SILICA INDUCES APOPTOSIS IN MACROPHAGES AND THE RELEASE OF INTERLEUKIN-1-ALPHA AND INTERLEUKIN-1-BETA

Resident adherent peritoneal cells selectively released high amounts o f interleukin-1 (IL-1) activity when treated with silica. The use of a nti-IL-1 antisera showed that both IL-1alpha and IL-1beta were present in supernatants of silica-treated macrophages. In contrast, intracell ular IL-1 activity was totally neutralized by anti-IL-1alpha antibodie s and was easily converted into the mature IL-1alpha form by autolysis in cytoplasmic extracts. Anion exchange chromatography clearly separa ted the two IL-1 species present in supernatants of silica-stimulated macrophages. Natural IL-1beta was further characterized by chromatofoc alization; it had an apparent isoelectric point, pI, in the range 8.3- 8.6. In agreement with previous findings showing that IL-1beta was rel eased only by apoptotic cells, we have found that silica-treated macro phages underwent apoptosis. This was demonstrated by the characteristi c laddering electrophoretic pattern of DNA extracted from silica-treat ed cells and by the morphology of macrophage nuclei stained with the D NA-specific dye DAPI. In addition, quantification of apoptotic cells w as performed by a flow cytometric analysis based on the reduction of c ellular DNA content exhibited by apoptotic cells. Treatment of macroph ages with silica, therefore, results in an active process that promote s the processing and liberation of IL-1beta.