Macrophage and monocyte IL-1 beta regulation differs at multiple sites. Messenger RNA expression, translation, and post-translational processing

Maturation of blood monocytes into macrophages is accompanied by a number of functional changes including decreased IL-1 beta release in response to LPS. This limitation has previously been ascribed to transcriptional regulation. However, in seeming conflict with the observed depression in IL-1 beta mRNA levels, recent work demonstrates increased intracellular IL-1 beta in macrophages. Therefore, the present study sought to explain these differences by comparing IL-1 beta production from autologous alveolar macrophage and blood monocyte pairs at multiple regulatory sites, including endotoxin responsiveness, mRNA expression, protein translation, and post-translational processing. Macrophages did not differ from monocytes in endotoxin sensitivity, but when analyzed by both ELISA and Western blot, were confirmed to have limitations in IL-1 beta release. Gene expression studies demonstrated that at 4 h, macrophage IL-1 beta steady state mRNA levels were 3-fold lower than the monocyte's. However, total IL-1 beta protein production, as measured by [35S]methionine labeling with immunoprecipitation, demonstrated three- to sixfold higher amounts in macrophages at comparable time points. The enhanced protein production in the face of relatively low mRNA levels suggests that macrophages translate IL-1 beta mRNA more efficiently. Furthermore, characterization of IL-1 beta release into supernatants revealed that whereas monocyte release occurred early, represented 5 to 20% of the intracellular amounts, and contained largely processed IL-1 beta, macrophage release was delayed, represented 1 to 5% of the intracellular amounts, and contained primarily unprocessed IL-1 beta. Taken together, these data demonstrate that the limitations in alveolar macrophage IL-1 beta release occur due to slower export and conversion of 35- to 17-kDa protein and are not due to differences in sensitivity to endotoxin or to transcriptional control mechanisms.