Label‐free Quantitative Proteomics Versus Antibody‐based Assays to Measure Neutrophil‐Derived Enzymes in Saliva

PURPOSE: This study aims to validate label-free quantitative proteomics (LFQ) against antibody-based methods for quantifying established periodontal disease biomarkers in saliva. EXPERIMENTAL DESIGN: In an experimental gingivitis model, healthy volunteers (n = 10) provide saliva at baseline (d0), during the induction (d7, d14, d21) and resolution (d35) of gingival inflammation (total n = 50). Biomarker levels are analyzed by LFQ and time-resolved immunofluorometric assay (IFMA) or enzyme-linked immunosorbent assay (ELISA). Molecular matrix metalloproteinase (MMP)-8 forms are assessed by Western blot (WB) analysis. RESULTS: LFQ detects significantly (p < 0.05) elevated MMP-8 (d21vsd7, d35vsd7) and tissue inhibitor of matrix metalloproteinases (TIMP)-1 (d35vsd7). Latent MMP-8 (70-80 kDa) is present (d0-d35), but not active MMP-8 (50-60 kDa). LFQ and immunoassay data significantly correlate for MMP-8 (r = 0.36), myeloperoxidase (r = 0.39), polymorphonuclear leukocyte elastase (r = 0.33), and TIMP-1 (r = -0.24). CONCLUSION AND CLINICAL RELEVANCE: LFQ can quantify enzyme levels in saliva, however lacks the ability to measure enzymatic activity. WB analysis reveals that MMP-8 may not be activated during induction of gingival inflammation. Significant but weak correlations between IFMA or ELISA and LFQ suggest a limited capacity of available antibodies to reliably quantify salivary biomarkers for periodontal diseases. Novel "anti-peptide" antibodies designed by newer targeted mass spectrometry-based approaches can help to overcome these drawbacks.