Immunosuppressive properties of surfactant and plasma on alveolar macrophages

Alveolar macrophages have been shown to be major producers of the potent proinflammatory cytokines interleukin-1 beta and tumor necrosis factor-alpha, and of the antiinflammatory cytokine interleukin-1 receptor antagonist. During the adult respiratory distress syndrome the normally surfactant-coated alveolus becomes flooded with plasma proteins, altering the milieu of alveolar cells such as alveolar macrophages. To understand alveolar macrophage function during the adult respiratory distress syndrome, the individual and combined effects of surfactant and plasma on alveolar macrophage cytokine production was examined. A synthetic surfactant (Exosurf) and a bovine-derived surfactant (Survanta) both inhibited production of interleukin-1 beta, pro-interleukin-1 beta, tumor necrosis factor-alpha, and interleukin-1 receptor antagonist in a dose-dependent manner. This inhibition was noted when both endotoxin and heat-killed Staphylococcus aureus were used as stimuli. Autologous plasma also inhibited interleukin-1 beta and tumor necrosis factor-alpha release in a dose-dependent manner, but, unlike surfactant, plasma did not inhibit interleukin-1 receptor antagonist release. Similarly, the combination of plasma and surfactant inhibited interleukin-1 beta and tumor necrosis factor-alpha release but not interleukin-1 receptor antagonist release. In support of these data, interleukin-1 receptor antagonist was detectable in five of six bronchoalveolar lavage fluid samples from patients with adult respiratory distress syndrome at a mean concentration of 465 pg/ml; on the other hand, interleukin-1 beta was not detectable in any of these samples. These results indicate that the relative production of interleukin-1 beta, tumor necrosis factor-alpha, and interleukin-1 receptor antagonist can be altered depending on the local concentration of both surfactant and plasma.