Bronchiolitis obliterans syndrome in adults after allogeneic stem cell transplantation-pathophysiology, diagnostics and treatment.

INTRODUCTION: Transplant-related complications are common after allogeneic hematopoietic stem cell transplantation (allo-HSCT), including graft versus host disease (GVHD). The lungs are frequently affected during the course of allo-HSCT, and among the non-infectious pulmonary complications bronchiolitis obliterans syndrome (BOS) is the most common and considered the only diagnostic manifestation of pulmonary GVHD. BOS is an irreversible obstructive disease that affects the terminal bronchioles, and it is associated with high morbidity and mortality rates. Area covered: We discuss the features of chronic GVHD, including the pathophysiological and cytokine-mediated alteration in BOS. Early treatment, before structural and irreversible changes have occurred, is crucial to reduce disease morbidity and mortality. This is challenging, given the unspecific symptoms of early stage disease and the complexity of the disease pathophysiology, obstructing both the diagnostic workup and the initiation of treatment. We highlight the main issues regarding diagnostic challenges, and we discuss the treatment options with a focus on new therapeutic options and modalities. Expert commentary: BOS is one of the most serious late complications after allo-HSCT and remains a diagnostic and therapeutic challenge. Thus, new and more effective therapeutic alternatives are strongly warranted.