Researchers discovered that there are many human melanoma cell lines that have completely lost expression of STING


Immunotherapy has become a standard treatment approach for several types of cancer, including melanoma.

However, tumors can escape immune cell detection even with the use of immunotherapies.

In a new study published in Cancer Immunology Research, Moffitt Cancer Center researchers, in collaboration with the University of Miami’s Miller School of Medicine, describe a cellular mechanism that controls tumor cell recognition by immune cells.

Agents that activate the immune system in advanced melanoma have significantly improved outcomes, and many patients have long-term responses following immunotherapy treatment.

However, according to James Mulé, Ph.D., associate center director of Translational Science at Moffitt, “There remains a subset of melanoma patients treated with immune-based therapies who do not achieve clinical benefit.

Understanding the mechanisms underlying both successful and failed immune responses may help improve immunotherapeutic approaches.”

Moffitt researchers believe that one reason why patients may not respond well to immunotherapies is because their immune system does not recognize tumor cells properly.

They hypothesized that the STING protein signaling pathway may be a contributing factor to immune cell recognition.

The STING pathway is known to contribute to the activation of the immune system by stimulating the production of protein messengers called interferons.

Defects in STING signaling have been reported in several different types of cancer, including melanoma; however, the impact of these defects is unclear.

In order to improve our understanding of how defects in STING signaling may contribute to cancer, the Moffitt team conducted a series of laboratory experiments in human melanoma cell lines.

The researchers discovered that there are many human melanoma cell lines that have completely lost expression of STING and do not respond to signaling that activates the pathway.

They also found that some human melanoma cell lines maintain STING expression, but are still incapable of activating the signaling pathway.

This suggests that an unknown alternative mechanism may also be responsible for defects in STING signaling, other than loss of STING expression itself.

In order to determine how STING signaling functions under normal circumstances, the researchers conducted experiments in human melanoma cell lines that express a functional STING pathway.

They discovered that activation of STING results in production of the molecular messengers interferon-beta and CXCL10 that stimulate inflammation and an immune response.

Activation of STING also caused human melanoma cells to increase the expression level of proteins called MHC molecules on their cell surface that allow them to be recognized and targeted by immune cells called T cells.

Conversely, human melanoma cells that had dysfunctional STING signaling were much less able to be recognized by T cells.

These observations suggest that one mechanism by which tumor cells bypass immune detection may be through alterations in the STING pathway.

The researchers hope that their work will lead to an improved understanding of immune cell activation and better treatment approaches for patients.

“Further understanding of the regulation and function of STING in melanomas and other tumor types may lead to the development of strategies that target the STING pathway to improve the efficacy of adoptive cell therapy and other immunotherapies in patients who do not currently benefit from these interventions,” said Mulé.

Tumor immunotherapy has changed the therapeutic landscape for an increasing number of patients with cancer.1 Yet, many patients do not respond to treatment and the mechanisms of innate and acquired drug resistance are incompletely understood.2 

Solid tumors are generally characterized by the presence or absence of tumor-infiltrating lymphocytes.3 

Indeed, studies in metastatic melanoma patients treated with programmed cell death 1 (PD-1) inhibitors revealed an association between T-cell infiltration and clinical response to immune checkpoint blockade.4 

The homeostatic mechanisms regulating the development of this so-called T cell-inflamed tumor microenvironment are being elucidated and appear to depend on patterns of intracellular signaling within tumor cells as well as innate features of the host immune system.

The presence of high tumor cell mutation burden, enriched neoantigen T cell repertoire, availability of tissue resident basic leucine zipper ATF-like transcription factor 3 (Batf3)+ dendritic cells (DCs) and expression of an interferon-related pro-inflammatory gene expression profile have correlated with improved therapeutic responses to immunotherapy.59

Recently, the cyclic guanosine monophosphate (GMP)–adenosine monophosphate (AMP) synthase (cGAS) and stimulatory of interferon genes (STING) complex has been implicated as a key intracellular regulator of host T cell recruitment to the tumor microenvironment in melanoma.10 

cGAS is a DNA sensor that responds to genotoxic cell stress by binding to abnormal DNA in neoplastic cells and activating STING, which serves as an adaptor protein that triggers innate immunity through type I interferon gene expression, release of chemokines CXCL9 and CXCL10, and ultimately recruitment of T cells.10,11 

While cGAS-STING signaling explains the presence of tumor-infiltrating T cells, this pathway also enhances expression of several counter-regulatory immune parameters, including expression of PD-1 ligand 1 (PD-L1), accumulation of CD4+FoxP3+ regulatory T cells, and production of indoleamine 2,3-dioxygenase (IDO), all factors that inhibit host anti-tumor immunity.6,12 

Thus, effective immunotherapy requires both T cell recruitment to the tumor microenvironment and suppression of the homeostatic counter-regulatory pathways.

