Abstract 5539: Mesenchymal stem cell altruism mediates a novel HMGB1/NKG2D immune response in oral cancer

Abstract

Abstract Background: Although cancer immunotherapy is now becoming a standard treatment method in oral cancer, but only about 20% cases respond to checkpoint inhibitors [1]. For example, Pembrolizumab a PDL1 inhibitor has not been showing good results to target hypoxic cancer stem cells (CSC). It may be attributed to the fact that immune editing mechanism is suppressed in cancer due to deficient function of immune cells like NK cells, T cells[2]. Thus, immune privileged tumor microenvironment may pose a major challenge to immunotherapyagainst CSCs. In the hypoxic tumor microenvironment (TME), CSCs interacts withmesenchymal stem cells (MSCs)and reprogram these cells to pro-tumorigenic cells. The interaction between CSC and BM-MSC may confer a stemness state to BM-MSCs, called Altruistic stemness; a mechanism where individual fitness is sacrificed for group fitness [3]. We speculate thatBCG internalized MSC may exploit the CSC-MSC interaction to target CSCs in the hypoxic niche. Methods: 1. ABCG2+ CSCs were infected with BM-MSC+BCG. 2. Acid-fast bacilli staining demonstrated the presence of BCG intracellular to BM-MSCs. The cells were lysed, and then subjected to CFU assay to quantify BCG colonies to confirm BCG internaliztion. 3. In vitro, in a Boyden chamber assay, BCG-MSCs and SCC-25 oral cancer cell line derived ABCG2+ CSCs were grown. Also, NK cells, T reg cells and macrophages were grown in the bottom chamber of Bodyen chamber assay. In this manner, NKG2D+mediated immune response can be evaluated. 4. After a week of culture, ABCG2+ cells were recovered from the bottom chamber, and ELISA was performed to detect HMGB1 and p53. The number of NKG2D+ NK cells, F/80 macrophages and CD8+ T cells were detected by flow cytometry. Results: We found that BCG+BM-MSC delivery to ABCG2+oral CSCs led to a marked immune response characterized by increase NKG2D+NK cells, F4/80 macrophages and CD8+ T cells and decrease in T-regcells(CD4+/FoxP3/CD25-) in the Boyden chamber assay. We also found that BCG-infected MSCs underwent stem cell altruism, and secreted HMGB1. Inhibition of HMGB1 led to marked reduction in NKG2D+ NK cells. Conclusions: These results indicate the potential activation of the NKG2D-mediated immune response by MSC derived altruistic stem cells. Overall our findings suggest that mesenchymal stem cell altruism mediated HMGB1/NKGD2 immune response in oral cancer can serve as a novel immunotherapeutic technique.