Graft-versus-host disease (GVHD) is the most serious complication of allogeneic hematopoietic cell transplantation. Notch signals delivered during the first 48 hours after transplantation drive proinflammatory cytokine production in conventional T cells (Tconv) and inhibit expansion of regulatory T cells (Tregs). Short-term Notch inhibition induces long-term GVHD protection. However, it remains unknown whether Notch blockade blunts GVHD through its effects on Tconv, Tregs, or both, and what early Notch-regulated molecular events occur in alloantigen-specific T cells. To address these questions, we engineered T cell grafts to achieve selective Notch blockade in Tconv vs. Tregs and evaluated their capacity to trigger GVHD in mice. Notch blockade in Tconv was essential for GVHD protection, as GVHD severity was similar in recipients of wild-type Tconv combined with Notch-deprived vs. wild-type Tregs. To identify the impact of Notch signaling on the earliest steps of T cell activation in vivo, we established a new acute GVHD model mediated by clonal alloantigen-specific 4C CD4+ Tconv. Notch-deprived 4C T cells had preserved early steps of activation, IL-2 production, proliferation, and T helper polarization. In contrast, Notch inhibition dampened IFN-? and IL-17 production, diminished mTORC1 and ERK1/2 activation, and impaired transcription of a subset of Myc-regulated genes. The distinct Notch-regulated signature had minimal overlap with known Notch targets in T cell leukemia and developing T cells, highlighting the specific impact of Notch signaling in mature T cells. Our findings uncover a unique molecular program associated with pathogenic effects of Notch in T cells at the earliest stages of GVHD. Overall design: 4 samples per cohort (Notch blockade using Dll1/4 neutralizing antibodies vs isotype control antibodies - GD) were analyzed. Additional 4 samples contained 4C T cells retrieved from syngeneic recipients.