In this issue, Du and colleagues uncover that optineurin functions as a key regulator of IFNγ receptor (IFNGR1) stability in malignant cells. Loss of optineurin in colorectal cancer cells causes IFNGR1 degradation, leading to impaired IFNγ signaling, decreased MHC-I expression, and enhanced ability to evade adaptive immune control.
See related article by Du et al., p. 1826.
Cancer immunotherapy—harnessing the patient's immune system to fight tumors—has demonstrated remarkable effects in the treatment of multiple malignancies, including melanoma, non–small cell lung carcinoma, and bladder cancer (1). Yet, the majority of patients with cancer do not respond to current immune-based interventions. Identifying, understanding, and targeting the mechanisms that tumors use to escape immune control are hence fundamental to improve the efficacy of cancer immunotherapy in the clinic. Various genetic screens have identified that alterations in IFNGR signaling or in components of the antigen presentation machinery in cancer cells can mediate resistance to T-cell killing (2). Indeed, in patients with melanoma, inactivating mutations in these pathways have been associated with acquired resistance to treatment with the immune-checkpoint blockers anti–PD-1 (3) and anti-CTLA4 (4). Nevertheless, patients with colorectal cancer who are often unresponsive to immunotherapy rarely present these mutations (5), indicating that additional mechanisms promoting immune escape are engaged in these tumors. Du and colleagues (6) now identify a positive regulator of IFNGR1 expression, optineurin, whose selective absence in cancer cells enables tumor immune evasion.
Through proteomic analyses of human colorectal cancer specimens, Du and colleagues found that optineurin expression was frequently reduced in malignant tissues compared with their normal adjacent counterparts. Low optineurin levels were associated with poor prognosis in patients with colorectal cancer and with decreased response to anti–PD-1 treatment in patients with melanoma. Of note, intratumoral immune cells such as T cells, B cells, macrophages, or dendritic cells did not exhibit decreased optineurin protein levels compared with the same populations residing in normal colon tissue, highlighting the selectivity of this defect to cancer cells. The authors found that optineurin was also lost in colon adenomas that precede tumor formation, indicating that optineurin deficiency is an early event during colorectal carcinogenesis. Although the mechanisms leading to optineurin loss in malignant cells remain unknown, this defect did not appear to be colorectal cancer–specific, because reduced optineurin transcripts were also found in breast and lung cancer tissues.
In order to recognize and kill tumors, T cells use their T-cell receptor (TCR) to identify antigenic peptides presented by major histocompatibility complex class I (MHC-I) molecules on the surface of cancer cells. Du and colleagues observed that optineurin expression in colorectal tumors positively correlated with the levels of genes related to the human MHC-I complex, specifically HLA-A, HLA-B, and HLA-C, leading to the hypothesis that optineurin may play a role in antitumor immunity. By using subcutaneously injected MC38 and CT26 cancer cell lines, as well as a model of colitis-associated cancer, the authors found that lack of optineurin in cancer cells accelerated tumor growth in immunocompetent, but not immunodeficient, mice. Although the absolute numbers of intratumoral T cells were unaltered, MC38 tumors devoid of optineurin showed increased proportions of infiltrating CD8+ and CD4+ T cells producing effector molecules such as Granzyme B, TNFα, and IFNγ. Hence, the quality, rather than the quantity, of intratumoral T cells appears to be defined by competent optineurin expression in colorectal cancer cells. Supporting this notion, weak responses to anti–PD-L1 treatment were observed in mice bearing optineurin-defective MC38 tumors, compared with their optineurin-sufficient counterparts.
Du and colleagues then found that optineurin-deficient malignant cells exhibited low MHC-I levels, which enabled them to evade T-cell recognition. IFNGR signaling enhances MHC-I expression through STAT1-mediated induction of the transcription factor IRF1 that binds the promoter of MHC-I genes (7). Notably, the authors demonstrated that decreased MHC-I in optineurin-deficient cancer cells was driven by diminished IFNGR1 expression and, therefore, reduced signaling through this receptor. Comparable transcript levels of the gene encoding IFNGR1 were found in optineurin-expressing and optineurin-deficient cancer cells, suggesting that optineurin might control IFNGR1 abundance via post-translational mechanisms. Indeed, using immunoprecipitation experiments coupled to mass spectrometry analyses, the authors identified adaptor-related protein complex 3 subunit delta 1 (AP3D1), a protein known to be involved in the trafficking of vesicles to the lysosome, as a binding partner of both IFNGR1 and optineurin. Remarkably, their data further indicated that optineurin loss provoked AP3D1-directed lysosomal degradation of IFNGR1 (Fig. 1).
Post-translational modifications can dictate the localization, stability, and abundance of proteins, and palmitoylation—a covalent attachment of palmitic acid to cysteine residues—plays a key role in this process (8). Importantly, palmitoylation has been implicated in the regulation of antitumor immunity as it stabilizes PD-L1 in colorectal cancer cells (9). Du and colleagues elegantly determined that the binding of AP3D1 to IFNGR1 was palmitoylation-dependent on the cysteine 122 residue of IFNGR1, and they further demonstrated that the in vivo inhibition of palmitoylation using cerulenin—a fungal natural product—improved the therapeutic effects of anti–PD-L1 treatment in mice bearing optineurin-deficient tumors.
The study by Du and colleagues therefore reveals an intriguing mechanism used by colorectal cancer cells to evade adaptive immune recognition: the downregulation of optineurin that leads to IFNGR1 degradation, disruption of IFNγ sensing and signaling, and inability to augment MHC-I expression in response to this cytokine (Fig. 1). These novel findings raise multiple interesting questions: (i) How does oncogenic signaling affect optineurin expression in cancer cells at early stages of tumor development? (ii) Beyond IFNGR1, what other proteins or cellular processes may be regulated by optineurin in cancer cells? Optineurin has been shown to play a role in type I interferon production via TANK-binding kinase 1 (TBK1) activation (10). Because type I interferon promotes antitumor immunity via multiple mechanisms, it would be important to test if the decreased activation status of T cells in optineurin-deficient tumors could also be attributed to defects in local type I interferon expression. (iii) How does loss of optineurin in cancer cells inhibit intratumoral CD4+ T-cell effector function? Although this study provides an explanation for reduced CD8+ T-cell function in optineurin-deficient tumors due to lack of MHC-I, whether optineurin is also necessary for direct activation of CD4+ T cells via sustained MHC-II expression remains to be defined. (iv) Could treatment with cerulenin affect palmitoylation events in noncancer cells of the tumor microenvironment to further improve anticancer immune responses? Of note, Du and colleagues showed that blocking palmitoylation with cerulenin in bone marrow–derived macrophages results in enhanced MHC-I expression, suggesting that optineurin might also regulate the antigen-presenting capacity of myeloid cells in cancer hosts.
Finally, the major findings reported by Du and colleagues in this issue suggest that controlling detrimental palmitoylation of IFNGR1 at early stages of neoplastic transformation using pharmacologic approaches, or even dietary interventions that restrict palmitate intake, may be useful to prevent or delay colorectal tumor progression by enhancing immune surveillance.
J.R. Cubillos-Ruiz is a member of the Scientific Advisory Board of NextRNA Therapeutics, Inc. No disclosures were reported by the other author.