Abstract
Immune Checkpoint Inhibitors (ICIs) have become a mainstay in the treatment of various solid tumors. At present, commonly used predictive biomarkers include tumor mutation burden, programed death-ligand 1 expression levels, and microsatellite instability. However, these biomarkers face inherent limitations, such as the challenges associated with tumor tissue sampling and the inability to provide dynamic monitoring. In recent years, significant efforts have been undertaken for the precise characterization of circulating T-lymphocyte subsets, with their classification offering the potential to reflect the functional state of T cells and predict responses to ICI therapy. Its advantages in terms of sampling convenience and minimally invasive nature further highlight its feasibility as a dynamic monitoring tool. This review expounds on current research progress on the use of "circulating" T-lymphocyte subsets as predictors of ICI efficacy and discusses their reliability and potential as predictive tools.