SCI-TECH & AGRI

How insects evolve immune-memory against bacterial infections

We showed that evolution of immune-memory in an insect can happen quickly and quite often, says Deepa Agashe (third from right)  

Much like humans, insects too develop an immune-memory in response to infection, a team at the National Centre for Biological Sciences (NCBS), Bengaluru has found. In humans, for instance, natural infection or vaccination can lead to the formation of important immunological memory in the human immune system. In other words, once infected, the immune system becomes ready to deal with that particular antigen because of immune-memory. For long, it has been a point of debate whether insects have such a memory that can protect them against future infections. The present study shows that such a memory can evolve over generations in red flour beetles (Tribolium castaneum) infected with Bacillus thuringiensis (Bt). The results of the study were recently published in Proceedings of the Royal Society B.

The study was conducted in Deepa Agashe’s lab at NCBS and the experiment was designed by Imroze Khan, first author of the paper, and Dr. Agashe, the principal investigator. The team infected nearly 5,000 to 6,000 beetles in every generation. “Every insect had to be pierced at the right point and injected with a standard number of bacterial cells. It took a year to standardise this process,” says Dr. Khan, who is now a faculty at Ashoka University, Delhi. With a generation being approximately 45 days long, the study of 10 generations stretched over two years. “Every day Arun Prakash [one of the authors] and I had to infect 1,000 insects,” he adds when asked what was the most challenging part of the study. The beetle populations were exposed to a single large dose of live Bt antigens or exposed to dead bacteria followed by live infection.

Evolved immunity

After observing ten generations, the team found that the new generation of insects had evolved better defence against Bt antigen. The emerging populations showed either improved innate resistance or immune-memory, as opposed to control populations which were injected with a buffer solution and did not evolve any special ability to deal with the pathogen.

“This is the first real-time observation of the evolution of immune-memory in an insect; we showed that this can happen very quickly, and quite often,” says Dr Agashe, in an email to The Hindu. “If our results hold true for other insects (which remains to be tested), this suggests that immune-memory can be a very broad system of defence against pathogens,” she adds. Since insects do not have the kind of immune cells that humans have, for a long time, scientists did not think it was even possible for insects to develop an immune-memory.

In the past few years, multiple studies showed that insects do show some form of immune-memory, but how such memory evolves remained a puzzle. “We now have some clues about how fast and how reliably memory could evolve, what might be the mechanisms involved and when might immune-memory versus resistance be favoured by natural selection,” says Dr Agashe.

Together, Dr. Khan and Dr. Agashe’s labs are now trying to figure out the molecules responsible for immune-memory and resistance.