Page 68 - Haematologica Vol. 109 - July 2024

ARTICLE - Leukemia-induced interferon signature in stroma M.W.E. Smeets et al.
ALL cells induce an IFN-related gene signature in MSC, which was dependent on IFNα/b signaling, but independent of IFNγ. The IFN gene signature of MSC was leukemia-dependent since a mixed population of normal cord blood cells had a limited effect on expression levels of these genes in MSC, with an exception for monocyte-containing samples which induced, among others, CXCL10 expression. Monocytes, however, were absent in our BCP-ALL samples, strengthen- ing the leukemia-driven origin of the IFN signature in MSC. BCP-ALL samples with an ETV6-RUNX1 translocation were the most potent inducers of the IFN-related gene expression signature in MSC. This was true for both primary BCP-ALL samples as well as an ETV6-RUNX1 cell line. In correspon- dence, we observed that addition of IFNα/b inhibitors (but not an IFNγ inhibitor) to co-cultures of ETV6-RUNX1 BCP- ALL and MSC reduced the expression levels of IFN-related genes in the MSC compared to those in MSC that were co-cultured in the absence of these inhibitors.
ETV6 is known to be a transcriptional activator of a vari- ety of IFN genes,22 and in correspondence higher baseline expression levels of IFN-related genes have been found in ETV6-RUNX1-driven leukemia.23 We also noticed some higher baseline levels in ETV6-RUNX1-positive ALL than in B-other ALL cases, but more remarkable was the up- regulation of IFN-related genes in MSC upon exposure to ETV6-RUNX1-positive cells. This upregulation was partially dependent on close cell-cell contact, since we noticed that the expression was reduced when BCP-ALL cells and MSC were physically separated in transwell experiments. In line with this, Dander et al. showed that the ETV6-RUNX1 fusion can trigger modifications in adhesion molecule expression and adherence capacity of B-precursor cells.24
Members of the IFN family are known to combat viral infec- tions, modulate immune responses, and stimulate antitu- mor activities.25-27 We noted that BCP-ALL cells, irrespective of ETV6-RUNX1 status, downregulated expression of the MSC-IFNα/b receptor (IFNAR1) and secreted IFNα/b levels in co-culture with MSC. The most likely explanation is that this occurs due to receptor internalization upon ligand bind- ing, which is part of the functional process of IFN-pathway activation.28 Internalization of the IFNAR1/2-IFNα/b complex may be visualized by a fluorescence-based imaging method using a pH-sensitive dye that binds the receptor and will only fluoresce in acidic environments inside the cell.29,30 Recently, interferons were shown to elicit immune sup- pressive mechanisms in cancer, which may promote cancer progression and induce therapy resistance.31,32 Several stud- ies have investigated the expression profile of IFN-related genes in cells derived from distinct cancer types.33-36 A subset of IFN-related genes, including the genes we found upregulated in MSC, i.e., IFI27, OAS1, OAS3, MX1 and ISG15, was persistently overexpressed in tumor cells resistant to chemotherapy or radiotherapy.33,34,36 We here observed that these same genes were induced in non-malignant
stromal support cells, i.e., the MSC, upon interaction with ETV6-RUNX1-positive ALL cells. MSC provide a survival benefit to BCP-ALL cells and protect these cells against chemotherapeutic drugs.9-12 However, we did not observe that the IFN-induced gene signature causally contributed to these processes since neither silencing of individual IFN-related genes or the key pathway regulator STAT1 in MSC nor blockade of IFNα/b signaling counteracted this MSC-mediated benefit. Our data therefore imply that the ETV6-RUNX1 BCP-ALL-induced IFNα/b signature in MSC serves a different role, e.g., attracting other cells, which are beneficial in maintaining leukemia, to the leukemic niche. IFNα/b are involved in recruitment, function, maturation and/ or activation of immune cells, such as NK cells, monocytes and dendritic cells.37,38 Cytokine and chemokine production can be induced by IFNα/b and may help to recruit inflamma- tory monocytes and NK cells to the site of inflammation.31,38 In contrast, prolonged activation of IFNα/b responsive genes can also suppress normal immune cell functions. In solid tumors, migration of NK cells towards the tumor site was observed. Trapped in the tumor microenvironment these NK cells downregulated NKG2D, the receptor that is important in defense against cancer by the immune system.39 This net- work of trapped, but not functional (innate) immune cells, resulted in a condition promoting tumor cell proliferation, survival, and metastasis.39,40 However, increased IFN-related gene expression in MSC after contact with leukemic cells may also reflect an anti-leukemic response of MSC in the tumor microenvironment to attract other immune cells.31,41,42 A recent study by Kumar et al. showed that reduced pro- duction of IFNα/b, leading to decreased NK-cell surveillance, promotes development of B-ALL in vivo.42 Restoring this IFNα/b production allows for NK effector cells to reduce leukemia progression in mice that are prone to MYC-driven B-ALL. In high-risk acute myeloid leukemia, IFNα/b admin- istration after hematopoietic stem cell transplantation may be effective in preventing relapse.43 As interferons are often described as cytokines with a dual role,26,31,38 it still needs to be determined whether the observed IFNα/b response in our study is advantageous or detrimental to leukemic cells. In conclusion, our study reveals that IFNα/b but not IFNγ contribute to an IFN signature elicited by BCP-ALL in MSC. The induction of IFN-related genes in MSC did not affect the viability and level of drug resistance of BCP-ALL cells observed upon exposure to MSC. Our data warrant further studies into the role of the BCP-ALL-induced IFNα/b re- sponse, especially in the context of ETV6-RUNX1-positive ALL. This may increase our understanding of how leukemic cells manipulate the immune microenvironment, as recent- ly suggested for IFNα/b-dependent activation of NK cells and dendritic cell-presenting leukemia-specific antigens. This knowledge is important in the emerging field of cell- based immunotherapies that are being applied ever more frequently in BCP-ALL.3,42
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