Targeting the immune microenvironment during immunotherapy for solid tumors

ABSTRACT

JAK/STAT signaling is a central hub in cancer development, progression, immunosurveillance and response to immunotherapy. We discuss recent advances in the role of the JAK/STAT pathway in immunotherapies. We stress the importance of fully understanding how JAK/STAT modifies the immune response before implementing clinical trials combining JAK/STAT inhibitors with immunotherapy.

Introduction

The JAK/STAT signaling pathway is a primary mediator of cellular responses to inflammation, but it also has functions in cancer and is among the top 12 signaling pathways aberrantly regulated in cancer. The JAK/STAT pathway not only coordinates intercellular communication between tumor cells and their immune microenvironment, it also regulates intracellular oncogenic processes. The impact of activated JAK/STAT signaling within tumor cells includes increased cell proliferation, cell survival, stem-like properties, self-renewal, epithelial-mesenchymal transition, and immunosurveillance. Here, we discuss the impact of JAK/STAT signaling on immunotherapy and provide an update on the potential efficacy of JAK/STAT inhibitors combined with cancer immunotherapies.

The role of JAK/STAT during cancer immunotherapy

Programmed cell death-1 (PD-L1) is an immune checkpoint protein that inhibits the T-cell response against tumor cells, and has received a great deal of recent attention due to the increased use of cancer immunotherapies. When PD-L1 is expressed on the surface of tumor cells, it binds to the PD-1 receptors on activated T-cells. This engagement of the PD-1 receptor with PD-L1 causes the inhibition of a particular subtype of T cells, called cytotoxic T-cells, which reduces the anti-tumor response. Immune checkpoint inhibitors work by interrupting this interaction. Immune checkpoint inhibitors that target PD-1 prevent the inhibition of the cytotoxic T cells, and therefore, reengage the anti-tumor T-cell response. PD-L1 is a well-known target gene of the JAK/STAT3 pathway, and activation of STAT3 causes increased PD-L1 expression across various types of epithelial cancers, notably melanoma and lung cancer.^1,2 The JAK/STAT pathway might also influence the immune microenvironment independently of PD-L1. Activated STAT3 was shown to decrease dendritic cell differentiation, which decreased CD8 T-cell activation.³ In addition, interleukin (IL)-6 is a cytokine that is also a STAT3 target gene, and when IL-6 is secreted, it results in the recruitment of myeloid-derived suppressor cells, further suppressing the function of anti-tumor cytotoxic T-cells. Because JAK/STAT3 signaling promotes an environment that suppresses tumor immune cell functions, inhibition of this pathway might enhance immunity against tumor cells.

Interestingly, functional mutations in proteins within the JAK/STAT pathway have been observed in many solid tumor types including prostate, urinary, lung and endometrial cancers. In particular, Janus Kinase 1 (JAK1) mutations are associated with a change in the types of immune cells that infiltrate the tumor microenvironment. Moreover, the well-known oncogenic JAK2^V617F mutation observed in numerous cancer types has been linked to increased tumor cell expression of PD-L1, although the impact of these mutations on sensitivity to immunotherapies, or the usefulness of these mutations as biomarkers for response, is not very clear. Much more work is needed to determine if and which JAK/STAT pathway activating mutations influence immunotherapy efficacy.

An important, but unanswered question in the field is whether immune checkpoint inhibitors supplemented with drugs that inhibit the JAK/STAT pathway would be an effective therapy for a subset of cancer patients. JAK/STAT inhibitors might synergize with immunotherapies,¹ partly because the same immune checkpoint pathway is targeted at two different, but critical, nodes. A few trials addressing the combination of immunotherapy and JAK/STAT inhibitors are on-going, including a trial in metastatic colorectal cancer combining pembrolizumab with the STAT3 inhibitor, BBI-608.⁴

Pharmacological inhibition of the JAK/STAT pathway inhibition might also improve responses to other types of immunotherapy, such as chimeric antigen receptor (CAR) T-cell therapy. CAR T-cell therapy is a promising cancer immunotherapy that was shown to be effective in leukemia and lymphoma, and is now being explored in solid tumors. Unfortunately, CAR T-cell therapy can cause a negative side effect called cytokine release syndrome, which is characterized by a rapid and abundant secretion of cytokines including IFNγ and IL-6. Itacitinib, a JAK1 inhibitor, reduced the cytokine release caused by CAR T-cells, in both in vitro and in vivo models,⁵ and the results were so promising that a phase II clinical trial combining CAR T-cell therapy with itacitinib was started (NCT04071366). Efforts are also underway to determine why certain patients treated with CAR T-cell therapy experience cytokine release syndrome, with a focus on JAK/STAT signaling.

