Acute lung injury after allogeneic stem cell transplantation: from the clinic, to the bench and back again

Diffuse Alveolar Hemorrhage in Hematopoietic Cell Transplantation

Diffuse alveolar hemorrhage (DAH) is a morbid syndrome that occurs after autologous and allogeneic hematopoietic cell transplantation in children and adults. DAH manifests most often in the first few weeks following transplantation. It presents with pneumonia-like symptoms and acute respiratory failure, often requiring high levels of oxygen supplementation or mechanical ventilatory support. Hemoptysis is variably present. Chest radiographs typically feature widespread alveolar filling, sometimes with peripheral sparing and pleural effusions. The diagnosis is suspected when serial bronchoalveolar lavages return increasingly bloody fluid. DAH is differentiated from infectious causes of alveolar hemorrhage when extensive microbiological testing reveals no pulmonary pathogens. The cause is poorly understood, though preclinical and clinical studies implicate pretransplant conditioning regimens, particularly those using high doses of total-body-irradiation, acute graft-versus-host disease (GVHD), medications used to prevent GVHD, and other factors. Treatment consists of supportive care, systemic corticosteroids, platelet transfusions, and sometimes includes antifibrinolytic drugs and topical procoagulant factors. Therapeutic blockade of tumor necrosis factor-α showed promise in observational studies, but its benefit for DAH remains uncertain after small clinical trials. Even with these treatments, mortality from progression and relapse is high. Future investigational therapies could target the vascular endothelial cell biology theorized to contribute to alveolar bleeding and pathways that contribute to susceptibility, inflammation, cellular resilience, and tissue repair. This review will help clinicians navigate through the limited evidence to diagnose and treat DAH, counsel patients and families, and plan for future research.

Endothelial Dysfunction Syndromes after Allogeneic Stem Cell Transplantation

Allogeneic hematopoietic stem cell transplantation (allo-HSCT) remains the only therapy with a curative potential for a variety of malignant and non-malignant diseases. The major limitation of the procedure is the significant morbidity and mortality mainly associated with the development of graft versus host disease (GVHD) as well as with a series of complications related to endothelial injury, such as sinusoidal obstruction syndrome/veno-occlusive disease (SOS/VOD), transplant-associated thrombotic microangiopathy (TA-TMA), etc. Endothelial cells (ECs) are key players in the maintenance of vascular homeostasis and during allo-HSCT are confronted by multiple challenges, such as the toxicity from conditioning, the administration of calcineurin inhibitors, the immunosuppression associated infections, and the donor alloreactivity against host tissues. The early diagnosis of endothelial dysfunction syndromes is of paramount importance for the development of effective prophylactic and therapeutic strategies. There is an urgent need for the better understanding of the pathogenetic mechanisms as well as for the identification of novel biomarkers for the early diagnosis of endothelial damage. This review summarizes the current knowledge on the biology of the endothelial dysfunction syndromes after allo-HSCT, along with the respective therapeutic approaches, and discusses the strengths and weaknesses of possible biomarkers of endothelial damage and dysfunction.

Mesenchymal stem cells alleviate idiopathic pneumonia syndrome by modulating T cell function through CCR2-CCL2 axis

BACKGROUND

Idiopathic pneumonia syndrome (IPS) is a non-infectious fatal complication characterized by a massive infiltration of leukocytes in lungs and diffuse pulmonary injury after allogeneic hematopoietic stem cell transplantation (allo-HSCT). Conventional immunosuppressive treatments for IPS have poor therapeutic effects. Safe and effective treatments are not yet available and under explorations. Our previous study demonstrated that mesenchymal stem cells (MSCs) can alleviate IPS, but the mechanisms remain unclear.

METHODS

Co-cultured pre-activated T cells and MSCs in vitro to observe the changes in the CCR2-CCL2 axis. By establishing an IPS mouse model and administering MSCs to further verify the results of in vitro experiments.

RESULTS

Co-culture of pre-activated T cells with MSCs in vitro modulated the CCR2-CCL2 axis, resulting in quiescent T cells and polarization toward CCR2⁺CD4⁺ T cell subsets. Blocking CCR2-CCL2 interaction abolished the immunoregulatory effect of MSCs, leading to re-activation of T cells and partial reversion of polarizing toward CCR2⁺CD4⁺ T cells. In IPS mouse model, application of MSCs prolonged the survival and reduced the pathological damage and T cell infiltration into lung tissue. Activation of CCR2-CCL2 axis and production of CCR2⁺CD4⁺ T cells were observed in the lungs treated with MSCs. The prophylactic effect of MSCs on IPS was significantly attenuated by the administration of CCR2 or CCL2 antagonist in MSC-treated mice.

