OBJECTIVE:
Phenotypic changes of chondrocytes toward hypertrophy might be fundamental in the pathogenesis of OA, of which type X collagen (Col10) is a well-known marker. The purpose was to develop a specific immunoassay for blood quantification of a newly identified neo-epitope of type X collagen to assess its diagnostic value for radiographic knee osteoarthritis (OA).

METHODS:
A neo-epitope of Col10 was identified in urine samples from OA patients. A monoclonal antibody against the neo-epitope was produced in Balb/C mice. The enzyme responsible for the cleavage was identified. Immunohistochemical detection of this neo-epitope was performed on human OA cartilage. An immunoassay (Col10neo) was developed and quantified in two clinical studies: the C4Pain-003 and the NYU OA progression study. ROC curve analysis was carried out to evaluate the discriminative power of Col10neo between OA and RA.

RESULTS:
A neo-epitope specific mAb was produced. The Cathepsin K-generated neo-epitope was localized to the pericellular matrix of chondrocytes, while its presence was extended and more prominent in superficial fibrillation in the cartilage with advanced degradation. In the C4Pain study, a higher level of Col10neo was seen in subjects with greater KL grade. The group of the highest tertile of Col10neo included more subjects with KL3-4. In the NYU study, Col10neo was statistically higher in OA than control or RA. ROC curve analysis revealed area under the curve was 0.88 (95% CI 0.81-0.94).

CONCLUSION:
Our findings indicate that Col10neo linked to hypertrophic chondrocytes could be used as a diagnostic biochemical marker for knee OA.