Retinoic acid induced ultrastructural alterations in <i>Leishmania donovani</i> promastigotes

Visceral leishmaniasis (VL) or Kala-azar is the deadliest form of leishmaniasis due to its high mortality rate, caused by <i>Leishmania donovani</i> (<i>Ld</i>), a protozoan parasite of the trypanosomatidae family. Malnutrition is considered one of the major risk factors for VL, favoring parasite survival and infection establishment. Retinoic acid (RA) is an essential nutritional component to maintain our normal physiological functions and immunity. Our earlier findings showed direct leishmanicidal activity of RA by inhibiting the ergosterol pathway in the parasite. Thus, the study aims to investigate the effects of RA treatment on the morphology and ultrastructure of the parasite. Promastigotes of <i>Ld</i> parasites were treated with three different concentrations (1, 5, and 10 µg/ml) of RA to analyze RA-induced morphological changes and ultrastructural alterations in the parasite organelles by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), respectively. SEM analysis demonstrated significant morphological changes in the shape and size of the parasite, possibly inhibiting <i>Ld</i> microvesicles release following RA treatment, which are crucial for the pathogenesis of the parasite. TEM analysis revealed several intracellular structural alterations, particularly in the appearance of mitochondria, nucleus, and kinetoplast, along with the presence of autophagic vacuoles after RA treatment. These findings highlight the direct anti-parasite properties of RA, potentially offering new avenues for advancement in the VL treatment, along with strengthening the host immune response.