CD26 activity mediates the anti-inflammatory properties of anergic T cells in vivo. Untreated female mice (negative control; panels A,G) and IFN-γ–treated male mice (positive control; panels B,H) were injected with an equal volume of sterile saline containing no T cells. Alternatively, IFN-γ–treated male mice were injected with either untreated anergic B9 T cells (3-5 × 10⁶/mouse; panels C,I), or preincubated with an antimouse CD26 mAb (5 μg/mL; panels D,J), or with an isotype-matched control (5 μg/mL; panels E,K), or with 30nM human YY peptide (3-36) (panels F,L) for 30 minutes at room temperature and subsequently washed. The following day, fluorescently labeled HY-specific C6 T cells (5-7 × 10⁶) were injected intraperitoneally. Following a further 24-hour incubation, the presence of red-fluorescent T cells in the peritoneal membrane (panels A-F) and lavage (panels G-L), as well as the lymph nodes and spleen (data not shown), was assessed. The median number and range of labeled cells quantified in 10 randomly selected × 40-magnified fields of the peritoneal membrane of at least 3 different animals are shown below each image (panels A-D), which is taken from a representative experiment. The median percentage of labeled cells recovered in the peritoneal lavage of 3 different samples is specified below each representative histogram (in which the percentage of positive cells is indicated). * indicates statistically significant (P < .0001 for both panel D and J) compared with anergic T cells treated with isotype-matched control mAbs or with untreated anergic T cells; **, statistically significant (P < .0001, panel F; P < .05, panel L) compared with untreated anergic T cells.