In vivo models have demonstrated that interleukin-13 (IL-13) plays an important role in asthma; however, few studies have evaluated the effect of inhibition of IL-13 on established and persistent disease. In the present study, we have investigated the effect of a therapeutic dosing regimen with an anti-IL-13 monoclonal antibody (mAb) in a chronic mouse model of persistent asthma. BALB/c mice were sensitized to allergen [ovalbumin (OVA); on days 1 and 8] and challenged with OVA weekly from day 22. Anti-IL-13 mAb or vehicle dosing was initiated following two OVA challenges when disease was established. At this time, mice exhibited airway hyperresponsiveness (AHR), increased mucus production, inflammation, and initiation of subepithelial fibrosis compared with saline-challenged mice. Mice received four additional OVA challenges. Treatment with anti-IL-13 mAb inhibited AHR and prevented the further development of subepithelial fibrosis and progression of inflammation. Furthermore, mAb treatment reversed the mucus hyperplasia to basal levels. These effects were associated with an inhibition of cytokines, chemokines, and matrix metalloproteinase-9. These data demonstrate that neutralization of IL-13 can inhibit the progression of established disease in the presence of repeated allergen exposures.Asthma is characterized by the presence of reversible bronchoconstriction, increased sensitivity to specific and nonspecific bronchospasmic agents, and excessive mucus production. These clinical features are accompanied by an underlying pathology of inflammation and airway remodeling. The pathological changes are thought to contribute to the clinical symptoms of the disease (Fireman, 2003). Current therapies for asthma provide symptomatic control but do not halt the underlying disease, highlighting the unmet medical need. Patients are continually exposed to allergens or other bronchospasmic agents that may contribute to the maintenance or progression of persistent disease. Evidence to date suggests that interleukin-13 (IL-13) has an important role in asthma; it is associated with human disease, and preclinical models have demonstrated that it induces many of the features associated with human asthma.Elevated levels of IL-13 mRNA and protein (Huang et al., 1995) have been described in human disease pathogenesis, and polymorphisms in the IL-13 gene have been associated with asthma (Wills-Karp, 2000). Multiple studies have demonstrated that administration of recombinant murine (rm) IL-13 to the lungs of mice induces airway mucus hyperplasia, eosinophilia, and airway hyperresponsiveness (AHR) (Grunig et al., 1998;Wills-Karp et al., 1998;Singer et al., 2002;Kibe et al., 2003; Vargaftig and Singer, 2003a,b). These effects of IL-13 are reproduced in transgenic mouse systems where IL-13 overexpression is induced in a constitutive or inducible manner (Zhu et al., 1999Lanone et al., 2002). Chronic transgenic overexpression of IL-13 also induces subepithelial fibrosis and emphysema. Mice deficient in the IL-13 (and IL-4) signalin...