Clinical and parasitological studies on immunity to Plasmodium falciparum malaria in children.

Malaria remains one of the major health problems in many tropical countries. Plasmodium falciparum is the most common malaria parasite in Africa, and it causes much more severe and progressive illness than any of the other types of malaria parasite. Children living in sub-Saharan Africa are bearing the major burden of the disease and the mortality. Whatever parameter is used to measure the mortality or the morbidity from malaria, the true problem is likely to be underestimated. The pattern of morbidity and mortality depends on the transmission intensity; the more intensity of malaria transmission is increased, the earlier and more confined the age range of symptomatic malaria. The asymptomatic carrier status is common, and 60-80% of the children in highly endemic areas have P. falciparum parasitaemia at any given time. Consequently a case definition based on the mere presence of parasites in the blood is non-informative in terms of measuring morbidity. Recognizing that there are no specific diagnostic clinical parameters for malaria, but that fever is very common, and that morbidity is to some extent dependent on the parasite density, we described using a logistic regression model the probability of being sick from malaria in relation to body temperature and parasite density. Acquired clinical and parasitological immunity develop progressively over several years after repeated exposure to infection. Protection is acquired first against death or severe clinical disease, then against milder clinical attacks, but protection against infection is never complete. Clinical and parasitological immunity develop concomitantly, as demonstrated by relating the parasite densities to measured body temperature. However, the ability to control the disease and parasite density develops earlier than the ability to prevent the parasite infection. The individual immune mechanisms that are responsible for the acquired immunity remain uncertain, but classical transfer experiments with polyvalent gamma globulin from immune donors to non-immune individuals showed that antibodies play an important role. Potential targets for malarial vaccines include antigens on the surface of the sporozoites and the merozoites. Several protein antigens from P. falciparum have been characterized at the molecular level, and most of the characterized antigens have the common characteristic that they are recognized by immune sera from individuals living in malaria endemic areas. Working on the approach that potentially useful targets for protective vaccine development can be identified by correlating the naturally acquired immune responses with defined P. falciparum antigens, we examined antigens from both the sporozoite stage (CS-protein) and the blood stages (Pf155/RESA, GLURP, and MSP1), as well as P. falciparum induced neoantigens on the red blood cell (band-3 neoantigens). The relationship between the immune response to these defined P. falciparum antigens and clinical and parasitological protection was analysed in the individual age groups. The contribution of the antigen-specific immune response was evaluated, and a positive correlation of parasite density or probability of an episode of clinical malaria with antibody response to the individual antigens was identified in defined age groups. This correlation, however, did not span all age groups, and thus overall responses to defined antigens are not considered to be reliable indicators of protection. The findings may contribute to the understanding of immunological and clinical host responses to parasitaemia and to defined P. falciparum antigens. The studies on the impact of asexual stage infection and the human immune response led to studies on specific and non-specific responses to P. falciparum blood-stage parasites and observations on gametocytaemia. We demonstrated that pyrimethamine/sulfadoxine and chloroquine did not induce gametocytogenesis as suggested previously, but preformed gametocytes persisted after