Anti-CD3ε mAb treatment induces RAG2^R229Q thymic expansion and DN to DP transition, without positive selection.RAG2^R229Q mice were intraperitoneally injected at the 3rd and 13th day after birth with 25 μg of anti-CD3ε mAb or PBS and killed 2 months after last injection. (A top) Macroscopic aspect of a representative RAG2^R229Q thymus from mice treated with PBS or anti-CD3ε mAb. (Bottom) Length and weight of thymi from the 2 groups (n = 5); **P = .0079. (B) Absolute number of thymic cells from RAG2^R229Q mice injected with PBS (n = 17) and anti-CD3ε mAb (n = 20); ***P < .0001. (C left panel) Representative dot plots of thymocyte distribution from the indicated mice. (Right panel) Graph shows the frequency of DN, DP, CD4⁺TCRb⁺ SP, and CD8⁺TCRβ⁺ SP populations in PBS (n = 15) and anti-CD3ε mAb (n = 21) treated mice; ***P < .0001. (D) Representative dot plots of CD69 and TCRβ expression in DP population of indicated mice; the percentage of CD69⁺TCRβ^high cells is represented.