Activated T cells acquire MHC-II molecule during the ongoing immune response in vivo. (A) Acquisition of MHC-II molecules by activated T cells in vivo. Naive Tg CD45.1 Marilyn cells (10⁶) were injected intravenously into B6 mice that were immunized by injection of 10⁶ H-Y peptide–loaded A^b-GFP knockin DCs²¹  into the footpad. Two days later, CD69, A^b, and GFP expression were analyzed on the transferred T cells in the draining LN and the nondraining LN (CD45.1⁺TCR⁺) by 5-color FACS with doublet exclusion. (B) Proliferating T cells acquire MHC-II in vivo. Naive CFSE-labeled Tg CD45.1 Marilyn cells (10⁶) were injected intravenously into B6 mice and immunized as in panel A. Two days later, CD69 and A^b expression was analyzed in the draining LN on the transferred T cells (CD45.1⁺TCR⁺) by 5-color FACS with doublet exclusion. (C) The inhibition starts at day 1 and plateaus by day 2. H-Y peptide–loaded DCs (10⁶) were injected into the footpad of female B6 mice that contained (or not) a first cohort of 10⁶ naive CD45.2 CFSE-labeled Marilyn LN cells. At the indicated time points, a second cohort of 10⁶ CD45.1 CFSE-labeled naive Marilyn LN or 2 × 10⁶ memory Marilyn T cells was injected, and the CFSE profile assessed 6 days later. The percentage of divided TCR⁺CD45.1⁺ Marilyn cells in the second cohort is shown for cells in the presence of a first cohort relative to the percentage seen in its absence. Each point is an individual mouse.