Clara cells represent the predominant secretory cell within distal conducting airways of mammals and exhibit functional alterations with chronic lung disease. We previously demonstrated that Clara cell secretory protein (CCSP) deficiency results in enhanced susceptibility to environmental agents. The present study was undertaken to define changes in Clara cell secretory function associated with CCSP deficiency in knockout mice. Comparative morphometry of Clara cell ultrastructure revealed dramatic alterations in secretory apparatus between wild-type (WT) and CCSP knockout (CCSP-/-) mice. Secretory granules, which occupy greater than 2% of Clara cell cytoplasmic volume in WT mice, were completely absent among Clara cells of CCSP-/- mice. Moreover, Clara cells of CCSP-/- mice exhibited a > 95% reduction in rough endoplasmic reticulum and alterations to Golgi apparatus, relative to WT controls. Ultrastructural perturbations to Clara cells were associated with altered protein composition of airway lining fluid as revealed by two-dimensional gel analysis of bronchoalveolar lavage proteins, but were not associated with altered abundance or secretion of CC26, another Clara cell secretory protein. We conclude that CCSP is required for the appearance of Clara cell secretory granules and that functional changes to Clara cells that result from CCSP deficiency lead to alterations in the composition of epithelial lining fluid.

Clara cell secretory protein (CCSP) is one of the most abundant proteins present in airway lining fluid of mammals. In an effort to elucidate the function of CCSP, we established CCSP-null [CCSP(-/-)] mice and demonstrated altered sensitivity to various environmental agents including oxidant pollutants and microorganisms. Although CCSP deficiency itself may be central to the observed changes in environmental susceptibility, altered lung gene expression associated with CCSP deficiency may contribute to the observed phenotype. To determine whether CCSP deficiency results in altered lung gene expression, high-density cDNA microarrays were used to profile gene expression in the total lung RNA of wild-type and CCSP(-/-) mice. Genes that were differentially expressed between wild-type and CCSP(-/-) mice included a previously non-annotated expressed sequence tag (EST W82219) and immunoglobulin A (IgA), both of which were elevated with CCSP deficiency. mRNA expression of EST W82219 and IgA was localized in the lungs of wild-type and CCSP(-/-) mice to airway Clara cells and peribronchial lymphoid tissues, respectively. We conclude that CCSP deficiency is associated with 1) altered gene expression in Clara cells of the conducting airway epithelium and 2) alterations to peribronchial B lymphocytes. These findings identify new roles for Clara cells and their secretions in airway homeostasis.

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