Modulation of pulmonary macrophage superoxide release and tumoricidal activity following activation by biological response modifiers

Abstract

Following immunologic activation, pulmonary macrophages may prevent or cause regression of lung metastases by mechanisms which remain largely unknown. The studies described here were designed to determine if enhanced oxygen metabolite release was related to postactivation tumoricidal activity. We have shown that in vitro activation of Fischer 344 rat pulmonary macrophages by either free or liposome-encapsulated muramyl dipeptide leads to both enhanced release of superoxide anions and marked tumoricidal activity against syngenic (Fischer 13762), allogeneic (Schmidt-Ruppin RR 1022) and xenogeneic (Fibrosarcoma MCA-F) 125I-deoxyuridine-labeled target cells. This immune modulator did not, however, metabolically activate pulmonary macrophages as effectively as liposome-encapsulated lipopolysaccharide. A 24-h in vitro incubation with either 150 U or 300 U of interferon-gamma (3 X 10(6) U/mg) or 30 U, 150 U or 300 U of interferon-alpha (6 X 10(5) U/mg) caused a significant elevation in superoxide release above controls, whereas short-term exposure (2 or 4 h) had little or no effect. Free or encapsulated 6-O-stearoyl muramyl dipeptide, on the other hand, did increase superoxide levels at all 3 time periods. When either interferon-gamma or free or encapsulated muramyl dipeptide derivative were administered to intact rats by either i.v. injection, intratracheal instillation or osmotic minipump infusion, pulmonary macrophage tumoricidal activity was observed 96 h after cell harvesting. Zymosan-stimulated superoxide release, however, was not consistently elevated above control or empty liposome treatment following this course of in vivo activation. The data collectively suggest that in vivo pulmonary macrophage activation to a tumoricidal state and metabolic activation resulting in enhanced superoxide may be separable events.