Regulation of hierarchical clustering and activation of innate immune cells by dendritic cells.

An early granulomatous response, characterized by collections of white blood cells at foci surrounding pathogens, occurs after infection by many intracellular organisms, including Listeria, but how these clusters become organized and for what purpose remain poorly understood. Here, we showed that dendritic cell (DC) activation by Listeria nucleated rapid clustering of innate cells, including granulocytes, natural killer (NK) cells, and monocytes, to sites of bacteria propagation where interleukin-12 was expressed in the spleen. Clustered NK cells expressed interferon-gamma (IFN-gamma), which was necessary for the activation and maturation of colocalized monocytes to tumor necrosis factor- and inducible nitric oxide synthase-producing DCs (TipDCs). NK cell clustering was necessary for IFN-gamma production and required pertussis-toxin-sensitive recruitment, in part mediated by the chemokine receptor CCR5, and MyD88 adaptor-mediated signaling. Thus, spatial organization of the immune response by DCs between 6 and 24 hr ensures functional activation of innate cells, which restricts pathogens before adaptive immunity is fully activated.