The inability of sodium antimony gluconate (SAG)-unresponsive kala-azar patients to clear Leishmania donovani (LD) infection despite SAG therapy is partly due to an ill-defined immune-dysfunction. Since dendritic cells (DCs) typically initiate anti-leishmanial immunity, a role for DCs in aberrant LD clearance was investigated. Accordingly, regulation of SAG-induced activation of murine DCs following infection with LD isolates exhibiting two distinct phenotypes such as antimony-resistant (SbRLD) and antimony-sensitive (SbSLD) was compared in vitro. Unlike SbSLD, infection of DCs with SbRLD induced more IL-10 production and inhibited SAG-induced secretion of proinflammatory cytokines, up-regulation of co-stimulatory molecules and leishmanicidal effects. SbRLD inhibited these effects of SAG by blocking activation of PI3K/AKT and NF- B pathways. In contrast, SbSLD failed to block activation of SAG (20 μg/ml)-induced PI3K/AKT pathway; which continued to stimulate NF- B signaling, induce leishmanicidal effects and promote DC activation. Notably, prolonged incubation of DCs with SbSLD also inhibited SAG (20 μg/ml)-induced activation of PI3K/AKT and NF- B pathways and leishmanicidal effects, which was restored by increasing the dose of SAG to 40 mg/ml. In contrast, SbRLD inhibited these SAG-induced events regardless of duration of DC exposure to SbRLD or dose of SAG. Interestingly, the inhibitory effects of isogenic SbSLD expressing ATP-binding cassette (ABC) transporter MRPA on SAG-induced leishmanicidal effects mimicked that of SbRLD to some extent, although antimony resistance in clinical LD isolates is known to be multifactorial. Furthermore, NF- B was found to transcriptionally regulate expression of murine cglutamylcysteine synthetase heavy-chain (mγGCShc) gene, presumably an important regulator of antimony resistance. Importantly, SbRLD but not SbSLD blocked SAG-induced mcGCS expression in DCs by preventing NF- B binding to the mγGCShc promoter. Our findings demonstrate that SbRLD but not SbSLD prevents SAG-induced DC activation by suppressing a PI3K-dependent NF- B pathway and provide the evidence for differential host-pathogen interaction mediated by SbRLD and SbSLD. © 2010 Haldar et al.