IL-27 and the generation of CD8⁺ T-cell responses to peptide vaccines

It can be argued that the greatest increases in human longevity in the last 200 y have been the introduction of public sanitation and vaccination. Success in the latter has minimized the impact of a multitude of pathogens on human health. Historically, vaccine efficacy is equated with the generation of protective antibodies driven by immunization with attenuated pathogens or pathogen-derived toxins. Attenuated pathogens are not always without toxicity, which can lead to poor adoption of vaccines. Additionally, a major 21st century clinical goal is to generate therapeutic immunity to chronic viral infections and cancer. For these diseases, cytotoxic CD8⁺ T cells, rather than antibodies, could have more utility. As these clinical conditions are often characterized by suboptimal or even compromised systemic immunity, there is considerable interest in developing noninfectious approaches to elicit T-cell responses. The short peptides that T cells recognize, when presented at the cell surface by MHC molecules, have served as the backbone for T-cell vaccines. However, our knowledge of how to immunize for CD8⁺ T cells is limited; current peptide vaccines, which commonly use adjuvants that were designed to support the generation of antibodies, can promote CD8⁺ T-cell responses in the range of 1–5% of the CD8⁺ T-cell population. In contrast, CD8⁺ T-cell responses to viruses can reach 50% of the total CD8⁺ T-cell population in a host. By defining the critical components that promote subunit vaccines, such as the critical contribution of IL-27 as reported in PNAS by Pennock et al. (1), mechanistic insight into the regulation of CD8⁺ T-cell responses and the opportunity to increase vaccine efficacy can be realized.

Fig. 1.

Coordinated regulation of CD8⁺ T-cell expansion. IFN-1 promotes the production of IL-27 by antigen-presenting cells that directly induces the expression of Eomes and CD122 on recently activated CD8⁺ T cells, permitting their survival after primary or secondary expansion in response to peptide vaccines.

Numerous studies have shown that optimal CD8⁺ T-cell responses are highly dependent on the activation of dendritic cells (DCs) by helper CD4⁺ T cells. DC activation is often mediated by the stimulation of CD40 (2). Kedl’s group has previously shown a requirement for concurrent stimulation of pattern recognition receptors, such as Toll-like receptors (TLRs), along with CD40 for optimal CD8⁺ T-cell responses (3). This is a truly synergistic interaction, leading to the expansion of up to 40% of the CD8⁺ T cells in peripheral blood with specificity for the vaccine. The contribution of TLRs to the vaccine formulation has been shown to be highly dependent on type I IFN, which supports the expression of a critical costimulatory molecule, CD70, on DCs (4). However, a piece of the puzzle was missing: an inflammatory cytokine that would drive the transcriptional programs known to promote effector T cells' expansion and survival (5). Pennock et al. have found another piece of the subunit vaccine jigsaw: they identify that IL-27 provides a critical contribution to the expansion, differentiation, and survival of CD8⁺ T cells in response to peptide immunization.

IL-27 is a member of the proinflammatory IL-12 family, and its expression is induced in cells of the myeloid lineage, such as macrophages and DCs, by type 1 IFNs and TLR stimulation, thus linking it to the subunit vaccination approach used by Pennock et al. It has remarkably divergent and contrary, context-dependent, influences on T cells, but has been previously shown to contribute to effector CD8⁺ T-cell differentiation (6). Here, using a series of elegant bone marrow chimeras and sophisticated flow cytometric analyses, Pennock et al. show that IL-27 acts directly on CD8⁺ T cells and, consistent with other studies (7), promotes their survival (limiting activation-induced cell death rather than influencing initial expansion) at late stages of the primary response elicited by subunit vaccination. IL-27 also contributes to CD8⁺ T-cell differentiation, as in its absence, the primary CD8⁺ T-cell responders are poor producers of effector cytokines such as IFNγ. Furthermore, although memory CD8⁺ T cells surprisingly appear to form at equal proportions in the presence and absence of IL-27 signals, deficiency in IL-27 during the initial priming or during reactivation results in inefficient expansion on reencounter with antigen and ultimately weaker protection against pathogens. Interestingly, T cells expressing high affinity receptors are particularly susceptible to the absence of IL-27. Whether the parallel reduction in protection against infection found with IL-27Rα–deficient T cells is a consequence of a lower sensitivity to antigen or more related to the reduced levels of IFNγ produced by these cells remains to be clarified.

