Dual Activation‐Induced Marker Combinations Efficiently Identify and Discern Antigen‐Specific and Bystander‐Activated Human CD4 ⁺ T Cells

Identifying activated T lymphocytes and differentiating antigen-specific from bystander T cells is crucial for understanding adaptive immune responses. This study investigates the efficacy of activation-induced markers (AIMs) in distinguishing these cell populations. We measured the expression of commonly used AIMs (CD25, CD38, CD40L, CD69, CD137, HLA-DR, ICOS, and OX40) in an in vitro T-cell activation system and evaluated their sensitivity, specificity, and positive predictive value. We demonstrated that individual AIMs, while specific in detecting activated CD4⁺ T cells, poorly discriminate between antigen-specific and bystander activation, as assessed by a discriminative capacity (DC) score we developed. Our analysis revealed that dual AIM combinations significantly enhanced the ability to distinguish antigen-specific from bystander-activated T cells, achieving DC scores above 90%. These combinations also improved positive predictive value and specificity with a modest reduction in sensitivity. The CD25^hi/ICOS^hi combination emerged as the most efficient, with an average sensitivity of 84.35%, specificity of 99.7%, and DC score of 90.12%. Validation through T-cell cloning and antigen re-stimulation confirmed the robustness of our predictions. This study provides a practical framework for researchers to optimize strategies for identifying and isolating antigen-specific human CD4⁺ T lymphocytes and studying their phenotype, function, and T-cell receptor repertoire.