Immune tolerance versus cancer surveillance: an ongoing challenge

Abstract

Maintenance of anticancer immunity without compromising immune tolerance remains a challenge. Immunosuppressive regulatory T cells (Tregs) are the guardians of immune tolerance that underpin prevention of autoimmune responses. Tregs express checkpoint molecules that suppress autoreactive immune cells from being activated by endogenous (self) proteins. However, expression of these checkpoints on immune cells contributes to cancer cell evasion from the immune system while inhibition of checkpoint signaling in cancer immunotherapy induces autoimmune-related adverse events. Two instructive cytokines that regulate the opposing functions of immunosuppressive Tregs and anticancer responses mediated by cytotoxic natural killer cells include the interleukins, IL-2 and IL-12. An overview based on published data is described herein to highlight how manipulation of these cytokines may have dual action in oncologic and autoimmune models. This raises the possibility of exploiting mechanisms in cancer immunotherapy that could promote anticancer immunity while still maintaining immune tolerance. Given the role of peptides in protein-related interactions, it remains important in rational drug design to remain vigilant for potentially beneficial effects that could benefit both cancer surveillance and immune tolerance.

Main text

The immune system not only responds to foreign proteins (antigens) presented by pathogens and cancer cells, but also acts to suppress autoreactive immune cells from being activated by endogenous (self) proteins. The latter constitutes immune tolerance to self-antigens and underpins avoidance of tissue damage characteristic of autoimmune pathology. Hence, immune tolerance and anticancer immunity represent two sides of the same coin. To varying degrees, the same cytokines (secreted cellular proteins) and their receptors are involved in both processes but work in opposite directions. The ongoing challenge is how to manipulate the immune system in such a way that anticancer immunity is enhanced without compromising immune tolerance and the discovery of immunosuppressive immune cells has provided great insight into mechanisms underlying autoimmune diseases.

Seminal work undertaken by the Laureates of the Nobel Prize in Physiology or Medicine in 2025 (Mary E Brunkow, Fred Ramsdell and Shimon Sakaguchi) has led to the critical finding that in the absence of one cell type controlled by a single gene, that is, immunosuppressive FOXP3-expressing regulatory T cells (Tregs), maintenance of immune tolerance is abrogated. The principal function of Tregs is to maintain immune homeostasis and suppress excessive immune responses to self-antigens by autoreactive effector T cells. Tregs have the capability to suppress the function of various immune cell populations, including T cells, B cells, natural killer (NK) cells, and antigen-presenting cells such as dendritic cells (DCs). Hence, Tregs are gatekeepers of immune tolerance and strategies to boost their function to maintain self-tolerance and prevent autoimmunity are the subject of intense investigation.

The groundbreaking discovery of Tregs is linked to a breakthrough in the field of cancer immunotherapy over the same period that focused on blocking immune checkpoints between immune cells within the tumor microenvironment (TME) to enhance antitumor immunity. Tregs are characterized by a high level of expression of immune checkpoints that serve to limit anticancer immunity and cancer immunotherapy has had an intriguing history not dissimilar to immune tolerance. James Allison was awarded the Nobel Prize in 2018 for his work on the checkpoint molecule, cytotoxic T-lymphocyte associated protein 4 (CTLA-4), as a potential therapeutic target in cancer treatment. Together with the programmed cell death protein 1 (PD-1) checkpoint, these receptors on T cells facilitate evasion of cancer cells from immune surveillance and activation of PD-1-mediated signaling induces production of Tregs together with increased FOXP3 expression. Unfortunately, Treg-mediated prevention of immune responses against self-antigens is not sustained when these signaling pathways are blocked. For example, immune-related adverse events (irAEs) have commonly been linked to immune checkpoint inhibitor (ICI) therapy using either anti-CTLA-4 and/or anti-PD-1 antibodies. This is highlighted in the first systematic review of irAEs associated with ICI therapy recently published by Barron and colleagues.¹ Non-endocrine irAEs were observed in more than 50% of patients treated with ICIs in whom the primary source for their metastatic cancer was mainly melanoma or non-small cell lung cancer.¹ Importantly, the site of irAEs appears to bear no relationship to tumor type and requires treatment with immunosuppressive agents¹ that have the potential to promote cancer cell evasion from the immune system.

Immune responses are also regulated by cytokines that bind to receptors on the cell surface, which activate downstream signaling pathways. Whether positive or negative signaling occurs is context-dependent and influenced by the cytokine milieu, cell–cell interactions, tissue cytokine levels and the kinetics of cytokine receptor binding. Moreover, not all receptors on immune cells require a ligand to transduce signals within a cell and may act as a decoy to inhibit ligand-induced signaling events. Two instructive cytokines that play key roles in modulating both Treg function and anticancer immunity demonstrate the complexity of cytokine signaling. One cytokine, interleukin-2 (IL-2) exerts differential dose-dependent effects. For example, FOXP3 Tregs do not produce IL-2 themselves but are critically dependent on low tissue concentrations of IL-2 for their survival via signaling through the IL-2 receptor (IL-2R) complex.²

