Abstract
When stimulated in vitro with macrophage-activating factor or lipopolysaccharide, mouse peritoneal macrophages acquire the capacity to develop a strong respiratory burst when they are triggered by membrane-active agents. The presence of intracellular parasites of the genus Leishmania (L. enriettii, L. major) significantly inhibited such activity, as measured by chemiluminescence, reduction of cytochrome c and Nitro Blue Tetrazolium, and hexose monophosphate shunt levels. On the contrary, inert intracellular particles such as latex beads strongly increased the macrophage respiratory burst, suggesting that the Leishmania-linked inhibition resulted from a specific parasite effect. Impairment of macrophage oxidative metabolism by intracellular Leishmania spp. was a function of the number of infecting microorganisms and was more pronounced in macrophages infected with living than with dead parasites. Moreover, the metabolic inhibition was less apparent in L. enriettii-infected macrophages that were exposed to both macrophage-activating factor and lipopolysaccharide, i.e., conditions leading to complete parasite destruction. The mechanisms of respiratory burst inhibition by intracellular Leishmania spp. are unclear, but these observations suggest that such effects may contribute significantly to intracellular survival of the microorganisms.