Matrix Type Influences the Levels of Soluble Immune Checkpoints

Soluble immune checkpoints (sICs) are emerging as possible serum and plasma biomarkers in cancer and immune-mediated diseases, but little is known about the impact of the matrix type in sIC detection. This study aimed to assess whether sIC measurements are comparable between serum and EDTA-plasma samples.

A cohort of 38 healthy subjects was enrolled. A multiplex bead-based assay was used to evaluate a panel of 17 sICs (CD137, 4-1BBL, CD27, CTLA4/CD152, CD80, CD40, CD40L, GITR, GITRL, ICOSL, IDO, LAG3, PD-1, PD-L1, PD-L2, TIM3, and VISTA) in paired serum and plasma-EDTA samples. The detection frequencies, concentrations, and correlations of each sIC were analyzed by comparing the two matrices.

Soluble CD137, CD152, CD40, and LAG3 were detected more frequently in plasma, while soluble CD40L was detected predominantly in serum. The concentrations of soluble 4-1BBL, CD27, PD-1, VISTA were higher in plasma, while the concentrations of soluble PD-L2 were higher in serum. The concentrations of soluble CD80, GITR, GITRL, ICOSL, IDO, and TIM3 were comparable between serum and plasma. Soluble CD27, CD80, GITRL showed a significant positive, slight correlation between plasmatic and serum concentrations.

Except for soluble CD80, the detection of the other sICs by the bead-based assay was influenced by the matrix type. The evaluation of the best matrix for sICs should be considered before starting clinical studies.