Pleural macrophages differentially alter mesothelial cell growth and collagen production

Intrapleural tetracycline (TCN) results in pleural macrophage influx and pleural fibrosis; intrapleural carrageenan (CAR) induces macrophage influx without fibrosis. Because macrophage products can modulate mesothelial cell activity, we investigated the role of TCN- and CAR-induced pleural macrophages on mesothelial cell growth can collagen production. Rabbit pleural macrophages, isolated by plastic adherence 72 h after 20 mg/kg TCN or 10 mg CAR instilled intrapleurally, were cultured for 20 h. Macrophage-conditioned media (MCM) from TCN-or CAR-induce pleural macrophages (TCN MCM, CAR MCM, respectively), were added to non-confluent or confluent rat visceral pleural mesothelial cells and compared to the effects of TCN and CAR. Nonconfluent mesothelial cells were harvested 72 h later for hemacytometry cell counts. A 20-h pulse of [3H] proline (1 mu Ci, 30 Ci/mM) preceded 72-h-cell harvesting of confluent cells. Collagen content was determined in the cell fraction and cell media separately after bacterial collagenase exposure. Mesothelial cells exposed to TCN MCM were found to have decreased numbers when compared to all groups (P < 0.05) except CAR. Cell media collagen content was increased in all macrophage-conditioned-media and chemical-exposed groups compared with control, with TCN MCM having a larger increase than TCN alone (P < 0.05). We conclude that stimulated pleural macrophages release a factor(s) that alters mesothelial cell growth and collagen production and that TCN- and CAR-stimulated pleural macrophages are functionally different. These in vitro mesothelial cell alterations may be important in the genesis of TCN pleurodesis.