Translocation of Particles to the Tracheobronchial Lymph Nodes after Lung Deposition: Kinetics and Particle-Cell Relationships

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Abstract
One of the pathways for particle clearance from the lung involves the trans-location of particles from the alveoli to the tracheobronchial lymph nodes. Details of the mechanisms involved in this translocation process have not been well delineated. In the present study, we: 1) assessed the kinetics of particle transfer to the tracheobronchial lymph nodes in the rat over a 30-day period following the intrapulmonary deposition of 4 × 108, 1.9 μm, fluorescent polystyrene microspheres; 2) utilized multiparameter flow cytometric technology to quantitatively differentiate between “free” and cell-associated particles that accumulated in the lymph nodes; and 3) assessed the role of alveolar macrophages (AM) in carrying particles to the lymph nodes by comparing the distributions of particles in lavaged AM with distributions of particles found in nodal mononuclear phagocytes (NMP). The accumulation of particles, most of which were extracellular, in the nodes was biphasic with the most rapid phase occurring within the first 24 hr. Over the Day 1–30 period, the numbers of particles in the lymph nodes increased linearly to ≅1.2 × 106 microspheres, or ≅0.3% of the originally instilled lung burden. The percentages of the nodal particles that were associated with NMP over the course of the study were inversely proportional to nodal paniculate burdens, even though the percentage of cells with engulfed particles increased; the percentage of NMP asymptotically approached a maximum value over the range of nodal burdens of 6–12 × 105 particles. The distributions of the microspheres in the NMP were essentially identical on Days 1, 14, and 30. Major differences in the distributions of particles in lavaged AM and NMP were not consistent with the notion that the latter represented translocated AM.