suppressor T cell” was coined to describe T cells that were responsible for processes that were previously categorized as tolerance, antigen competition, or feedback regulation. Initially, suppressor cells were defined largely by their in vitro functional properties. However, the suppressor cell concept quickly fell out of favor due to the absence of distinct surface markers or transcription factors unique to this cell population along with the inability of suppressor cell investigators to identify genes in the putative I-J region in the immune response region of the murine major histocompatibility complex (MHC) implicated in suppressor cell development.2 As a result, suppressor cell research was largely abandoned for over a decade until it was resurrected by Sakaguchi et al3 who provided compelling evidence for the presence of CD4 + CD25+ suppressor cells. However, this time around, suppressor cells wore the euphemistic moniker T regulatory cells (Treg). Since Sakaguchi's report, hundreds of publications have described and analyzed CD4+ Treg and the overwhelming majority of these publications have focused on conventional CD4 + CD25 + Foxp3+ Treg. Because numerous review articles have addressed the biology and immunologic properties of CD4 + CD25 + Foxp3+ Treg, this review will focus on non-CD4 + CD25 + Foxp3+ Treg in the context of organ transplantation.

Back to Top | Article Outline

CD8+ Treg

CD8+ Treg Phenotypes

The phenotype of Gershon's original suppressor cell was defined by its expression of the CD8 (Lyt-2) surface marker.4 However, in the 40 years since their original description, CD8+ Treg have been categorized into several distinct phenotypes including: (a) Qa-1/HLA-restricted Treg5-7; (b) CD8 + CD122+ Treg8; (c) CD8 + CD28− Treg9; (d) CD8 + Foxp3+ Treg10; (e) CD8 + CD103+ Treg11; (f) CD8 + LAG-3 + Foxp3 + cytotoxic T lymphocyte-associated protein (CTLA-4)+ Treg12; (g) CD8 + IL-10 + CCR7 + CD45RO+ Treg13; (h) CD8 + CD45RClow Treg14; (i) CD8 + CD122 + PD-1+ Treg,15 and (j) CD8 + CD11chigh Treg16 (Table 1). CD8+ Treg can arise within the thymus as naturally occurring CD8+ Treg or they can be induced in peripheral tissues by a variety of maneuvers including donor-specific transfusion,21 injection of antigen into the anterior chamber (AC) of the eye,22 anti-idiotype immunization with allospecific T cells or MHC-derived peptides,23,24 or by in vivo immunization combined with blockade of CD40 costimulatory molecules.25 Expression of the IL-2 receptor β subunit, CD122, is crucial for CD8+ Treg development and function.7 Although CD122+ is also expressed on classical CD8+ memory T cells, expression of PD-1 on CD8 + CD122+ T cells is limited to Treg and distinguishes Treg from memory T cells.26 In addition to the surface markers mentioned above, CD8+ Treg can also express CD44 and the natural killer (NK) cell inhibitory marker, Ly49.

TABLE 1

TABLE 1

Back to Top | Article Outline

CD8+ Treg Mechanisms of Suppression

CD8+ Treg suppression can be mediated by either contact-dependent27 or contact-independent mechanisms28 (Table 2). Contact-dependent suppression by CD8+ Treg involves the direct killing of CD4+ T effector cells by perforin-mediated cytolysis29,30 or Fas ligand (FasL)-induced apoptosis.16 In addition to suppressing effector T cells, CD8 + Foxp3+ Treg are capable of inducing the de novo generation of CD4 + Foxp3+ Treg by a process that is contact-dependent and requires the production of soluble TGF-β,31 and is reminiscent of a phenomenon that has been previously described as “infectious tolerance.” When CD8+ Treg are in contact with CD4+, T cell suppression is supported by IFN-γ, indoleamine 2,3 dioxygenase, and fibroleukin-2.28 However, indoleamine 2,3 dioxygenase can also mediate suppression when CD8+ Treg are not in direct contact with CD4+ effector T cells by inhibiting T cell proliferation.28 CD8+ Treg can also suppress immune effector responses by elaborating a variety of soluble factors, such as TGF-β and IL-10, which inhibit T cell activation and proliferation.32,33

TABLE 2

TABLE 2

Recent investigations on CD8+ Treg have demonstrated the importance of the nonclassic MHC Ib molecules Qa-1 (mouse) and HLA-E (human) in the induction of CD8+ Treg.7 Engagement of Qa-1-Qdm peptides expressed on CD4+ Th1 cells with its receptor, NKG2A/CD94 on CD8+ T cells leads to the generation of CD8+ Treg.7 Qa-1–restricted CD8+ Treg are believe to arise in the thymus and play a central role in the maintenance of self-tolerance.7 They can also be induced in the periphery by T-cell vaccination34-36 or by introducing alloantigens into the AC of the eye.37,38

