OPSONIC RECEPTOR FUNCTION IS REDUCED ON THE SURFACE OF NEWBORN ALVEOLAR MACROPHAGES

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Abstract
Neonates have an increased susceptibility to infections. Because optimal phagocytosis of offending organisms by alveolar macrophages (AM) requires recognition and attachment of opsonized organisms to the AM cell membrane, the expression of opsonic receptors on the surface of newborn and adult rat AM was investigated using immunologic techniques, cell culture, and flow cytometry. We investigated the expression of Fc, C3b, fibronectin, and lectin receptors on newborn (1 to 5 wk) AM and compared them with those of adult AM. The expression of Fc receptors (FcR) was significantly less on the surface of newborn AM, particularly during the first week of life, as determined by their binding of aggregated IgG, E(IgG), and opsonized 125I-Listeria monocytogenes. A similar depressed receptor function was observed for C3b, fibronectin, and some lectins. The possible effect of cell size on receptor expression was examined by morphometry and flow cytometry. The results indicated that, while mean AM size was approximately 12% smaller during the first week of life, it attained adult levels by the second week. Thus, a difference in size did not account for either the magnitude of decreased receptor expression or the diminished intensity of staining for surface-aggregated IgG that was detectable for up to 5 wk. Binding of a variety of lectins to the AM surface was decreased during the first week after birth, but approximated adult levels by 4 wk. By contrast, soybean and peanut agglutinin, lectins that bind to D-GalNAC moieties, showed a transient increase in binding to AM during the second and third weeks of life. The observed diminished receptor function could not be accounted for on the basis of an inhibitor in the bronchoalveolar lavage. Instead, the results indicated that there is a diminished density of opsonic receptors on the surface of newborn rat AM, particularly during the postnatal week.

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