Bronchiolitis Obliterans Syndrome (BOS)

Bronchiolitis obliterans syndrome (BOS) is a clinical syndrome of progressive obstructive lung disease characterized by fibrotic constriction of small airways (constrictive bronchiolitis), occurring most commonly after lung transplantation as a major form of chronic lung allograft dysfunction (CLAD), and after allogeneic hematopoietic stem cell transplantation (HSCT) as pulmonary manifestation of chronic graft-versus-host disease (cGVHD). The term 'bronchiolitis obliterans' (BO) refers to the histopathologic pattern of fibrotic obliteration of bronchiolar lumens, while BOS is the clinical syndrome diagnosed by spirometric criteria without requiring tissue confirmation. Etiologic categories: (1) Post-lung transplantation BOS — the most common cause, occurring in 50-70% of lung transplant recipients within 5 years post-transplant, the leading cause of late mortality after lung transplant. Considered to result from chronic rejection. (2) Post-HSCT BOS — pulmonary manifestation of cGVHD, occurring in 5-10% of allogeneic HSCT recipients, usually 6-24 months post-transplant. (3) Toxic inhalation: sulfur mustard gas, nitrogen oxides, ammonia, smoke from fires, diacetyl (popcorn flavoring, 'popcorn lung'), volatile organic compounds. (4) Severe viral respiratory infections: especially adenovirus (post-infectious BOS classically described in children), respiratory syncytial virus (RSV), influenza, parainfluenza, measles. (5) Connective tissue diseases: rheumatoid arthritis (most common, 1-2% of patients), Sjögren syndrome, less commonly polymyositis, systemic sclerosis. (6) Drug-induced (rare): penicillamine, gold salts, sulfasalazine. (7) Idiopathic (rare).

Pathogenesis of post-transplant BOS: complex multifactorial process involving alloimmune and non-alloimmune mechanisms. Alloimmune mechanisms: (1) Cellular rejection — recurrent acute cellular rejection episodes are major risk factor for BOS development. (2) Antibody-mediated rejection — donor-specific antibodies (DSA), particularly against HLA class II antigens, contribute to chronic rejection. (3) Lymphocytic bronchiolitis — small airway centered inflammation. Non-alloimmune mechanisms: (1) Gastroesophageal reflux disease (GERD) with microaspiration of acid and bile acids — causes chronic airway injury (early fundoplication may reduce BOS). (2) Respiratory infections — viral (CMV, community respiratory viruses) and bacterial (Pseudomonas aeruginosa colonization in cystic fibrosis recipients). (3) Ischemia-reperfusion injury at transplantation. (4) Primary graft dysfunction. (5) Air pollution exposure. The histopathology of BO/BOS is constrictive bronchiolitis: peribronchiolar fibrosis with submucosal lamellar fibrosis, smooth muscle hypertrophy, fibroblast proliferation, partial or complete obliteration of bronchiolar lumens. The lesions are patchy, often involving terminal and respiratory bronchioles, with relative sparing of larger airways and alveolar parenchyma. Clinical presentation: insidious onset of dyspnea on exertion (may be subtle and progress slowly), dry cough, decreased exercise tolerance, decreased FEV1 on spirometry. Acute exacerbations with infections may accelerate decline. Diagnosis: BOS is diagnosed by spirometric criteria, defined by ISHLT 2019 update of CLAD criteria. BOS criteria: persistent (3 weeks or more) decline in FEV1 of 20% or more from post-transplant baseline (best 2 post-transplant FEV1 values averaged), in absence of other identifiable causes (pneumonia, pleural effusion, anastomotic complication, restrictive allograft syndrome). FEV1/FVC less than 0.7 with TLC preserved (obstructive pattern, distinguishing from restrictive allograft syndrome RAS). BOS staging: BOS 0 (FEV1 greater than 90% of baseline), BOS 0p (potential, FEV1 81-90% with new respiratory symptoms), BOS 1 (FEV1 66-80%), BOS 2 (FEV1 51-65%), BOS 3 (FEV1 50% or less). Imaging: HRCT shows mosaic attenuation (heterogeneous lung density due to air trapping in obstructed lobules), expiratory air trapping (key feature, requires expiratory CT), bronchiectasis (especially central airways), bronchial wall thickening, with relatively normal interstitium (distinguishing from restrictive allograft syndrome which has fibrotic changes). Bronchoalveolar lavage may show neutrophilic inflammation. Lung biopsy (transbronchial often inadequate, surgical biopsy more reliable) shows constrictive bronchiolitis but is not required for diagnosis as biopsy yield is low. Workup also includes evaluation for other causes of FEV1 decline: pulmonary infections (bronchoscopy with cultures and viral PCRs), bronchial anastomotic complications, GERD evaluation (impedance-pH testing, upper endoscopy), donor-specific antibody testing, evaluation for restrictive allograft syndrome (CT for fibrosis, decreased TLC).

