Extracellular galectin-3 preferentially through CD44 and PD-1 binds activated T cells and marks memory-phenotype T cell subsets

Background Galectin-3 (Gal-3) is a multifunctional β-galactoside-binding lectin implicated in immune regulation, yet its association with T cell phenotypes under physiological and inflammatory conditions remains incompletely defined. Methods We investigated Gal-3 expression and binding on murine and human T cells across developmental stages, tissues, sex, genetic background, circadian time, and inflammatory disease settings. Results In mice, Gal-3-associated CD4 + and CD8 + T cells were dynamically regulated by age, anatomical location, strain, sex, and circadian rhythm. Gal-3 + T cells displayed increased activation markers, enhanced TCR-associated signaling molecules, elevated cytolytic mediators, increased proliferation, and greater apoptosis. Gal-3 + T cells were enriched within central and effector memory compartments and expressed higher levels of co-inhibitory receptors, including PD-1, with ImageStream analysis confirming Gal-3 co-expressed with PD-1. Recombinant Gal-3 preferentially bound activated, but not resting T cells, and associated with CD44 and CD3, suggesting activation-dependent acquisition of extracellular Gal-3. Re-analysis of human thymocyte and Long COVID single-cell RNA-seq datasets further supported an association between LGALS3 expression and activated/exhausted T cell states. Finally, intracellular staining indicated that T cells are unlikely to be a major source of Gal-3. Conclusion Our findings identify Gal-3 as a context-dependent marker and potential regulator of activated, antigen-experienced T cells, which promotes T cell apoptosis.