Microbiol. infections remain a major problem for bone marrow (BM) transplant (BMT) recipients (35, 45). Mortality from opportunistic fungal infections exceeds 50% in most studies and has been reported to be as high as 95% in allogeneic BMT recipients with sp. contamination, despite aggressive antifungal therapy (4). Studies in vitro and in animal models have indicated that this innate defenses are primarily responsible for the elimination of inhaled conidia from the lungs (10, 13, 19, 38). Early fungal clearance is usually mediated by a dual phagocytic system involving both alveolar macrophages and recruited polymorphonuclear leukocytes capable of efficiently opposing fungal infectivity at the level of conidia or hyphal forms (37). However, the killing of phagocytosed conidia by mononuclear cells is usually a slow process that occurs with a low killing rate and depends on the immunocompetence of effector monocytes (19). Moreover, the finding that the conidiocidal activity of monocytes c-Fms-IN-8 in both clinical disease c-Fms-IN-8 and experimental chronic granulomatous disease is largely unaffected (26) reveals the unique importance of neutrophil activity against germinating conidia and hyphae in the control of aspergillosis. Human studies have shown that prolonged neutropenia is one of the most important factors predisposing to invasive aspergillosis (35, 45). However, the efficacy of immunotherapies aimed at both shortening the duration of neutropenia and restoring neutrophil antifungal activity has been limited by problems associated with the transfusion therapy, including the still uncertain efficacy of colony-stimulating factors (34) and the limited persistence of the transfused cells (16). It appears that strategies aimed at keeping the infection in check until the recovery of adequate innate antifungal activity are needed for prompt handling of the fungus by the host. Recent studies have highlighted the therapeutic potential of killer antiidiotypic antibodies in several fungal infections (23). Antiidiotypes to a monoclonal antibody (MAb) specifically reacting with killer toxins (KT) from and are characterized by a broad antimicrobial spectrum (30) and are lethal to pathogenic microorganisms expressing specific cell wall receptors (KTR). Polyclonal antibodies, MAbs, or single-chain recombinant killer antiidiotypic antibodies appear to have fungicidal activity in vitro and to confer active and passive protection in vivo on mice with candidiasis or pneumocystosis (6, 22, 31, 39). Although the impact of natural killer antibodies, as well as of the overall antibody response, on antifungal immune resistance is not completely clear, the use of antibodies is usually emerging as an effective adjunct therapy for fungal diseases (40). To assess the therapeutic potential of killer antiidiotypic antibodies against contamination, we used a mouse model of T-cell-depleted allogeneic BMT with invasive pulmonary aspergillosis (IPA). We have already shown that these mice failed to develop antifungal T-helper type 1 resistance, an activity that could be efficiently restored upon treatment with T-helper type 2 cytokine antagonists (25). We found that a killer antiidiotypic MAb, the K10 MAb, that potently inhibited hyphal development and metabolic activity in vitro had in vivo therapeutic CACNB3 efficacy against IPA. MATERIALS AND METHODS Mice. Female, 8- to 10-week-old, inbred C3H/HeJ and DBA/2 mice were obtained from c-Fms-IN-8 Charles River Breeding Laboratories (Calco, Italy). All mice were kept in small sterile cages (four animals in each cage) and fed with sterile food and water at the animal facilities of the University of Perugia, Perugia, Italy. Procedures involving animals and their care c-Fms-IN-8 were conducted in conformity with national and international laws and guidelines. Irradiation. C3H/HeJ mice were exposed to a single lethal dose of 9 Gy from an 18-mV photon beam linear accelerator (Clinac 600/C Varian; Cernusco, Milan, Italy) with a focus-to-skin distance of 75 cm and a dose of 0.7 Gy/min (20). Without BMT, the mice died within 14 days. Preparation of T-cell-depleted BM cells. BM cells were prepared as previously described, with minor modifications (32). Donor BM cells were collected into phosphate-buffered saline (PBS) by flushing the shafts of the femurs and tibias of DBA/2 mice, which are known to be highly susceptible to IPA (9). The cells were suspended, and clumps of debris were allowed to settle out. The cells were washed three times with PBS and resuspended at a final concentration of 3 108 cells per ml. The cells were then fractionated by differential agglutination with soybean agglutinin as previously described (32). Briefly, the BM cell suspension was incubated in polystyrene tubes with soybean agglutinin at 2 mg/ml for 5 min at room heat. The cells were gently layered on top of a 5% bovine serum albumin answer in 8 ml of PBS in 15-ml conical tubes. After 15 min.