Elevated Levels of sLAG-3 as a Possible Biomarker in Graves' Disease With and Without Thyroid Eye Disease: A Prospective Observational Case-Control Study.

Background and Objectives: The pathogenesis of thyroid eye disease (TED) is driven by interactions between orbital fibroblasts and immune cells. Lymphocyte-activation gene 3 (LAG-3) is an immune checkpoint molecule with a similar structure to the T lymphocyte CD4 receptor but with higher affinity for MHC class II, and LAG-3-MHC class II interaction inhibits T lymphocyte activity. Lymphocytes shed LAG-3, generating soluble LAG-3 (sLAG-3), whose function is unclear. We investigated sLAG-3 involvement in Graves' disease (GD) and GD-associated TED pathogenesis. Materials and Methods: Patients with GD-associated TED (n = 47) and GD without TED (n = 35) were enrolled alongside 37 healthy controls (HCs). Peripheral blood serum sLAG-3 levels were measured using enzyme-linked immunosorbent assays and compared across the three groups. The effect of intravenous glucocorticosteroid (IVGC) treatment (12 weeks) on sLAG-3 concentrations in patients with GD-associated TED was monitored, and associations of sLAG-3 levels with clinical characteristics were analyzed. Disease activity before and after IVGC treatment was assessed using Clinical Activity Score. Results: Relative to those in HCs, serum sLAG-3 levels were significantly higher in GD patients both with (p < 0.001) and without (p = 0.0129) TED. No significant difference in sLAG-3 levels was observed between the two patient groups (p = 1.000), and no significant change in sLAG-3 levels was detected in patients with TED after IVGC therapy (p = 0.0536). Conclusions: The higher sLAG-3 levels in patients compared to HCs suggest that sLAG-3 dysregulation may contribute to GD and GD with orbitopathy development and the pathomechanisms underlying these conditions. Metalloproteinase-mediated cleavage of LAG-3 from the lymphocyte surface enables T lymphocyte proliferation and activation, while released sLAG-3 may enhance the immune response. Further studies of sLAG-3's mechanisms of action are needed to establish clear cut-off values and to define the diagnostic role of sLAG-3 in GD diagnosis.