ANTI-MALARIA ANTIBODY-PRODUCING B-CELL FREQUENCIES IN ADULTS AFTER A PLASMODIUM-FALCIPARUM OUTBREAK IN MADAGASCAR

The central highlands of Madagascar offer a unique opportunity to expl ore the malaria immune memory, as the last murderous epidemic in the s tudy area occurred 8 years ago. Quantification of the circulating memo ry B lymphocytes reacting to Plasmodium falciparum was assessed among 14 Madagascans by using a limiting dilution assay, applied to the EL4 culture system, which leads to activation, proliferation and different iation into antibody-secreting cells (ASC) of most peripheral B cells. This system allowed us to observe, without any malaria-specific resti mulation, a geometric mean frequency of one anti-P. falciparum ASC amo ng 2992 circulating B cells, except for one Madagascan who did not hav e any detectable ASC. A geometric mean frequency of one anti-P. falcip arum ASC among 1403 was obtained for six malaria hyperimmune Camerooni ans, but conversely, no anti-malaria ASC was detected in the blood of six malaria non-immune French control subjects. Anti-P. falcipaium ASC frequencies and serum specific antibodies were strongly related. Our results indicate that anti-malaria ASC are still present in peripheral blood of Madagascan subjects, who have not been exposed to P. falcipa rum for several years. These responder B cells reflect the malaria B c ell memory acquired during the last epidemic.