This explains why the presence of T cells, per se, is insufficient to mediate tumor regression in the absence of immune checkpoint inhibition. In addition, re-establishing cGAS-STING activation in tumors with deficient type 1 interferon responses has been suggested as an important strategy for converting lymphoid-deficient tumors (i.e., “cold” tumors) into T cell-inflamed tumors (i.e., “hot” tumors).13,14

Oncolytic viruses are viral vectors that preferentially replicate in tumor cells, inducing immunogenic cell death (ICD) and promoting host anti-tumor immune responses.13 

The preferential replication in tumor cells is based on several features, including deficiencies in tumor cell anti-viral machinery elements and defective type 1 interferon signaling as compared to normal, non-neoplastic cells.15

 Additionally, oncolytic viruses are thought to enhance ICD through release of danger-associated molecular pattern (DAMP) factors and soluble tumor-associated antigens that cooperate to induce innate and adaptive immune responses, although this has not been confirmed for most oncolytic viruses.13 

The ability of oncolytic viruses to recruit T cells to the tumor microenvironment and promote ICD with release of tumor-associated antigens suggests that oncolytic immunotherapy is especially well suited for converting T-cell-deficient tumors into T cell-inflamed tumor microenvironments, which should further enhance systemic immunotherapy.16

The first oncolytic virus approved in the U.S. for the treatment of cancer is talimogene laherparepvec (T-VEC), an attenuated herpes simplex virus, type 1 (HSV-1) encoding granulocyte-macrophage colony stimulating factor (GM-CSF), which was approved based on an improvement in durable and objective response rates in patients with advanced melanoma.17 

Treatment with T-VEC has also been associated with infiltration by melanoma-specific CD8+ T cells.18 Furthermore, HSV-1 is known to trigger cGAS-STING and initiate strong type 1 interferon production.19

 In addition, Batf3+ DC is known to be especially competent at presentation of HSV-1-related antigens.20 Thus, oncolytic viruses, such as T-VEC, might provide a more natural way to utilize innate elements of the anti-viral response to kill tumor cells in a more immunogenic manner while promoting systemic anti-tumor immunity.

A better understanding of how oncolytic viruses induce ICD might also suggest new targets for combination therapy in melanoma and potentially other tumors permissive to oncolytic virus infection.21

Thus, in this report, we sought to explore the molecular factors involved with T-VEC-mediated ICD in melanoma cells and determine which intracellular factors are important for promoting viral replication and promoting anti-tumor immunity.

We hypothesized that T-VEC would induce ICD through release of defined DAMPs and would promote T cell recruitment to established melanomas through type 1 interferon-related factors, including CXCL9 and CXCL10, as well as a pro-inflammatory gene signature profile. In addition, we found that specific components of the anti-viral machinery, such as STING, were critical for both T-VEC permissive replication and induction of host anti-tumor immunity.

Tumors which have low levels of STING show minimal response to anti-PD-1 therapy but respond to T-VEC treatment. Further, T-VEC treatment induced a systemic anti-tumor specific CD8+ T cell response and increased immune inflammatory gene signature both in injected and contralateral tumors, leading to regression of un-injected tumors.

These data support the role of T-VEC in tumors with low STING levels and confirms how T-VEC mediates melanoma ICD and triggers innate and adaptive anti-tumor immunity.

More information: Rana Falahat et al, STING Signaling in Melanoma Cells Shapes Antigenicity and Can Promote Antitumor T-cell Activity, Cancer Immunology Research (2019). DOI: 10.1158/2326-6066.CIR-19-0229

Provided by H. Lee Moffitt Cancer Center & Research Institute


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