P. falciparum Infection May Confer Long-Term Protection

Artificially induced immunity against malaria re-infection may persist for more than two years

MONDAY, April 25 (HealthDay News) -- Artificially induced immunity against infection with Plasmodium falciparum (P. falciparum) may last for 2.5 years or longer, according to a study published online April 25 in The Lancet.

Meta Roestenberg, M.D., from the Radboud University Nijmegen Medical Centre in the Netherlands, and colleagues investigated the duration of experimentally induced protection against P. falciparum. Previously immune volunteers were rechallenged 28 months after immunization, with the bites of five mosquitoes infected with P. falciparum, and they were compared with newly recruited malaria-naive volunteers. Blood-stage parasitemia was detected by microscopy, parasitemia kinetics was evaluated with real-time quantitative polymerase chain reaction (rtPCR), and clinical signs and symptoms were recorded during the 21-day follow-up period. The in vitro production of interleukin 2 and interferon γ by effector memory T cells was investigated and cellular immune responses were examined in volunteers and controls.

The investigators found that, after rechallenge, four of the six immune volunteers were microscopically negative, and no blood-stage parasites were detected by rtPCR. The remaining two volunteers showed delayed patent parasitemia on days 15 and 18, respectively. A sustained P. falciparum-specific pluripotent effector memory T-cell response was demonstrated in protected volunteers. Several mild to moderate adverse events were seen in the four protected volunteers, with headache being the most commonly reported symptom. Adverse events in the two patients with delayed patency and those in the control group were similar.

"Artificially induced immunity lasts longer than generally recorded after natural exposure, providing a new avenue of research into the mechanisms of malaria immunity," the authors write.

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