Counterintuitive to what was presented above, a decrease in or loss of PD-L1 expression caused by JAK/STAT inhibition might reduce the efficacy of immunotherapy rather than enhance it. When the JAK/STAT pathway is fully functional, it responds to the actions of IFNγ from the tumor microenvironment, by providing signals to tumor cells to inactivate the antitumor T-cells and upregulating PD-L1, among other mechanisms mentioned above, allowing the tumor cells to escape their cytotoxic effects. However, it was recently reported that loss-of-function mutations in JAK1 and JAK2 were found in cases of acquired resistance to pembrolizumab in melanoma.⁶ This may be explained by a lack of PD-L1 expression, caused by the inability of tumor cells to respond to IFNγ. This work was supported by a separate study in non-small cell lung cancer, in which JAK2-inactivating mutations were associated with lower PD-L1 expression in tumor cells, enabling immune tolerance.⁷ Although inactivating JAK2 mutations are rare in solid tumors among certain populations, they are more common in lung tumors from African Americans,⁸ making this more important to determine the impact of these mutations on immunotherapy. Thus, it stands to reason that pharmacological inhibition of JAK/STAT signaling would have a similar effect as inactivating mutations, and reduce the response to immune checkpoint inhibitors. JAK/STAT pathway mutations might also impact other types of immunotherapies. Loss of JAK2 by genomic deletion in melanoma was identified in non-responders to the anti-CTLA-4 therapy, ipilimumab, suggesting that loss of JAK2 loss and the IFNγ response was associated with resistance.⁹ Therefore, a combination of JAK/STAT pathway inhibitors with immunotherapy needs additional investigations.

The crosstalk between different STAT proteins also requires consideration. In certain tumor microenvironments, specific signals can alter both the duration and intensity of STAT1 and STAT3 activation. STAT1 differs from STAT3 by blocking cell cycle progression and inhibiting angiogenesis within the tumor, as well as inducing anti-proliferative and pro-apoptotic responses in tumor cells. In contrast, STAT3’s role in tumor progression and therapy resistance continues to emerge, as does the impact of STAT3 as a suppressor of immune cells’ functions within the tumor microenvironment. Interestingly, several proteins that are upregulated by tumor cells upon STAT3 inhibition are well-known STAT1 targets (e.g., CXCL10, CCL5, ICAM1), suggesting that a reciprocal regulation between STAT3 and STAT1 occurs within tumor cells. In addition, IFNβ, which activates STAT1 and STAT2 to a complex with IRF9, creates the transcription factor complex ISGF3. Signaling through IFNβ/P-ISGF3 can induce mesenchymal-to-epithelial transition, differentiation into a less aggressive epithelial cell state, decrease tumor cell invasion and migration, and reduce stem cell-like properties.¹⁰ STAT5 also contributes to immune cell functions. In contrast to STAT3, when STAT5 is activated, it increases antitumor immune functions particularly within NK cells. Thus, inhibition of STAT5 could have negative effects anti-tumor immunity and promote tumor progression.

Summary

JAK/STAT signaling is a central hub bridging inflammation with cancer development, progression, and immunosurveillance. While JAK/STAT inhibitors are being explored as therapeutic agents to sensitize tumors to various immunotherapies, caution is needed until the molecular underpinnings of how modulating the JAK/STAT pathway influences response to immunotherapy are fully defined.

Funding Statement

This work was supported by a grant from the National Cancer Institute [R01CA239093].

Disclosure statement

No potential conflict of interest was reported by the author(s).