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Intercellular adhesion molecule I (ICAM-1) expression by rat fetal alveolar epithelial cells in vitro. • 1784

ICAM-1 is an important molecule in immune and inflammatory interactions that is expressed abundantly on normal type I cells in the adult lung. ICAM-1 expression by adult alveolar epithelial cells in vitro is a function of cell differentiation that is greatly influenced by cell shape. Previously we found that there is low level expression of ICAM-1 on primitive cuboidal epithelial cells in the developing rat lung, and that this expression increases dramatically in neonatal life during alveolarization, suggesting that ICAM-1 expression on maturing alveolar epithelial cells (AEC) is a developmentally regulated process. To further investigate the mechanisms involved in this process, AEC were isolated from 21 day fetal rats (term=22 day) by enzymatic cell dispersion and differential adherence. Epithelial cell purity was consistently >90%. Adult type II AEC were used for comparison. After 2 days, adult AEC that had spread in culture expressed abundant ICAM-1 that remained stable through 7 days in culture. Although both adult and fetal cells had assumed a similar flat, squamous shape at 2 days, fetal AEC expressed dramatically less ICAM-1 by Western blot analysis and immunofluorescence microscopy. However, by 4 days in culture, ICAM-1 expression had increased markedly in the fetal AEC, and was similar to that in adult cells. Thus, fetal AEC recapitulate the differentiation-related induction of ICAM-1 expression by isolated cells in vitro found with cells from adult animals. However, ICAM-1 expression in fetal AEC is determined by additional AEC features beyond cell shape and cell-cell interactions.