<i>Leishmania donovani</i> Exploits Tollip, a Multitasking Protein, To Impair TLR/IL-1R Signaling for Its Survival in the Host

Abstract

IL-1R/TLR signaling plays a significant role in sensing harmful foreign pathogens and mounting effective innate and adaptive immune responses. However, the precise mechanism by which <i>Leishmania donovani,</i> an obligate intramacrophagic pathogen, breaches IL-1R/TLR signaling and host-protective immunity remains obscure. In this study, we report the novel biphasic role of Toll-interacting protein (Tollip), a negative regulator of the IL-1R/TLR pathway, in the disease progression of experimental visceral leishmaniasis. We observed that during early hours of infection, <i>L. donovani</i> induced phosphorylation of IRAK-1, resulting in the release of Tollip from the IL-1R-associated kinase (IRAK)-1 complex in J774 macrophages, which then acted as an endocytic adaptor on cell surface IL-1R1 and promoted its lysosomal degradation. In the later stage, Tollip shuttled back to IRAK-1, thereby inhibiting IRAK-1 phosphorylation in association with IRAK-M to neutralize downstream TLR signaling in infected macrophages. Moreover, during late infection, <i>L. donovani</i> enhanced nuclear translocation and recruitment of transcription factors early growth response protein 2, NF erythroid 2-related factor 2, and Ahr on Tollip promoter for its induction. Small interfering RNA-mediated silencing of Tollip in infected macrophages significantly enhanced NF-κB activation and induced host-defensive IL-12 and TNF-α synthesis, thereby reducing amastigote multiplication. Likewise, abrogation of Tollip in <i>L. donovani-</i>infected BALB/c mice resulted in STAT-1-, IRF-1-, and NF-κB-mediated upregulation of host-protective cytokines and reduced organ parasite burden, thereby implicating its role in disease aggravation. Taken together, we conclude that <i>L. donovani</i> exploited the multitasking function of Tollip for its own establishment through downregulating IL-1R1/TLR signaling in macrophages.