Children who live in places where malaria is rampant often suffer repeated bouts of infection with the mosquito-borne parasite but are less likely to become feverish in later episodes of the disease. This happens not because their immune system becomes more adept at protecting them from the parasite but appears to result from certain immune cells being lost and turning dysfunctional, according to research that has just been published.
A team of American and Ugandan researchers monitored a group of 78 children in a rural part of south-eastern Uganda where malaria is widespread. These children were followed from their infancy, with regular check-ups to look for malaria.
Critical age In this group, the incidence of symptomatic malaria, where the child developed fever, peaked at about two and a half years of age and then declined, with a corresponding increase in asymptomatic infections without fever.
By the time the children were four years old, the ones who caught malaria repeatedly had lower levels of certain immune cells, the ‘Vδ2 γδ T cells,’ in their blood, the scientists reported in a paper jupublished today (August 28) in the journal Science Translational Medicine .
The Vδ2 cells are capable of secreting a substance that is toxic to the malaria parasite when they pick up signs of its presence.
These cells, which are innately reactive to the parasite even without prior exposure, “may help protect infants and young children from malaria but at the cost of creating excessive inflammation which results in fevers and other symptoms,” said Margaret Feeney of the University of California at San Francisco, the paper's corresponding author, in an email.
The Vδ2 cells also release molecular alarm signals, known as cytokines. The researchers found that such cells from children with numerous prior malaria episodes were less responsive to an infection.
Children with lower levels of cytokine-producing Vδ2 cells had a higher risk of harbouring more parasites in their blood, but nevertheless were more likely to be asymptomatic. The progressive loss and dysfunction of those immune cells could conceivably interfere with effective clearance of the infection, the paper noted.
However, it is likely that other immune mechanisms, such as antibodies and conventional T cells, play a role in keeping parasite numbers in check, according to Dr. Feeney.
Adults who live in malaria endemic regions too have asymptomatic episodes of the disease, observed V. Arun Nagaraj, a researcher at the Indian Institute of Science in Bangalore who studies the malaria parasite. It needed to be checked whether the immune mechanism identified in the paper was involved in adults as well.
