Failure of the phagocytic oxidative response to protect human monocyte-derived macrophages from infection by Leishmania donovani .

Abstract

In order to determine whether Leishmania donovani survives within macrophages by interfering with the phagocytic oxidative burst or later oxidative responses, we examined the interaction of the promastigote and amastigote stages of the parasite with human monocyte-derived macrophages. In a chemiluminescence assay, the maximal oxidative response to promastigotes in the presence of fresh or heat-inactivated (56°C, 30 min) serum occurred later but was comparable in magnitude to that with opsonized zymosan. In the absence of serum, the maximal response was 25 ± 5% that of the opsonized zymosan control. The uptake of parasites by macrophages was correspondingly reduced in the absence of serum. In an NBT assay, 88% of macrophages exposed to promastigotes and 98% of macrophages exposed to opsonized zymosan contained aggregates of reduced formazan, indicating that they had undergone an oxidative burst. The oxidative response did not protect macrophages from infection by promastigotes. Twelve hours after exposure to promastigotes at a parasite-to-macro-phage ratio of 20:1, 54 ± 8% of macrophages were infected with a mean of 225 ± 66 parasites per 100 macrophages. Amastigotes also initiated an oxidative burst, although the response was less than that observed with promastigotes. Leishmania-infected phagocytes did not produce an ongoing chemiluminescence response. Twenty-four hours after infection with promastigotes, the response to opsonized zymosan was less than that of noninfected controls, but at 72 hr the response of infected monolayers was greater than or equal to that of control monolayers. L. donovani does not survive in macrophages by interfering with the phagocytic oxidative burst, does not produce a continuing oxidative response, and only transiently alters the oxidative response of macrophages to subsequent particulate stimulation.