The immune system has evolved complex mechanisms that allow rapid and destructive responses to microbial intruders while sparing the host’s own tissues. Regulation of this delicate balance depends mainly on the immune system’s two major types of T cell, which are distinguished by the protein — either CD4 or CD8 — that is expressed on their surface. They are called CD4 T cells and CD8 T cells, respectively. The task of CD8 T cells has generally been assumed to be to kill cells infected with microbial invaders and to destroy foreign or abnormal cells. However, writing in Nature, Saligrama et al.1 report another role: CD8 T cells can inhibit self-reactive CD4 T cells and quell autoimmune disease in a mouse model of multiple sclerosis.

In previous work2, researchers from the present group showed that, if people with coeliac disease were exposed to gluten proteins (a major type of trigger, called an allergen, in this autoimmune disorder), it activated not only CD4 T cells that could specifically recognize gluten, as expected, but also a subset of CD8 T cells. Exactly what the latter were doing, however, was unclear. Saligrama and colleagues now report their investigation into whether a similarly coordinated T-cell response might be detected in experimental autoimmune encephalomyelitis (EAE), which is a mouse model of multiple sclerosis. This autoimmune model can be induced by injecting the protein myelin oligodendrocyte glycoprotein (MOG), a component of the fatty coating of nerve cells called myelin, into mice. The authors identified populations of both CD4 and CD8 T cells (among other immune cells) that proliferated vigorously after immunization with MOG, generating clones of cells (Fig. 1a).

Figure 1 | A regulatory function for T cells that express the protein CD8. Saligrama et al.1 studied a model of multiple sclerosis called experimental autoimmune encephalomyelitis (EAE), which is induced by injecting the protein myelin oligodendrocyte glycoprotein (MOG) into mice. a, Immune cells called CD8 and CD4 T cells express T-cell antigen receptors (TCRs) on their surface that recognize peptide fragments called antigens. If antigen recognition occurs, the T cell proliferates. In EAE, as the condition progressed, this was accompanied by the proliferation of both CD4 T cells (dark red) that recognize MOG and CD8 T cells (dark orange) that recognize an unknown target. b, Using yeast cells, the authors screened a library of peptides bound to mouse major histocompatibility complex (MHC) molecules, which enabled the identification of peptides that are recognized by the TCRs of the CD8 T cells. c, The vaccination of mice with both MOG and these peptides diminished the severity of EAE, compared with that seen in control animals. This effect was associated with the proliferation of CD8 T cells that express the proteins CD44, CD122 and Ly49. These cells suppress the proliferation of CD4 T cells in a process that involves the recognition of antigens presented by MHC proteins.

For each of these mobilized populations, Saligrama et al. identified the T-cell antigen receptors (TCRs, the proteins on T cells that recognize foreign or self peptide fragments known as antigens), and attempted to identify antigens that the TCRs could recognize. Such recognition causes T-cell activation and proliferation. The authors found that the CD4 T cells in question recognized MOG-derived peptide fragments and so were primed to specifically attack myelin-coated nerve cells and cause disease. But the CD8 T-cell clones did not recognize MOG, and none of about 350 myelin-derived peptides tested could activate their TCRs. So how were these cells being activated?

Read the paper: Opposing T cell responses in experimental autoimmune encephalomyelitis

To find out, the authors generated a library of roughly 108 different peptides, each embedded in a class Ia major histocompatibility complex (MHC) protein — an essential component of the immune system that displays antigens to T cells. The authors used TCRs from CD8 T cells as the bait with which to capture the corresponding antigens in this peptide–MHC library (Fig. 1b). This approach to defining TCR-binding peptides has not been widely adopted because it is tricky to generate large libraries that display individual peptide–MHC complexes with enough structural fidelity and stability to allow sensitive and efficient antigen screening3. Saligrama and colleagues’ technique overcomes these obstacles. The strength of their approach can be appreciated by a glance at the numbers: a screen of around 5 × 108 different peptide–MHC complexes, using a single TCR, identified a dozen peptide–MHC complexes that could bind the TCR, akin to finding a small needle in a large haystack.

