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The molecular mechanism of antimony-resistant Leishmania donovani (Sb R LD)-driven up-regulation of IL-10 and multidrug-resistant protein 1 (MDR1) in infected macrophages (Mϕs) has been investigated. This study showed that both promastigote and amastigote forms of Sb R LD, but not the antimony-sensitive form of LD, express a unique glycan with N-acetylgalactosamine as a terminal sugar. Removal of it either by enzyme treatment or by knocking down the relevant enzyme, galactosyltransferase in Sb R LD (KD Sb R LD), compromises the ability to induce the above effects. Infection of Mϕs with KD Sb R LD enhanced the sensitivity toward antimonials compared with infection with Sb R LD, and infection of BALB/c mice with KD Sb R LD caused significantly less organ parasite burden compared with infection induced by Sb R LD. The innate immune receptor, Toll-like receptor 2/6 heterodimer, is exploited by Sb R LD to activate ERK and nuclear translocation of NF-κB involving p50/c-Rel leading to IL-10 induction, whereas MDR1 up-regulation is mediated by PI3K/Akt and the JNK pathway. Interestingly both recombinant IL-10 and Sb R LD up-regulate MDR1 in Mϕ with different time kinetics, where phosphorylation of PI3K was noted at 12 h and 48 h, respectively, but Mϕs derived from IL-10 −/− mice are unable to show MDR1 up-regulation on infection with Sb R LD. Thus, it is very likely that an IL-10 surge is a prerequisite for MDR1 up-regulation. The transcription factor important for IL-10-driven MDR1 up-regulation is c-Fos/c-Jun and not NF-κB, as evident from studies with pharmacological inhibitors and promoter mapping with deletion constructs.visceral leishmaniasis | antimony resistance | glycoconjugate | antimony efflux K ala-azar or visceral leishmaniasis, which is caused by the protozoan parasite Leishmania donovani (LD), is (re)emerging and spreading worldwide, essentially because of humanmade and environmental changes, immunosuppression, and drug resistance (1, 2). To combat the disease, organic pentavalent antimonials were introduced in the Indian subcontinent almost 9 decades ago (3) with dramatic clinical success. However, the toxicity, together with the high treatment failure rate (up to 65%) and the emergence of resistance in Bihar State, India, made the drug obsolete in the Indian subcontinent (4, 5). Nevertheless, it is still used as a first-line treatment in Africa and Latin America (6). Interestingly, 78% of the recent clinical isolates from the hyperendemic zone of Bihar State still showed in vitro resistance to antimonials (7), which might be explained by an increased fitness of the corresponding parasites (8).Recently, we have demonstrated three unique characteristics of antimony-resistant LD (Sb R LD). First, it shows a higher concentration of terminal glycoconjugates (N-acetylgalactosamine residues) on its surface than antimony-sensitive LD (Sb S LD) (7).Second, on infection of macrophages (Mϕs), Sb R LD induces a surge of IL-10 (7, 9). Third, we also observed that Sb R LD modulates host cells to overexpress the ATP-b...