Lubricin or proteoglycan-4 (PRG-4) is a mucinous glycoprotein lubricating the cartilage and maintaining normal tissue function and cell-homeostasis. Altered O-glycoforms of lubricin has been found in osteoarthritis (OA) synovial fluid (SF). Here, we utilized a Fluorescent Immuno-Lectin Assay (FILA) to measure the levels of lubricin glycoforms in plasma/SF and their potential as biomarkers for disease. Five different lectins were used in the assay: Macrophage Galactose-type lectin (MGL), Sambucus Nigra Agglutinin (SNA), Maackia Amurensis Agglutinin (MAA), Peanut Agglutinin (PNA), and Galectin-3 (Gal-3). Our results showed that the levels of lubricin glycoforms in late-stage knee OA plasma (typically <10 μg/ml) are 1-3 order of magnitude lower than in OA SF (typically> 100 μg/ml). Furthermore, plasma lubricin glycans displayed higher level of sialylation, while SF derived lubricin displayed higher level of Tn-antigens. In OA patients we found decreased level in plasma of SNA binding lubricin (p=0.0023) compared to controls. This lectin binds 6 linked sialic acid. In addition, we found that lubricin glycoforms correlated both with Body Mass Index (BMI) and age, especially in regards of sialylation as measured by both MAA and SNA. Our data suggest that glycosylation of lubricin is different comparing SF with plasma. Moreover, the glycosylation of plasma lubricin is altered in OA patients compared controls.

NGK authored a patent involving the use of lubricin for diagnostics assigned to the company Lynxon AB, and NGK holds equity in Lynxon AB. GJ and TS authored patents related to rhPRG4. GJ and TS hold equity in Lubris BioPharma LLC. TS was also paid consultancy fees by Lubris BioPharma. The remaining authors declare no competing interests. The other authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Financial contribution to NGK from the Swedish state under the agreement between the Swedish government and the county council, the ALF-agreement (ALFGBG-722391), the Swedish Research Council (621-2013-5895), Petrus and Augusta Hedlunds foundation (M-2016-0353), and AA insurance research fund (dnr 150150). GJ was funded from the National Institute of Health R01AR067748. A stipend for ARA was provided from OsloMet.

I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained.

Yes

The details of the IRB/oversight body that provided approval or exemption for the research described are given below:

Plasma samples and their controls were from a biobank maintained by Sahlgrenska University hospital (Gothenburg, Sweden). All patients and participants had given informed consent. Ethical permission for patient screening was obtained from the ethical board in Gothenburg, Sweden (ethical application 172-15). Analysis of OA biobank samples in Norway has been ethically approved by the Sor-ost regional ethics commitee (470919) and handling of personal data approved by Norwegian Agency for Shared Services in Education and Research (431575).

I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals.

Yes

I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance).

Yes

I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable.

Yes

Analytical data about individual patient FILA results are included in the Supplementary Material. Inquiries about additional patient meta data can be directed to the corresponding author.