Leonard Chess and Hong Jiang are in the Department of Medicine, Division of Rheumatology, Columbia University, New York, New York 10032-3702, USA. lc19@columbia.edu
The idea that Qa-1-dependent CD8+ suppressor T cells regulate peripheral immune responses was unpopular during the era of 'suppressor skepticism' in the late 1980s, but this has now been resurrected.
Qa-1 is a major histocompatibility complex (MHC) class Ib molecule that is known best to the current generation of immunologists as the natural ligand for the CD94-NKG2 complex expressed on natural killer (NK) cells. In that function, Qa-1−peptide complexes bind CD94-NKG2 and regulate NK cell activity. An older generation of immunologists may recall that Qa-1 was originally characterized serologically using allosera during the late 1970s and was found to be a key molecule expressed on a subset of lymphocytes that preferentially induce CD8+ T suppressor cells1. Additional evidence for this idea lay dormant until the last decade, when a series of experiments defined a pathway of immunoregulation mediated by Qa-1-dependent CD8+ T cells2,
3. The paper by Hu et al. in this issue of Nature Immunology now provides the first direct in vivo evidence confirming on the molecular level that Qa-1 indeed has a crucial function in the regulatory pathway mediated by CD8+ T cells4.
Many self-reactive clones escape thymic selection and emerge in the periphery as potentially pathogenic autoreactive cells. These self-reactive cells are regulated by a variety of intrinsic mechanisms induced by antigen activation, including apoptosis, anergy and regulatory cytokines. In addition, there is emerging evidence that suppressor cells, which abort the differentiation of antigen-activated cells2,
5, are ultimately required to control immune responses. The idea of suppressor T cells arose in the laboratory of Richard Gershon at Yale in the late 1960s from studies of immune tolerance to foreign antigens6. These early studies were extended in many laboratories documenting that suppressor cells were involved in the regulation of immune responses to foreign and self antigens and in the control of autoimmunity7. Initially progress was rapid and evidence emerged that suppression was mediated by the Lyt 2,3 (later called CD8) T cells and these cells required inductive signals from Qa-1-expressing Lyt 1 (CD4) cells1. However, progress in defining the molecular basis of suppression was hampered by the fact that molecular immunology was in its infancy. For example, monoclonal antibodies were not yet discovered and the nature of the T cell receptor (TCR) was unknown, as was the precise structure and function of MHC molecules in restricting T cell activity. In addition, most cytokines now known to regulate immune functions were largely unknown. Thus, the time was not ripe for deciphering the complex molecular pathways involved in immune regulation. Consequently, interest in T cell suppression models waned by the mid-1980s and the very idea of suppressor cells was questioned8. Recent interest in immunosuppression was driven mainly by the discovery of naturally occurring NKT and CD4+ regulatory cells5,
9, as well as studies resurrecting the idea of the involvement of Qa-1-restricted CD8 suppressor cells in controlling the adaptive immune response to both foreign and self antigens2.
The revival of the CD8 suppressor pathway was initiated in part by two studies showing that CD8+ T cells participate in resistance to the autoimmune disease model experimental allergic encephalomyelitis (EAE)10,
11. The first episode of EAE induced by myelin basic protein (MBP) rendered B10PL mice highly resistant to the reinitiation of EAE by secondary immunization. Mice that recovered from EAE and were then depleted of CD8+ T cells developed EAE after being reimmunized with MBP9. Furthermore, mice depleted of CD8+ T cells during the initial induction of EAE and allowed to recover normal numbers of CD8+ T cells were not resistant and developed EAE again after rechallenge with MBP. Thus, CD8+ T cells require priming during the first episode of EAE to regulate CD4+ T cells triggered by secondary MBP stimulation in vivo. Moreover, when CD8-deficient mice were bred with the EAE-susceptible PL/J strain, the CD8-deficient mice developed more chronic EAE than did the wild-type PL/J mice10. These experiments provided evidence that CD8+ T cells are key in both inducing resistance to autoimmune EAE and abrogating recurrent relapsing episodes of pathogenic autoimmunity in vivo.
Since then, a series of studies were done to elucidate the cellular and molecular mechanisms of CD8+ T suppressors3,
12,
13. One of the crucial elements involved in the regulatory pathway is Qa-1. It was initially shown that a subset of the activated Qa-1-expressing Lyt 1 T cells were required to induce differentiation of CD8+ T cells into suppressor cells, which in turn downregulated CD4+ T cell activity1. Like many elements of the CD8+ T suppressor pathway, Qa-1 was one of the 'babies' inadvertently 'thrown out with the bath water' during a period of skepticism in the 1980s. The Qa-1 issue was revived in a more defined experimental system in 1995. The key observations were that CD8+ T cells controlled the CD4 T cell response to superantigens in vivo and that the specific inhibition of CD4+ TCR V8+ target T cells in vitro by the CD8+ T cells could be blocked by antibody to Qa-1 (ref. 12). Further support came from experiments showing that MBP-activated Qa-1+ CD4+ T cells used as a T cell vaccine induced regulatory CD8+ T cells in vivo that protected mice from developing EAE and that this protection could be abrogated by CD8+ T cell depletion in vivo13. Furthermore, CD8+ T cell hybridomas derived from the EAE-protected mice preferentially recognized V8+ but not V6+ MBP-reactive T cell clones in vitro. Specific recognition was blocked by antibodies to Qa-1, CD8 and TCR13. This led to the idea that the TCR expressed on the CD8+ T suppressor cells recognize Qa-1−self peptide complexes presented by the activated CD4+ T cells. Furthermore, it was shown that the CD8+ T cells protect animals from EAE by selectively downregulating the potentially encephalitogenic T cells3.
