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CD200 belongs to cell adhesion molecules of the immunoglobulin superfamily. It lacks intracellular signaling motifs and exerts immunosuppressive effect in various tissues. We have reported previously that CD200 is predominantly associated with the capillary network in the alveolar septum of adult rats. The alveolar endothelial cells express CD200, which is confined to their luminal cell membrane facing the blood-air barrier. Our present results show that lung CD200 protein increases gradually with advancing age, being maximally expressed in the early postnatal (P) period. CD200 protein expression, however, declines at P5 but increases again after P7, reaching the adult level at P21. In developing lungs in fetal and neonatal stages, double-immunofluorescence staining has confirmed intense CD200 immunoreactivity delineating the vascular profiles in the double layers of the alveolar capillaries; this staining becomes diffuse and patchy with time. Unlike in adult lungs, immunoelectron microscopy has revealed that CD200 expression in fetal and early postnatal lungs is localized over the entire luminal cell membrane and in the cytoplasm of the endothelia. CD200 expression is progressively redistributed to a specific luminal domain of alveolar endothelia during pulmonary microvascular maturation. In neonatal rats treated with dexamethasone, the amount of lung CD200 significantly increases and is also elevated with time. Upregulation of endothelial CD200 has further been confirmed in isolated pulmonary microvascular endothelial cells treated with dexamethasone. Thus, lung CD200 is developmentally regulated, possibly under hormonal influence. [ABSTRACT FROM AUTHOR]