NOS2 and meningitis: Microscopic examination of brains from mice that were infected with DENV-3 (MG20) revealed the presence of inflammatory infiltrates and edema in infected WT and Nos2−/− mice on the 5th d.p.i. However, on the 8th d.p.i., the brains of the infected WT mice showed an intense endothelial response with perivascular polymorphonuclear cells, mononuclear leukocyte cell migration, meningitis and intense vacuolization that suggested neuronal degeneration, necrosis, and apoptosis.