Back to Top | Article Outline

CD8+ Treg and Transplantation

The majority of studies on Treg have focused on CD4 + CD25+ Treg and it has been assumed by many that CD8+ Treg were minor players in the maintenance of self-tolerance and in the promotion of allograft survival. However, recent evidence suggests that CD8 + CD122+ naturally occurring Treg play a crucial role in immune homeostasis and contribute to allograft survival15,26,39-41 (Table 3). In fact, there is compelling evidence that CD8+ Treg are more potent than conventional CD4 + CD25+ Treg in suppressing pancreatic islet allograft rejection.39 In 1 study, CD8 + CD122+ Treg underwent swifter expansion, produced more IL-10, and suppressed T cell proliferation in vitro more effectively than their CD4 + CD25+ Treg counterparts.39 Importantly, adoptively transferred CD8 + CD122+ Treg were able to prolong the survival of pancreatic islet allografts, whereas similar adoptive transfers of CD4 + CD25+ Treg were unable to prolong allograft survival.39 CD8 + CD28− Treg can promote the generation of tolerogenic dendritic cells (DCs) and are expanded in heart allograft recipients.44 A mouse study of allogenic stem cell transplantation revealed that in addition to the conventional role of CD4+ Foxp3+ Treg in this model, CD8 + Foxp3+ Treg were able to prevent graft-versus-host disease mortality.57

TABLE 3

TABLE 3

CD8+ Treg may also be one of the key elements sustaining immune privilege in the eye. Nominal antigens or alloantigens introduced into the AC of the eye elicit the generation of antigen-specific CD8+ Treg through a process called AC-associated immune deviation.61 Interestingly, 2 populations of Treg are induced after AC injection of alloantigens, CD4+ Treg and CD8+ Treg.22 The CD8+ Treg that are generated in the spleen after AC injection of antigen suppress both ocular and systemic immune responses by elaborating TGF-β, IL-10, and IFN-γ.61,62 Additionally, CD8+ Treg are also induced in situ by cells lining the AC of the eye. Corneal endothelial cells express membrane-bound TGFβ2, which directly inhibits the activation of effector CD8+ T cells in situ and also induces the conversion of effector CD8+ T cells to Treg63 that elaborate TGFβ1. The pigmented epithelial cells of the iris also induce the generation of CD8+ Treg by B7-1/B7-2 interaction with CTLA-4 on effector T cells.64,65 It bears noting that injection of donor-specific alloantigenic cells into the AC of the eye produces AC-associated immune deviation and dramatically enhances corneal allograft survival.66,67

While most studies have focused on CD4 + CD25 + Foxp3+ Treg in promoting allograft survival, there is a sizeable body of evidence that CD8+ Treg can also enhance the survival of skin,25,42 heart,21,25,28,44,45 pancreatic islet,25,39 and kidney53 allografts and heart25 and corneal24 xenografts, and hematopoietic allografts57 (Table 3).

Back to Top | Article Outline

T Cell Receptorαβ + CD4CD8 Double Negative Treg

Double Negative Treg Phenotypes

In 1989, Strober and colleagues68 described and cloned a population of spleen cells that did not express either CD4 or CD8, but could suppress T cell proliferation in vitro. The CD4, CD8 T cell population was termed double negative (DN) T cells. The DN T cells express the αβ T cell receptor (TCR), yet do not express CD4, CD8, or NK cell markers and represent 1% to 5% of the αβ TCR positive T cells in mice and humans.17,69 Although no specific marker has been identified for DN Treg, a lack of Foxp3 expression and patterns of surface markers have been reported70 (Table 1). The origin of DN T cells is still unclear. Some studies suggest that DN αβ TCR+ T cells can be derived from CD8+ T cells,71 whereas others have shown that CD4+ T cells can convert to DN Treg both in vitro and in vivo.18 Cloned DN Treg from TCR transgenic mice are identified as CD25 + CD30 + CD28low,17 whereas DN Treg arising from CD4+ precursors are CD28 + CD25 + CD44 + CD69 + .18 Isolated human DN Treg are CD27 + CD28lowCD25 − .19 Subsequent studies reported that DN Treg displayed antigen-specific suppressive activity both in vitro and in vivo.17 The DN Treg not only suppress CD8+ T cell responses but also inhibit CD4+ T cells,43 B cells,55 NK cells,58 and DCs.72

Back to Top | Article Outline

DN Treg Mechanisms of Suppression and Impact on Transplantation

The DN Treg have a unique capacity to acquire the entire MHC-alloantigen complex from antigen presenting cells' (APCs) cell membrane via a cell-contact-dependent interaction with the TCR on the DN Treg via a process termed “trogocytosis” and to express the captured MHC-alloantigen complex on their cell surface.73 The effector CD8+ T cell binds to the captured MHC-alloantigen complex that is now presented on the DN Treg, which culminates in the transmission of a death signal to the CD8+ T cells.73-75 Accordingly, suppression by DN Treg is antigen-specific (Figure 1).

FIGURE 1

FIGURE 1

There is evidence that the death signal delivered by the DN Treg to the CD8+ T cells in mice is through Fas/FasL,75 whereas other studies suggest a role for perforin-mediated cytolysis18 (Table 2). The suppression of CD8+ T cells by DN T cells via recognition of alloantigens and the killing of CD8+ effector T cells is reminiscent of the “veto cell” concept that was proposed 35 years ago.76 Adoptive transfer of DN Treg after a cardiac allograft in mice prolonged graft survival as well as augmented the Foxp3+ Treg population.46 There is also evidence that murine DN Treg regulate immune responses at the level of antigen-presenting DCs. Gao and co-workers72 demonstrated that murine DN Treg expressed high levels of CTLA-4 and downregulated costimulatory molecules CD80 and CD86 on antigen-presenting mature DCs. Moreover, DN Treg killed syngeneic antigen-loaded DCs or allogeneic DCs through a Fas-FasL pathway.

