Host Immune Responses to Antigens of Malaria Parasites

National Institute Of Allergy And Infectious Diseases

Investigators

Linked publications & trials

Abstract

Studies on asexual stage immunity to P. falciparum 1)Evaluate the merozoite antigen PfRH5 as a vaccine candidate. Our collaborators at Oxford University (Dr. Simon Draper et al.) are pursuing this protein as a vaccine candidate and we have contributed to studies using our standardized growth-inhibition assay (GIA). A clinical trial using recombinant PfRH5 protein showed 20% reduction in parasite growth. This is the first time that positive results have been seen with a blood stage vaccine in a challenge trial. We have completed GIA studies from this trial, showing that the GIA results correlated with the reduced parasite growth rate. For the first time these results provide a marker to show what level of GIA should be necessary to obtain protection in humans. This has been published in Med 2)Two versions of this PfRH5 vaccine are now being tested in field trials conducted by the Institute for Health Initiatives in Tanzania. We have shown high levels of GIA activity in immunized infants and this will be pursued in future efficacy trials. 3) We have initiated a detailed study of immune responses to PfRH5 as well as other associated molecules which are part of the invasion complex with this molecule (PfCyRPA and PfRipr). Antibody titers in Malians exposed to these proteins by repeated infection are very low. To determine interaction (synergistic, additive, or antagonistic) between preexisting anti-malaria immunity and vaccine induced anti-RH5 antibodies, we performed GIA with mixtures of purified Malian total IgGs and several different human anti-Rh5 polyclonal and monoclonal Abs provided by Dr. Simon Draper. Overall interactions were additive; however, a proportion of Malian IgGs showed synergistic effects when mixed with anti-RH5 mAbs, and a proportion of other Malian IgGs showed sub-additive interactions with polyclonal antibodies. The importance of these findings is that we did not observe substantial antagonistic effects which were previously seen with AMA1 vaccines. (Published in Cell Report Med). 4) Collaborative projects on anti-malarial monoclonal antibodies. We have started a collaboration with Drs. Peter Crompton/Joshua Tan to aid in characterizing human monoclonal antibodies to whole parasites and recombinant proteins of malaria using B cells from Malians as the source of the antibodies. We are also collaborating with Dr. Richard Eastman (NCATS) on selecting and characterizing additional monoclonal antibodies and nanobodies to PfRH5 and other parasite molecules using various human and camelid libraries. 5) Tracking antimalarial drug use and efficacy is essential for monitoring the current spread of antimalarial drug resistance. However, available methods for assessing patient drug levels and tablet quality are often inaccessible. We have addressed this problem by developing aptamer-based fluorescent sensors for the rapid, specific detection of the antimalarial compounds piperaquine and mefloquine, two slow-clearing partner drugs in first-line artemisinin-based combination therapies (ACTs). DNA aptamers were identified that bind piperaquine and mefloquine with high selectivity over similarly structured small molecules. The aptamers were isolated from a library of single-stranded DNA molecules using a capture-SELEX technique then adapted into structure-switching aptamer fluorescent sensors. Sensor performance was optimized for the detection of drug from human serum and crushed tablets. The patient sample platform was validated against an LC-MS standard drug detection method in healthy volunteer and malaria patient samples. This assay provides a rapid and inexpensive method for tracking antimalarial drug use and quality. This work has been published in Science Transl. Med. Studies on parasite sexual stages and transmission blocking vaccine candidates: 1) Search for and evaluate new possible transmission blocking vaccine candidates. In collaboration with PATH/MVI, we have evaluated a number of potential transmission blocking vaccine candidates and vaccine formulations by SMFA. We have tested antibodies to significant numbers of sexual stage and mosquito vaccine candidates to compare their activity using quantitative measurements of antibody concentration and SMFA. 2) We have also initiated studies with Dr. Taka Tsuboi at Ehime University in Japan to identify other transmission blocking vaccine candidates. This work is in progress. In addition, we are working with these investigators to map the important transmission blocking epitopes in Pfs230. We have identified a peptide that will elicit transmission reducing antibodies after immunization of mice. 3) We have compared the number of oocysts with the number of sporozoites in the mosquito salivary gland. We have shown that there is a strong concordance of oocysts and salivary gland sporozoites in the SMFA. This work has now been published. 4) We continue a collaboration with Dr. Jonathan Lovell (New York State University at Buffalo) to characterize antibody interaction using various sexual stage antigens and vaccine formulations. 5) Evaluation of human/humanized monoclonal antibodies (mabs) to several sexual stage parasite proteins. We are working with various investigators (including Dr. Patrick Duffy of LMIV) to compare mabs to Pfs25 and other sexual stage vaccine candidates by SMFA. 6) We are continuing studies to examine the transcriptional pattern of P. falciparum gametocytes during differentiation in culture using RNASeq. We are currently analyzing the data to provide a detailed transcriptome of this differentiation process. We are seeking genetic targets which determine sex assignment of the gametocytes. 7)The differentiation of malaria gametocytes often takes place within the bone marrow of the vertebrate host. We have initiated a project to explore the interaction of developing gametocytes with myeloid cells within the bone marrow and we have surprisingly found that co-culture with human myeloid cells greatly enhances the numbers of oocysts in the mosquito. This important finding is being pursued to identify the mechanisms involved. 8) We are continuing a series of experiments to determine what serum-derived factors are necessary for successful differentiation of gametocytes. 9) We are part of a consortium of investigators led by Dr. Sumi Biswas of Oxford University and funded by the EU to develop a transmission blocking vaccine.