The Key Role of IL-6–Arginase Cascade for Inducing Dendritic Cell–Dependent CD4+ T Cell Dysfunction in Tumor-Bearing Mice

Evaluation of immune dysfunction during the tumor-bearing state is a critical issue in combating cancer. In this study, we initially found that IL-6, one of the cachectic factors, suppressed CD4 + T cell–mediated immunity through downregulation of MHC class II by enhanced arginase activity of dendritic cells (DC) in tumor-bearing mice. We demonstrated that administration of Ab against IL-6R (anti–IL-6R mAb) greatly enhanced T cell responses and inhibited the growth of tumor in vivo. We also found that IL-6 upregulated the expression of arginase-1 and arginase activity of DC in vitro. Tumor-infiltrating CD11c + DC exhibited upregulated mRNA expression of arginase-1 but reduced expression of MHC class II in parallel with the increase in serum IL-6 levels at the late stage in tumor-bearing hosts. However, the administration of anti–IL-6R mAb into tumor-bearing mice inhibited both the downmodulation of MHC class II and the upregulation of arginase-1 mRNA levels in DC. Furthermore, we noted that N ω -hydroxy-L-arginine or L-arginine, an arginase-1 inhibitor, blocked the reduction in MHC class II levels on CD11c + DC during the tumor-bearing state. Finally, we demonstrated that the administration of N ω -hydroxy-L-arginine at the peritumor site significantly enhanced CD4 + T cell responses and inhibited tumor growth. Thus, IL-6–mediated arginase activation and the subsequent reduction in MHC class II expression on DC appeared to be critical mechanisms for inducing dysfunction of the immune system in the tumor-bearing state. Blockade of the IL-6–arginase cascade is a promising tool to overcome the dysfunction of antitumor immunity in tumor-bearing hosts.