In vivo switching between variant surface antigens in human Plasmodium falciparum infection.

A semi-immune individual was retrospectively found to have maintained an apparently monoclonal and genotypically stable asymptomatic infection for months after clinical cure of a Plasmodium falciparum malaria episode. Before the attack, the individual had no antibodies to variant surface antigens (VSAs) expressed by an isolate (isolate A) obtained at the time of the episode or by a genotypically identical isolate (isolate B) obtained from the same individual 3 months later. Six weeks after the attack, a strong isolate A-specific VSA antibody response had developed in the complete absence of isolate B-specific antibodies. In contrast, plasma obtained 7 months after the attack contained high levels of VSA antibodies recognizing both isolates. This is the first direct evidence of in vivo switching between VSAs in human P. falciparum infection. Our results suggest that VSA switching is an important survival strategy of P. falciparum, enabling the parasite to persist despite protective, parasite-specific immune responses.