CD40, a costimulatory molecule expressed on macrophages, induces expression of interleukin 12 (IL-12) in uninfected macrophages and IL-10 in macrophages infected with Leishmania major. IL-12 suppresses, whereas IL-10 enhances, L. major infection. The mechanisms that regulate this difference in CD40-induced cytokine production remain unclear, but it is known that L. major depletes cholesterol. Here we show that cholesterol influenced the assembly of distinct CD40 signalosomes. Depletion of membrane cholesterol inhibited the assembly of an IL-12-inducing CD40 signalosome containing the adaptors TRAF2, TRAF3 and TRAF5 and the kinase Lyn and promoted the assembly of an IL-10-inducing CD40 signalosome containing the adaptor TRAF6 and the kinase Syk. Thus, cholesterol depletion might represent an immune-evasion strategy used by L. major.

In vitro infection of murine peritoneal macrophages with the protozoan Leishmania donovani has been found to alter the signaling parameters of the host. The present study indicates that the enhancement of intracellular ceramide level in macrophages after infection is a major event relating to macrophage dysfunction. We have previously demonstrated that increased ceramide synthesis in host macrophages was involved in the dephosphorylation of extracellular signal-regulated kinase (ERK). In the present study, we further show that downregulation of ERK by ceramide was found to be associated with the inhibition of activated protein 1

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These results provide a mechanistic understanding of GA-mediated protection against leishmanial parasites within the host.

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