CONCLUSIONS

We demonstrated an important role of CCR2-CCL2 axis in modulating T cell function which is one of the mechanisms of the prophylactic effect of MSCs on IPS.

Noninfectious pulmonary complications of haematopoietic stem cell transplantation

Haematopoietic stem cell transplantation (HSCT) is an established treatment for a variety of malignant and nonmalignant conditions. Pulmonary complications, both infectious and noninfectious, are a major cause of morbidity and mortality in patients who undergo HSCT. Recent advances in prophylaxis and treatment of infectious complications has increased the significance of noninfectious pulmonary conditions. Acute lung injury associated with idiopathic pneumonia syndrome remains a major acute complication with high morbidity and mortality. On the other hand, bronchiolitis obliterans syndrome is the most challenging chronic pulmonary complication facing clinicians who are taking care of allogeneic HSCT recipients. Other noninfectious pulmonary complications following HSCT are less frequent. This review provides a clinical update of the incidence, risk factors, pathogenesis, clinical characteristics and management of the main noninfectious pulmonary complications following HSCT.

Noninfectious pulmonary complications following haematopoietic stem cell transplantation is a major cause of morbidity and mortality in this patient population. There are recent advances in the diagnosis and management of these conditions.http://bit.ly/2FgsIYG

Current Approach to Non-Infectious Pulmonary Complications of Hematopoietic Stem Cell Transplantation

Hematopoietic stem cell transplantation is an established treatment for patients with a wide range of malignant and nonmalignant conditions. Noninfectious pulmonary complications still remain a leading cause of morbidity and mortality in these patients. Treating hematopoietic stem cell transplantation recipients with noninfectious pulmonary complications is still challenging, and the current treatment armamentarium and strategies are not adequate for patients receiving hematopoietic stem cell transplantation. Further trials are needed for a better description of the pathogenesis and the complete diagnostic criteria as well as for the development of effective therapeutic approaches for the management of noninfectious pulmonary complications of the hematopoietic stem cell transplantation. This review outlines the incidence, risk factors, pathogenesis, and clinical spectrum and discusses the current approaches to the management of noninfectious pulmonary complications of Hematopoietic stem cell transplantation.

Inert Gas Washout in Bronchiolitis Obliterans Following Hematopoietic Cell Transplantation

BACKGROUND

Bronchiolitis obliterans syndrome (BOS) is a leading cause of chronic graft-vs-host disease (cGvHD) and is associated with mortality after allogeneic hematopoietic stem cell transplantation (alloHSCT). The nitrogen multiple breath washout test (N₂-MBW) measures ventilation inhomogeneity, a biomarker of central and peripheral airway obstruction. The aim of this study was to examine ventilation inhomogeneity according to cGvHD score and histologically defined bronchiolitis obliterans (BO).

METHODS

This single-center prospective cross-sectional study included 225 adults (mean age, 52.8 years; median, 5.4 years [interquartile range, 2.0-11 years]) after alloHSCT. Outcomes were global (lung clearance index [LCI]) and acinar ventilation inhomogeneity index (S_ACIN) from N₂-MBW. Patients were categorized into five groups: (1) no cGvHD and no obstruction (cGvHD overall score 0 and FEV₁/FVC ≥ 70) (2) cGvHD and no obstruction (cGvHD overall score 1-3 and FEV₁/FVC ≥ 70), (3) BOS with or without cGvHD (if available, no BO on histologic examination, and FEV₁/FVC < 70), (4) histologically proven BO, and (5) diffuse parenchymal lung disease other than BO.

RESULTS

The LCI and S_ACIN differed significantly between groups (P < .001) and increased progressively according to cGvHD score. In BO, the LCI and S_ACIN were elevated in 95.5% and 81.8% of patients, respectively, whereas FEV₁/FVC was abnormal in only 56.5% of patients, respectively.

CONCLUSIONS

N₂-MBW is highly sensitive for detecting abnormal lung function in patients following alloHSCT. LCI and S_ACIN seem to be promising biomarkers of lung involvement in cGvHD.

Infection with a respiratory virus before hematopoietic cell transplantation is associated with alloimmune-mediated lung syndromes

BACKGROUND

Alloimmune-mediated lung syndromes (allo-LSs) are life-threatening complications after hematopoietic cell transplantation (HCT). Respiratory virus (RV) has been suggested to play a role in the pathogenesis.

