Serological CTX-II does not measure the same as urinary CTX-II

Type II collagen is the most abundant protein of articular cartilage. The urinary cross-linked C-terminal telopeptide of type II collagen (uCTX-II) is a matrix metalloproteinase (MMP) cleaved fragment and may be the most well-validated biomarker in osteoarthritis. The aim was to develop a serological immunoassay of CTX-II (sCTX-II) and evaluated both sCTX-II and uCTX-II levels in a cross-sectional osteoarthritis cohort.

The biological relevance of sCTX-II was validated in bovine cartilage explants cultured in the presence of Oncostatin M and tumor necrosis factor alpha (OT) or OT supplemented with GM6001 for 3 weeks. Serum and urine samples from an osteoarthritis cohort were assayed using sCTX-II and uCTX-II, respectively. Spearman's correlation was performed to evaluate the correlation between sCTX-II and uCTX-II. The association between the level of biomarkers and clinical variables was also investigated.

The supernatant analyzed in sCTX-II showed significant higher CTX-II levels in the end phases of explant culture compared to the vehicle group. The release of CTX-II was completely suppressed by GM6001. The sCTX-II levels in serum were not associated with uCTX-II in urine although sCTX-II levels in urine were significantly correlated with uCTX-II. uCTX-II correlated with age and gender while sCTX-II did not. sCTX-II cannot demonstrate any clinical relevance in a cross-sectional OA cohort as uCTX-II did.

The sCTX-II assay can reflect the MMP-mediated type II collagen degradation in bovine cartilage explants. However, sCTX-II and uCTX-II assays show different patterns suggesting the presence of CTX-II in blood may be different from that of urine.