The pivotal role of receptor tyrosine kinases (RTKs) in regulation of cellular growth and differentiation has triggered considerable interest in the identification of novel members of this ubiquitous protein family. Screening techniques, which were not dependent on function, brought about the isolation of numerous novel RTKs for which the ligands were not known. Many were EPH-like RTKs, which comprise the largest RTK family known to date (1-19).In contrast to the PCR-based approaches used for most other EPH-like RTKs, human EPH-like kinase (HEK) was identified on the cell surface of a human pre-B-cell line with monoclonal antibody (mAb) IIIA4 (2). HEK protein was affinity purified on a mAb IIIA4 column (2), and its amino acid sequence revealed homology with EPH and the predicted amino acid sequences of other EPH-like RTKs [ELK (3), ECK (4), and ERK (5)]. The sequences of the mouse (MEK4) and chicken (CEK4) homologues of HEK have also been reported (6, 7). To date, at least 28 members of the EPH subfamily have been identified in diverse vertebrate species including zeThe publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.

Eph and its homologues form the largest subfamily of receptor tyrosine kinases. Normal expression patterns of this subfamily indicate roles in differentiation and development, whereas The receptor tyrosine kinases (RTKs) are transmembrane molecules which transduce signals from the extracellular environment into the cytoplasm. They include well-studied regulators of cell proliferation and differentiation, such as c-Kit and the receptors for epidermal growth factor, platelet-derived growth factor, and macrophage colony-stimulating factor (1). Signaling by an RTK is initiated when its ligand binds to the extracellular domain of the receptor. This leads to the formation of receptor dimers, which activate their catalytic domains by reciprocal phosphorylation on tyrosine residues (2). Once activated, RTKs can bind and phosphorylate specific intracellular proteins that act as second messengers.

BackgroundRH5 is the leading vaccine candidate for the Plasmodium falciparum blood stage and has shown impact on parasite growth in the blood in a human clinical trial. RH5 binds to Ripr and CyRPA at the apical end of the invasive merozoite form, and this complex, designated RCR, is essential for entry into human erythrocytes. RH5 has advanced to human clinical trials, and the impact on parasite growth in the blood was encouraging but modest. This study assessed the potential of a protein-in-adjuvant blood stage malaria vaccine based on a combination of RH5, Ripr and CyRPA to provide improved neutralizing activity against P. falciparum in vitro.MethodsMice were immunized with the individual RCR antigens to down select the best performing adjuvant formulation and rats were immunized with the individual RCR antigens to select the correct antigen dose. A second cohort of rats were immunized with single, double and triple antigen combinations to assess immunogenicity and parasite neutralizing activity in growth inhibition assays.ResultsThe DPX® platform was identified as the best performing formulation in potentiating P. falciparum inhibitory antibody responses to these antigens. The three antigens derived from RH5, Ripr and CyRPA proteins formulated with DPX induced highly inhibitory parasite neutralising antibodies. Notably, RH5 either as a single antigen or in combination with Ripr and/or CyRPA, induced inhibitory antibodies that outperformed CyRPA, Ripr.ConclusionAn RCR combination vaccine may not induce substantially improved protective immunity as compared with RH5 as a single immunogen in a clinical setting and leaves the development pathway open for other antigens to be combined with RH5 as a next generation malaria vaccine.