M.L. Ng

Rapid purification of recombinant dengue and West Nile virus envelope Domain III proteins by metal affinity membrane chromatography

Arthropod-borne flaviviruses such as dengue virus (DENV) and West Nile virus (WNV) pose significant health threats to the global community. Due to escalating numbers of DENV and WNV infections worldwide, development of an effective vaccine remains a global health priority. As flavivirus envelope Domain III (DIII) protein is highly immunogenic and capable of inducing neutralizing antibodies against wild-type virus, it is both a potential protein subunit vaccine candidate and a suitable diagnostic reagent. Here, we describe the use of metal affinity membrane chromatography as a rapid and improved alternative for the purification of recombinant DIII (rDIII) antigens from DENV serotypes 1-4 and WNV - New York, Sarafend, Wengler and Kunjin strains. Optimum conditions for the expression, solubilization, renaturation and purification of these proteins were established. The purified proteins were confirmed by MALDI-TOF mass spectrometry and ELISA using antibodies raised against the respective viruses. Biological function of the purified rDIII proteins was confirmed by their ability to generate DIII-specific antibodies in mice that could neutralize the virus.

Understanding aggregation-based assays: nature of protein corona and number of epitopes on antigen matters

The development of assays that exploit aggregation of gold nanoparticles (NPs) has been widely studied for detection of biomolecules in diagnostics. These assays are often based on antibody–antigen interactions to mediate aggregation of NPs. However, the protein parameters underlying the performance of these assays are not well understood. In this study, we systematically examine how the nature of the protein corona on the NPs, formed from either antibody or antigen, and how the number of binding sites or epitopes on the antigen affect aggregation. We selected two small antigen proteins: 13 kDa recombinant dengue viral envelope domain III protein with a polyhistidine tag (DIII-His), and 19 kDa vascular endothelial growth factor A (VEGFA), to form protein corona around NPs and study the aggregation induced by their monoclonal and polyclonal antibodies. We then reciprocated the systems to form protein corona with the antibodies and compared the aggregation induced by the antigens. We showed that the nature of the protein corona matters, as the corona formed from antigens had lower limits of detection and elicited greater degrees of NP aggregation compared to the corona formed from antibodies. Furthermore, the number of epitopes on the antigen matters, as polyclonal antibodies, which target multiple epitopes on the antigen, were able to induce aggregation for both antigen- and antibody-corona systems. In contrast, monoclonal antibodies that target a single epitope on the antigens induced aggregation for the antigen-corona system only. Our results showed that an understanding of the antibody–antigen system is crucial for establishing guidelines for rational selection of proteins in the design of aggregation-based assays with NPs.

Emergence of human West Nile Virus infection in Sri Lanka

BackgroundWest Nile virus (WNV) has emerged as one of the most common causes of epidemic meningoencephalitis worldwide. Most human infections are asymptomatic. However, neuroinvasive disease characterized by meningitis, encephalitis and/or acute flaccid paralysis is associated with significant morbidity and mortality. Although outbreaks have been reported in Asia, human WNV infection has not been previously reported in Sri Lanka.MethodsSera and cerebrospinal fluid (CSF) from 108 consecutive patients with a clinical diagnosis of encephalitis admitted to two tertiary care hospitals in Colombo, Sri Lanka were screened for WNV IgM antibody using enzyme-linked immunosorbent assay. Positive results were confirmed using plaque reduction neutralization test (PRNT). Patient data were obtained from medical records and by interviewing patients and care-givers.ResultsThree of the 108 patients had WNV IgM antibody in serum and one had antibody in the CSF. The presence of WNV neutralizing antibodies was confirmed in two of the three patients using PRNT. Two patients had presented with the clinical syndrome of meningoencephalitis while one had presented with encephalitis. One patient had CSF lymphocytic pleocytosis, one had neutrophilic pleocytosis while CSF cell counts were normal in one. CSF protein showed marginal increase in two patients.ConclusionsThis is the first report of human WNV infection identified in patients presenting with encephalitis or meningoencephalitis in Sri Lanka. There were no clinical, routine laboratory or radiological features that were distinguishable from other infectious causes of meningoencephalitis.

Optimized sequential purification protocol for in vivo site-specific biotinylated full-length dengue virus capsid protein

Dengue virus (DENV) capsid (C) protein is one of the three structural proteins that form a mature virus. The main challenge impeding the study of this protein is to generate pure non-truncated, full-length C proteins for structural and functional studies. This is mainly due to its small molecular weight, highly positively charged, stability and solubility properties. Here, we report a strategy to construct, express, biotinylate and purify non-truncated, full-length DENV C protein. A 6× His tag and a biotin acceptor peptide (BAP) were cloned at the N-terminus of C protein using overlapping extension-polymerase chain reaction method for site-specific biotinylation. The final construct was inserted into pET28a plasmid and BL-21 (CodonPlus) expression competent cell strain was selected as there are 12% rare codons in the C protein sequence. Strikingly, we found that our recombinant proteins with BAP were biotinylated endogenously with high efficiency in Escherichia coli BL-21 strains. To purify this His-tagged C protein, nickel-nitriloacetic acid affinity chromatography was first carried out under denaturing condition. After stepwise dialysis and concurrent refolding, ion exchange-fast protein liquid chromatography was performed to further separate the residual contaminants. To obtain C protein with high purity, a final round of purification with size exclusion chromatography was carried out and a single peak corresponding to C protein was attained. With this optimized sequential purification protocol, we successfully generated pure biotinylated full-length DENV C protein. The functionality of this purified non-truncated DENV C protein was examined and it was suitable for structural and molecular studies.