The functional deficiency that arises in the absence of IL-27 is associated with reduced expression of eomesodermin (Eomes) and CD122, a receptor subunit shared between the IL-2 and IL-15 receptors. The current study did not perform compensation assays for either Eomes or CD122, thus leaving the causal contribution of these molecules undefined. However, these are logical candidates. Eomes is a member of the T-box transcription family and serves as a critical player in the acquisition of effector functions by CD8⁺ T cells and cooperates with T-bet in regulating the development of memory cells from the effector pool (8). CD122 mediates IL-15 and IL-2 signaling, cytokines that have been linked to CD8⁺ T-cell memory formation and effector activity, respectively. The authors make a provocative argument that CD122’s absence could lead to metabolic perturbations in T-cell function as IL-2 promotes the glycolytic switch that is necessary for both effector cell expansion and function (9), and IL-15 promotes the reciprocal switch to fatty acid oxidation that is necessary for the development of functional memory cells (10). Thus, IL-27 may be working to coordinate the metabolic state of the expanding T cells. It will be of interest to determine whether downstream signaling elements from the IL-27 receptor promote the expression of Eomes and CD122 and other molecules that regulate the bioenergetic activity of CD8⁺ T cells.

Regardless of the mechanism, the stage has been set to test whether inclusion of exogenous IL-27 during vaccination can further improve the efficacy of the responding T-cell populations. Pertaining to this, the combination of anti-CD40 and IL-27 has limited ability to drive primary CD8+ T-cell responses to peptide vaccination compared to CD40 and TLR stimulation. This indicates that, although IL-27 is necessary for T-cell responses, it might not be sufficient, meaning other factors that play important roles in driving T-cell responses are likely elaborated by TLR stimulation, either alone or with concomitant CD40 stimulation. Cytokines such as type-1 IFN and IL-12, and costimulatory molecules such as CD70 [which interestingly

The current study by Pennock et al. argues for the inclusion of IL-27– or IL-27–inducing agents, such as TLR agonists or IFN-1, along with CD40 agonists for subunit vaccines.

has also recently been demonstrated to induce Eomes expression and the expression of IL-7R (11)], are good candidates. The role of other stimulatory molecules is supported by Pennock et al.’s observation that IL-27 is redundant during CD8⁺ T-cell responses to Listeria or Vaccinia infection. The logical conclusion from this is that infectious agents elicit sufficient quantities of a different cofactor that compensates for the loss of IL-27. IL-12 is strongly produced by both Vaccinia and Listeria and potently induces T-bet (12), which theoretically could overcome an insufficiency in Eomes. Comparison of peptide and pathogen immunizations may reveal the critical cofactors involved and identify whether there is interconnectivity between IFNs, CD70, and IL-27, allowing further development of this vaccination approach. Alternatively, in many contexts, IL-27 is anti-inflammatory: it redirects the differentiation of T cells; can induce immunoregulatory activity in DCs; and can induce the expression of programmed death ligand 1 (PD-L1) (13–15). These activities could restrain the ability of IL-27 to increase the magnitude of the CD8⁺ T-cell response. Future studies will no doubt segregate these issues and define when and where the pleiotropic effects of IL-27 are influential, which in turn may help refine the use of IL-27 clinically.

In the short term, the current study by Pennock et al. argues for the inclusion of IL-27– or IL-27–inducing agents, such as TLR agonists or IFN-1, along with CD40 agonists for subunit vaccines. If such combinations lead to the “viral”-magnitude responses that are achieved by clinically effective adoptive cell transfer therapy for cancer, then we might be on the road to reinvigorating peptide-based vaccines as a broadly available cancer therapy. In the longer term, mobilization of IL-27 may be an important prognostic immunological correlate for vaccination trials in HIV and cancer, helping to constrain the size and duration of such trials.