High levels of transforming growth factor-beta are present in the epithelial lining fluid of the normal human lower respiratory tract.

Transforming growth factor-beta (TGF-beta), a mediator capable of modulating a broad range of effects on the behavior of many normal cells, was found in high concentrations in the epithelial lining fluid (ELF) of the normal human lower respiratory tract. Although plasma contained small amounts of TGF-beta, the concentrations of TGF-beta in normal ELF were in the 200 to 300 pM range, more than 15-fold higher. This ELF TGF-beta had similar physical characteristics to purified human platelet TGF-beta, competed with platelet TGF-beta for its receptor on A549 carcinoma cells, and stimulated the anchorage-independent growth of NRK cells in soft agar in the presence of epidermal growth factor. Furthermore, ELF TGF-beta suppressed diploid lung fibroblast proliferation in a dose-dependent fashion similar to platelet TGF-beta. In the context of these observations and with the known biologic properties of this molecule, TGF-beta in ELF has the potential to play a role in a variety of cellular processes in the lower respiratory tract.