Background Adaptor proteins associated with the T cell receptor (TCR) play critical roles in regulating immune responses by Translating receptor engagement into intracellular signals. T cell Receptor Associated Transmembrane Adaptor 1 (TRAT1) has been implicated in modulating TCR complex stability, but its functional role in human effector and regulatory CD4⁺ T cell subsets remains poorly understood. This study aimed to elucidate the role of TRAT1 in regulating T cell activation and differentiation, particularly in helper T cells function and regulatory T cells. Methods Primary human CD4⁺ T cells, including thymus-derived and induced regulatory T cells (Treg), were genetically modified by CRISPR/Cas9-mediated gene deletion or retro-/lentiviral overexpression of TRAT1. Functional assays, flow cytometry, cytokine quantification, and RNA sequencing were performed to evaluate modulation of T cell functions. Mechanistic studies included pathway inhibition using small molecules and phospho-protein analysis. The influence of TRAT1 on Treg function was further assessed in a CAR Treg context in an immune organoid model of allo-rejection. Results Thymus-derived, TGFb-induced and FOXP3-transgenic Treg displayed reduced expression of TRAT1 compared to effector T cells, which showed pronounced up-regulation of TRAT1 following activation. In effector T cells, deletion of TRAT1 led to increased signaling through the phosphoinositide 3-kinase pathway resulting in enhanced proliferation and increased expression of activation markers. However, this was accompanied by reduced production of interleukin-17, which was linked to elevated activity of STAT6 as shown by inhibition experiments using small molecule inhibitors. Overexpression and CRISPR/Cas9-mediated knockout of TRAT1 in Treg enhanced suppression of CD4⁺ target cells via up-regulation of LAP/GARP but reduced suppression of CD8⁺ target cells, an effect confirmed in HLA-A2-specific CAR Treg in a human organoid model of allo-rejection. Conclusions TRAT1 acts as a dual regulator of human CD4⁺ T cell function, limiting effector activation through modulation of intracellular signaling and supporting regulatory T cell-mediated suppression. These findings reveal a novel mechanism of immune regulation with potential implications for the development of cell-based immunotherapies.

The Nobel Prize in Physiology or Medicine 2025 was awarded to Mary E. Brunkow, Fred Ramsdell and Shimon Sakaguchi “for their discoveries concerning peripheral immune tolerance.”