Serum-soluble SDC1 in septic patients is rich in heparan sulfate, which affects the ELISA quantification values

Soluble syndecan (sSDC) 1, which is released into the blood by shedding of the SDC1 ectodomain, is a potent biomarker of various inflammatory diseases and cancers. However, the characteristics of serum sSDC1 and methods for its quantification have not been thoroughly investigated. We investigated the properties of sSDC1 in the serum of patients with sepsis and their effects on measurement values.

Serum sSDC1 levels in patients with and without sepsis were determined using dot blot analysis and enzyme-linked immunosorbent assay (ELISA). The molecular characteristics of serum sSDC1 were evaluated using western blotting and immunoprecipitation.

Serum sSDC1 ELISA values were more clearly able to distinguish between septic and non-septic patients than dot blot intensities. Serum sSDC1 mainly exists as the derivatives of the SDC1 ectodomain with molecular masses of 150-200 and 75 kDa, which were more prevalent in septic patients than non-septic patients. The former derivatives showed significant susceptibility to heparinase III. The presence of high-molecular-weight sSDC1 (>200 kDa), immunoprecipitated with an anti-heparan sulfate (HS) antibody, was also characteristic of the serum of patients with sepsis. The 150-200 and 75 kDa forms may have been derived from this high-molecular-weight HS-rich sSDC1. Importantly, the serum sSDC1 ELISA values for patients with sepsis showed a significant increase after treatment with heparinase III.

sSDC1 molecules generated by sepsis-induced pathological shedding may be rich in HS and heterogeneous compared to those generated by physiological shedding. The HS content affected the ELISA quantification values of serum sSDC1.