The DN Treg can also suppress B-cell and NK-cell responses. Ma and coworkers55 found that adoptively transferred DN Treg prolonged rat heart xenograft survival in mice and induced B cell apoptosis via a perforin-dependent process. As might be expected, antidonor IgM and IgG antibody titers were significantly diminished in recipients of adoptively transferred DN Treg.55 In a model of murine allogenic bone marrow transfer, DN Treg supported graft survival through suppressing NK cells by perforin and Fas-FasL–dependent pathways.58

In contrast to murine DN Treg, human DN Treg do not kill effector T cells.77 Although cell-cell contact is required for human DN Treg to function, the suppressive activity of human DN Treg is not Fas/FasL or perforin-mediated. Suppression by DN Treg is reversible, and the function of previously suppressed effector T cells can be restored once DN Treg are removed. Soluble factors, such as IL-10 and TGF-β, are not involved in suppression mediated by DN Treg.77 In a study of 40 human patients who received a hematopoietic stem cell allograft, the percent of DN Treg in the peripheral blood was inversely correlated with risk of graft rejection.59

The DN Treg have been shown to enhance the survival of skin,17,18,43 pancreatic islet,18,49 and heart allografts46-48 and heart xenografts55,56 and hematopoietic stem cell allografts58,59 (Table 3).

Back to Top | Article Outline

Type 1 Regulatory T Cells

Type 1 Treg Phenotypes

Two categories of CD4+ Treg have been described: conventional CD4 + CD25 + Foxp3+ Treg78 and type 1 Treg (Tr1 cells).79 The Tr1 cells are found in both humans and mice and are characterized by their copious secretion of IL-10 and their lack of Foxp3 expression. Human and mouse Tr1 clones coexpress CD49b and lymphocyte activation gene 3 (LAG-3), which distinguishes Tr1 cells from other CD4+ T cells, including Th1, Th2, Th17, and Foxp3+ Treg20 (Table 1). The Tr1 cells are induced in vivo after chronic antigenic stimulation in the presence of IL-1080 or in vitro by activating naive T cells through their TCR in the presence or IL-10 alone or IL-10 in combination with immunosuppressive drugs, such as dexamethasone or rapamycin (RPM).50,79,81

A growing body of evidence indicates that IL-27 produced by tolerogenic DCs is a crucial differentiation factor for the development of IL-10–producing Tr1 cells in mice82-84 and humans.85 However, Jin and coworkers86 have recently shown that IL-6 can induce the differentiation of murine IL-10–producing Tr1-like Treg from naive CD4+ T cells in the absence of IL-27, suggesting that IL-6 produced in inflammatory conditions might serve as a feedback mechanism for generating Tr1 cells that dampen inflammation and restore homeostasis.

Back to Top | Article Outline

Tr1 Mechanisms of Suppression and Impact on Transplantation

The Tr1 cells mediate immune suppression by secreting IL-10 and by killing antigen-presenting cells via a perforin/granzyme-dependent process that requires MHC class I recognition87 (Table 2). The Tr1 also suppress Th17 cells in murine model of colitis by an IL-10–dependent process.88

The first indication that Tr1 cells might contribute to allograft tolerance came from severe combined immunodeficiency patients who developed long-term tolerance to stem cell allografts and expressed cells with Tr1-like properties.89 Subsequent studies showed that Tr1 cells were associated with the induction of mixed chimerism in patients receiving hematopoietic stem cell transplants.60 Investigations in murine allograft models have demonstrated a role for Tr1 cells in tolerance in pancreatic islet transplantation.50,51 In humans, Tr1 cells can also contribute to transplantation tolerance as Tr1 cells have been detected in patients who spontaneously developed tolerance to kidney or liver allografts54 and also occur in mice treated with IL-10 in combination with RPM a means of establishing tolerance for pancreatic islet transplants.50 In human subjects, Tr1 cells have been associated with tolerance in kidney,54 pancreatic islet,52 and liver54 allografts (Table 3).

Back to Top | Article Outline

Effect of Immunosuppressive Agents on Treg Generation and Function

Immunosuppressive drugs, such as cyclosporine A (CsA) and RPM, have made organ transplantation a feasible and effective therapeutic option for treating end-stage organ failure. By contrast, the clinical application of Treg in promoting the long-term survival of organ transplants is gaining traction but much remains to be improved before it becomes a reality. It is important to address the impact of immunosuppressive drugs in the context of various organ transplantations because they may have varied effects depending on dose, combination treatments, or the type of organ affected. A key issue is whether conventional antirejection agents can be combined with Treg for promoting graft acceptance or if immunosuppressive drugs have untoward effects on Treg function. Cyclosporine A and RPM have been used successfully to prevent transplant rejection. Although both CsA and RPM target IL-2, they have remarkably different effects on the generation and function of CD4 + CD25 + Foxp3+ Treg. Cyclosporine A inhibits IL-2 transcription and synthesis, whereas RPM acts downstream by blocking T-cell responses to IL-2. Using a murine heart allograft model, Coenen and co-workers found that CsA treatment resulted in a sharp reduction in peripheral CD4 + CD25 + Foxp3+ Treg, whereas RPM treatment did not reduce the generation of these Treg.90 Moreover, RPM, but not CsA, induces de novo generation of CD4 + CD25 + Foxp3+ Treg which potentiate murine skin allograft survival in an alloantigen-specific manner.91 Similar findings were seen in another skin allograft model where RPM treatment supported Treg and CsA antagonized Treg expansion.92