The dependency of Tregs on IL-2 is seen in the autoimmune-related condition, psoriasis, where administration of low-dose IL-2 not only significantly improves the clinical manifestations of this disease but also leads to increased Treg numbers.³ In contrast, efficacy of higher IL-2 concentrations in the treatment of cancer is linked to its ability to activate cytotoxic lymphocytes.³ The IL-2R complex signals via beta (CD122) and gamma (CD132) chains and addition of the alpha subunit (CD25) to form the trimeric IL-2R (IL-2Rα/β/γ) results in higher affinity for IL-2 than dimeric IL-2Rβ/γ.⁴ Tregs express high levels of CD25, which act to deprive the TME of IL-2. Modified IL-2 therapies that bind only to CD122/CD132 to limit Treg activation show encouraging efficacy in preliminary clinical trials although a significant proportion of patients experience irAEs.^{4 5}

The functional heterogeneity of Tregs is also linked to the expression of the inhibitory transmembrane lymphocyte activation gene-3 (LAG-3) protein.⁶ LAG-3 effects on Treg biology depend on whether IL-2-responsive CD25⁺ Tregs or IL-2-producing effector T cells are predominantly affected. Notably, anti-LAG-3 combined with anti-PD-1 improves outcomes in patients with advanced melanoma notwithstanding increased irAEs over anti-PD-1 alone.⁷ Importantly, LAG-3-mediated regulation of peripheral tolerance is cell-type and tissue-type dependent presenting both challenges and opportunities for therapeutic targeting.⁶

The other instructive cytokine is IL-12. NK cells contribute to cancer surveillance and IL-2 primes NK cells by upregulating the expression of a receptor chain, β2, required for IL-12-mediated signaling. Of all five members of the IL-12 superfamily of cytokines (IL-12, IL-23, IL-27, IL-35 and IL-39) whose biological functions directly or indirectly affect various autoimmune diseases, the role of the specific member, IL-12, is perhaps the most complex. IL-12 exists as two-related proteins that are distinguished by their pairing of subunit molecules, p40 and p35. One isoform, IL-12p40, is comprised of paired p40 subunits whereas IL-12p70 is made up of p40 paired with a p35 subunit. The composite IL-12 isoforms have different receptor binding requirements and can antagonize one another with respect to Treg functionality.² This occurs, at least in part, because their biological activities are determined by both the tissue concentration of each isoform as well as the ratio between the two isoforms, including the expression levels of their respective receptor subunits.

Exposure of human immune cells to anti-PD-1 antibody in the laboratory promotes the expression of IL-12p40, but not the IL-12p70 isoform, associated with the production of interferon-gamma (IFN-γ).⁸ Notably, the efficacy of anti-PD-1 cancer therapy is attributed to a positive feedback loop between IL-12p40 and IFN-γ produced by DCs and T cells, respectively.⁹ However, a paradox exists in so far as autoimmune diseases such as systemic lupus erythematosus and psoriasis are linked to the expression of IL-12p40 and IFN-γ.³

Although antigen-presenting cells are responsible for the production of the IL-12 isoforms, increased expression of IL-12p40 has also been observed in T cells isolated from patients with cancer but not healthy donors.² Furthermore, a small lipidic peptide has recently been reported that selectively enhances the production of IL-12p70, but not IL-12p40, in cultured T cells obtained from healthy individuals, associated with inhibition of IL-12p40 and IFN-γ production by DCs and T cells, respectively.² This compound induces IL-2 production and promotes the expansion of FOXP3-expressing Tregs in vitro² accompanied by increased expression of the β2 chain of the IL-12p70 receptor complex in NK cells.⁸ While IL-2 enhances both Treg cell functionality and activation of NK cells, peptide-mediated NK cell activation, including β2 expression, can occur in an IL-2-independent manner.⁸ Hence, on the one hand, IL-2 produced by T-cell receptor-activated CD4⁺ T cells exposed to peptide becomes available to maintain Treg cell functionality. On the other hand, the “IL-2-like” action of the compound as reflected by increased expression of β2 in the absence of IL-2, may serve to promote responsiveness of NK cells to IL-12p70 and, thereby, enhance NK cell-mediated cytotoxicity.

Interestingly, suppression of lung cancer relies on the expression of the receptor chain β2, that is, essential for IL-12p70-mediated signaling.¹⁰ Notably, the peptide suppresses lung cancer and melanoma in immunocompetent murine models as well as psoriasis at the same dose,^{3 8} indicating dual action in oncologic and autoimmune models. These observations raise the possibility of exploiting mechanisms that enhance anticancer immunity without necessarily compromising immune tolerance, which offers a new paradigm in immune checkpoint research.

Conclusions

In summary, exactly how Tregs behave in different autoimmune conditions remains to be elucidated. In addition, while further research directed at suppression of Treg cell activity within the TME and delivery of the right cytokine to the right location will undoubtedly be informative, this may prove challenging because metastatic cancer is often located at multiple sites. Efforts to increase the expression of cytokine receptors in a cytokine-independent manner could be helpful by maintaining cytokine responsiveness at tissue concentrations less likely to induce an autoimmune response. Computational modeling of peptide-protein binding is also increasingly being used in rational drug design given that peptides are estimated to be involved in 15–40% of all protein-related interactions. Hence, it will be important to remain vigilant to the possibility of off-target, potentially beneficial effects given that opposing actions could benefit both cancer surveillance and immune tolerance.