Net absorption of serum proteins across primary human alveolar epithelial cell (hAEpC) monolayers

Specific information on transport of serum proteins in distal human lung is lacking. Here, we determined transport rates of serum proteins across an in vitro model of human alveolar epithelial barrier. Primary hAEpC were isolated from non‐tumor lung tissue obtained from patients undergoing lung resection and cultured for 7–9 days on Transwell filters until confluent polarised monolayers were formed (TEER > 1000 Ω ×cm 2 ). Bi‐directional transport studies were conducted using human serum albumin (HSA), transferrin (TF) and immunoglobulin G (IgG), labeled with iodine‐125 or FITC. Results showed asymmetric transport of all human serum proteins studied. Apparent permeability coefficients (absorptive vs. secretive cm/sec×10 −7 ) were: HSA (2.45±1.02 vs. 0.21±0.31), TF (0.88±0.15 vs. 0.30±0.03) and IgG (0.36±0.22 vs. 0.15±0.16). Analysis of upstream and downstream fluids collected after 4 h flux studies revealed no significant instability of the label. These data indicate that net absorption of HSA, TF and IgG occurs across the human alveolar barrier. We speculate that transcytotic processes for these serum proteins may play an important role in lung fluid balance and defence. Funded by: Novo Nordisk, NIH (KJK, HL 38658 and HL 64365)