Fruit fly study reveals a toxic relationship between lead and immunity - IndiaBioscience


Lead is a heavy metal that was once used extensively in paints, gasoline, batteries, plumbing etc. Now, a new study by Indian researchers shows that in addition to its many known toxic effects, lead may lower immunity as well. Through a series of experiments on fruit flies, the researchers have demonstrated a link between lead exposure and susceptibility towards bacterial infections.

Lead, a metal that made us question the safety of our favourite instant noodles not too long ago, may have damaging effects on the immune system, as shown by a recent study by Hena Firdaus and her team from the Central University of Jharkhand. Using fruit flies as a model system, Firdaus’s team showed that higher lead levels can make flies more susceptible to bacterial infections, possibly due to a decrease in the number of blood cells (hemocytes).

Lead is one of the most toxic naturally occurring metals, for both humans and the environment. We can get exposed to lead through paints, contaminated food, water, dust, smelting, gasoline, aviation fuel etc. Even though acute lead poisoning occurs only when the level of exposure exceeds 100 µg/​dl, the Centre for Disease Control (CDC), USA, has recently declared levels greater than 2 µg/​dl as toxic. Lead exposure can result in anaemia, unequal size of red blood cells, hypertension, kidney failure and nervous toxicity.

Fruit flies (Drosophila melanogaster) are often used as model systems since they are easy to work on and can be used to understand complex phenomena in other organisms. Fruit flies also have a significant similarity in their heavy metal tolerance mechanism with humans, which can be studied using genetic approaches.

Previous studies have shown evidence of DNA damage in flies due to exposure to heavy metals resulting in a delay in development. Firdaus and her team confirmed this finding with the help of a growth-based assay wherein they exposed fruit fly larvae to lead in their diets. Dietary lead not only hindered development in fruit fly from eggs into adults but also caused a drop in the rate of survival. Moreover, many of the flies exposed to high lead concentrations tended to die prematurely due to bacterial infections. In particular, the researchers found that the lead-exposed flies were more susceptible to B.subtilis, a fly pathogen, when compared to flies fed a lead-free diet.

Hemocytes are analogues of blood cells in Drosophila which play an important role in controlling cellular immunity and immune responses. They do this by clearing microbial pathogens and engulfing other parasites. ​“As lead continually enters into the environment due to many unavoidable natural activities, it will be interesting to see what kind of effect on hemocytes, if any, can be found in lead-fed Drosophila, ” said Ashim Kumar Basak from the Institute of Genetic Engineering, who was not a part of this study.

Interestingly, the researchers found that hemocyte count drops when fruit fly larvae are exposed to dietary lead. Hemocytes also have certain developmental functions which may be tied to the high mortality seen in lead-fed fruit flies. Firdaus points out that, ​“So far studies have not talked about hemocyte levels decreasing due to lead-based stress in Drosophila which connects our data to the other major finding of a compromised immune system in lead-exposed flies.”

This research also supports previous studies investigating the effects of lead exposure in zebrafish which also showed compromised immunity. When asked about their next steps, Firdaus said, ​“In future, we hope to conduct more experiments on Drosophila to study the mechanism behind declining hemocytes.”

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