High Affinity Antibodies to Plasmodium falciparum Merozoite Antigens Are Associated with Protection from Malaria

Malaria kills almost 1 million people every year, but the mechanisms behind protective immunity against the disease are still largely unknown. In this study, surface plasmon resonance technology was used to evaluate the affinity (measured as k(d)) of naturally acquired antibodies to the Plasmodium falciparum antigens MSP2 and AMA1. Antibodies in serum samples from residents in endemic areas bound with higher affinities to AMA1 than to MSP2, and with higher affinities to the 3D7 allele of MSP2-3D7 than to the FC27 allele. The affinities against AMA1 and MSP2-3D7 increased with age, and were usually within similar range as the affinities for the monoclonal antibodies also examined in this study. The finding of MSP2-3D7 type parasites in the blood was associated with a tendency for higher affinity antibodies to both forms of MSP2 and AMA1, but this was significant only when analyzing antibodies against MSP2-FC27, and individuals infected with both allelic forms of MSP2 at the same time showed the highest affinities. Individuals with the highest antibody affinities for MSP2-3D7 at baseline had a prolonged time to clinical malaria during 40 weeks of follow-up, and among individuals who were parasite positive at baseline higher antibody affinities to all antigens were seen in the individuals that did not experience febrile malaria during follow up. This study contributes important information for understanding how immunity against malaria arises. The findings suggest that antibody affinity plays an important role in protection against disease, and differs between antigens. In light of this information, antibody affinity measurements would be a key assessment in future evaluation of malaria vaccine formulations

Restriction in IgM expression-II. The V<SUB>H</SUB> regions of murine anti-lactose antibodies

A comparison of the anti-lactose IgM response among individual animals of five inbred strains of mice has been made with a view to possible germ-line restriction in IgM diversity. The VH regions of the antibodies were isolated and the individual preparations examined by isoelectrical focusing (IEF) in 7 M urea. The main findings were the complexity of the IEF patterns as well as their similarity among individuals of the same strain and their near-identity to the patterns for the VH regions derived from normal IgM. These observations parallel results found earlier with VH preparations of equine anti-lactose IgM. This homology of behavior between the two systems suggests a cautious generalization about IgM responses to the effect that limited longevity of the B cell clones and germ-line restriction of VH in IgM expression remain tenable notions

Restriction in igm expression-V. fine structure analysis in the anti-lactose system

A methodology for the analysis of the fine specificity of monoclonal anti-lactose IgM and IgG antibodies is described using structural variants of the homologous lactoside epitope. These variants are used as inhibitors of the binding of a reference ligand N-(5-dimethylaminonaphthalene-l-sulfonyl)-p-aminophenyl-&#946;-lactoside. Excitation of the antibody with bound ligand at 295 nm leads to resonance energy transfer to and fluorescence emission by the ligand. Titration of the antibody-ligand mixture with the inhibitor and measurement of the emission at 550 nm provide the data for the calculation of the binding constants of the inhibitors. A comparison of two IgM and two IgG antibodies showed that the higher affinity of the IgG antibodies arises from their specific interaction with both hexosides of the lactoside in contrast to IgM antibodies which do not engage the non-terminal hexoside as effectively. The quantitative significance of this difference is a differential free energy contribution of about -3 kcal/mole to the binding of lactoside by IgG. A finer discrimination between homologous and several cross-reactive molecules is evident with IgG antibody compared to IgM. The former exhibits about 100-fold greater difference in their binding constants than does IgM. These differences applied to biologically relevant multivalent interactions, where functional affinity governs complex formation, suggest a possible explanation for the IgM to IgG conversion characteristic of the humoral immune response