In mice, AR was expressed selectively in TCR-activated Th2 cells [27] but not Th1 ( Figure S1 ) or naive CD4 T cells. This was a pre-committed, intrinsic property of the Th2 cells, as Th2 but not Th1 cells expressed AR even when in vitro-derived mouse Th1 and Th2 cell lines were activated together in the same culture (data not shown). However, most human CD4 (and CD8) T cells expressed AR in response to PMA plus ionomycin stimulation ( Figure 1A ). We therefore examined in more detail which human T cell subsets were responsible for AR production.