Immunosuppression by preventing the egress of T cells from lymphoid organs can be achieved by FTY720 (Fingolimod), a sphingosine-1-phosphate receptor agonist. In a murine corneal allograft model, graft survival was prolonged by topical treatment with CsA or FTY720, with an increase in CD4+ Treg found in the FTY720-treated mice.93 A model using adoptive transfer for allograft rejection found that either RPM or FTY720 treatment significantly enhanced conversion of CD4 + CD25 + Foxp3+ Treg.94

Histone deacetylase inhibitors (HDACs) may be a novel route for promoting Treg presence by preventing the conversion of Treg into effector T cells through enhancing the access to Foxp3 within the chromatin. An HDAC, suberoylanilide hydroxamic acid, synergized with low doses of tacrolimus to prolong cardiac allograft survival in mice by promoting expression of Treg molecules, Foxp3 and CTLA-4, as well as increasing apoptosis of T effector cells.95 Furthermore, if the cardiac graft was introduced into Foxp3-deficient recipient mice, suberoylanilide hydroxamic acid treatment was still able to marginally increase allograft survival, suggesting that non-Foxp3 regulatory cells are also impacted by HDACs. Another HDAC, trichostatin A, in combination with donor-cell transfusion increased CD4 + CD25 + Foxp3+ Treg numbers and promoted mouse pancreatic islet graft survival.96

There are still many clinical drugs that could potentially impact Treg numbers and functions. A high-throughput screen, flow cytometry based assay using Foxp3-GFP reporter mice evaluated the in vitro effects of 640 FDA-approved drugs and found that after 3 days in culture 75 drugs significantly increased Treg numbers.97 This study measured Foxp3+ T cell numbers, thus it warrants future investigations to determine if the same drugs, or a different combination of drugs, have similar impacts on non-Foxp3+ Treg.

Few studies have been conducted with regard to the impact of immunosuppressive agents on the generation or function of non-CD4 + CD25 + Foxp3+ Treg. In vitro studies using human peripheral blood mononuclear cells found that RPM caused an increase in the numbers of CD103 + CD8+ alloreactive T cells with immunosuppressive properties, whereas CsA had no significant effect on the percentage of these cells and prednisolone diminished the numbers of these cells.98 CD8 + CD28- Treg function was improved in rheumatoid arthritis patients that received TNF-α inhibitor therapy, whereas patients treated with methotrexate had no effects on the defective CD8 + CD28− Treg activity normally found in these patients.99 There is a dearth of published reports on the effects of immunosuppressive agents on Tr1 Treg and DN Treg activity in the context of allograft and xenograft survival. Thus, there are significant gaps in our knowledge about the effects of immunosuppressive agents and the non-CD4 + CD25 + Foxp3+ Treg.

Back to Top | Article Outline

CONCLUSIONS

Since Sakaguchi's discovery of CD4 + CD25+ Treg, over 25,000 publications have dealt with the general topic of Treg. The overwhelming majority of these publications have focused on the role of CD4 + CD25+ Treg. However, in recent years, there has been a growing awareness of the importance of CD8+ Treg, DN Treg, and Tr1 cells in controlling autoimmune diseases and in enhancing allograft survival. The presence of multiple populations of Treg is a reflection of the remarkable redundancy and plasticity of the immune system. The importance of CD4 + CD25 + Treg for immune homeostasis is well recognized. Deficiencies in Foxp3 expression invariably lead to lymphoproliferative and multiorgan autoimmune diseases in both humans and mice. It has been suggested that CD4 + CD25+ Treg are generated in response to the initial priming stage of the immune response and act to limit immune-mediated inflammation that inflicts damage to juxtaposed tissues in various organs. By contrast, CD8+ Treg, and perhaps DN Treg, are generated from previously activated T cells. In both cases, Treg act to suppress immune-mediated inflammation and restore immune homeostasis. In certain conditions, organ allografts and xenografts benefit from the development of these nonconventional Treg. Harnessing CD4 + CD25+, CD4− CD8+, DN Treg, and Tr1 cells as a comprehensive means of enhancing the survival of allografts and xenografts in patients at high risk of rejecting their grafts is an appealing goal that is still in its early stages of development.

