Increased group 2 Innate lymphoid cell (ILC2) activity in pediatric familial mediterranean fever patients with spondyloarthropathy.

Familial Mediterranean fever (FMF), an autoinflammatory disease caused by MEFV mutations, frequently coexists with immune-mediated conditions such as spondyloarthritis (SpA). However, the contribution of innate lymphoid cells (ILCs) to FMF-associated SpA remains poorly defined. Our objective was to characterize the phenotype and functional activity of ILCs in pediatric patients with FMF, SpA, and overlap FMF/SpA. Peripheral blood mononuclear cells (PBMCs) and synovial fluid mononuclear cells (SFMCs) from FMF (n = 15), SpA (n = 11), FMF/SpA (n = 13), and age-matched healthy controls (n = 14) were analyzed by flow cytometry to quantify ILC subsets. Plasma and synovial cytokines were measured by ELISA, and expression of ILC-associated cytokines in PBMCs and sorted ILCs was assessed by quantitative PCR. Total ILC frequencies and subset distributions in PBMCs and SFMCs were comparable across groups (p > 0.05). However, FMF/SpA patients demonstrated increased IL4 and IL5 expression in sorted ILCs (p < 0.05), consistent with a type 2-skewed ILC activation profile. PBMC thymic stromal lymphopoietin (TSLP) expression was elevated in SpA (p < 0.01). Plasma IL-6, IL-8 and IP-10 concentrations were higher in the full cohort of FMF/SpA patients; however, these differences were sensitive to outlier exclusion and indicate substantial inter-individual heterogeneity. Synovial fluid from SpA patients showed higher IP-10 and IL-17 levels (p < 0.05). Pediatric FMF/SpA is characterized by altered ILC functional polarization rather than numerical expansion, with a prominent type 2 ILC signature. In contrast, SpA displays synovial IP-10 and IL-17 enrichment and increased PBMC TSLP expression. These findings suggest distinct innate immune activation axes in overlap versus isolated SpA and support further investigation of ILC-mediated pathways in FMF-associated spondyloarthritis in larger cohorts.