Treg cells, IDO, and HO-1 enzymes lead to a decrease of effector cell proliferation and induce apoptosis by diverse mechanisms, resulting in a decrease in pro-inflammatory cytokines production (Hori and Sakaguchi, 2004; Bettelli et al., 2005; Fontenot and Rudensky, 2005), which is the type of immune response produced during late infection in our murine model (Hernandez-Pando et al., 1996). The gene discussed is IDO1; the disease is infection.