Interleukin 13 has long been a frustrating target for lupus researchers. The cytokine sits at the center of allergic inflammation and tissue fibrosis, and earlier studies suggested it might drive some of the immune chaos of systemic lupus erythematosus. But the molecule circulates at such vanishingly small concentrations that conventional laboratory assays simply could not see it reliably. A new cross-sectional study of 313 patients in Spain, published in BMC Rheumatology, has now measured the cytokine with an ultrasensitive single-molecule technique and arrived at a surprising verdict: serum interleukin 13 has nothing to do with the classic hallmarks of lupus, yet it tracks closely with blood cell counts and an atherogenic lipid profile.
The research team, led by investigators at four tertiary centers including Hospital Universitario de Canarias, recruited adults with confirmed systemic lupus erythematosus between September 2023 and February 2024. Every participant fulfilled at least four American College of Rheumatology classification criteria. From 400 preselected patients, 45 were excluded for not meeting the criteria, 20 for incomplete consent, and 10 for inadequate serum samples, leaving a final cohort of 313. Most were women, reflecting the disease’s strong female predominance, with a mean age of 52 years and a mean disease duration of 19 years.
What set this study apart was the depth of characterization. Each patient underwent assessment of disease activity using the SLEDAI-2K, SLE-DAS, and related indices, cumulative damage with the SLICC Damage Index, and remission status under the DORIS definition. The researchers also collected complete hematological and lipid profiles, insulin resistance indices derived from the HOMA2 model, autoantibody panels, interferon-alpha and interferon-gamma measurements, complement levels, and ultrasound assessments of carotid intima-media thickness, plaques, and arterial stiffness.
The key technological advance was the Simoa platform, short for Single Molecule Array. This technique detects proteins in the femtogram range, far below the sensitivity of the enzyme-linked immunosorbent assays used in most earlier cytokine studies. In this cohort, serum interleukin 13 had a median concentration of 103 femtograms per milliliter, with an interquartile range of 63 to 151. Only one patient had an undetectable value. The assay’s limit of detection ranged from 0.001 to 0.007 picograms per milliliter, and its lower limit of quantification was 0.136 picograms per milliliter, comfortably below the concentrations observed.
Against this precise backdrop, the absence of associations became meaningful. In multivariable linear regression analyses with log-transformed interleukin 13 as the dependent variable, demographic features, disease activity indices including SLEDAI-2K, SLE-DAS, DORIS, and LLDAS, damage scores, overall disease duration, autoantibody profiles, and interferon or complement levels all showed no significant relationship with the cytokine. This held even though most patients were clinically quiet at assessment: 45 percent were inactive, 46 percent had mild activity, and only 9 percent had moderate-to-high activity, with 68 percent in DORIS remission and 80 percent in a low disease activity state.
The associations that did emerge clustered around hematology and metabolism. Neutrophil counts and platelet counts were both inversely and independently associated with interleukin 13 after adjustment, meaning patients with higher cytokine levels tended to have lower counts of these blood cells. Higher red cell distribution width was consistently linked to higher interleukin 13 in both univariable and adjusted models, while hemoglobin, mean corpuscular volume, and other red cell indices showed no relationship. The authors suggest that interleukin 13 may mark immune pathways involved in peripheral destruction or clearance of blood cells, processes that could contribute to neutropenia and thrombocytopenia in lupus, though they emphasize that causality cannot be inferred from a cross-sectional design.
The lipid findings were equally striking. Higher HDL cholesterol and apolipoprotein A1 levels were independently associated with lower interleukin 13, while a higher LDL to HDL ratio, a higher apolipoprotein B to apolipoprotein A1 ratio, and a higher atherogenic index were associated with higher cytokine levels, both before and after adjustment. Notably, no association appeared with triglycerides, a contrast with earlier studies in diabetic nephropathy and metabolic syndrome where interleukin 13 correlated mainly with triglycerides. The authors propose that lipid disturbances in lupus, driven by chronic immune activation, affect HDL metabolism and reverse cholesterol transport differently from the insulin-resistance-driven dyslipidemia of metabolic disease.
Vascular measurements added a final twist. Carotid intima-media thickness, carotid plaque, pulse wave velocity, and SCORE2 cardiovascular risk categories showed no significant association with the cytokine. But the augmentation index, a measure of arterial stiffness derived from pulse wave analysis, was inversely and independently associated with interleukin 13, remaining significant in an additional model adjusted for smoking, prednisone use, and antinuclear antibody status. Since interleukin 13 was linked to an adverse lipid profile, its inverse relationship with augmentation index suggests the cytokine’s cardiovascular effects may be at least partially independent of circulating lipids, possibly reflecting anti-inflammatory and tissue-protective actions within the vascular wall, including reduced endothelial activation and macrophage polarization toward an M2 phenotype.
A few secondary signals deserve mention. Anti-nucleosome autoantibody positivity was the only autoantibody association that survived adjustment, and higher daily prednisone dose was independently associated with increased interleukin 13, while smoking and statin use were inversely associated in univariable models. The authors caution, however, that because multiple associations were examined without formal correction for multiplicity, the reported p-values are nominal and the findings should be treated as exploratory and hypothesis-generating until externally validated.
The study’s limitations frame how far its conclusions can travel. There was no healthy control group, so the study cannot say whether lupus patients have higher or lower interleukin 13 than the general population, particularly since no previous study has quantified the cytokine with the same Simoa platform in healthy donors. The cross-sectional design rules out causal inference, the cohort’s high proportion of patients in remission means flares and longitudinal changes remain unexplored, and mechanisms at the tissue or cellular level cannot be excluded even where serum associations are absent. Still, as the first ultrasensitive measurement of this elusive cytokine in a large, comprehensively characterized lupus cohort, the study redraws the map: interleukin 13 appears less a barometer of lupus activity and more a marker of hematologic and metabolic pathways that may shape cardiovascular risk in this population.
Subject of Research: Ultrasensitive measurement of serum interleukin 13 and its associations with disease features in systemic lupus erythematosus
Article Title: Ultrasensitive serum levels of interleukin 13 in systemic lupus erythematosus patients: a cross-sectional study
Article References: Ferraz-Amaro, I., Quevedo-Abeledo, J. C., Tejera-Segura, B., García-Barrera, E., Villar, L. M., Ocejo-Vinyals, J. G., Largo, R., & González-Gay, M. Á. (2026). Ultrasensitive serum levels of interleukin 13 in systemic lupus erythematosus patients: a cross-sectional study. BMC Rheumatology, 10(1), Article 80. https://doi.org/10.1186/s41927-026-00694-0
Image Credits: AI Generated
DOI: 10.1186/s41927-026-00694-0
Keywords: systemic lupus erythematosus, interleukin 13, Simoa, cytokines, lipid profile, atherosclerosis, autoantibodies, cross-sectional study, neutrophils, platelets, arterial stiffness, cardiovascular risk
News Source: Ophelia Keating. (October 30, 2026). Ultrasensitive test reveals interleukin 13 links to lipids, not lupus activity. Scienmag.
