Abstract Body (Do not enter title and authors here): Background. Immune dysregulation and chronic inflammation have been documented in children and adults who have undergone Fontan palliation for single ventricle heart disease (SV). Each surgical stage introduces distinct hemodynamic, immunologic and inflammatory stressors, yet the evolution of systemic immune activation across these stages remains ill-defined. Additionally, it remains unclear whether chronically elevated circulating cytokines predisposes Fontan patients to co-morbidities, such as protein-losing enteropathy (PLE).

Hypothesis. SV patients have elevated circulating levels of classical and non-classical cytokines that may contribute to development of Fontan-associated co-morbidities.

Methods. Global plasma proteomics (SomaScan 11K platform) was performed in 156 SV samples (Pre-Norwood n=25, Norwood n=32, Glenn n=35, Fontan n=64) and 19 healthy controls (median age 10.4 years, IQR 7.1-15.3). A targeted analysis of a 168-cytokine panel was performed and expression level between groups were compared using t-test with an adjusted p-value (Wilcoxon) for multiple comparisons.

Results. Among SV patients undergoing surgical palliation, the greatest number (31/168 cytokines with adjusted p-value of <0.01) of differentially expressed plasma cytokines occurs in patients before and after Norwood. Plasma cytokines also differ significantly between Fontan patients and similarly aged healthy pediatric controls (Figure 1A). Circulating protein expression is also notably altered in Fontan patients, with and without active PLE, with a notable increase in insulin growth factor binding protein 2 (IGFBP2) isoforms and apolipoprotein F (APOF, Figure 1B).

Conclusions. Dysregulation of classical and non-classical plasma cytokines in pediatric SV patients underscores consistent immune and inflammatory disturbances across all stages of palliation and persist following Fontan completion. Furthermore, alterations in the circulating proteome may provide insights into pathogenesis of Fontan-associated co-morbidities. Further investigations are needed to elucidate the relationship between plasma proteome and clinical outcomes in SV patients to inform future recommendations for monitoring and risk-stratification.