LAUSR

Simple Summary Different components of the tumor microenvironment, such as cancer-associated fibroblasts and tumoral mesenchymal stromal cells, play a major role in cancer progression, metastatic spread and resistance to chemo-immunotherapy. Neuroblastoma tumor-associated mesenchymal stromal cells (NB-TA-MSC) have been extensively characterized for their pro-tumorigenic properties, while their immunosuppressive potential, especially against NK cells, has not been investigated. Herein, we show for the first time that primary young/proliferating NB-TA-MSC, but not senescent ones, are resistant to direct cell lysis by activated NK cells, inhibiting proliferation, cytolytic activity and functional markers of freshly isolated NK cells. NB-TA-MSC express the neuroblastoma marker GD2, the most common target for NB immunotherapy; thus, they represent priority targets whose elimination is essential for improved NB immunotherapy. From a future perspective, in vivo eradication or disabling of NB-TA-MSC could be achieved using monoclonal antibodies directed against targets different from GD2 or inducing their senescence. Abstract Neuroblastoma tumor-associated mesenchymal stromal cells (NB-TA-MSC) have been extensively characterized for their pro-tumorigenic properties, while their immunosuppressive potential, especially against NK cells, has not been thoroughly investigated. Herein, we study the immune-regulatory potential of six primary young and senescent NB-TA-MSC on NK cell function. Young cells display a phenotype (CD105+/CD90+/CD73+/CD29+/CD146+) typical of MSC cells and, in addition, express high levels of immunomodulatory molecules (MHC-I, PDL-1 and PDL-2 and transcriptional-co-activator WWTR1), able to hinder NK cell activity. Notably, four of them express the neuroblastoma marker GD2, the most common target for NB immunotherapy. From a functional point of view, young NB-TA-MSC, contrary to the senescent ones, are resistant to activated NK cell-mediated lysis, but this behavior is overcome using anti-CD105 antibody TRC105 that activates antibody-dependent cell-mediated cytotoxicity. In addition, proliferating NB-TA-MSC, but not the senescent ones, after six days of co-culture, inhibit proliferation, expression of activating receptors and cytolytic activity of freshly isolated NK. Inhibitors of the soluble immunosuppressive factors L-kynurenine and prostaglandin E2 efficiently counteract this latter effect. Our data highlight the presence of phenotypically heterogeneous NB-TA-MSC displaying potent immunoregulatory properties towards NK cells, whose inhibition could be mandatory to improve the antitumor efficacy of targeted immunotherapy.