Treatment of post-lung transplant BOS: BOS is challenging to treat with limited efficacy of available therapies. Goals: stabilize lung function decline, treat reversible contributing factors, prevent infections, optimize quality of life, evaluate for re-transplantation if eligible. (1) Optimization of immunosuppression: typically a triple-drug regimen (calcineurin inhibitor + antimetabolite + corticosteroid). Switch from cyclosporine to tacrolimus may improve outcomes. Addition of mTOR inhibitor (sirolimus or everolimus) replacing antimetabolite has shown benefit in some studies. Pulses of methylprednisolone for acute decline. Antithymocyte globulin (ATG) or alemtuzumab as rescue for refractory rejection. (2) Azithromycin: 250 mg orally three times weekly is now standard adjunct therapy based on multiple studies showing stabilization of FEV1 in 30-50% of patients (Vos et al., Verleden et al.). Mechanism includes antimicrobial activity against airway pathogens, anti-inflammatory effects (reduces neutrophilic airway inflammation), and modulation of mucus production. Should be considered in all patients with declining FEV1 or BOS. (3) Fluticasone-azithromycin-montelukast (FAM) triple therapy: combination has been associated with FEV1 stabilization in some studies. Inhaled corticosteroids alone have limited efficacy. (4) Extracorporeal photopheresis (ECP): immunomodulatory therapy with leukapheresis followed by 8-MOP and UVA light treatment of cells, then reinfusion. Multiple studies show stabilization or improvement of FEV1 in 30-60% of patients with BOS. Considered standard of care for refractory BOS in many centers. Typical regimen: weekly for 1-3 months, then bi-weekly, then monthly maintenance. (5) Aggressive management of GERD: proton pump inhibitor (PPI) twice daily, dietary and lifestyle modifications, fundoplication (Nissen fundoplication) for documented GERD, especially with positive pH/impedance studies — has been associated with stabilization of FEV1 in some studies. (6) Treatment of infections: aggressive treatment of bacterial, viral, fungal infections; chronic suppressive therapy for chronic Pseudomonas colonization with inhaled tobramycin or aztreonam. (7) Treatment of donor-specific antibodies: plasmapheresis, IVIG, rituximab, bortezomib for documented antibody-mediated rejection. (8) Anti-inflammatory therapies: montelukast (CysLT1 antagonist), pirfenidone and nintedanib are being studied. (9) Re-transplantation for severe BOS (BOS 3) with progressive decline despite maximal medical therapy in eligible candidates with good functional status — outcomes after re-transplant inferior to first transplant but provide additional years of survival. (10) Pulmonary rehabilitation, oxygen therapy, palliative care for advanced disease. Treatment of post-HSCT BOS (cGVHD): similar principles with optimization of immunosuppression for cGVHD (corticosteroids, calcineurin inhibitors, ECP, ibrutinib, ruxolitinib), azithromycin (BUT noted FDA warning regarding increased mortality in HSCT patients with BOS, so use cautiously), supportive care, and hematopoietic re-transplant rare. Prognosis: post-lung transplant BOS associated with significant mortality (50-70% 5-year mortality after BOS diagnosis), depending on rate of decline and stage at diagnosis. Slow stable BOS has better prognosis than rapidly progressive disease.