Characteristics of alveolar macrophages following the deposition of a low burden of iron oxide in the lung

Rats were exposed to aerosols of iron-59 oxide (mass median aerodynamic diameter, MMAD = 1.6 μm, sigma/sub g/ = 3.0) at a nominal concentration of 20 mg/m3 for 2 h to determine how a low lung burden (approx.30 μg) of innocuous particles affects the size of the alveolar macrophage (AM) pool. The functional status of the AM was also determined as assessed in vitro by their ability to exclude Trypan blue, adhere to plastic substrate, and bind phagocytize sheep erythrocytes opsonized with immunoglobulin G (SRBC-IgG). Iron oxide deposition did not bring about significant changes in cell types or numbers of AM lavaged, AM viabilities, or the plastic substrate adherence characteristics of the AM. As of 1 d post exposure, however, the ability of AM to phagocytize SRBC-IgG increased. Phagocytosis was maximally enhanced 3-7d post exposure and returned to control levels by 20 d after exposure. The increase in phagocytic activity correlated with an increase in AM avidities for SRBC-IgG. The kinetics of subsidence of the phagocytic response did not parallel the alveolar clearance rate of the deposited particles [t/sub 1/2/ (biol) 53 d]. These studies show the deposition of a low lung burden of a noncytotoxic dust can transiently enhance FCγ-receptor-mediated particle binding and phagocytosis by AM