None of the peptides identified after numerous rounds of screening came from mouse proteins, and the authors call them ‘surrogate’ peptides, to indicate that they probably stand in for self peptides normally present in the body. To determine how T cells that recognize these peptides contribute to EAE, the authors immunized mice with a mixture of these surrogate peptides and MOG. CD8 T cells that recognized the peptides used for vaccination proliferated and suppressed the proliferation of the CD4 cells that promote EAE, possibly by recognizing peptide–MHC complexes displayed on MOG-reactive CD4 T cells (Fig. 1c). The peptides that these CD8 T cells recognize in the body are not yet defined. Cells such as these CD8 T cells that can suppress an immune response are called regulatory cells. Whether the initial proliferation of these regulatory CD8 T cells also depends on peptides presented to them by CD4 cells, or indeed by other types of immune cell, is unknown.

Progress in identifying and sequencing the TCRs that are expressed by specific T-cell clones has led to the development of important TCR-based therapeutics. However, the identity of the peptide–MHC complexes that bind to those TCRs has generally been elusive. An essential feature of Saligrama and colleagues’ approach involves modifying the peptides to increase their affinity for class Ia MHCs, ensuring that most of the peptides in the library are bound to MHCs. These modifications might also increase the strength of binding of peptide–MHC complexes to TCRs, and hence their ability to stimulate regulatory CD8 T cells. An analogy might be seen with CD4 regulatory T cells. Earlier research4 suggested that mutations that increase the binding of peptides to MHC class II proteins and the T-cell-activating effect of self peptides derived from insulin resulted in peptide–MHC complexes that stimulated the differentiation of insulin-specific CD4 regulatory T cells. Perhaps the strength of binding of peptide–MHC complexes to TCRs has a similar stimulatory effect on CD8 regulatory T cells.

One downside of the authors’ approach could be a bias towards studying cells that recognize peptide–MHC complexes of the class Ia type in particular. Other studies5 have found that CD8 regulatory T cells similar to those found here also recognize complexes of peptides with MHC proteins from class Ib. It would be interesting to know whether the CD8 regulatory T cells identified by Saligrama et al. consist of two different cellular lineages, which act in a complementary way to block autoimmunity by monitoring class Ia or class Ib MHCs expressed in different tissues.

Multiple sclerosis enters a grey area

The authors’ further investigation of the CD8 T-cell population that dampened the CD4 autoimmune response boiled down to studies of whether or not these CD8 T cells are a specialized lineage of cells that is distinct from ‘effector’ CD8 T cells — those that are genetically programmed to respond to microbial intruders. The authors found that regulatory activity was invested in a small subpopulation (less than 5%) of CD8 T cells, which express a specific triad of proteins (CD44, CD122 and Ly49) on their surface6. RNA analysis of this subpopulation indicates a profile that is distinct from that of most typical CD8 effector cells, and that shares features with what are termed natural killer T cells and with CD8 regulatory cells identified in other autoimmune settings5.

Like CD4 T cells, CD8 T cells might be divided into an effector-cell lineage that targets microbes, and a regulatory-cell lineage that subdues self-reactive CD4 T cells. The tracing of isolated cells by their surface markers has been instrumental in defining the regulatory lineage of CD4 T cells. Saligrama and colleagues’ approach might prove equally revealing in efforts to define a regulatory lineage of CD8 T cells.

Finally, Saligrama et al. observed coordinated mobilization of CD4 and CD8 T cells in people with recent-onset multiple sclerosis, suggesting that their findings in mice might also apply to humans. The identification of cell-surface markers that could be used to reliably isolate putative human CD8 regulatory T cells should offer insight into whether such cells contribute to human autoimmune disease. Moreover, defining the antigens that such cells recognize could pave the way to new clinical treatments.