Characterization of alternative sPD-1 isoforms reveals that ECD sPD-1 signature predicts an efficient antitumor response

Soluble PD-1 is a dissociated form of membrane PD-1 broadly present in cancer, infections, or autoimmune diseases. However, the clinical significance of sPD-1 remains controversial due to the uncertainty of its isoforms, origin, and production mechanism. Here, using antibodies specifically binding to the intracellular domain of PD-1, we identified two sPD-1 isoforms in cancers at the protein level: FL sPD-1 containing both the extra- and intracellular domains of PD-1, and ECD sPD-1 containing only the extracellular fragment. Subsequently, we tracked their origin and found that in tumor models, both sPD-1 isoforms were generated by activated CD8 T cells highly expressing membrane PD-1. However, ECD sPD-1 was released from live PD-1⁺T cells by matrix metalloproteinases, while FL sPD-1 production was accompanied by PD-1⁺T cell death. Therefore, only ECD sPD-1 predicts effective immune response and better tumor outcome. Our study distinguished sPD-1 isoforms and highlighted ECD sPD-1 as a prognostic biomarker in cancer.