Kenneth E. Palmer

Scaleable manufacture of HIV-1 entry inhibitor griffithsin and validation of its safety and efficacy as a topical microbicide component

To prevent sexually transmitted HIV, the most desirable active ingredients of microbicides are antiretrovirals (ARVs) that directly target viral entry and avert infection at mucosal surfaces. However, most promising ARV entry inhibitors are biologicals, which are costly to manufacture and deliver to resource-poor areas where effective microbicides are urgently needed. Here, we report a manufacturing breakthrough for griffithsin (GRFT), one of the most potent HIV entry inhibitors. This red algal protein was produced in multigram quantities after extraction from Nicotiana benthamiana plants transduced with a tobacco mosaic virus vector expressing GRFT. Plant-produced GRFT (GRFT-P) was shown as active against HIV at picomolar concentrations, directly virucidal via binding to HIV envelope glycoproteins, and capable of blocking cell-to-cell HIV transmission. GRFT-P has broad-spectrum activity against HIV clades A, B, and C, with utility as a microbicide component for HIV prevention in established epidemics in sub-Saharan Africa, South Asia, China, and the industrialized West. Cognizant of the imperative that microbicides not induce epithelial damage or inflammatory responses, we also show that GRFT-P is nonirritating and noninflammatory in human cervical explants and in vivo in the rabbit vaginal irritation model. Moreover, GRFT-P is potently active in preventing infection of cervical explants by HIV-1 and has no mitogenic activity on cultured human lymphocytes.

Domain-swapped structure of the potent antiviral protein griffithsin and its mode of carbohydrate binding.

Summary The crystal structure of griffithsin, an antiviral lectin from the red alga Griffithsia sp., was solved and refined at 1.3 Å resolution for the free protein and 0.94 Å for a complex with mannose. Griffithsin molecules form a domain-swapped dimer, in which two β strands of one molecule complete a β prism consisting of three four-stranded sheets, with an approximate 3-fold axis, of another molecule. The structure of each monomer bears close resemblance to jacalin-related lectins, but its dimeric structure is unique. The structures of complexes of griffithsin with mannose and N-acetylglucosamine defined the locations of three almost identical carbohydrate binding sites on each monomer. We have also shown that griffithsin is a potent inhibitor of the coronavirus responsible for severe acute respiratory syndrome (SARS). Antiviral potency of griffithsin is likely due to the presence of multiple, similar sugar binding sites that provide redundant attachment points for complex carbohydrate molecules present on viral envelopes.

The Molecular Biology of Mastreviruses

Publisher Summary This chapter discusses the molecular biology of mastreviruses, drawing analogies where necessary from research on curtoviruses and begomoviruses. The genomic organization of mastreviruses, curtoviruses, and begomoviruses is illustrated in the chapter. In comparison with mastreviruses, curto- and begomoviruses seem to have a far more complex set of genes and regulatory mechanisms. This may only reflect the fact that these viruses have been more comprehensively investigated—nevertheless, it is clear that all members of the Geminiveridae have common life strategies, but in some cases these viruses have gained extra genes to help them accomplish similar functions, perhaps in a more efficient manner. Mastreviruses have a simpler genomic organization than other geminiviruses, with only three functional genes compared with up to eight genes in some Old World begomoviruses: these are (1) rep, which may be alternatively spliced to produce two different protein products, Rep and RepA, (2) a movement protein gene, and (3) a coat protein gene.

