Inflammatory cytokines mediate the gut microbiota-EGPA subtype link: a Mendelian randomization study

Limited studies have explored the potential link between gut microbiota and eosinophilic granulomatosis with polyangiitis (EGPA), and the specific inflammatory cytokines involved in EGPA remain controversial. Additionally, the causal relationships between gut microbiota, inflammatory cytokines, and EGPA subtypes, as well as whether inflammatory cytokines mediate these effects, are not well understood.

We aim to investigate the causal effects of gut microbiota and inflammatory cytokines on the subtypes of EGPA and to elucidate the mediating role of inflammatory cytokines in this relationship.

We conducted a two-sample Mendelian randomization (MR) analysis using data from large-scale genome-wide association studies (GWAS) on gut microbiota, 91 inflammatory cytokines, and EGPA subtypes (ANCA/MPO ( +) EGPA and ANCA/MPO ( -) EGPA). A bidirectional MR analysis was performed to exclude the influence of EGPA subtypes on gut microbiota. Then, we explored the mediating role of inflammatory cytokines. The primary analysis employed the inverse-variance-weighted method, supplemented by MR-Egger, weighted median, weighted mode, and simple mode analyses. We used the Steiger test to address reverse causality, Cochran's Q statistic to evaluate heterogeneity, and the MR-PRESSO outlier detection test to identify and mitigate horizontal pleiotropy.

For ANCA/MPO ( +) EGPA, we identified 12 gut microbiota (5 positive, 7 negative) and three circulating inflammatory cytokines (1 positive, 2 negative) with causal effects. Specifically, the genus Ruminococcus1 was a potential risk factor (OR, 1.052; 95% CI, 1.019-1.087, P = 0.002), while Eubacterium nodatum showed a protective effect (OR, 0.979; 95% CI, 0.966-0.991, P = 0.0009). IL-1α was associated with an increased risk (OR, 1.029; 95% CI, 1.010-1.049, P = 0.003), whereas leukemia inhibitory factor had a mitigating effect (OR, 0.963; 95% CI, 0.942-0.985, P = 0.001). For ANCA/MPO ( -) EGPA, we identified seven gut microbiota (2 positive, 5 negative) and six circulating inflammatory cytokines (5 positive, 1 negative) with causal effects. The family Bacteroidaceae showed a protective effect (OR, 0.939; 95% CI, 0.894-0.986, P = 0.01), whereas Ruminococcus1 was a risk factor (OR, 1.057; 95% CI, 1.009-1.108, P = 0.02). IL-2 (OR, 1.043; 95% CI, 1.009-1.077, P = 0.011), FGF 19 (OR, 1.044; 95% CI, 1.016-1.073, P = 0.002), and CXCL11 (OR, 1.035; 95% CI, 1.010-1.060, P = 0.005) were linked to increased risk. We also observed that the genus Parasutterella may mediate risk for ANCA/MPO ( +) EGPA by reducing leukemia inhibitory factor, accounting for a 21.4% mediating effect. No significant heterogeneity of instrumental variables or horizontal pleiotropy was detected.

This pioneering MR study bridges a significant gap by elucidating the relationship between gut microbiota, inflammatory cytokines, and EGPA subtypes. It highlights novel mechanisms in autoimmune disease and suggests new therapeutic targets for clinical intervention. Future studies should focus on further elucidating the detailed mechanisms involved. Key Points • The underlying mechanism of EGPA remains elusive, leading to a high relapse rate after treatment, but without adequate attention from studies • Our research bridges this knowledge gap by investigating the novel mechanism regarding gut microbiota-inflammatory cytokines-EGPA subtypes • Mendelian randomization and mediation analysis offer robust approaches to uncover potential mechanisms, mitigating the biases inherent in observational studies • This is the first MR to identify potential novel drug targets for EGPA, and suggest that the genus Parasutterella may heighten the risk for ANCA/MPO ( +) EGPA by modulating the levels of leukemia inhibitory factor.