Fig. 1.

Flk2-Cre and IL7R-Cre efficiently labeled tissue-resident lymphoid populations. (A) Genetics of the ‘Switch’ models. Flk2 or IL7R regulatory elements drive Cre recombinase expression. Flk2-Cre or IL7r-Cre mice were crossed to Rosa26^mTmG mice expressing a dual color reporter expressing either Tomato (Tom) or GFP. (B,C) Schematics of Cre-mediated reporter switching in the ‘FlkSwitch’ (B) and ‘IL7rSwitch’ (C) models. Expression of Cre results in an irreversible switch from Tomato to GFP expression. Once a cell expresses GFP, it can only give rise to GFP-expressing progeny. These models represent Cre-driven labeling in young adult steady-state hematopoiesis of circulating traditional mature myeloid and lymphoid cells. (D-H) TLCs were highly labeled in both FlkSwitch and IL7rSwitch lineage tracing models. Representative flow cytometric analysis of reporter expression across traditional circulating B cells and TLCs, all pre-gated on singlets, lymphocytes and live cells. (D) Traditional B cells (Ter119⁻ CD11b⁻ Gr-1⁻ CD3⁻ B220⁺) in the peripheral blood and different tissue-resident lymphoid populations in adult mice. (E) B1a (Lin⁻ IgM⁺ CD5⁺ CD11b^mid) cells in the peritoneal cavity. (F) MZB cells (Lin⁻ B220⁺ IgM⁺ CD21⁺ CD23⁻) in the spleen. (G) ILC2 (Lin⁻ CD45⁺ KLRG⁺ Sca-1⁺ CD25⁺) in the lung. (H) Tregs (Lin⁻ CD3⁺ CD4⁺ TCRB⁺ CD25⁺ FOX-P3⁺) in the lung. Tomato and GFP expression are highlighted by red and green boxes, respectively, in FlkSwitch and IL7RSwitch models. Values indicate mean frequencies±s.e.m. of gated Flk2-Cre and IL7R-Cre marked GFP⁺ populations. Data are representative of 4-5 mice per cohort in three independent experiments.