Research Article

Unresolved endoplasmic reticulum stress engenders immune-resistant, latent pancreatic cancer metastases

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Science  15 Jun 2018:
Vol. 360, Issue 6394, eaao4908
DOI: 10.1126/science.aao4908

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Chronic stress as a survival tactic

Most patients with pancreatic ductal adenocarcinoma (PDA) develop liver metastases after surgical removal of their primary tumor. These metastases are thought to potentially arise from quiescent disseminated cancer cells, likely present at the time of surgery, which evade elimination by the immune system. Pommier et al. explored how these quiescent cells survive by analyzing mouse models and tissue samples from patients with PDA. They found that disseminated cancer cells do not express a cell surface molecule that triggers killing by T cells. This phenotypic feature is linked to their inability to resolve endoplasmic reticulum stress. When this stress is resolved, the disseminated cells begin proliferating and form metastases.

Science, this issue p. eaao4908

Structured Abstract

INTRODUCTION

Pancreatic ductal adenocarcinoma (PDA) is the fourth most common cause of death from cancer worldwide and has a 5-year survival rate of 6%. Patients who have had their primary PDA surgically resected often develop metastatic disease, despite intra-operative examination of the liver confirming the absence of macrometastatic lesions. These observations lead to the conclusion that latent metastases, detectable only microscopically, were present in these patients and were responsible for the postoperative development of metastatic disease.

RATIONALE

Latent metastases with the potential for outgrowth had been considered to be lesions in which cancer cell proliferation is balanced by immune-mediated cancer cell death, but a more recent explanation invokes quiescent, single disseminated cancer cells (DCCs). Single, nonreplicating DCCs have been observed in several cancer types, but whether quiescence is enforced by the microenvironment or is cancer cell–autonomous is not known. Immunity, both innate and adaptive, also is likely to have a role in the selection and/or maintenance of latent DCCs. This has long been suspected on the basis of the clinical observation of donor-derived cancer in immune-suppressed recipients of allografts. However, there is an unexplained paradox of immunity preventing the outgrowth of latent metastases but not eliminating them.

RESULTS

We studied the metastatic process in the context of an ongoing adaptive immune response because of the occurrence of cancer cell–specific immunity in human and mouse PDA. Livers from patients and mice with PDA contained single DCCs with an unusual phenotype of being negative for cytokeratin 19–negative (CK19)and major histocompatibility complex class I (MHCI). The absence of the expression of MHCI in DCCs and the occurrence of cancer-specific CD8+ T cells in the genetically engineered mouse model of PDA, and possibly in patients with PDA, suggested that DCCs may be selected by an anticancer immune response during the metastatic process.

To investigate this hypothesis, we created a mouse model that would allow us to determine how DCCs develop, their relationship to metastatic latency, and the role of immunity. Intraportal injection of immunogenic PDA cells into preimmunized mice seeded livers only with single, nonreplicating DCCs lacking MHCI and CK19, whereas naïve recipients of PDA cells had macrometastases. We found that a subpopulation of PDA cells with the phenotype of DCCs was present in vitro and that those cells are the precursors of DCCs in vivo. We found that T cells select DCCs by eliminating MHCI+ proliferating cancer cells.

To identify the cell-autonomous “switch” regulating the developmental state of the metastases, we preformed single-cell RNA sequencing of PDA cells with the DCC phenotype. This transcriptomic analysis demonstrated an endoplasmic reticulum (ER) stress response. Moreover, DCCs showed a lack of activation of the IRE1α (inositol-requiring enzyme 1α) pathway of the unfolded protein response, whereas the PERK (protein kinase RNA-like ER kinase) pathway was activated, suggesting that DCCs cannot resolve ER stress. Relieving ER stress pharmacologically with a chemical chaperone or genetically by overexpression of spliced XBP1, in combination with T cell depletion, stimulated outgrowth of macrometastatic lesions containing PDA cells expressing MHCI and CK19.

CONCLUSION

We find that a PDA-specific adaptive immune response selects DCCs, in which the ER stress response accounts for both quiescence and resistance to immune elimination. Accordingly, outgrowth of DCCs to macrometastases requires not only relief from the cancer cell–autonomous ER stress response, but also suppression of systemic immunity. Thus, the ER stress response is a cell-autonomous reaction that enables DCCs to escape immunity and establish latent metastases.

Unresolved ER stress allows disseminated cancer cells to escape the T cell response.

Quiescent cancer cells that exhibit unresolved ER stress lack expression of MHCI. They avoid killing by T cells and become latent disseminated cancer cells (DCCs). Resolution of ER stress allows DCCs to regain proliferative capacities and grow as overt metastases, only if the T cell response is disrupted, because they also regain the expression of MHCI. Ecad, E-cadherin.

Abstract

The majority of patients with pancreatic ductal adenocarcinoma (PDA) develop metastatic disease after resection of their primary tumor. We found that livers from patients and mice with PDA harbor single disseminated cancer cells (DCCs) lacking expression of cytokeratin 19 (CK19) and major histocompatibility complex class I (MHCI). We created a mouse model to determine how these DCCs develop. Intraportal injection of immunogenic PDA cells into preimmunized mice seeded livers only with single, nonreplicating DCCs that were CK19 and MHCI. The DCCs exhibited an endoplasmic reticulum (ER) stress response but paradoxically lacked both inositol-requiring enzyme 1α activation and expression of the spliced form of transcription factor XBP1 (XBP1s). Inducible expression of XBP1s in DCCs, in combination with T cell depletion, stimulated the outgrowth of macrometastatic lesions that expressed CK19 and MHCI. Thus, unresolved ER stress enables DCCs to escape immunity and establish latent metastases.

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