Summary:Respiratory syncytial virus (RSV) is widely recognized as a leading cause of pneumonia, with substantial mortality, in bone marrow transplant recipients. We tested the efficacy of a systemic monoclonal antibody (MAB) preparation possessing a high titer of anti-RSV neutralizing antibody, palivizumab (Synagis) for prophylaxis and therapy of RSV infection in cytoxan (CY) immunosuppressed cotton rats, a model in which the efficacy of a polyclonal anti-RSV product (Respigam) has been demonstrated. Both prophylaxis and therapy with this MAB were highly effective in reducing pulmonary viral replication. However, multiple sequential therapeutic doses of MAB were necessary to control rebound viral replication in continually suppressed animals. Bone Marrow Transplantation (2002) 29, 117-120. DOI: 10.1038/sj/bmt/1703326 Keywords: RSV; BMT; immunotherapy; palivizumab; Synagis; cotton rat Respiratory syncytial virus (RSV) is widely recognized as one of the major causes of pneumonia in immunocompromised cancer patients. 1 The most striking impact of this pathogen to be identified over the last decade has been its propensity to lead to severe pneumonia during the early period post stem cell transplantation, with a high morbidity and mortality. 1-3 A recent case review reported three deaths among seven patients who developed RSV infections in the post-engraftment period, suggesting a continued significant impact of this pathogen. 4 Those authors also reported an association between RSV infection and both primary and secondary graft failure in four patients. The opinions or assertions contained herein are those of the author(s) and are not to be construed as official or reflecting the views of the Department of Defense, the United States Air Force, or the Uniformed Services University.The cotton rat model has been a useful tool in the study of RSV pathogenesis and therapy for over 30 years. 5 We recently reported a modification of the use of that model where, by prolonged high-dose cytoxan immunosuppression, we replicated the persistent RSV pneumonia seen in stem cell transplant recipients. 6 In that paper, we demonstrated the ability of a human commercial polyclonal high anti-RSV activity IgG preparation (RSVIG or Respigam) to control RSV replication when given either prophylactically or therapeutically. We return to that model to test the use of the next generation of the product, the humanized anti-RSV monoclonal antibody (MAB) palivizumab (Synagis). Materials and methods AnimalsWeanling inbred cotton rats (Sigmodon hispidus) were obtained from Virion Systems Incorporated. Animals were housed in large polycarbonate cages in small groups, and were provided water and rat chow ad libitum. ImmunosuppressionAnimals were treated with intraperitoneal (i.p.) injections of cyclophosphamide (CY) at a dose of 50 mg/kg three times weekly for at least 21 days prior to viral challenge. Immunosuppressive therapy was continued until the end of each experiment. Prior and current use of this regimen has consistently led to the develo...