Translocation of Particles by Macrophages from Lung to Lymph Nodes in Rats

Abstract

The mechanism by which particles deposited in the alveolar region of the lung are translocated to tracheobronchial lymph nodes (TBLN) remains unclear. In the present study, we investigated the ability of rat pulmonary alveolar macrophages (PAM) to contribute to this process. Rats were instilled intratracheally with 5 x 10⁹ fluorescent microspheres about 1.3 μm diameter and groups were killed at 1, 2, 4 and 8 days after instillation. Two-micron plastic sections of the TBLN revealed that approximately 90% of the microspheres in the TBLN were within macrophages at each time point. The distribution of the numbers of microspheres per TBLN macrophage did not change between 1 and 8 days even though the total number of particles in the TBLN increased linearly during this time. Had the microspheres accumulated in the TBLN as free particles, it seems likely that there would have been a shift toward larger numbers of microspheres per macrophage with time. We also instilled pulmonary alveolar macrophages (PAM), approximately half of which were labeled with red fluorescent microspheres and half with green fluorescent microspheres, into rat lungs. After three days the TBLN of the rats contained numerous macrophages containing only one color of microsphere. The use of the two colors of microspheres precluded the possibility that the microspheres were translocated as free particles and were phagocytized in the TBLN by resident macrophages. These results suggest that in the rats, a considerable number of microspheres was translocated from lung to TBLN within PAM.