OBJECTIVE

We studied the relation between RV DNA/RNA detection in the upper/lower airways before HCT and the occurrence of allo-LSs.

METHODS

We retrospectively analyzed all HCT recipients between 2004 and 2014, in whom real-time PCR for RV was performed in nasopharyngeal aspirates (NPAs) and bronchoalveolar lavage (BAL) fluid before HCT. The main outcome of interest was the presence of an allo-LS, which was defined as idiopathic pneumonia syndrome or bronchiolitis obliterans syndrome. Other outcomes were overall survival and treatment-related mortality. We used Cox proportional hazard models, logistic regression models, and Fine-Gray competing risk regression for analyses.

RESULTS

One hundred seventy-nine children (median age, 6.8 years) were included. RVs were found in 61% (41% in BAL fluid/NPAs and 20% in NPAs only). Rhinovirus was the most frequently detected RV (42%). Allo-LSs occurred in 13%. RV positivity in BAL fluid was a predictor for allo-LSs (hazard ratio, 3.8; 95% CI, 1.4-10.7; P = .01), whereas RV positivity in NPAs only was not. No other predictors were found. Grade II to IV acute graft-versus-host disease related to steroid treatment shows a trend toward a protective effect (odds ratio, 0.16; 95% CI, 0.0-1.3; P = .08). Allo-LSs significantly increased treatment-related mortality (52% ± 10% in allo-LSs and 20% ± 4% in non-allo-LSs, P = .007).

CONCLUSIONS

These results show that pre-HCT BAL fluid RV positivity was a predictor for allo-LSs. Screening for RVs before HCT might identify patients at risk for allo-LSs. This could have implications for prevention and treatment and might subsequently influence the outcomes of HCT.

The lung function score and its components as predictors of overall survival and chronic graft-vs-host disease after allogeneic stem cell transplantation

Aim

To retrospectively assess if the modified lung function score (LFS) and/or its components, forced expiratory volume within the first second (FEV₁) and diffusion capacity for carbon monoxide corrected for hemoglobin level (cDLCO), predict overall survival (OS) and chronic graft-vs-host-disease (cGvHD).

Methods

We evaluated 241 patients receiving allogeneic hematopoietic stem cell transplantation (allo-HSCT) at the University of Regensburg Transplant Center between June 1998 and July 2005 in relation to their LFS, FEV₁ and cDLCO, before and after HSCT.

Results

Decreased OS after allo-HSCT was related to decreased pre-transplantation values of FEV₁<60% (P = 0.040), cDLCO<50% of predicted value (P = 0.025), and LFS≥III (P = 0.037). It was also related to decreased FEV₁ at 3 and 12 months after HSCT (P < 0.001 and P = 0.001, respectively) and increased LFS at 3 and 12 months after HSCT (P = 0.028 and P = 0.002, respectively), but not to changes of cDLCO. A higher incidence of cGvHD was related to decreased FEV₁ at 6, 12, and 18 months (P = 0.069, P = 0.054, and P = 0.009, respectively) and increased LFS at 12 months (P = 0.002), but not to changes in cDLCO.

Conclusions

OS was related to both LFS and FEV₁, but cGvHD had a stronger relation to FEV₁ than to cDLCO or LFS. FEV₁ alone offered more information on the outcome after allo-HSCT than LFS or cDLCO, suggesting limited value of LFS for the patients’ assessment after allo-HSCT.

Host impairments in patients with neoplastic diseases

Healthy individuals possess an immune system comprising physical barriers, innate and acquired immunity as well as the indigenous microflora that populate the body surfaces. The immune system maintains constant vigilance over the body at the cellular level as well as at the interface between the host integument and the resident microflora. However, neoplastic diseases and their treatment often lead to impaired immunity resulting in an increased risk of infections due to viruses, bacteria, fungi, and protozoa. This chapter explores the various aspects of host impairment focusing on the components of immunity and the interplay between them to explain why it is that these patients succumb to infections per se. In so doing, we hope that the reader will be better equipped to understand the risks patients face so as to anticipate potential infectious complications and implement appropriate measures to help attain successful remission of the neoplastic diseases and maintain the best quality of life for the patient.

