Tumour cell-induced senescence-like state of macrophages plays a pivotal role in tumour stable growth after tumour cell emergence in immunocompetent condition

Abstract Background: The cancer stem cell theory proposes that tumour formation in vivo is driven only by specific tumour-initiating cells having stemness; however, clinical trials conducted to test drugs that target tumour stemness have provided unsatisfactory results thus far. Recent studies showed clear involvement of immunity in tumours; however, the requirements of tumour-initiation followed by stable growth in immunocompetent individuals remain largely unknown. Methods: To clarify this, we used two similarly induced glioblastoma lines, 8B and 9G. They were both established by overexpression of an oncogenic H-RasL61 in p53-deficient neural stem cells. In immunocompromised animals in an orthotopic transplantation model using 1000 cells, both show tumour-forming potential. On the other hand, although in immunocompetent animals, 8B shows similar tumour-forming potential but that of 9G’s are very poor. This suggests that 8B cells are tumour-initiating cells in immunocompetent animals. Therefore, we hypothesised that the differences in the interaction properties of 8B and 9G with immune cells could be used to identify the factors responsible for its tumour forming potential in immunocompetent animals and performed analysis. Results: Different from 9G, 8B cells induced senescence-like state of macrophages around tumours. We investigated the senescence-inducing factor of macrophages by 8B cells and found that it was IL-6. Such senescence-like macrophages produced Arginase-1, an immunosuppressive molecule known to contribute to T cell hyporesponsiveness. The senescence-like Mφs highly expressed CD38, a nicotinamide adenine dinucleotide (NAD) glycohydrolase associated with NAD shortage in senescent cells. The addition of nicotinamide mononucleotide (NMN), a NAD precursor, in vitro inhibited to induction of macrophage senescence-like phenotype and inhibited Arginase-1 expression resulting in retaining T cell function. Moreover, exogenous in vivo administration of NMN after tumour inoculation inhibited tumour-initiation followed by stable growth in the immunocompetent mouse tumour model. Conclusions: We identified one of the requirements for tumour-initiating cells in immunocompetent animals. In addition, we have shown that tumour growth can be inhibited by externally administered NMN against macrophage senescence-like state that occurs in the very early stages of tumour-initiating cell development. This therapy targeting the immunosuppressive environment formed by macrophage senescence-like state is expected to be a novel promising cancer therapeutic strategy.