Serum serotonin in rheumatoid arthritis patients: Relation to rheumatoid factor positivity, clinical manifestations and fibromyalgia

Tryptophan degradation as a systems phenomenon in inflammation - an analysis across 13 chronic inflammatory diseases

BACKGROUND

Chronic inflammatory diseases (CIDs) are systems disorders that affect diverse organs including the intestine, joints and skin. The essential amino acid tryptophan (Trp) can be broken down to various bioactive derivatives important for immune regulation. Increased Trp catabolism has been observed in some CIDs, so we aimed to characterise the specificity and extent of Trp degradation as a systems phenomenon across CIDs.

METHODS

We used high performance liquid chromatography and targeted mass spectrometry to assess the serum and stool levels of Trp and Trp derivatives. Our retrospective study incorporates both cross-sectional and longitudinal components, as we have included a healthy population as a reference and there are also multiple observations per patient over time.

FINDINGS

We found reduced serum Trp levels across the majority of CIDs, and a prevailing negative relationship between Trp and systemic inflammatory marker C-reactive protein (CRP). Notably, serum Trp was low in several CIDs even in the absence of measurable systemic inflammation. Increases in the kynurenine-to-Trp ratio (Kyn:Trp) suggest that these changes result from increased degradation along the kynurenine pathway.

INTERPRETATION

Increases in Kyn:Trp indicate the kynurenine pathway as a major route for CID-related Trp metabolism disruption and the specificity of the network changes indicates excessive Trp degradation relative to other proteogenic amino acids. Our results suggest that increased Trp catabolism is a common metabolic occurrence in CIDs that may directly affect systemic immunity.

FUNDING

This work was supported by the DFG Cluster of Excellence 2167 "Precision medicine in chronic inflammation" (KA, SSchr, PR, BH, SWa), the BMBF (e:Med Juniorverbund "Try-IBD" 01ZX1915A and 01ZX2215, the e:Med Network iTREAT 01ZX2202A, and GUIDE-IBD 031L0188A), EKFS (2020_EKCS.11, KA), DFG RU5042 (PR, KA), and Innovative Medicines Initiative 2 Joint Undertakings ("Taxonomy, Treatments, Targets and Remission", 831434, "ImmUniverse", 853995, "BIOMAP", 821511).

Serum serotonin levels are elevated in patients with increased risk of rheumatoid arthritis

Background

Even though serotonin (5-HT) has been ascribed immunomodulatory features, very little is known about its role in chronic inflammatory diseases. Serotonin is implicated in inflammation and increased levels have been associated with progression of bone erosions in RA.

Objective

To investigate serum serotonin levels in patients with increased risk of rheumatoid arthritis (RA) and patients with recent-onset disease. Moreover, we aimed to determine the prognostic value of serotonin for arthritis development and the disease course.

Methods

Two prospective observational patient cohorts were studied; anti-citrullinated protein antibody (ACPA) -positive patients with musculoskeletal pain without clinical arthritis (n = 82) and patients with early RA (n = 412). Serotonin levels were measured by enzyme-linked immunosorbent assay (ELISA) in baseline serum samples from both cohorts, and longitudinally in at-risk individuals.

Results

Compared to healthy controls (median 65 ng/ml), serotonin levels were significantly higher in both at-risk individuals (median 111 ng/ml, p < 0.0001) and patients with early RA (median 135 ng/ml, p < 0.0001). No significant differences were found between at-risk individuals and patients with early RA. At-risk individuals progressing to arthritis had similar levels as those not progressing, and no significant differences were seen over time. Baseline levels in early RA did not associate with mean 28-joint disease activity scores during 3 years follow-up.

Conclusion

Serum serotonin levels are elevated both at, and prior to, onset of RA. However, increased serotonin is not prognostic for arthritis development or disease course.

Fibromyalgia interferes with disease activity and biological therapy response in inflammatory rheumatic diseases

Fibromyalgia is one of the numerous comorbidities that may accompany inflammatory rheumatic diseases. Concomitant fibromyalgia in inflammatory rheumatic conditions can interfere with symptomatology, disease activity and overall management plan. The aim of the present narrative review article was to discuss the current evidence on (i) the prevalence/frequency of comorbid fibromyalgia in inflammatory rheumatic conditions, (ii) the role of fibromyalgia on disease activity, (iii) the impact of concomitant fibromyalgia on biological disease-modifying antirheumatic treatment outcomes and (iv) potential effectiveness of biological disease-modifying antirheumatic drugs on fibromyalgia-related symptoms among patients with inflammatory rheumatic diseases. A literature search was conducted through PubMed/MEDLINE Cochrane and Web of Science databases by using relevant keywords and their combinations. Studies representing different geographical areas of the world revealed that frequency rates of fibromyalgia are higher in inflammatory rheumatic diseases than those in the general population. Comorbid fibromyalgia interferes not only with the disease activity scores but also with the treatment outcomes and management plan. Further evidence is warranted in order to determine the potential benefits of biological disease-modifying antirheumatic drugs on fibromyalgia-related symptoms in patients with inflammatory rheumatic diseases.