TNF and infection: Furthermore, human cytomegalovirus infected M1 macrophages secrete IL-6, IL-12, TNF-α, and IFN-γ whereas M2 macrophages produce only low levels of TNF-α, IFN-γ, and IL-10 (Romo et al., 2011) The emerging complexity of interactions between NK cells and macrophages makes the understanding of the pathways implicated in vivo in steady state or during infection an ongoing challenge.