Advances in critical care of the pediatric hematopoietic stem cell transplant patient

Hematopoietic stem cell transplant (SCT) remains a curative option for a variety of malignant and non-malignant disorders in children. Following transplant a proportion of SCT recipients become critically ill and need intensive care. Critical illness may occur in the setting of transplant complications such as graft versus host disease (GVHD), idiopathic pneumonia syndrome (IPS), veno-occlusive disease (VOD) and transplant associated thrombotic microangiopathy (TA-TMA). Hence, familiarity with recent advances in the transplant process and complications is crucial for the intensivist. This article will highlight common complications encountered in the critically ill SCT recipient.

Effect of weight on outcomes of children undergoing hematopoietic cell transplantation

Chemotherapy dosing in hematopoietic cell therapy (HCT) conditioning regimens is based on patient weight. We hypothesized that potential underdosing or overdosing of patients with significant deviation of weight from normal might alter HCT outcomes, such as early mortality, overall or organ-specific toxicity, and/or relapse. We therefore conducted a retrospective analysis of 400 children between the ages of 2 and 18 years who underwent HCT for malignant or nonmalignant disease at Boston Children's Hospital over a 10-year period. Using the Centers for Disease Control and Prevention standard weight classification schema, we found no evidence to suggest a difference in survival or in time to engraftment or in relapse in patients with malignant disease. In the subgroups of patients either receiving autologous HCT or with underlying malignancy, combined overweight and obese patients had a higher rate of any organ, but not organ-specific, Grade 3-5 toxicity compared with the normal weight group. The study was not powered to detect a difference between underweight and normal weight patients. These data suggest that multiple outcome measures over the first year after HCT are unaffected by weight.

Cytokine profiles in various graft-versus-host disease target organs following hematopoietic stem cell transplantation

Previous studies using genetic-deficient murine models suggest that different T-helper subsets may contribute to different types of tissue damages in graft-versus-host disease (GvHD). However, there is limited information available on the distribution of T-helper cytokines in the various GvHD target tissues. In the current study, an acute GvHD murine model was set up to directly assess the in situ cytokine profiles in various GvHD tissue lesions; in addition, we also studied GvHD tissues from patients who had undergone bone marrow transplantation procedures. We observed that interferon-γ (IFN-γ was dominant in murine liver and gastrointestinal tissue lesions, whereas IFN-γ and interleukin 17 (IL-17) were abundant in murine skin lesions. Furthermore, in human GvHD tissues, interleukin 4 (IL-4) and IFN-γ were predominant in liver lesions and colon lesions, respectively, while no specific cytokine was prevalent in human GvHD skin lesions. In addition, a low ratio of CD4(+) T helper (Th) versus CD8(+) T cytotoxic (Tc) cells in human GvHD tissue lesions, especially in the liver, was detected, and this contrasts with the situation in murine GvHD tissues where CD4(+) Th cells were predominant. Dual staining for CD markers and cytokine expression showed that IFN-γ-secreting T cells were enriched in all murine GvHD target tissue lesions, and Tc1 and Tc2 cells were predominant in human GvHD colon and liver sections, respectively. However, IFN-γ(+) Th1, IL-17(+) Th17, IFN-γ(+) Tc1, and IL-17(+) Tc17 cells were slightly more frequent in human skin lesions compared to IL-4(+) Th2 and IL-4(+) Tc2 cells. To sum up, these results suggest that differences in cytokine imbalances may significantly contribute to tissue-specific pathogenesis in GvHD target organs, and CD8(+) Tc cells may play an important role in human GvHD induction.

Incidence, clinical features, and risk factors of idiopathic pneumonia syndrome following hematopoietic stem cell transplantation in children

BACKGROUND

Idiopathic pneumonia syndrome (IPS) is a severe complication that can occur after hematopoietic stem cell transplantation (HSCT) and is often associated with a fatal outcome despite intensive supportive care.

PROCEDURE

To assess the incidence and risk factors of IPS, we reviewed 251 consecutive patients (median age, 7.0 years) who received HSCT at the Department of Pediatrics, Nagoya University Hospital, between January 1990 and July 2009.

RESULTS

Twenty of 251 (cumulative incidence of IPS at 2 years after HSCT, 8.0%; 95% confidence interval (CI), 5.1-12.4%) patients developed IPS. The median duration from HSCT to diagnosis of IPS was 67 days (range, 12-486 days). Patients with IPS had significantly higher 5-year transplant-related mortality compared to patients without IPS (52% (95% CI, 19-77%) vs. 13% (95% CI, 5-25%), P < 0.001), and the probability of 5-year overall survival was significantly worse for patients with IPS (42% (95% CI, 25-64%) vs. 68% (95% CI, 59-76%), P = 0.01). By multivariate analysis, high risk in underlying disease (HR, 2.5; 95% CI, 1.0-6.7; P = 0.05) and a busulfan-containing regimen (HR, 3.5; 95% CI, 1.3-9.9; P < 0.01) were identified as the independent risk factors for developing IPS.