Back to Top | Article Outline

REFERENCES

1. Gershon RK, Cohen P, Hencin R, et al. Suppressor T cells. J Immunol. 1972; 108 (3): 586–590.
2. Moller G. Do suppressor T cells exist? Scand J Immunol. 1988; 27 (3): 247–250.
3. Sakaguchi S, Sakaguchi N, Asano M, et al. Immunologic self-tolerance maintained by activated T cells expressing IL-2 receptor alpha-chains (CD25). Breakdown of a single mechanism of self-tolerance causes various autoimmune diseases. J Immunol. 1995; 155 (3): 1151–1164.
4. Cantor H, Shen FW, Boyse EA. Separation of helper T cells from suppressor T cells expressing different Ly components. II. Activation by antigen: after immunization, antigen-specific suppressor and helper activities are mediated by distinct T-cell subclasses. J Exp Med. 1976; 143 (6): 1391–40.
5. Hu D, Ikizawa K, Lu L, et al. Analysis of regulatory CD8 T cells in Qa-1-deficient mice. Nat Immunol. 2004; 5 (5): 516–523.
6. Jiang H, Chess L. Qa-1/HLA-E-restricted regulatory CD8+ T cells and self-nonself discrimination: an essay on peripheral T-cell regulation. Hum Immunol. 2008; 69 (11): 721–727.
7. Kim HJ, Cantor H. Regulation of self-tolerance by Qa-1-restricted CD8(+) regulatory T cells. Semin Immunol. 2011; 23 (6): 446–452.
8. Rifa'i M, Kawamoto Y, Nakashima I, et al. Essential roles of CD8 + CD122+ regulatory T cells in the maintenance of T cell homeostasis. J Exp Med. 2004; 200 (9): 1123–1134.
9. Strioga M, Pasukoniene V, Characiejus D. CD8+ CD28- and CD8+ CD57+ T cells and their role in health and disease. Immunology. 2011; 134 (1): 17–32.
10. Mahic M, Henjum K, Yaqub S, et al. Generation of highly suppressive adaptive CD8(+)CD25(+)FOXP3(+) regulatory T cells by continuous antigen stimulation. Eur J Immunol. 2008; 38 (3): 640–646.
11. Uss E, Rowshani AT, Hooibrink B, et al. CD103 is a marker for alloantigen-induced regulatory CD8+ T cells. J Immunol. 2006; 177 (5): 2775–2783.
12. Boor PP, Metselaar HJ, Jonge S, et al. Human plasmacytoid dendritic cells induce CD8(+) LAG-3(+) Foxp3(+) CTLA-4(+) regulatory T cells that suppress allo-reactive memory T cells. Eur J Immunol. 2011; 41 (6): 1663–1674.
13. Wei S, Kryczek I, Zou L, et al. Plasmacytoid dendritic cells induce CD8+ regulatory T cells in human ovarian carcinoma. Cancer Res. 2005; 65 (12): 5020–5026.
14. Xystrakis E, Dejean AS, Bernard I, et al. Identification of a novel natural regulatory CD8 T-cell subset and analysis of its mechanism of regulation. Blood. 2004; 104 (10): 3294–3301.
15. Dai H, Wan N, Zhang S, et al. Cutting edge: programmed death-1 defines CD8 + CD122+ T cells as regulatory versus memory T cells. J Immunol. 2010; 185 (2): 803–807.
16. Chen Z, Han Y, Gu Y, et al. CD11c(high)CD8+ regulatory T cell feedback inhibits CD4 T cell immune response via Fas ligand-Fas pathway. J Immunol. 2013; 190 (12): 6145–6154.
17. Zhang ZX, Yang L, Young KJ, et al. Identification of a previously unknown antigen-specific regulatory T cell and its mechanism of suppression. Nat Med. 2000; 6 (7): 782–789.
18. Zhang D, Yang W, Degauque N, et al. New differentiation pathway for double-negative regulatory T cells that regulates the magnitude of immune responses. Blood. 2007; 109 (9): 4071–4079.
19. Fischer K, Voelkl S, Heymann J, et al. Isolation and characterization of human antigen-specific TCR alpha beta + CD4(−)CD8- double-negative regulatory T cells. Blood. 2005; 105 (7): 2828–2835.
20. Gagliani N, Magnani CF, Huber S, et al. Coexpression of CD49b and LAG-3 identifies human and mouse T regulatory type 1 cells. Nat Med. 2013; 19 (6): 739–746.
21. Liu J, Liu Z, Witkowski P, et al. Rat CD8+ FOXP3+ T suppressor cells mediate tolerance to allogeneic heart transplants, inducing PIR-B in APC and rendering the graft invulnerable to rejection. Transpl Immunol. 2004; 13 (4): 239–247.
22. Streilein JW, Niederkorn JY. Characterization of the suppressor cell (s) responsible for anterior chamber-associated immune deviation (ACAID) induced in BALB/c mice by P815 cells. J Immunol. 1985; 134 (3): 1381–1387.
23. Picarda E, Bezie S, Venturi V, et al. MHC-derived allopeptide activates TCR-biased CD8+ Tregs and suppresses organ rejection. J Clin Invest. 2014; 124 (6): 2497–2512.
24. Wang J, Jiang S, Shi H, et al. Prolongation of corneal xenotransplant survival by T-cell vaccination-induced T-regulatory cells. Xenotransplantation. 2008; 15 (3): 164–173.
25. Guillonneau C, Hill M, Hubert FX, et al. CD40Ig treatment results in allograft acceptance mediated by CD8CD45RC T cells, IFN-gamma, and indoleamine 2,3-dioxygenase. J Clin Invest. 2007; 117 (4): 1096–1106.
