Proliferation and number of Clara cell 10-kDa protein (CC10)-reactive epithelial cells and basal cells in normal, hyperplastic and metaplastic bronchial mucosa

Clara cell 10-kDa protein (CC10) is an inhibitor of phospholipase A(2) and binds to phosphatidylinositol. It may therefore interfere with intracellula r signal transduction. Bronchial CC10-reactive cells have been described by several authors. In contrast to the bronchiolar CC10-containing Clara cell , which is a progenitor cell of terminally differentiated airway epithelium , the role of bronchial CC10-reactive cells remains to be elucidated. We as sessed the number of bronchial CC10-reactive cells in relation to cytokerat in (CK) expression and proliferative activity in normal, hyperplastic and s quamous metaplastic epithelium. Sixty-five human bronchial mucosal specimen s were investigated immunohistochemically for CK expression (CK7, CK13 and CK5/6), proliferative activity (MIB-1) and number of CC10-reactive epitheli a. The proliferation fraction of CC10-reactive cells was assessed with doub le staining for MIB-1 and CC10. The proliferation index of the epithelium d iffered significantly between normal, hyperplastic and metaplastic epitheli um. The number of CC10-reactive cells was inversely related to the epitheli al proliferation. Bronchial CC10-reactive cells showed no proliferative act ivity as assessed using immunohistochemical double staining for CC10 and MI B-1. In contrast to normal and hyperplastic epithelium, squamous metaplasia disclosed CK5/6 in all epithelial layers, a loss of CK7 and a gain of CK13 . We conclude that CC10-reactive cells have no progenitor role in the bronc hial mucosa. However. because the proliferative activity is inversely relat ed to the number of CC10-reactive cells, the CC10 protein may play a role i n the regulation of epithelial repair. Squamous metaplasia most likely orig inates from basal cells.