CONCLUSION

The prophylactic strategies for IPS in patients with these risk factors were warranted.

Effect of total body irradiation on late lung effects: hidden dangers

PURPOSE

In our ongoing investigation into the consequences of a radiological terrorism or nuclear dispersion event, we assessed whether a dose range that is believed to be sub-threshold for the development of lung endpoints results in late pathological changes and, secondarily, whether those late changes affect the lung's ability to respond to subsequent challenge.

MATERIALS AND METHODS

C57BL/6J mice received total body irradiation (0.5-10 Gy) and were followed for 6-18 months after irradiation. At 12 and 15 months, a subset of mice was exposed to a second challenge (aerosolised lipopolysaccharide [LPS]).

RESULTS

Cytokines shown to be upregulated early (hours) following irradiation (interleukin [IL]6, keratinocyte chemoattractant [KC], IL1B, and IL1R2) demonstrated increases in messenger ribose nucleic acid (mRNA) expression at late time points, beginning at nine months. Although persistent, dose-dependent increases in T cell counts were seen, no other overt changes in pathophysiology were observed. Nonetheless, animals that were exposed to a secondary challenge at late time points demonstrated an increased inflammatory cell recruitment and persistence in response relative to controls.

CONCLUSIONS

We propose that, following doses that elicit little change in pathophysiology, sub-clinical radiation-induced injury increases the lungs' susceptibility to a secondary challenge, possibly through a radiation-induced alteration in the immune defense system.

Alloimmune lung injury induced by local innate immune activation through inhaled lipopolysaccharide

BACKGROUND

Alloimmune lung injury, characterized by perivascular lymphocytic inflammation, lymphocytic bronchiolitis (LB), and obliterative bronchiolitis (OB), causes substantial morbidity and mortality after lung transplantation and bone marrow transplantation (BMT), but little is known regarding its pathogenesis. We have developed and pursued the hypothesis that local activation of pulmonary innate immunity through toll-like receptor (TLR)-4 is critical to the development of posttransplant alloimmune lung injury.

METHODS

We developed a fully major histocompatibility complex-mismatched murine BMT model without systemic graft-versus-host disease, and challenged mice with aerosolized lipopolysaccharide (LPS), a prototypic TLR4 agonist, to determine the effect upon pulmonary alloimmune lung injury.

RESULTS

LPS-exposed allogeneic BMT recipient mice developed histological and biological features of LB and OB, which were not observed in non-LPS-exposed allogeneic controls or syngeneic LPS-exposed mice. LPS-induced lymphocytic lung inflammation was dependent upon intact TLR4 signaling in donor-derived hematopoietic cells but not recipient structural lung cells, demonstrating a distinct function for TLR4 on hematopoietic cells in mediating alloimmunity.

CONCLUSIONS

We demonstrate a critical role for localized, environmentally induced innate immune activation in promoting alloimmune lung injury. Local inhibition of TLR4 signaling in pulmonary resident hematopoietic cells represents a novel and potentially important therapeutic target to prevent posttransplant rejection.

Decreased asbestos-induced lung inflammation and fibrosis after radiation and bone marrow transplant

The effect of lung irradiation on subsequent inflammatory or fibrotic lung injuries remains poorly understood. We postulated that irradiation and bone marrow transplantation might impact the development and progression of lung remodeling resulting from asbestos inhalation. Our objective was to determine whether irradiation and bone marrow transplantation affected inflammation and fibrosis associated with inhaled asbestos exposure. Inflammation, cytokine production, and fibrosis were assessed in lungs of naïve and sex-mismatched chimeric mice exposed to asbestos for 3, 9, or 40 days. Potential engraftment of donor-derived cells in recipient lungs was examined by fluorescence in situ hybridization and immunohistochemistry. Compared with asbestos-exposed naïve (nonchimeric) mice, chimeric mice exposed to asbestos for 3, 9, or 40 days demonstrated significant abrogation of acute increases in asbestos-associated inflammatory mediators and fibrosis. Donor-derived cells trafficked to lung but did not significantly engraft as phenotypic lung cells. Irradiation and bone marrow transplantation alters inflammatory and fibrotic responses to asbestos, likely through modulation of soluble inflammatory mediators.

