Abstract
BACKGROUND: Paediatric noncystic fibrosis bronchiectasis (NCFB) is a chronic respiratory condition characterised by airway infection, chronic inflammation, mucociliary dysfunction and structural lung damage. Emerging evidence highlights the importance of the local immune response and microbial environment in driving disease progression. OBJECTIVES: This systematic review aimed to summarise contemporary evidence on bronchoalveolar lavage (BAL)-derived cytokine levels, immune cell phenotypes and microbiota in paediatric NCFB, with a focus on their role in disease pathogenesis, phenotype classification and potential to inform targeted interventions. METHODS: The review followed PRISMA guidelines and was registered with PROSPERO (CRD42024520391). Six electronic databases were searched for studies investigating BAL cytokines, immune cells or microbiota in paediatric NCFB. Data extraction followed a pre-defined template and methodological quality was assessed using the RoB 2.0 and AXIS tools. RESULTS: 20 studies were included. Methodological and clinical heterogeneity precluded meta-analysis. Across studies, neutrophils consistently dominated the cellular profile, closely associated with pathogen detection, particularly Haemophilus influenzae and BAL-derived immunological biomarkers. Cytokine profiling demonstrated consistent elevations of pro-inflammatory mediators, notably interleukin-8, with variable associations to disease severity and pathogen-specific immune responses. Microbiota analyses, though limited, suggest that paediatric NCFB is not defined by a distinct microbial signature of the lower airways. CONCLUSION: Current evidence supports a model in which neutrophilic inflammation, driven by dysregulated cytokine responses and potentially sustained by microbial dysbiosis, is central to the pathophysiology of paediatric NCFB. Key gaps remain regarding dysbiosis, adaptive immunity and the longitudinal trajectories of immune mediators. Addressing these may support biomarker development and targeted therapies.