Soluble immunosuppressive checkpointome of mesenchymal stromal cells: Immunoregulatory mechanisms and a conceptual framework for potency assessment

Figure 1 Three routes by which immune checkpoint proteins enter the extracellular space. A: Proteolytic ectodomain shedding. Membrane-anchored checkpoints are cleaved within the juxtamembrane stalk by a disintegrin and metalloproteinase 10 (ADAM10), ADAM17 or matrix metalloproteinases, releasing an active ectodomain while reducing surface density; B: Alternative splicing. Transcripts lacking the transmembrane exon yield an isoform that is secreted constitutively, without any requirement for sheddase activity and without depleting the surface pool; C: Vesicle-associated release. The checkpoint remains membrane-anchored but is displayed on small extracellular vesicles. Because conventional sandwich immunoassays capture epitopes irrespective of the physical state of the analyte, a single reported concentration typically represents the sum of all three pools. ADAM10: A disintegrin and metalloproteinase 10; MMPs: Matrix metalloproteinases; MSC: Mesenchymal stromal cell; sEV: Small extracellular vesicle.