Lubricin, encoded by the gene PRG4, is the principal
lubricant in articulating joints. We immunized mice genetically deficient for
lubricin (Prg4-/-) with purified human
lubricin, and generated several mAbs. We determined each mAb's binding
epitope, sensitivity, and specificity using
biologic samples and recombinant
lubricin sub-domains, and we also developed a competition ELISA assay to measure
lubricin in synovial fluid and blood. We found the mAbs all recognized
epitopes containing O-linked
oligosaccharides conjugated to the
peptide motif KEPAPTTT. By western blot, the mAbs detected
lubricin in 1 μl of synovial fluid from several animal species, including human. The mAbs were specific for
lubricin since they did not cross-react with other synovial fluid constituents from patients with
camptodactyly-arthropathy-coxa vara-pericarditis syndrome (CACP), who genetically lack this
protein. The competition ELISA detected
lubricin in blood samples from healthy individuals but not from patients with CACP, indicating blood can be used in a diagnostic test for patients suspected of having CACP.
Lubricin epitopes in blood do not represent degradation fragments from synovial fluid. Therefore, although blood
lubricin levels did not differentiate patients with inflammatory
joint disease from healthy controls,
epitope-specific anti-
lubricin mAbs could be useful for monitoring disease activity in synovial fluid.