Antimony Resistant Leishmania donovani but Not Sensitive Ones Drives Greater Frequency of Potent T-Regulatory Cells upon Interaction with Human PBMCs: Role of IL-10 and TGF-β in Early Immune Response

In India the sand fly, Phlebotomus argentipes, transmitted parasitic disease termed kala-azar is caused by Leishmania donovani (LD) in humans. These immune-evading parasites have increasingly developed resistance to the drug sodium antimony gluconate in endemic regions.Lack of early diagnosis methods for the disease limits the information available regarding the early interactions of this parasite with either human tissues or cell lineages. We reasoned that peripheral blood mononuclear cells (PBMCs) from healthy human beings could help compare some of their immune signatures once they were exposed for up to 8 days, to either pentavalent antimony sensitive (SbS-LD) or resistant (SbR-LD) Leishmania donovani isolates.At day 2, PBMC cultures exposed to SbS-LD and SbR-LD stationary phase promastigotes had four and seven fold higher frequency of IL-10 secreting monocyte-macrophage respectively, compared to cultures unexposed to parasites. Contrasting with the CD4+CD25−CD127− type-1 T-regulatory (Tr1) cell population that displayed similar features whatever the culture conditions, there was a pronounced increase in the IL-10 producing CD4+CD25+CD127low/− inducible T-regulatory cells (iTregs) in the PBMC cultures sampled at day 8 post addition of SbR-LD.Sorted iTregs from different cultures on day 8 were added to anti-CD3/CD28 induced naïve PBMCs to assess their suppressive ability. We observed that iTregs from SbR-LD exposed PBMCs had more pronounced suppressive ability compared to SbS-LD counterpart on a per cell basis and is dependent on both IL-10 and TGF-β, whereas IL-10 being the major factor contributing to the suppressive ability of iTregs sorted from PBMC cultures exposed to SbS–LD. Of note, iTreg population frequency value remained at the basal level after addition of genetically modified SbR-LD lacking unique terminal sugar in surface glycan.Even with limitations of this artificial in vitro model of L. donovani-human PBMC interactions, the present findings suggest that SbR-LD have higher immunomodulatory capacity which may favour aggressive pathology.