Lung and pleural "free-cell responses" to the intrapulmonary deposition of particles in the rat

Abstract

Deposition of a high lung burden of particulate carbon or latex in the mouse elicits a biphasic macrophagic response, with the early increase in the size of the population of the alveolar macrophages (AM) being mainly due to an influx of mononuclear phagocytes from the blood compartment and the later phase being due to the egression of interstitial macrophages (IM). In the present study, we investigated the lung's free-cell response in the rat to the intrapulmonary instillation of microspheres and determined the contributions of newly arrived blood monocytes to the macrophagic response over a 30-d period. As of 24 h after deposition, the lavageable population size increased approximately fivefold. Most of the increase in the free cell population was due to a recruitment of polymorphonucleated leukocytes (PMN), although the size of the AM population nearly doubled. Only about 13% of the AM on d 1 were positive for myeloperoxidase activity, suggesting that recruitment of mononuclear phagocytes from the blood compartment plays a limited role in the early expansion of the AM pool in the rat. As of 14 and 30 d after particle deposition, lavaged AM numbers continued to be elevated; the enlarged sizes of the AM populations at these times could be explained, in part, by a continuing influx of blood monocytes. In a parallel component of the study, we also investigated how the pleural cell population might be affected during the lung's free-cell response to the particulate burden. No acute influx of PMN into the pleural space (PS) accompanied the early free-cell response; the chemotactic factors initially involved in recruitment of the PMN into the lung apparently did not reach the PS in significant or biologically active concentrations, or they were inoperable in the pleural compartment. On the other hand, the numbers of macrophages in the PS were significantly elevated on d 1 and 14 after particle deposition in the lung. The macrophagic response in the PS subsided by d 30, but at this latter sacrifice time, pleural mast-cell and lymphocyte numbers were significantly increased and decreased, respectively. These results indicate that the deposition of particles into the lung can lead to numerical alterations in the cell types composing pleural cell populations.