Lung cancer-initiating cells avoid immune recognition

Steel, JC ORCiD: 0000-0003-3608-7542Background: Increasing evidence supports the concept of a unique population of cells within a tumor with stem cell-like characteristics. These cancer-initiating cells (CICs) are thought to be responsible for tumor organization, maintenance, progression and recurrence, as well as contributing to radiation and chemotherapy resistance. In this study we outline a new characteristic of CICs, one in which the CIC can subvert the host immune response. We pose that this characteristic is essential for the establishment of a tumor in an immunocompetent host and may represent a mechanism in which lung tumors may gain resistance to immune based therapies. Methods: We examined whether lung CICs could be enriched from murine and human cell lines through the use of tumorsphere culture assays. We vaccinated mice against the HPV E7 antigen and challenged mice with HPV E6/7 expressing murine lung cancer cells (TC-1) enriched for CICs. We also examined ex-vivo the capacity of CD8 and NK cells derived from vaccinated mice to lyse CICs. We examined by flow cytometry the expression of MHC-I and NK activating ligands. We examined the effect of interferon gamma (IFN-γ) on MHC-I expression by CICs and determined whether increased MHC-I by CICs would inhibit tumor formation in an immunocompetent animal. Results: We showed that both murine and human lung cancer cells can be grown and maintained in tumorsphere culture conditions. The resulting cells were enriched for CICs as evidenced by increased OCT4, NANOG, and/or SOX2 expression. Additionally, tumorsphere cultures of murine tumor lines had increased tumor take in immunocompetent animals, however this was significantly less than that seen in immunodeficient mice; indicating that true CICs must be able to thwart the immune response to establish tumors. In order to further examine this, we vaccinated mice with HPV E7 peptides and challenged with TC-1 non-CICs or CICs. We showed that mice challenged with CICs had no survival advantage compared to non-vaccinated animals; whereas those animals challenged with non-CICs exhibited a vaccine induced survival advantage. Further we showed ex-vivo that CICs were resistant to lysis from CD8+ T-cells compared to the non-CICs. Next, we showed that both murine and human lung CICs have down-regulated expression of MHC-I as well as a number of ligands for NK cell activating receptors, essentially making them “opaque” to the immune system and therefore less susceptible to CTL or NK cell lysis. Further, we demonstrate that IFN-γ increases MHC-I expression on CICs and restores sensitivity to CTL killing. Finally, we demonstrate that decreased MHC-I expression is a critical component of the ability of CICs to establish tumor in immunocompetent hosts. Conclusion: Increasing evidence indicates that CICs are critical players in the development and establishment of cancers. In this study we demonstrated that these cells also subvert tumor immune surveillance and play a key role in the resistance of tumors to cancer vaccines through their ability to escape host immune responses. Our results demonstrated that modulation of MHC-I on CICs can alter their susceptibility to T-cell mediated lysis thus opening a new target for cancer vaccine strategies