-We have previously demonstrated that adoptive transfer of alveolar macrophages from allergy-resistant rats to alveolar macrophage-depleted allergic rats prevents airway hyperresponsiveness development, suggesting an important role for alveolar macrophages in asthma pathogenesis. Given that ovalbumin sensitization can modulate alveolar macrophage cytokine production, we investigated the role of sensitized and unsensitized alveolar macrophages in an asthma model. Alveolar macrophages from unsensitized or sensitized Brown Norway rats were transferred to alveolar macrophage-depleted sensitized rats 24 h before allergen challenge. Airway responsiveness to methacholine and airway inflammation were measured the following day. Methacholine concentration needed to increase lung resistance by 200% was significantly higher in alveolar macrophage-depleted sensitized rats that received unsensitized alveolar macrophages compared with alveolar macrophage-depleted sensitized rats that received sensitized alveolar macrophages. Tumor necrosis factor levels in bronchoalveolar lavage fluid of sensitized rats that received unsensitized alveolar macrophages were significantly lower compared with rats that received sensitized alveolar macrophages. Interestingly, alveolar macrophages of unsensitized animals showed higher phagocytosis activity compared with alveolar macrophages of sensitized rats, suggesting that sensitization modulates alveolar macrophage phagocytosis function. Our data suggest an important role of allergen sensitization on alveolar macrophage function in asthma pathogenesis. airway hyperresponsiveness; cytokine; phagocytosis THE INCIDENCE OF allergic respiratory diseases increased dramatically over the last two decades. The genetic predisposition to develop asthma is now well recognized (20). Numerous studies have also shown the importance of inflammatory cells, such as mast cells, lymphocytes, eosinophils, and neutrophils, as well as Th2 type cytokines in asthma (6,19,32). Interleukin (IL)-4 and IL-13, two Th2 cytokines, are important in immunoglobulin (Ig) class switching to IgE, whereas IL-4 and IL-10 play a crucial role in Th2 cell commitment (9,16,50). In contrast, Th1 cytokines (interferon-␥ and IL-12) protect against asthma development in children (48). Other cytokines, such as tumor necrosis factor (TNF), have been demonstrated to amplify the effect of asthmatic inflammation (29,42). Recently, a role of regulatory T cells (Th3 cells, T R cells, CD4 ϩ CD25 ϩ cells, and natural killer T cells) has been suggested in the control of asthma and allergy, but the specific mechanisms are still unknown (2).Alveolar macrophages (AM) are predominant immune effector cells in the alveolar spaces and conducting airways and play a key role in the immunological homeostasis of the lung (8, 30, 43). Although they are well known to suppress T cell activation and antigen presentation activities of dendritic cells (22,41), their role in asthma is still debated. Using the liposome-mediated macrophage suicide technique (47), we have...