Birgitta Versluijs

110 Introduction Introduction Allogeneic hematopoietic cell transplantation (HCT) is a curative treatment for several malignant and nonmalignant childhood diseases. Its success is limited by toxic events. Pulmonary complications contribute to posttransplantation morbidity and mortality. Noninfectious causes, such as alloimmune-mediated lung syndromes (allo-LSs), are res- ponsible for a significant proportion of posttransplantation lung injury in pediatric HCT recipients. 1 There is growing interest in the role of the microbiome in patients with graft-versus-host disease (GVHD). Pretransplanta-tion conditioning regimens disrupt the intestinal bar- rier, and the gut flora shows major changes during HCT, causing dysregulation of intes- tinal immune homeostasis, which can eventually lead to acute graft-versus-host disease (aGVHD). 2,3 Little is known about the respiratory microbiome and its relation to health and disease. 4 Studies in patients undergoing lung transplantation or allogeneic HCT suggest that the presence of common cold viruses early after transplantation is associated with either graft rejection (in lung transplantation) or development of alloimmune lung disease (in HCT). 5,6 On this basis, we hypothesize that early presence of viruses in the respiratory tract can cause tissue damage, resulting in activation of the alloimmune system. Howe- ver, distinguishing allo-LSs from progressive viral infection remains a point of contro- versy. To assess the effect of common cold viruses on the development of allo-LSs, we perfor- med a retrospective analysis to relate the presence of viral DNA/RNA in either nasopha- ryngeal aspirates (NPAs), bronchoalveolar lavage (BAL) fluid, or both to various outcome parameters, such as allo-LSs and survival. Methods Study design and patients We included all consecutive pediatric patients receiving their first allogeneic HCT from January 2004 to October 2013 who underwent routine BAL and nasal aspiration accor- ding to our previously described pre-HCT screening protocol, which consisted of chest high-resolution computed tomography (HRCT), pulmonary function tests (PFTs; in children >5 years of age), nasal aspiration for viral tests, and BAL for viral, bacterial, and fungal diagnostics. 7 Clinical data were collected prospectively, starting before conditio- ning, and registered in the clinical database. Minimum follow-up for surviving patients was 6 months. Patients were included and data were collected after written informed 7