KL-6, an epitope of MUC1 mucin expressed on type II pneumocytes and bronchiolar epithelia in humans, is a sensitive serum marker for interstitial pneumonia. However, an in vivo model for KL-6 has not been established because no KL-6 epitope is expressed in animals other than humans and apes. To investigate whether KL-6 is detectable in human MUC1-expressing (hMUC1-exp) mice and whether KL-6 level reflects the degree of lung injury, we examined serum and bronchoalveolar lavage fluid (BALF) levels of KL-6 and surfactant protein-D (SP-D) in either lipopolysaccharide (LPS)- or bleomycin (BLM)-induced lung injury models. KL-6 was expressed on type II pneumocytes and bronchiolar epithelial cells in naive hMUC1-exp mice. Serum KL-6 levels in these mice were comparable to those in humans, and KL-6 levels in BALE were significantly higher than those in sera. In the LPS model, KL-6 levels in sera and BALF were slightly increased, although SP-D levels were markedly increased. During the inflammatory phase in the BLM model, KL-6 levels in sera were greatly increased, but those in BALF were decreased. Serum KL-6 levels were positively correlated with BALE albumin levels, a representative marker for increased the alveolar-capillary permeability. SP-D levels in sera and BALE were significantly increased compared to the corresponding levels in the LPS model. The increase in serum KL-6 levels appeared to be associated with the disruption of alveolar-capillary barrier after BLM-induced lung injury. This hMUC1-exp mouse can be used for assessment of KL-6 in vivo during lung injury. (C) 2013 Elsevier Inc. All rights reserved.