A novel role in the removal of blood-borne foreign bodies for pulmonary capillaries in the guinea pig.

In certain mammals, the lung plays an important role in removing blood-borne foreign bodies by means of the numerous macrophages disposed in pulmonary capillaries. The present ultrastructural study demonstrates that in the guinea pig, the lung also plays a significant role in this respect, but that in this species, foreign body elimination takes place by another, hitherto undescribed process. In the guinea pig, the pulmonary capillaries are characterized by numerous neutrophils that adhere to endothelial cells even under normal conditions. At 30 min after intravenous injection of glutaraldehyde-fixed erythrocytes, large numbers of these foreign bodies were found to be ingested by these neutrophils. At 6 h after injection, the erythrocyte-carrying neutrophils had disappeared from the vascular lumen, but endothelial cells had begun vigorously to engulf the fixed erythrocytes by extending membranous processes which completely enwrapped them. Since endothelial cells lack lysosomes, there was no morphologic evidence of erythrocyte digestion within their cytoplasm. It is evident that the erythrocytes then passed through the endothelium, since by 48 h after injection, most of them were incorporated and digested by macrophages proliferating in the alveolar interstitium. The increase in macrophages was confirmed by acid phosphatase histochemistry. These observations indicate that in the guinea pig, the lung plays an important role in clearing blood-borne foreign bodies by the sequential involvement of intracapillary neutrophils, capillary endothelial cells and interstitial macrophages.