CD4+ T cells continuously monitor antigen levels in their environment and adjust their expansion accordingly, as shown on page 1555. Obst and colleagues demonstrate that when antigen runs out, CD4+ T cell proliferation stops dead in its tracks.

CD8+ T cells have been argued to undergo multiple rounds of cell division and to acquire effector function after only a brief encounter with antigen. Whether CD4+ T cells behave similarly remains controversial. Previous studies—mostly performed in vitro—have produced evidence both for and against a need for prolonged antigen stimulation for sustained CD4+ cell proliferation.

Obst et al. tackled this question in vivo by making a mouse in which expression of a CD4+ T cell epitope could be turned on and off using an antibiotic-inducible promoter. Using this on–off model, they showed that antigen withdrawal resulted in the rapid cessation of CD4+ T cell proliferation, even in the presence of an inflammatory stimulus. CD4+ T cells stimulated in vivo for 48 hours divided only once if transferred into an antigen-free host. This suggested that CD4+ T cells, unlike CD8+ T cells, require continued presence of antigen for continued activation.

The need for persistent antigen by CD4+ T cells may reflect a threshold of signaling molecules that is required for continued activation of the cell. Experiments to test this idea are underway. The authors also plan to test CD8+ T cell activation in a similar system to determine whether the rules governing the proliferation of these cell subsets are truly distinct.