Background <p>Immune dysregulation plays a pivotal role in the pathogenesis of bronchiolitis. This study aimed to investigate the role of immune checkpoint molecules and regulatory cytokines in relation to disease severity.</p> Methods <p>A prospective cohort of 151 pediatric patients aged 1 month to 2 years was enrolled and classified into control, mild, moderate, and severe bronchiolitis groups. Immune checkpoint molecules (CTLA-4, TIM-3, TIGIT, GARP) and cytokines (IL-2Rα, 4-1BB, TGF-β1, LAG-3, galectin-9) were evaluated in CD4 ⁺ FOXP3⁺ (Treg) and CD4 ⁺ FOXP3⁻T cells, and in plasma using flow cytometry and ELISA.</p> Results <p>CD4⁺ T cell levels decreased with increasing disease severity. Treg frequencies were elevated in mild cases but decreased in moderate and severe cases. CTLA-4 and TIM-3 expression increased on both Treg and non-Treg CD4⁺ T cells in moderate and severe groups. Soluble PD-1, TIM-3, LAG-3, TGF-β1, and 4-1BB levels were significantly elevated in severe bronchiolitis.</p> Conclusion <p>Disease severity in bronchiolitis is associated with immune checkpoint dysregulation and an immunosuppressive environment. The observed alterations in T cell subsets and increased expression of CTLA-4 and TIM-3 highlight the potential of these molecules as biomarkers of disease progression.</p> Impact <p><UnorderedList Mark="Bullet"> <ItemContent> <p>This study demonstrates that the severity of pediatric bronchiolitis is associated with the dysregulation of immune checkpoint molecules.</p> </ItemContent> <ItemContent> <p>It adds novel insight into the immunopathogenesis of bronchiolitis by focusing on early immune biomarkers beyond conventional inflammatory parameters.</p> </ItemContent> <ItemContent> <p>These findings may support risk stratification and contribute to the development of targeted therapies in future pediatric bronchiolitis management.</p> </ItemContent> </UnorderedList></p>