MEK1/2 Signalling establishes an alloreactive effector T cell program through SMAD4 during cardiac allograft rejection

BACKGROUND: Acute allograft rejection is largely dependent on T cell-driven immune responses; however, the molecular programs that establish and maintain effector T cell states during rejection remain incompletely defined. METHODS: Single-cell transcriptomic analysis was conducted to characterize the activation of signalling pathways in T cells during acute allograft rejection. The function of MEK1/2 signalling was evaluated both in vitro and in vivo using mice with targeted deletion of Mek1/2 in T cells and pharmacological inhibition. Phosphoproteomic analysis, immunofluorescence staining, and western blotting were conducted to investigate downstream signalling mechanisms, particularly those involving SMAD4. RESULTS: Effector CD4 and CD8 T cells exhibited enhanced MEK/ERK pathway activation throughout the rejection process. MEK1/2 inhibition, either via genetic deletion or pharmacological blockade, attenuated T cell activation and effector activity, leading to reduced rejection and prolonged graft survival. Phosphoproteomic analysis identified SMAD4 as a key downstream effector of MEK1/2 signalling. Mechanistically, MEK1/2 deficiency reduced SMAD4 nuclear translocation in activated T cells, accompanied by diminished effector T cell responses. CONCLUSIONS: MEK1/2 signalling regulates T cell activation and effector phenotypes via SMAD4 nuclear translocation in heart transplant rejection. Therapeutic intervention targeting this pathway may contribute to alloimmune response regulation and improve transplant outcomes.