LT expression is induced by Ag-specific activation of CD4⁺ thymocytes.

(A) RANKL, CD40L, LTα and LTβ mRNAs were quantified in DP and CD4⁺ thymocytes from OTII:Rag2^−/− and Rip-mOVA:OTII:Rag2^−/− mice: means and SEM are from 3 experiments, each with 2 mice per group. (B) LTα mRNA and cell surface LT were assessed for unstimulated and anti-CD3/CD28-activated CD4⁺ thymocytes from OTII:Rag2^−/− or Marilyn:Rag2^−/− mice: data representative of 3 experiments. (C) LTα mRNA was quantified in CD4⁺ thymocytes from OTII:Rag2^−/− mice co-cultured with unloaded (none) or OVAp-loaded mTECs: data representative of 2 experiments. (D) LTα mRNA was quantified in CD4⁺ thymocytes from OTII:Rag2^−/− mice isolated 1.5 days after injection of PBS or OVAp: data representative of 3 experiments. (E) β-casein, CRP and RANK mRNAs were quantified in mTECs from WT, LTα^−/− mice and OTII:Rag2^−/− mice 5 days after injection of PBS or OVAp. (F) LTα mRNA was quantified in DP and CD4⁺ thymocytes from CD80/86^−/− mice: means and SEM are derived from 2 experiments, each with 2 mice per group. (G) Graphs show distributions of medullary areas (mm²) in WT, CD80/86^−/− and CD28^−/− thymi (left), and thymi from DT-treated WT and Foxp3-DTR mice (right): significance relative to WT. (H) Positive selection induces CD40L and RANKL expression in thymocytes. After migrating into the medulla, CD4⁺ thymocytes scan the surface of mTECs for the presence of auto-Ag–MHCII complexes. Ag-specific and CD28-CD80/86 dependent interactions between CD4⁺ thymocytes and mTECs induce the expression of LT in CD4⁺ thymocytes and RANK in mTECs, thereby completing the signaling axes required for promoting mTEC expansion and maturation.