26. Li S, Xie Q, Zeng Y, et al. A naturally occurring CD8(+)CD122(+) T-cell subset as a memory-like Treg family. Cell Mol Immunol. 2014; 11 (4): 326–331.
27. Barbon CM, Davies JK, Voskertchian A, et al. Alloanergization of human T cells results in expansion of alloantigen-specific CD8(+) CD28(−) suppressor cells. Am J Transplant. 2014; 14 (2): 305–318.
28. Li XL, Menoret S, Bezie S, et al. Mechanism and localization of CD8 regulatory T cells in a heart transplant model of tolerance. J Immunol. 2010; 185 (2): 823–833.
29. Lu L, Cantor H. Generation and regulation of CD8(+) regulatory T cells. Cell Mol Immunol. 2008; 5 (6): 401–406.
30. Lu L, Kim HJ, Werneck MB, et al. Regulation of CD8+ regulatory T cells: interruption of the NKG2A-Qa-1 interaction allows robust suppressive activity and resolution of autoimmune disease. Proc Natl Acad Sci U S A. 2008; 105 (49): 19420–19425.
31. Lerret NM, Houlihan JL, Kheradmand T, et al. Donor-specific CD8+ Foxp3+ T cells protect skin allografts and facilitate induction of conventional CD4+ Foxp3+ regulatory T cells. Am J Transplant. 2012; 12 (9): 2335–2347.
32. Endharti AT, Rifa IM, Shi Z, et al. Cutting edge: CD8 + CD122+ regulatory T cells produce IL-10 to suppress IFN-gamma production and proliferation of CD8+ T cells. J Immunol. 2005; 175 (11): 7093–7097.
33. Mangalam AK, Luckey D, Giri S, et al. Two discreet subsets of CD8 T cells modulate PLP(91–110) induced experimental autoimmune encephalomyelitis in HLA-DR3 transgenic mice. J Autoimmun. 2012; 38 (4): 344–353.
34. Gaur A, Ruberti G, Haspel R, et al. Requirement for CD8+ cells in T cell receptor peptide-induced clonal unresponsiveness. Science. 1993; 259 (5091): 91–94.
35. Koh DR, Fung-Leung WP, Ho A, et al. Less mortality but more relapses in experimental allergic encephalomyelitis in CD8−/− mice. Science. 1992; 256 (5060): 1210–1213.
36. Varthaman A, Khallou-Laschet J, Clement M, et al. Control of T cell reactivation by regulatory Qa-1-restricted CD8+ T cells. J Immunol. 2010; 184 (12): 6585–6591.
37. Cone RE, Chattopadhyay S, Sharafieh R, et al. The suppression of hypersensitivity by ocular-induced CD8(+) T cells requires compatibility in the Qa-1 haplotype. Immunol Cell Biol. 2009; 87 (3): 241–248.
38. D'Orazio TJ, Mayhew E, Niederkorn JY. Ocular immune privilege promoted by the presentation of peptide on tolerogenic B cells in the spleen. II. Evidence for presentation by Qa-1. J Immunol. 2001; 166 (1): 26–32.
39. Dai Z, Zhang S, Xie Q, et al. Natural CD8 + CD122+ T cells are more potent in suppression of allograft rejection than CD4 + CD25+ regulatory T cells. Am J Transplant. 2014; 14 (1): 39–48.
40. Guillonneau C, Picarda E, Anegon I. CD8+ regulatory T cells in solid organ transplantation. Curr Opin Organ Transplant. 2010; 15 (6): 751–756.
41. Wan N, Dai H, Wang T, et al. Bystander central memory but not effector memory CD8+ T cells suppress allograft rejection. J Immunol. 2008; 180 (1): 113–121.
42. Sireci G, Barera A, Macaluso P, et al. A continuous infusion of a minor histocompatibility antigen-immunodominant peptide induces a delay of male skin graft rejection. Immunobiology. 2009; 214 (8): 703–711.
43. Ford MS, Young KJ, Zhang Z, et al. The immune regulatory function of lymphoproliferative double negative T cells in vitro and in vivo. J Exp Med. 2002; 196 (2): 261–267.
44. Colovai AI, Mirza M, Vlad G, et al. Regulatory CD8 + CD28- T cells in heart transplant recipients. Hum Immunol. 2003; 64 (1): 31–37.
45. Kapp JA, Honjo K, Kapp LM, et al. TCR transgenic CD8+ T cells activated in the presence of TGFbeta express FoxP3 and mediate linked suppression of primary immune responses and cardiac allograft rejection. Int Immunol. 2006; 18 (11): 1549–1562.
46. Zhang ZX, Lian D, Huang X, et al. Adoptive transfer of DNT cells induces long-term cardiac allograft survival and augments recipient CD4(+)Foxp3(+) Treg cell accumulation. Transpl Immunol. 2011; 24 (2): 119–126.
47. Chen W, Ford MS, Young KJ, et al. Infusion of in vitro-generated DN T regulatory cells induces permanent cardiac allograft survival in mice. Transplant Proc. 2003; 35 (7): 2479–2480.
48. Hill M, Thebault P, Segovia M, et al. Cell therapy with autologous tolerogenic dendritic cells induces allograft tolerance through interferon-gamma and epstein-barr virus-induced gene 3. Am J Transplant. 2011; 11 (10): 2036–2045.
49. Zhang D, Zhang W, Ng TW, et al. Adoptive cell therapy using antigen-specific CD4(−)CD8(−)T regulatory cells to prevent autoimmune diabetes and promote islet allograft survival in NOD mice. Diabetologia. 2011; 54 (8): 2082–2092.
