PURPOSE

Survival of the Leishmania parasite within monocytes hinges on its ability to effectively nullify their microbicidal effector mechanisms. Accordingly, this study aimed to delineate this biological niche in patients with visceral leishmaniasis (VL).

METHODS

In monocytes, the redox status, antigen presenting capacity, expression of Toll-like receptors (TLRs), co-stimulatory molecules (CD80/86) and generation of intracellular cytokines (IL-8, IL-1β, IL-10 and LAP-TGF-β1) was measured by flow cytometry, levels of circulating cytokines (IL-1β, IL-6, TNF-α, IL-8, IL-4, IL-13, IL-10 and GM-CSF) by ELISA and arginase activity by spectrophotometry.

RESULTS

Within monocytes, generation of an oxidative burst was markedly attenuated as evident by decreased generation of nitric oxide and reactive oxygen species, concomitant with raised levels of thiols. This was accompanied by lowered frequency of TLR4(+) monocytes, but the arginase activity remained unaltered. Pathogen persistence was enhanced by the predominance of anti-inflammatory cytokines within monocytes, notably IL-10. Alongside, development of adaptive immunity was severely attenuated as manifested by a pronounced impairment of antigen presentation and co-stimulation evident by down regulation of CD54, HLA-DR and CD86. Treatment corrected the redox imbalance and reversed the impaired antigen presentation.

CONCLUSIONS

In VL, monocyte functions were severely impaired facilitating parasite persistence; anti-leishmanial chemotherapy mediated parasite elimination through modulation of the macrophage microenvironment by restoring its redox status and antigen presenting capacity.