Soluble LAG-3 Identifies a Dynamic Early T Cell Activation Window in self-reactivity, Type 1 Diabetes, and Broader Immune Responses.

Aims/hypothesisType 1 diabetes is a complex autoimmune disorder in which autoreactive CD4 and CD8 T cells destroy pancreatic beta-cells, resulting in insulin deficiency and hyperglycemia. Although genetic susceptibility, particularly certain HLA alleles, contributes to disease risk, not all genetically predisposed individuals develop Type 1 diabetes. Screening first degree relatives (FDRs) for islet autoantibodies (GAD65, IAA, IA-2, ZnT8) helps detect autoimmune activity. However, these serum markers arise only after T-helper cell activation, limiting early intervention opportunities. Since protein antigen recognition by B cells requires T-helper cell assistance through linked recognition, T cell activation precedes B cell activation and autoantibody production. Activation of these T cells leads to shedding of the immune-regulatory (activation) surface protein LAG-3 (Lymphocyte Activation Gene-3 or CD223), generating its soluble form, sLAG-3, that is detectable in circulation. We hypothesized that sLAG-3 may serve as an early biomarker of autoimmune activity preceding islet autoantibody development in type 1 diabetes. MethodsPlasma sLAG-3 levels were measured longitudinally in female diabetes-prone NOD mice and analyzed in relation to islet antigen-specific CD4 T cell expansion and diabetes onset. To mechanistically link autoreactive T cell activation to sLAG-3 release. Naive autoreactive C6.6.9 TCR-transgenic (TCR-Tg) CD4 T cells were adoptively transferred into NOD.SCID mice and longitudinal assessment for plasma sLAG-3, beta-cell antigen specific CD4 T cell tetramer profiles, and circulating insulin (Ins2) mRNA to determine ongoing beta-cell stress. In parallel, sLAG-3 levels were analyzed from different human cohorts, including FDRs of individuals with type 1 diabetes, using cross-sectional and longitudinal approaches. ResultsIn murine models, elevated sLAG-3 correlated with expansion of islet-specific CD4 T cells that preceded hyperglycemia and diabetes onset. In the adoptive transfer model, early increases in sLAG-3 and circulating Ins2 mRNA marked immune activation and emerging beta-cell stress prior to overt diabetes. In our human cohorts, sLAG-3 was detectable in autoantibody-negative and single-autoantibody-positive FDRs, with higher levels observed in progressors compared to non-progressors, and associated with high-risk HLA genotypes. Conclusions/interpretationThese findings identify sLAG-3 as a candidate biomarker of early T cell activation in type 1 diabetes that may precede islet autoantibody development. Integration of sLAG-3 with antigen-specific T cell and beta-cell stress markers could improve early risk stratification and inform preventive strategies before substantial loss of beta-cell. Prospective longitudinal studies aligned to seroconversion are required to validate sLAG-3 as a surrogate marker of early disease activity. Research in contextO_ST_ABSWhat is already known about this subject?C_ST_ABSO_LIBefore the clinical onset of hyperglycemia, type 1 diabetes is characterized by a prolonged preclinical phase in which autoreactive B and T cells mediate progressive beta-cell destruction. C_LIO_LICurrent risk stratification strategies rely mainly on genetic susceptibility (genomic DNA) and the detection of islet autoantibodies in plasma/serum. C_LIO_LIIslet autoantibodies arise only after CD4 T cell activation and therefore do not capture the earliest stages of immune dysregulation. C_LIO_LIConsequently, biomarkers that directly reflect early pathogenic T cell activity prior to, or independent of, seroconversion remain limited and insufficiently validated. C_LI What is the key question?Can plasma sLAG-3 levels, beta-cell antigen-specific CD4 T cell tetramer expression, and circulating Ins2 mRNA serve as very early biomarkers of autoimmune activity in type 1 diabetes and serve to better inform risk stratification, thereby informing preventive intervention strategies for the clinician? What are the new findings?O_LIsLAG-3 increases transiently during early antigen-specific CD4 T cell activation stage, precedes hyperglycemia in mouse models, and is elevated in autoantibody-negative and single-autoantibody-positive first-degree relatives who later progress to type 1 diabetes. C_LIO_LIsLAG-3 was associated with beta-cell antigen-specific CD4 T cell expansion, assessment of stress induced beta cell Ins2 mRNA release and high-risk HLA genotypes, indicating early autoimmune activation rather than established disease. C_LI How might this impact clinical practice in the foreseeable future?These findings support sLAG-3 as a candidate early biomarker of T cell activation, before or at the earliest stages of islet autoantibody development in some at-risk individuals. Integration of plasma sLAG-3 with beta-cell antigen specific CD4 T cell profiling and insulin mRNA measurements could complement current autoantibody-based screening, improve risk stratification, and enable earlier preventive interventions to preserve beta-cell function in patients at-risk for type 1 diabetes.