Anti-lubricin Monoclonal Antibodies Created Using Lubricin-knockout Mice Immunodetect Lubricin in Several Species and in Patients with Healthy and Diseased Joints

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.