big-h3/TGF-bi is a secreted protein capable of binding to both extracellular matrix and cells. Human genetic studies recently revealed that in the tgfbi gene encoding for big-h3, three single nucleotide polymorphisms were significantly associated with type 1 diabetes (T1D) risk. Pancreatic islets express big-h3 in physiological conditions, but this expression is reduced in b-cell insult in T1D. Since the integrity of islets is destroyed by autoimmune T lymphocytes, we thought to investigate the impact of big-h3 on T-cell activation. We show here that big-h3 inhibits T-cell activation markers as well as cytotoxic molecule production as granzyme B and IFN-g. Furthermore, big-h3 inhibits early T-cell receptor signaling by repressing the activation of the early kinase protein Lck. Moreover, big-h3-treated T cells are unable to induce T1D upon transfer in Rag2 knockout mice. Our study demonstrates for the first time that T-cell activation is modulated by big-h3, an islet extracellular protein, in order to efficiently avoid autoimmune response.Type 1 diabetes (T1D) is a polygenic autoimmune disease characterized by smoldering inflammatory response directed against the insulin-producing b-islet cells of the pancreas. Both CD4 + and CD8 + T cells are involved in the destruction of b-cells (shed by a prior insult). CD4 + T cells are insulin reactive, and CD8 + T cells play a major role as b-cell killers (1,2). Although both in vitro and in vivo evidence point to a role for regulatory T cells in T-cell-mediated regulation controlling diabetes in mouse models (3), the influence of the inflammatory environment in which they function remains poorly understood. The cross talk between the inflammatory milieu and the immune response during insulitis is likely to be communicated in large part through the extracellular matrix (ECM). big-h3 (also known as TGF-bi) is a secreted protein found in the ECM and it has an N-terminal secretory signal (aa 1-23), four FAS1 homologous internal domains, and a cell attachment site (RGD) at its C terminus (4). big-h3 binds to the ECM through interaction of the YH motif in its FAS1 domains with collagens I, II, IV, and VI (5,6). Its FAS1 domain interacts with its a3b1, aVb3, and aVb5 integrins on the cell surface. Previous studies demonstrated that recombinant big-h3 could preserve the integrity and enhance the function of cultured pancreatic islet cells (7). Conversely, big-h3 knockout (KO) islet function was compromised in vivo after transplantation in recipients with diabetes (8). These studies point out that big-h3 might play an active role in the protection of b-cells against T-cell cytotoxic insult in T1D. The effect of big-h3 on T-cell function and activation relative to its roles in islet b-cell-ECM destruction in T1D in mice has never been investigated and it is the focus of our project. RESEARCH DESIGN AND METHODS MiceFemale C57Bl/6 NOD mice were bred and housed in specific pathogen-free conditions. T1D was chemically induced inC57Bl/6 using a standard low-dose (35 mg/kg) streptozotoci...