Griffithsin has antiviral activity against hepatitis C virus

ABSTRACT Hepatitis C virus (HCV)-infected patients undergoing liver transplantation universally experience rapid reinfection of their new liver graft. Current treatment protocols do not prevent graft reinfection and, in addition, an accelerated disease progression is observed. In the present study, we have evaluated a novel strategy to prevent HCV infection using a lectin, griffithsin (GRFT) that specifically binds N-linked high-mannose oligosaccharides that are present on the viral envelope. The antiviral effect of GRFT was evaluated in vitro using the HCV pseudoparticle (HCVpp) and HCV cell culture (HCVcc) systems. We show here that preincubation of HCVpp and HCVcc with GRFT prevents infection of Huh-7 hepatoma cells. Furthermore, GRFT interferes with direct cell-to-cell transmission of HCV. GRFT acts at an early phase of the viral life cycle by interfering in a genotype-independent fashion with the interaction between the viral envelope proteins and the viral receptor CD81. The capacity of GRFT to prevent infection in vivo was evaluated using uPA+/+-SCID mice (uPA stands for urokinase-type plasminogen activator) that harbor human primary hepatocytes in their liver (chimeric mice). In this proof-of-concept trial, we demonstrated that GRFT can mitigate HCV infection of chimeric mice. Treated animals that did become infected demonstrated a considerable delay in the kinetics of the viral infection. Our data demonstrate that GRFT can prevent HCV infection in vitro and mitigate HCV infection in vivo. GRFT treatment of chronically infected HCV patients undergoing liver transplantation may be a suitable strategy to prevent infection of the liver allograft.

Broad-Spectrum In Vitro Activity and In Vivo Efficacy of the Antiviral Protein Griffithsin against Emerging Viruses of the Family Coronaviridae

ABSTRACT Viruses of the family Coronaviridae have recently emerged through zoonotic transmission to become serious human pathogens. The pathogenic agent responsible for severe acute respiratory syndrome (SARS), the SARS coronavirus (SARS-CoV), is a member of this large family of positive-strand RNA viruses that cause a spectrum of disease in humans, other mammals, and birds. Since the publicized outbreaks of SARS in China and Canada in 2002-2003, significant efforts successfully identified the causative agent, host cell receptor(s), and many of the pathogenic mechanisms underlying SARS. With this greater understanding of SARS-CoV biology, many researchers have sought to identify agents for the treatment of SARS. Here we report the utility of the potent antiviral protein griffithsin (GRFT) in the prevention of SARS-CoV infection both in vitro and in vivo. We also show that GRFT specifically binds to the SARS-CoV spike glycoprotein and inhibits viral entry. In addition, we report the activity of GRFT against a variety of additional coronaviruses that infect humans, other mammals, and birds. Finally, we show that GRFT treatment has a positive effect on morbidity and mortality in a lethal infection model using a mouse-adapted SARS-CoV and also specifically inhibits deleterious aspects of the host immunological response to SARS infection in mammals.

Investigation of Griffithsin's Interactions with Human Cells Confirms Its Outstanding Safety and Efficacy Profile as a Microbicide Candidate

Many natural product-derived lectins such as the red algal lectin griffithsin (GRFT) have potent in vitro activity against viruses that display dense clusters of oligomannose N-linked glycans (NLG) on their surface envelope glycoproteins. However, since oligomannose NLG are also found on some host proteins it is possible that treatment with antiviral lectins may trigger undesirable side effects. For other antiviral lectins such as concanavalin A, banana lectin and cyanovirin-N (CV-N), interactions between the lectin and as yet undescribed cellular moieties have been reported to induce undesirable side effects including secretion of inflammatory cytokines and activation of host T-cells. We show that GRFT, unlike CV-N, binds the surface of human epithelial and peripheral blood mononuclear cells (PBMC) through an exclusively oligosaccharide-dependent interaction. In contrast to several other antiviral lectins however, GRFT treatment induces only minimal changes in secretion of inflammatory cytokines and chemokines by epithelial cells or human PBMC, has no measureable effect on cell viability and does not significantly upregulate markers of T-cell activation. In addition, GRFT appears to retain antiviral activity once bound to the surface of PBMC. Finally, RNA microarray studies show that, while CV-N and ConA regulate expression of a multitude of cellular genes, GRFT treatment effects only minimal alterations in the gene expression profile of a human ectocervical cell line. These studies indicate that GRFT has an outstanding safety profile with little evidence of induced toxicity, T-cell activation or deleterious immunological consequence, unique attributes for a natural product-derived lectin.

