Date Published: February 5, 2019
Publisher: Public Library of Science
Author(s): Ghufran Ahmed, Ajit K. Thakur, Sanjay K. Chaturvedi, Pushkar Shivam, Fauzia Jamal, Manish K. Singh, Sanjiva Bimal, Subhankar K. Singh, Sunil K. Choudhary, Pradeep Das, Shyam Narayan, Farhat Afrin.
The arsenic contamination of ground water in visceral leishmaniasis (VL) endemic areas in Bihar, India leads to human exposure through drinking water. Possibly, the consumed arsenic (As) accumulates in the tissues of VL patients, who subsequently internalize intracellular amastigotes to confer resistance against chemotherapy to the parasite, leading to modulation in the host’s immune response. This hypothesis appears to be consistent with the in vitro findings that in arsenic-exposed parasites, the mitochondrial membrane potential became depolarized, whereas the reduced thiol and lactate production was overexpressed with enhanced glucose consumption; therefore, the reduced thiol possibly supports an immunosuppressive state in the host cells. This observation was well supported by the down-regulated expression of pro-inflammatory cytokines (IL-2, IL-12, IFN-γ, and TNF-α) with a suppressed anti-leishmanial function of macrophage (NO, ROS). In contrast, the pathophysiological mechanism of VL has received ample support by the promotion of Th2 cytokines (IL-4 and IL-10) in the presence of arsenic-exposed Leishmania parasites (LdAS). Dysfunction of mitochondria and the overexpression of lactate production raise the possibility of the Warburg effect being operative through the up-regulation of glucose consumption by parasites to enhance the energy production, possibly augmenting virulence. Therefore, we surmise from our data that arsenic exposure to Leishmania donovani modulates the immune response and infection pattern by impairing parasite function, which may affect the anti-leishmanial effect in VL.
The Leishmania species are obligate intracellular protozoa of the kinetoplastids family that cause various clinical manifestations of leishmaniasis. The most severe form is visceral leishmaniasis (VL) caused by Leishmania donovani, which is fatal if left untreated . Although leishmaniasis is widespread in 98 countries and more prevalent in tropical and sub-tropical regions, ≥90% of VL cases are reported from six countries, Bangladesh, Brazil, Ethiopia, India, Sudan and South Sudan, with a disease burden of 0.2–0.4 million and an annual mortality rate of 0.02–0.04 million worldwide [2,3,4]. The emergence of drug resistance to most of the available anti-leishmanial drugs has created a new level of obstacles in the treatment of VL . The widespread distribution of arsenic in the ground water of VL endemic areas is one of the major contributing factors to antimony resistance . Arsenic is a highly toxic metalloid, released into the water of Indian subcontinent, and its co-existence with antimony resistance in the VL endemic area of Bihar, India has led to multiple challenges for public health [7,8]. The consumption of arsenic contaminated water by the inhabitants of this region could have resulted in the induction of changes in parasites through arsenic exposure in their hosts. Arsenic is toxic to most cells, including microorganisms that have evolved multiple mechanisms to detoxify the effect of arsenic and have survived well in such harsh environmental conditions [9,10]. In connection to arsenic entry in parasites, arsenic exposure induces resistance in Leishmania parasites  as arsenicals and antimonials are transported by the same aquaglyceroporin (AQP1) channels [11,12] of microbes. It has previously been reported that the level of thiol has been increased several fold in arsenic-exposed and antimony-resistant Leishmania parasites that bind to and are sequestered in intracellular vesicles [13,14]. The mitochondria have a pivotal role in adenosine triphosphate (ATP) generation, and arsenic exerts its toxic effect directly on mitochondria. The metalloid lowers the rate of the Krebs cycle and alters the expression of most of the mitochondrial enzymes to depolarize the membrane potential and cause oxidative phosphorylation [15,16].
In view of published report of arsenic contamination in the ground drinking water up to 1861 ppb in endemic area of visceral leishmaniasis, Bihar, India , residents may consume high concentration of arsenic. Although a majority of the arsenic may be cleared from the blood within hours , due to the regular intake of high concentrations of the element, arsenic deposition can be expected in the liver, spleen and bone marrow, leading to increased levels compared to that in blood , where the virulent Leishmania donovani form resides during infection. Therefore, during this in vitro study, the authors have standardized the laboratory culture of Leishmania donovani parasites with a lower concentration (0.5 to 1.5 mg/L) of arsenic, which was found to be optimal for immunomodulation and infection establishment.