Parasite specific immune responses in humans exposed to Plasmodium falciparum malaria

University dissertation from Stockholm : Stockholm University

Abstract: Malaria is one of the most serious diseases worldwide with regard to mortality and morbidity and the most severe form in humans is caused by Plasmodium falciparum. The development of a vaccine against this disease is an important goal in malaria research and extensive efforts are made to identify antigens suitable for inclusion in such vaccines. The selected antigens must have the capacity to induce protective immune responses in the vaccinated population. Thus, knowledge of the mechanisms and antigens involved in eliciting protective immunity in donors naturally exposed and immune to P.falciparum is required. In this thesis P.falciparum malaria induced human immune responses have been studied in two ways.First, by immortalizing B-cells from malaria exposed and immune donors, in vitro cell lines continuously producing human monoclonal antibodies (HuMAbs) were established to obtain reagents for studying antigens relevant for induction of protective immunity. The asexual erythrocytic stages of the parasite cycle are the cause of the clinical manifestations seen in malaria and the studies were focused on antigens expressed during these stages. Two different antigens detected by HuMAbs are described in more detail. Both are present on all erythrocytic parasite stages and also associated with the membrane of the infected host cell, but at different maturation stages of the parasite. Chemical analysis revealed that one of the HuMAbs (33G2) recognized a family of cross reactive protein antigens, while the other (A52A6) detected a lipid antigen with a complex structure. Results from immunofluorescence microscopy suggested an involvement of this latter antigen in the parasite invasion into the host eryhrocytic cell and both antibodies were efficient inhibitors of this process in in vitro cultures of P.falciparum.Taken together, the properties of these HuMAbs obtained from a naturally exposed and immune donor, suggest that they may be important in vivo and thus should be useful tools for understanding the mechanisms of parasite host cell invasion as well as reagents for antigenic epitopes of potential interest for inclusion in a malaria vaccine.Secondly, it was of importance to establish whether a human population will differ in their individual immune responses to malaria antigens, due to their genetic background. Therefore, B- and T-cell immune responses against defined epitopes of Pfl55/RESA, an antigen generally considered to be a good vaccine candidate, were investigated in P.falciparum exposed related donors (i.e. monozygotic twinpairs, dizygotic twin pairs and siblings) and unrelated controls. The results revealed a clear genetic regulation of these responses.The measured immune responses are dependent on the activation of T-cells of the "helper" subtype, which must recognize antigen in the context of MHC class II molecules. Thus, MHC class II molecules function as restriction elements and it was of importance to investigate if the individual immune responses were restricted by certain class MHC class II haplotypes. However, no such correlations were found, neither when unrelated nor when related donors were studied.It was concluded that although the immune responses against Pfl55/RESA were genetically regulated, it was not possible to correlate them to any MHC class II products, suggesting that other non-MHC linked genetic factors were involved.