Cytokine profiling of molecular endotypes of knee osteoarthritis: insights from the IMI-APPROACH cohort

Molecular endotypes that decipher the heterogeneity of osteoarthritis (OA) have been described. This exploratory study aimed to further the molecular understanding of three previously identified biomarker-based endotypes of knee OA through cytokine profiling. Fifteen pro- and anti-inflammatory cytokines were measured in serum at the six-, 12-, and 24-month follow-up visits of 277 knee OA participants from IMI-APPROACH using Luminex multiplexed immunoassays. Longitudinal differences in cytokine levels between previously defined endotype subgroups; (i) structural damage to bone and cartilage, (ii) low tissue turnover, and (iii) connective tissue inflammation were estimated with linear mixed-effects models, adjusting for patient-specific random effects, age, sex, and BMI. Within-patient stability of the cytokines over 18 months was compared to 19 tissue turnover biomarkers of which defined the endotypes. Compared to tissue-turnover biomarkers measured in IMI-APPROACH, the panel of 15 cytokines demonstrated increased fluctuations over time with lower within-patient stability and less discriminatory abilities between the endotype. Only the anti-inflammatory cytokine interleukin-1 receptor antagonist (IL-1ra) was consistently and differentially elevated in the inflammatory endotype subgroup (n = 92). At the 12-month visit, a mean change of IL-1ra of 36% (95% CI: 19%, 54%; p-value < 0.001) was found for the inflammatory endotype relative to the structural damage endotype, and 40% (95% CI: 22%, 59%; p-value < 0.001) at month 24. At the 24-month visit, a mean change of IL-1ra of 21% (95% CI: 10%, 31%; p-value = 0.047) was found for the inflammatory endotype relative to the low tissue turnover endotype. Participants with the highest quartile expression of IL-1ra within the inflammatory endotype (n = 23) exhibited higher BMI (p = 0.035, Mann-Whitney U test) and worsened Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) function (p = 0.035, Mann-Whitney U test) compared to the lowest quartile of IL-1ra expression. This study found that the majority of the cytokines exhibited considerable fluctuations over time with no endotype-specific cytokine profiles. This study indicates that while the included cytokines are important for the understanding of OA pathology, they may not be stable reflections of the endotypic profiles of KOA over time.