CD4 and infection: Our results demonstrated that 100% of the Nos2–/– mice survived infection with the DENV-3 (MG20) without exhibiting any clinical signs until the 15th d.p.i. Moreover, flow cytometric analysis revealed that CD4+ T cells, CD8+ T cells and macrophages were present in the brains of infected mice and that the CD4+ T cells were responsible for producing IFN gamma, which most likely induced the production of NOS2 by the WT mouse macrophages in response to DENV infection.