The lung microbiome in hematopoietic stem cell transplantation: immune interactions, clinical consequences, and emerging interventions.

Hematopoietic stem cell transplantation (HSCT) offers curative potential for hematologic malignancies and immune disorders, yet pulmonary complications remain major contributors to non-relapse morbidity and mortality. Traditionally attributed to immune suppression and graft-versus-host disease (GvHD), these complications are increasingly recognized to involve disruption of pulmonary microbial communities. A growing body of clinical and experimental evidence indicates that HSCT-associated perturbations in the lung microbiome, driven by conditioning, antimicrobials, immune injury, and infection, are associated with distinct post-transplant pulmonary phenotypes and, in some cohorts, with mortality risk. Whether these microbial shifts represent causal contributors to lung injury or contextual biomarkers of immune vulnerability remains unresolved, and this distinction carries direct implications for microbiome-targeted intervention. Dysbiotic shifts in the lung have been associated with both infectious and non-infectious complications, including idiopathic pneumonia syndrome, bronchiolitis obliterans syndrome, and fibrotic lung disease. Gut-lung microbial crosstalk may amplify or reflect systemic immune dysfunction, though the directionality of this relationship remains incompletely characterized. Multi-omics approaches, integrating metagenomics, metatranscriptomics, and metabolomics, are beginning to define the host-microbiome interaction signatures that distinguish injury subtypes and predict outcomes. This review synthesizes mechanistic insights into lung microbiome-immune interactions after HSCT, critically appraises the methodological constraints on the current evidence base, and evaluates microbiome-based interventions, including fecal microbiota transplantation, inhaled postbiotics, and precision antimicrobials, as candidate strategies for respiratory protection in transplant recipients, while acknowledging that prospective interventional evidence in this population remains limited.