Soluble CTLA-4 as a context-dependent immune checkpoint: biology, biomarker potential, and therapeutic implications.

Cytotoxic T-lymphocyte-associated antigen 4 (CTLA-4) exists not only as a membrane-bound immune checkpoint but also as a naturally secreted soluble isoform (sCTLA-4), generated primarily through exon 3 skipping. This review focuses specifically on splice-derived sCTLA-4 and examines its biological and clinical significance. sCTLA-4 is shaped by immune state, cellular source, and tissue microenvironment. Mechanistically, sCTLA-4 can restrain B7/CD28-dependent immune activation, while recent preclinical evidence suggests a preferential effect on type 1 immune responses. In autoimmune and inflammatory diseases, longitudinal changes in sCTLA-4 often parallel changes in immune activity, although its performance as a disease-specific biomarker remains limited. In cancer, associations with tumor burden, prognosis, and treatment response are more variable and appear to depend strongly on the surrounding immune context. Thus, circulating sCTLA-4 may be better viewed as a state-dependent immunoregulatory signal than as a universal standalone biomarker. Current evidence does not clearly establish whether its elevation is causal, compensatory, or epiphenomenal. Key translational priorities include detection methods that distinguish sCTLA-4 isoforms and cellular sources, standardized longitudinal sampling, biomarker panels that integrate sCTLA-4 with defined immune phenotypes, and therapeutic strategies tailored to specific immune contexts while preserving peripheral tolerance.