TECs elicit IFN-γ secretion by allospecific T cells. (A-C) Balb/c T cells (H-2^d; wild-type, WT) were primed to H-2^b alloantigens on B6 splenic pAPCs (▨) and subsequently added to TEC1-2 cells (H-2^b). IFN-γ secretion (A) and TEC1-2 apoptosis (Ann⁺PI⁻ cells in panels B and C) were assessed after 72 hours of this mixed epithelial-lymphocyte (MELR) culture. As controls, B6 T cells (H-2^b, primed to H-2^d alloantigens), were used ⊡). The addition of unprimed B6 or Balb/c T-cells to TEC1-2 is denoted as (□). Experiments were performed 3 times. Bars depict mean ± SD; *P < .05. (C) In the graph, TEC1-2 apoptosis was assessed after addition of a blocking anti–IFN-γ antibody during MELR, and, on the right, by the use of GKO (Balb/c-IFN-γ^−/−; ▪) T cells (H-2^d, primed to H-2^b), 3 animals per group, *P < .05. (D) Primary TECs are sufficient to prime naïve allogeneic T cells in vitro. Reaggregation cultures of CD45⁻I-A^{b int+high} adult TECs (Figure S1) and CFSE-labeled naïve syngeneic T-cells (H-2^b, left) or allogeneic T-cells (H-2^d, middle) were used to detect ex vivo a graft-versus-host reaction. Top row shows histograms of CFSE fluorescence in CD4⁺ and CD8⁺ responder cells. Bottom row is the dot blot analysis of intracellular IFN-γ expression and CFSE labeling. FI indicates fluorescence intensity. As controls, CD45⁺, I-A^{b high} cells were used as hematopoietic pAPCs (E). The experiment was repeated twice, with identical results.