Background largely considered as mere sources of energy, metabolomics have revealed a role for lipid species in a number of biological processes. Among them, free fatty acids (FFA) are known to modulate cytokine transcription, expression of adhesion molecules and are involved in the production of proinflammatory and pro-resolving lipid compounds, such as prostaglandins and leukotrienes or protectins and resolvins, respectively. Recently, FFA were reported to modulate the function of CD4+ T-cells and macrophages in osteoarthritis. Thus, we hypothesize that altered FFA levels may underlie the initiation or perpetuation of pathogenic mechanisms in immune-mediated rheumatic diseases.
Objectives to evaluate whether altered FFA levels can be found in RA patients linked to clinical features as well as CD4+ T-cell response.
Methods serum levels of FFA (palmitic, stearic, palmitoleic, oleic, linoleic, γ-linoleic, arachidonic –AA–, linolenic, eicosapentaenoic –EPA– and docosahexaenoic –DHA–) were quantified by LC-MS/MS after methyl-tert-butylether (MTBE)-extraction in 124 RA patients and 56 healthy controls (HC). Total FFA serum levels were quantified by means of a colorimetric assay. CD4+ phenotype was studied by flow cytometry. TNFα, IL-8, VEGF, GM-CSF, IFNγ, IL-17, MCP-1, IP-10, leptin and resistin serum levels were quantified by immunoassays. The effect of FFA on IFNγ production by PBMC was evaluated in vitro.
Results lower levels of palmitic (p<0.0001), palmitoleic (p=0.002), oleic (p=0.010), arachidonic (p=0.027), EPA (p<0.0001) and DHA (p<0.0001) were found in RA patients, some FFA being altered in patients at disease onset (n=18). Altered FFA exhibited distinct chemical properties, leading us to think that individual FFA within a given class followed distinct patterns and similar patterns were found among different groups of FFA. Total FFA levels did not differ between patients and controls (p=0.157), thereby supporting the existence of different profiles within the FFA pool. Cluster analysis identified a FFA profile (hallmarked by increased stearic and decreased EPA and DHA) overrepresented in RA patients compared to HC (p=0.002), being associated with clinical features (RF: p=0.029, shared epitope (2 copies): p<0.001 and erosions: p=0.003), increased IFNγ expression in CD4+ T-cells (p=0.002) and a Th1-biased serum milieu (IFNγ: p<0.001, MCP-1: p<0.0001 and IP-10: p=0.006). No associations with regulatory T cells or Th17 subsets were found. In vitro assays demonstrated that stearic/DHA/EPA-imbalanced FFA could underlie IFNγ production by CD4+ T-cells, in both resting and PHA-stimulated conditions. Finally, changes on FFA levels were associated with clinical response and CD4+ Th1 shift upon TNFα-blockade.
Conclusions an altered FFA profile can be found in RA patients associated with clinical characteristics of aggressive disease, Th1-skewed response and poor clinical outcome upon TNFa-blockade. These results support the relevance of lipidomic studies in RA and provide a rationale for new therapeutic targets as well as for FA supplementation and stratification in RA.
Disclosure of Interest None declared