Abstract
Metastatic and relapsed Ewing sarcoma typically afflicts the adolescent population and is largely fatal. These bone tumors are most commonly driven by the fusion oncoprotein EWS-FLI1. Ewing tumors demonstrate significant intra-tumoral heterogeneity, and individual tumor cells can express highly variable and dynamic levels of EWS-FLI1. Recent studies revealed that the EWS-FLI1 oncoprotein level (high versus low expression) greatly influences the behavior of Ewing tumor cells. As compared to cells with high EWS-FLI1, Ewing cells in the EWS-FLI1 low state demonstrate an increased propensity for metastasis. In light of these observations, we sought to determine how tumor cell EWS-FLI1 level influences the anti-tumor cell immune response. Since ICAM-1, which can promote tumor cell/T-cell interaction and T-cell activation, is highly expressed on EWS-FLI1 low cells, we hypothesized that EWS-FLI1 low cells would be more susceptible to T-cell mediated tumor cell apoptosis as compared to cells with high EWS-FLI1. Unexpectedly, we found that EWS-FLI1 low cells are more resistant to T-cell mediated apoptosis than EWS-FLI1 high cells. We investigated the potential mechanisms by which EWS-FLI1 level might influence the T-cell anti-tumor response, and discovered that low EWS-FLI1 expression results in upregulation of PD-L1 and PD-L2, both important ligands for the PD-1 immune checkpoint receptor on T-cells. We demonstrated that blocking PD-1 results in a greater increase of T-cell mediated killing of EWS-FLI1 low tumor cells as compared to cells with higher EWS-FLI1 expression. Our studies suggest that Ewing cells in the EWS-FLI1 low expression state may serve as a niche of tumor immune-evasion.
Highlights
Ewing sarcoma is a cancer of the bone or soft tissue that is driven by EWS-ETS family member fusion oncoproteins, with the majority of tumors harboring an EWS-FLI1 fusion [1]
Since we found that EWS-FLI1 low cells demonstrate an enhanced transcriptional response to IFN-γ, we chose to examine the expression of PD-L1 and PD-L2, both negative regulators of T-cell function whose transcription can be induced by IFN-γ [30]
We examined the impact of PD-1 blockade on the susceptibility of EWS-FLI1 high and low tumor cell susceptibility to T-cell mediated apoptosis and found that PD-1 blockade enhances T-cell mediated tumor cell apoptosis to a greater degree for the EWS-FLI1 low cells (Figure 6C). These results suggest that despite having high ICAM-1 expression, EWS-FLI1 ‘low’ cells are able to evade T-cell mediated apoptosis by simultaneously upregulating factors that negatively impact the T-cell anti-tumor response, such as PD-L1 and PD-L2 (Figure 7)
Summary
Ewing sarcoma is a cancer of the bone or soft tissue that is driven by EWS-ETS family member fusion oncoproteins, with the majority of tumors harboring an EWS-FLI1 fusion [1]. Clinical trials investigating various single agent and combination cytotoxic chemotherapies have not resulted in improved outcomes for patients with advanced Ewing sarcoma. Pediatric tumors, including Ewing sarcoma, have demonstrated less immune cell infiltration into the tumor and appear to have a more immunosuppressive microenvironment than many adult tumors [5,6,7]. Despite this observation, significant immune cell infiltration and peri-tumoral inflammation are seen at key points during solid tumor therapy, such as at the time of radiation and/or chemotherapy induced tumor cell apoptosis [8, 9]. Despite much effort by multiple laboratories, it has not yet been possible to develop a syngeneic/ transgenic mouse model of Ewing sarcoma [12], and as a result, very little is known about the Ewing sarcoma immune-microenvironment and the tumoral factors that promote immune evasion
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