Figure 2: Role of arginase in immunosuppression. During parasitic infections, mouse macrophage-specific arginase-1 induces T cell suppression due to arginase-mediated depletion of L-arginine in the T cell environment, which leads to CD3ζ chain down-regulation. Additionally, PD-1/PD-L pathway may be also involved in the T cell suppression. Furthermore, human granulocyte arginase can also induce immunosuppression by a similar mechanism. Upon human polymorphonuclear granulocytes (PMN) cell death, arginase is liberated and high activities of this enzyme accumulate extracellularly during purulent inflammatory reactions. PMN arginase induces a profound suppression of T cell and NK cell proliferation and cytokine synthesis.