These experiments suggest that the CD8 T cell regulatory pathway comprises a series of sequential cellular events (Fig. 1). A characteristic feature of the Qa-1-dependent CD8+ T cells is that they require priming by the activated CD4+ T cells during the primary immune response to regulate the secondary immune response. This distinguishes the CD8+ T cell regulatory pathway from other cellular regulatory pathways, including the NKT and the CD4+CD25+ regulatory T cells, which exist predominately as naturally occurring suppressor cells and function mainly during the early and primary phases of the immune response. The suppression mediated by naturally occurring CD4+CD25+ T cells and NK cells is not specific and does not require specific priming. In contrast, the CD8+ T cell pathway is involved mainly in regulation of the adaptive secondary or later phases of immunity. These distinct regulatory pathways in concert allow the immune system to more efficiently control peripheral immunity2.
Figure 1. The CD8 T cell regulatory pathway comprises a series of sequential cellular events.
It is initiated by the activation of naive CD4+ T cells during the primary immune response, in which the TCR on CD4+ T cells interact with MHC class II−peptide complexes presented by conventional antigen-presenting cells. This initial interaction induces Qa-1−self peptide complexes to be differentially expression on the surface of certain CD4+ T cells. The Qa-1−self peptide complex serves as the inducer and targets structures recognized by the TCR on regulatory CD8+ T cells. Thus, the Qa-1−self peptide complex expressed by activated CD4+ T cells triggers the TCR on regulatory CD8+ T cells. These CD8+ T cells then differentiate into effector cells, which in turn suppress CD4+ T cells expressing the same target Qa-1−self peptide complex.
The creation of the Qa-1-deficient mice and the demonstration that these mice have defects in immunoregulation mediated by CD8+ T cells represents a substantial advance in our understanding of how Qa-1-dependent CD8+ T cells control peripheral immunity4. In particular, Hu et al. show that the effect of Qa-1 on regulatory CD8+ T cells in the control of autoimmune disease in vivo is only found during the secondary, not the primary, immune response. The authors convincingly demonstrate that the enhanced response in Qa-1-deficient mice to self proteolipid protein (PLP) peptide is associated with the failure to develop resistance to EAE that normally develops in wild-type mice after immunization with PLP peptide. Furthermore, failure of resistance to EAE is associated with the escape of Qa-1-deficient CD4 cells from CD8+ T cell suppression, which could be restored by lentiviral-based expression of the syngeneic allele encoding Qa-1. These results form the heart of the functional data demonstrating the in vivo relevance of Qa-1 in the control of autoimmunity. Furthermore, the authors also show that Qa-1-deficient mice fail to control outgrowth of herpes simplex virus 1−induced CD4+ T helper type 1 cells that induce ocular keratitis and blindness. These T helper type 1 CD4+ T cells are specific for herpes simplex virus peptides cross-reactive with self corneal tissues and thus these results confirm the findings in the relevant EAE autoimmune disease model.
Many biological features of the Qa-1 molecule make it particularly interesting with respect to its involvement as a restricting element in immunoregulation. Qa-1 is preferentially expressed on activated, but not resting, T cells. Moreover, because Qa-1 expression on activated T cells is short lived, this may exclude resting T cells from downregulation by Qa-1-dependent CD8+ T cells. Furthermore, Qa-1 is of limited polymorphism with the potential to present self and foreign peptides to CD8+ T cells. The main self peptide presented by Qa-1 is Qdm, a hydrophobic peptide (AMAPRTLLL) derived from the leader sequence of certain MHC class Ia molecules14. However, during T cell activation, Qa-1 can also bind other hydrophobic self peptides that may serve as target antigens for the CD8+ T cells as described above. Because the Qa-1−self peptide complexes can interact with the CD94-NKG2 receptors expressed on NK or classical CD8+ T cells to either positively or negatively regulate function, Qa-1 may not only serve as the target of CD8 suppression but also regulate CD8 suppressor T cell function through the CD94-NKG2 receptors.
Finally, the Qa-1-dependent regulatory CD8 pathway has begun to be 'translated' from mice to humans with the in vitro finding that human CD8+ T cells can be induced to differentiate into regulatory cells whose function is dependent on HLA-E, the human homolog of Qa-1 (refs. 2,15). These data support the idea that clinically relevant methods that enhance this pathway in humans may prove useful in preventing and treating human autoimmune disease.
8+ target T cells in vitro by the CD8+ T cells could be blocked by antibody to Qa-1 (ref. 
TCR