50. Battaglia M, Stabilini A, Draghici E, et al. Rapamycin and interleukin-10 treatment induces T regulatory type 1 cells that mediate antigen-specific transplantation tolerance. Diabetes. 2006; 55 (1): 40–49.
51. Gagliani N, Jofra T, Stabilini A, et al. Antigen-specific dependence of Tr1-cell therapy in preclinical models of islet transplant. Diabetes. 2010; 59 (2): 433–439.
52. Huurman VA, Velthuis JH, Hilbrands R, et al. Allograft-specific cytokine profiles associate with clinical outcome after islet cell transplantation. Am J Transplant. 2009; 9 (2): 382–388.
53. Zhou J, Appleton SE, Stadnyk A, et al. CD8+ gammadelta T regulatory cells mediate kidney allograft prolongation after oral exposure to alloantigen. Transpl Int. 2008; 21 (7): 679–687.
54. VanBuskirk AM, Burlingham WJ, Jankowska-Gan E, et al. Human allograft acceptance is associated with immune regulation. J Clin Invest. 2000; 106 (1): 145–155.
55. Ma Y, He KM, Garcia B, et al. Adoptive transfer of double negative T regulatory cells induces B-cell death in vivo and alters rejection pattern of rat-to-mouse heart transplantation. Xenotransplantation. 2008; 15 (1): 56–63.
56. Chen W, Ford MS, Young KJ, et al. Role of double-negative regulatory T cells in long-term cardiac xenograft survival. J Immunol. 2003; 170 (4): 1846–1853.
57. Beres AJ, Haribhai D, Chadwick AC, et al. CD8+ Foxp3+ regulatory T cells are induced during graft-versus-host disease and mitigate disease severity. J Immunol. 2012; 189 (1): 464–474.
58. He KM, Ma Y, Wang S, et al. Donor double-negative Treg promote allogeneic mixed chimerism and tolerance. Eur J Immunol. 2007; 37 (12): 3455–3466.
59. McIver Z, Serio B, Dunbar A, et al. Double-negative regulatory T cells induce allotolerance when expanded after allogeneic haematopoietic stem cell transplantation. Br J Haematol. 2008; 141 (2): 170–178.
60. Serafini G, Andreani M, Testi M, et al. Type 1 regulatory T cells are associated with persistent split erythroid/lymphoid chimerism after allogeneic hematopoietic stem cell transplantation for thalassemia. Haematologica. 2009; 94 (10): 1415–1426.
61. Niederkorn JY. See no evil, hear no evil, do no evil: the lessons of immune privilege. Nat Immunol. 2006; 7 (4): 354–359.
62. Cone RE, Li X, Sharafieh R, et al. The suppression of delayed-type hypersensitivity by CD8+ regulatory T cells requires interferon-gamma. Immunology. 2007; 120 (1): 112–119.
63. Yamada Y, Sugita S, Horie S, et al. Mechanisms of immune suppression for CD8+ T cells by human corneal endothelial cells via membrane-bound TGFbeta. Invest Ophthalmol Vis Sci. 2010; 51 (5): 2548–2557.
64. Sugita S, Keino H, Futagami Y, et al. B7+ iris pigment epithelial cells convert T cells into CTLA-4+, B7-expressing CD8+ regulatory T cells. Invest Ophthalmol Vis Sci. 2006; 47 (12): 5376–5384.
65. Sugita S, Ng TF, Lucas PJ, et al. B7+ iris pigment epithelium induce CD8+ T regulatory cells; both suppress CTLA-4+ T cells. J Immunol. 2006; 176 (1): 118–127.
66. Niederkorn JY. Anterior chamber-associated immune deviation and its impact on corneal allograft survival. Curr Opin Organ Transplant. 2006; 11: 360–365.
67. Niederkorn JY, Mellon J. Anterior chamber-associated immune deviation promotes corneal allograft survival. Invest Ophthalmol Vis Sci. 1996; 37 (13): 2700–2707.
68. Strober S, Dejbachsh-Jones S, Van Vlasselaer P, et al. Cloned natural suppressor cell lines express the CD3 + CD4-CD8- surface phenotype and the alpha, beta heterodimer of the T cell antigen receptor. J Immunol. 1989; 143 (4): 1118–1122.
69. Zhang ZX, Young K, Zhang L. CD3 + CD4-CD8- alphabeta-TCR+ T cell as immune regulatory cell. J Mol Med. 2001; 79 (8): 419–427.
70. Juvet SC, Zhang L. Double negative regulatory T cells in transplantation and autoimmunity: recent progress and future directions. J Mol Cell Biol. 2012; 4 (1): 48–58. PubMed PMID: 22294241.
71. Balomenos D, Rumold R, Theofilopoulos AN. The proliferative in vivo activities of lpr double-negative T cells and the primary role of p59fyn in their activation and expansion. J Immunol. 1997; 159 (5): 2265–2273.
72. Gao JF, McIntyre MS, Juvet SC, et al. Regulation of antigen-expressing dendritic cells by double negative regulatory T cells. Eur J Immunol. 2011; 41 (9): 2699–2708.
73. Ford McIntyre MS, Young KJ, Gao J, et al. Cutting edge: in vivo trogocytosis as a mechanism of double negative regulatory T cell-mediated antigen-specific suppression. J Immunol. 2008; 181 (4): 2271–2275.
74. Thomson CW, Lee BP, Zhang L. Double-negative regulatory T cells: non-conventional regulators. Immunol Res. 2006; 35 (1–2): 163–178.
75. Young KJ, Zhang L. The nature and mechanisms of DN regulatory T-cell mediated suppression. Hum Immunol. 2002; 63 (10): 926–934.
