Leishmaniasis, a disease caused by the Leishmania spp. parasite, affects 1 million people annually worldwide. Specific Leishmania species, such as Leishmania donovani, cause chronic spleen, liver and bone marrow infection which if left untreated leads to a fatal visceral infection resulting in 25,000-65,000 deaths globally each year. Current visceral leishmaniasis treatments are expensive, have severe side effects, and with rising drug resistance and no vaccine, there is an unmet need for novel therapeutics.
Intracellular pathogens, such as Leishmania spp. manipulate host cell survival and death signalling pathways to survive, replicate and disseminate. We hypothesise that identifying the exact cell death modalities hijacked by the pathogen informs specific therapeutic targeting to reduce parasite burden and ultimately, treat visceral leishmaniasis.
To this end, we used L. donovani in vivo and in vitro infection models including gene-targeted mice and therapeutic compounds targeting host cell apoptotic and pyroptotic machinery. We determined the parasitic burden, cell death and protein expression through microscopy, live-cell imaging, flow cytometry, immunohistochemistry and immunoblotting upon infection with L. donovani.
Our results suggest there is no role for pyroptosis as mice deficient in either Gasdermin‑D, the executioner protein for pyroptosis, showed no difference in parasite burden compared to C57Bl/6 controls. Additionally, targeting intrinsic apoptosis using BCL-2, MCL‑1, BCL-XL inhibitors also did not affect parasite burdens in vitro.
However, inducing extrinsic apoptosis of infected primary macrophages with IAP inhibitors resulted in both host cell and concurrent parasite death. L. donovani infected mice treated with IAP inhibitors displayed reduced leishmaniasis symptoms, and the splenic parasite reservoir was decreased due to apoptosis of macrophages - the parasite reservoir cells. Importantly, combining IAP inhibitors with the standard therapy for visceral leishmaniasis, Amphotericin-B, enabled the dosage and thus toxicity of both therapies to be reduced while maintaining significant reduction in splenic parasite burden.
Taken together, our data indicates that targeting host extrinsic apoptotic pathways using clinical stage IAP inhibitors may be a valid therapeutic option for visceral leishmaniasis.