Griffithsin Protects Mice from Genital Herpes by Preventing Cell-to-Cell Spread

ABSTRACT Griffithsin, which binds N-linked glycans on gp120 to prevent HIV entry, has the most potent HIV-1 inhibitory activity described for any antiviral lectin and is being developed for topical preexposure prophylaxis. The current studies were designed to further assess its potential by exploring its activity against herpes simplex virus 2 (HSV-2), a cofactor for HIV acquisition, in vitro and in a murine model. Safety was evaluated by examining its impact on epithelial barrier integrity in polarized cultures and testing whether repeated intravaginal dosing potentiates the susceptibility of mice to genital herpes. Griffithsin displayed modest inhibitory activity against HSV-2 if present during viral entry but completely blocked plaque formation if present postentry, reduced plaque size, and prevented cell-to-cell spread. These in vitro findings translated to significant protection against genital herpes in mice treated with 0.1% griffithsin gel. Griffithsin, but not placebo gel, prevented viral spread (visualized with a luciferase-expressing virus), significantly reduced disease scores, and resulted in greater survival (P < 0.05, log rank test). Protection persisted when HSV-2 was introduced in seminal plasma. Although griffithsin triggered a small decline in transepithelial electrical resistance in polarized cultures, this did not translate to any significant increase in the ability of HIV to migrate from the apical to the basolateral chamber nor to an increase in susceptibility to HSV-2 in mice treated with griffithsin gel for 7 days. These findings demonstrate that griffithsin inhibits HSV-2 by a unique mechanism of blocking cell-to-cell spread and support its further development for HIV and HSV-2 prevention.

Molecular characterisation of a distinct South African cassava infecting geminivirus

SummaryAfrican cassava mosaic virus (ACMV) and East African cassava mosaic virus (EACMV) are whitefly-transmitted geminiviruses (WTGs) which are widespread in cassava in Africa and cause serious yield losses. Recently, a new geminivirus affecting cassava in South Africa (SACMV) has been reported. In this work SACMV was found to have DNA-A and DNA-B components. Comparisons of amino acid sequences of the putative coat protein, and nucleotide sequences of the common region and a 687-bp DNA B fragment of SACMV with other WTGs, showed that SACMV clustered with the Old World subgroup of the Begomovirus genus of geminiviruses. Despite its bipartite nature, SACMV was most closely related to monopartite TYLCVs, but was sufficiently different to justify designating it as a distinct virus. In serological studies, SACMV grouped biologically with EACMV isolates.

Genetically engineered Tobacco mosaic virus as nanoparticle vaccines

Tobacco mosaic virus (TMV) is an RNA virus that typically infects plants but has recently been adapted for vaccine development, owing to the suitability of the virions for modifications as nanoparticles. TMV also has a simple functional structure of a 6.4 Kb (+)-strand RNA encapsidated by a single coat protein, which permits facile genetic manipulation. In this review, we describe recent advances in the manipulation of TMV for the development of several different types of vaccines, including ones that induce antibody and T-cell responses that are protective in pathogen and tumor challenge animal models. Lastly, we describe how TMV self-assembly properties are being used to make a new mammalian RNA pseudovirus, that has unique characteristics for RNA and protein antigen delivery to antigen-presenting cells.

Generation of maize cell lines containing autonomously replicating maize streak virus-based gene vectors

SummaryVirion sense gene replacement derivatives of maize streak virus (MSV) were constructed with selectable marker expression cassettes based on the bialaphos resistance gene (bar) and the CaMV 35S promoter. The effect on replication of increasing the genomic size was tested by including: (1) the 550-bp maize adh I intron and 68-bp TMV Ω RNA leader sequences upstream of the bar genes; and (2) a fusion between the bar and E. coli glutathione reductase (gor) genes. Three recombinant viral vectors ranging in size from 2.7 kb to 4.8 kb replicated efficiently in biolistically transfected cells of suspension cultured Black Mexican sweetcorn (BMS) cells. Deletions greater than 39 bp 3′ of the stemloop sequence in the LIR adversely affected replicon release. Transformed bialaphos-resistant BMS cell lines were generated with all three vectors containing the bar gene: between 38 and 60% of cell lines contained replicating viral episomes. The replicons were structurally stable, replicated to copy numbers of over 500 per haploid genome, and were detected for more than one year after introduction. We noted significant enhancement of bar gene expression, both at the protein and RNA levels, associated with the presence of episomal vector DNA. The maize adhI intron and TMV Ω RNA leader sequences did not seem to have a significant effect on bar gene expression from replicating constructs, although expression from controls was enhanced. The results suggest that MSV-based constructs would provide a useful system for long-term gene amplification in cereal cell culture systems.