76. Miller RG. An immunological suppressor cell inactivating cytotoxic T-lymphocyte precursor cells recognizing it. Nature. 1980; 287 (5782): 544–546.
77. Voelkl S, Gary R, Mackensen A. Characterization of the immunoregulatory function of human TCR-alphabeta + CD4- CD8- double-negative T cells. Eur J Immunol. 2011; 41 (3): 739–748.
78. Hori S, Nomura T, Sakaguchi S. Control of regulatory T cell development by the transcription factor Foxp3. Science. 2003; 299 (5609): 1057–1061.
79. Groux H, O'Garra A, Bigler M, et al. A CD4+ T-cell subset inhibits antigen-specific T-cell responses and prevents colitis. Nature. 1997; 389 (6652): 737–742.
80. Roncarolo MG, Gregori S, Battaglia M, et al. Interleukin-10-secreting type 1 regulatory T cells in rodents and humans. Immunol Rev. 2006; 212: 28–50.
81. Volchenkov R, Karlsen M, Jonsson R, et al. Type 1 regulatory T cells and regulatory B cells induced by tolerogenic dendritic cells. Scand J Immunol. 2013; 77 (4): 246–254.
82. Awasthi A, Carrier Y, Peron JP, et al. A dominant function for interleukin 27 in generating interleukin 10-producing anti-inflammatory T cells. Nat Immunol. 2007; 8 (12): 1380–1389.
83. Fitzgerald DC, Zhang GX, El-Behi M, et al. Suppression of autoimmune inflammation of the central nervous system by interleukin 10 secreted by interleukin 27-stimulated T cells. Nat Immunol. 2007; 8 (12): 1372–1379.
84. Stumhofer JS, Silver JS, Laurence A, et al. Interleukins 27 and 6 induce STAT3-mediated T cell production of interleukin 10. Nat Immunol. 2007; 8 (12): 1363–1371.
85. Murugaiyan G, Mittal A, Lopez-Diego R, et al. IL-27 is a key regulator of IL-10 and IL-17 production by human CD4+ T cells. J Immunol. 2009; 183 (4): 2435–2443.
86. Jin JO, Han X, Yu Q. Interleukin-6 induces the generation of IL-10-producing Tr1 cells and suppresses autoimmune tissue inflammation. J Autoimmun. 2013; 40: 28–44.
87. Magnani CF, Alberigo G, Bacchetta R, et al. Killing of myeloid APCs via HLA class I, CD2 and CD226 defines a novel mechanism of suppression by human Tr1 cells. Eur J Immunol. 2011; 41 (6): 1652–1662.
88. Huber S, Gagliani N, Esplugues E, et al. Th17 cells express interleukin-10 receptor and are controlled by Foxp3(−) and Foxp3+ regulatory CD4+ T cells in an interleukin-10-dependent manner. Immunity. 2011; 34 (4): 554–565.
89. Bacchetta R, Bigler M, Touraine JL, et al. High levels of interleukin 10 production in vivo are associated with tolerance in SCID patients transplanted with HLA mismatched hematopoietic stem cells. J Exp Med. 1994; 179 (2): 493–502.
90. Coenen JJ, Koenen HJ, van Rijssen E, et al. Rapamycin, not cyclosporine, permits thymic generation and peripheral preservation of CD4+ CD25+ FoxP3+ T cells. Bone Marrow Transplant. 2007; 39 (9): 537–545.
91. Gao W, Lu Y, El Essawy B, et al. Contrasting effects of cyclosporine and rapamycin in de novo generation of alloantigen-specific regulatory T cells. Am J Transplant. 2007; 7 (7): 1722–1732.
92. Lim DG, Koo SK, Park YH, et al. Impact of immunosuppressants on the therapeutic efficacy of in vitro-expanded CD4 + CD25 + Foxp3+ regulatory T cells in allotransplantation. Transplantation. 2010; 89 (8): 928–936.
93. Liu Y, Jiang J, Xiao H, et al. Topical application of FTY720 and cyclosporin A prolong corneal graft survival in mice. Mol Vis. 2012; 18: 624–633.
94. Wu T, Zhang L, Xu K, et al. Immunosuppressive drugs on inducing Ag-specific CD4(+)CD25(+)Foxp3(+) Treg cells during immune response in vivo. Transpl Immunol. 2012; 27 (1): 30–38.
95. Zhang X, Han S, Kang Y, et al. SAHA, an HDAC inhibitor, synergizes with tacrolimus to prevent murine cardiac allograft rejection. Cell Mol Immunol. 2012; 9 (5): 390–398.
96. Sugimoto K, Itoh T, Takita M, et al. Improving allogeneic islet transplantation by suppressing Th17 and enhancing Treg with histone deacetylase inhibitors. Transpl Int. 2014; 27 (4): 408–415.
97. Mao R, Xiao W, Liu H, et al. Systematic evaluation of 640 FDA drugs for their effect on CD4(+)Foxp3(+) regulatory T cells using a novel cell-based high throughput screening assay. Biochem Pharmacol. 2013; 85 (10): 1513–1524.
98. Uss E, Yong SL, Hooibrink B, et al. Rapamycin enhances the number of alloantigen-induced human CD103 + CD8+ regulatory T cells in vitro. Transplantation. 2007; 83 (8): 1098–1106.
99. Ceeraz S, Hall C, Choy EH, et al. Defective CD8 + CD28+ regulatory T cell suppressor function in rheumatoid arthritis is restored by tumour necrosis factor inhibitor therapy. Clin Exp Immunol. 2013; 174 (1): 18–26.
Copyright © 2015 Wolters Kluwer Health, Inc. All rights reserved.