Lymph node NK cells (green) form long-lasting contacts with dendritic cells (red).

In recent years, intravital imaging has shown that T cells dart rapidly around lymph nodes in search of their rare cognate antigens, making fleeting contacts with DCs as they go. An encounter with a cognate antigen delivers a stop signal that triggers stable T cell–DC interactions, during which the T cell presumably receives activating signals.

Bajénoff and colleagues used the same technology to show that NK cells move at a more leisurely pace than T cells—a pace that is unaltered by infection. The reason for this lethargy is a matter of speculation, but might reflect both the clonal nature of NK cells and the diversity of ligands that activate these cells. In other words, unlike T cells, NK cells needn't look far for their activating signals.

Also unlike T cells, NK cells form long-lasting contacts with DCs in the absence of antigen. These prolonged interactions, the authors suggest, might provide the NK cells with survival signals. Indeed, recent studies showed that interleukin (IL)-15—an essential NK cell survival factor—must be presented in trans, with the cytokine binding to its receptor on one cell type (the DC) and acting on another (the NK cell).

Infection with Leishmania major caused lymph node NK cells to secret the T helper type 1 (Th1)-promoting cytokine interferon- (IFN-) and congregate with CD4⁺ T cells—a positioning consistent with the requirement for NK cell–derived IFN- in the development of a protective Th1 response against L. major.