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...
