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Treatment of murine macrophages with interferon‐γ inhibits their ability to bind leishmania promastigotes

The binding of leishmania promastigotes to macrophages pretreated with interferon‐γ (IFN‐γ) was compared to binding to untreated (resident) cells. IFN‐ γ‐treated macrophages bound fewer leishmania promastigotes than did untreated cells. The decreased binding was apparent over a wide dose range of parasite inocula when he assays were performed in the absence of exogenous complement. This decrease was specific to leishmania, since treated and untreated macrophages bound comparable amounts of immunoglobulin G‐ and complement‐coated sheep red blood cells. Decreased parasite binding occurred early in the macrophage activation pathway. Pretreatment of macrophages with IFN‐7 for as little as 6 h, a time insufficient to induce other macrophage activation parameters, significantly reduced their ability to bind leishmania promastigotes. To determine the mechanism of this decreased phagocytosis by activated cells, macrophages were pretreated with specific inhibitors before the addition of leishmania. The binding of promastigotes to untreated (resident) macrophages was inhibited by approximately 50% by reagents that blocked either of two macrophage receptors, complement receptor type 3 (Mac‐1) or a leishmania species‐specific lectin‐like receptor. Binding to IFN‐γ ‐treated macrophage populations, in contrast, was substantially inhibited only by antibody to Mac‐1. Saccharides that were 50% inhibitory in the resident cell population, decreased binding by less than 10% in activated cells. The lack of saccharide inhibition by IFN‐γ ‐treated cells was also reflected in an inability of activated macrophages to bind to beads coated with purified leishmania lipophosphoglycan (LPG). These LPG‐coated beads bound well to resident macrophages but poorly to activated cells. Thus, leishmania bind to macrophages by two distinct mechanisms, one that utilizes Mac‐1 and a second mechanism that does not depend on complement and is saccharide inheritable. These two binding mechanisms are distinct and differentially regulated in resident and activated cells.