A pediatric case of transfusion-related acute lung injury following bone marrow infusion

TRALI is a rare but serious complication associated with transfusion, and known to occur following infusion of all types of plasma-containing blood products. However, only one adult case of TRALI after allogenic marrow graft has been reported. In this study, we present a pediatric case possibly associated with allogenic marrow infusion. A 10-yr-old girl was referred to our hospital for the treatment of acute myeloid leukemia. She underwent allogenic bone marrow transplantation from her HLA-2-loci-mismatched mother. During conditioning, she suffered from bacterial sepsis, but it had improved with antibiotics until day 0 of transplantation. Two h after starting the marrow infusion, she developed severe hypoxia. We discontinued the infusion and started steroids, which improved her respiratory condition. However, she developed respiratory failure again after resuming infusion of the graft. Despite intensive care with mechanical ventilation, the patient died of endotoxin shock five days after transplantation. Although we could not identify the antibody which might have been involved in the respiratory distress, the clear temporal relationship between marrow infusion and respiratory distress suggested that similar acute lung injury to TRALI might have occurred following allogenic marrow infusion in the present case.

Eicosanoid regulation of pulmonary innate immunity post-hematopoietic stem cell transplantation

Hematopoietic stem cell transplantation (HSCT) is a therapeutic option for a number of malignant and inherited disorders. However, the efficacy of this therapy is limited by a number of serious infectious and noninfectious complications. Pulmonary infections represent a significant cause of morbidity and mortality post-HSCT and can occur both pre- and post-hematopoietic reconstitution. Susceptibility to Gram-negative bacterial infections despite full hematopoietic engraftment suggests that innate immunity remains impaired months to years post-HSCT. This review will describe the process and complications of HSCT and will summarize what is known about innate immune reconstitution post-HSCT. Data from the literature as well as our own laboratory will be presented to suggest that an eicosanoid imbalance characterized by over-production of prostaglandins and under-production of leukotrienes leads to impaired lung phagocyte function post-HSCT. Of therapeutic interest, strategies which limit production of prostaglandins can improve pulmonary host defense in animal HSCT models, which suggests that this may also be beneficial for human HSCT recipients.

IFNgamma differentially controls the development of idiopathic pneumonia syndrome and GVHD of the gastrointestinal tract

Although proinflammatory cytokines are key mediators of tissue damage during graft-versus-host disease (GVHD), IFNgamma has previously been attributed with both protective and pathogenic effects. We have resolved this paradox by using wild-type (wt), IFNgamma(-/-), and IFNgammaR(-/-) mice as donors or recipients in well-described models of allogeneic stem cell transplantation (SCT). We show that donor-derived IFNgamma augments acute GVHD via direct effects on (1) the donor T cell to promote T helper 1 (Th1) differentiation and (2) the gastrointestinal (GI) tract to augment inflammatory cytokine generation. However, these detrimental effects are overwhelmed by a protective role of IFNgamma in preventing the development of idiopathic pneumonia syndrome (IPS). This is the result of direct effects on pulmonary parenchyma to prevent donor cell migration and expansion within the lung. Thus, IFNgamma is the key cytokine differentially controlling the development of IPS and gastrointestinal GVHD after allogeneic SCT.

Incidence and outcome of idiopathic pneumonia syndrome in pediatric stem cell transplant recipients

Idiopathic pneumonia syndrome (IPS) is a rare complication following stem cell transplant (SCT) and its incidence among pediatric SCT recipients is not known. To assess the incidence of IPS, we retrospectively reviewed the incidence of IPS at our center. IPS is defined as the presence of multilobar infiltrates by chest radiograph or computed tomography scan, need for supplemental oxygenation with declining pulse oximetry and no identifiable pulmonary infection. Between July 1999 and August 2005, 11 of 93 children who received a fully ablative allogeneic SCT (11.8%) developed IPS. All 11 patients had normal pulmonary evaluation before transplant. IPS developed at a median of 17 days (range 8-42 days) after transplant. Recipients of unrelated donor transplant had increased risk of developing IPS. There was a significant association between acute or hyperacute graft-versus-host disease (GVHD) and IPS (P=0.035). All patients had significant hypoxia and five patients required assisted ventilation. IPS was the cause of death in two patients. Although there was complete resolution of respiratory symptoms in the other nine patients, overall transplant-related mortality was significantly higher among patients with IPS (64 vs 17%, P=0.002). IPS is a relatively common complication in pediatric SCT recipients and acute GVHD is an important associated factor.