Human adenovirus-vectored foot-and-mouth disease vaccines: establishment of a vaccine product profile through in vitro testing.

Next generation, foot-and-mouth disease (FMD) molecular vaccines based on replication deficient human adenovirus serotype 5 viral vectored delivery of FMD capsid genes (AdFMD) are being developed by the United States Dept. of Homeland Security and industry partners. The strategic goal of this program is to develop AdFMD licensed vaccines for the USA National Veterinary Stockpile for use, if needed, as emergency response tools during an FMD outbreak. This vaccine platform provides a unique opportunity to develop a set of in vitro analytical parameters to generate an AdFMD vaccine product profile to replace the current lot release test for traditional, inactivated FMD vaccines that requires FMDV challenge in livestock. The possibility of an indirect FMD vaccine potency test based on a serological alternative was initially investigated for a lead vaccine candidate, Adt.A24. Results show that serum virus neutralization (SVN) based serology testing for Adt.A24 vaccine lot release is not feasible, at least not in the context of vaccine potency assessment at one week post-vaccination. Thus, an in vitro infectious titer assay (tissue culture infectious dose 50, TCID50) which measures FMD infectious (protein expression) titer was established. Pre-validation results show acceptable assay variability and linearity and these data support further studies to validate the TCID50 assay as a potential potency release test. In addition, a quantitative physiochemical assay (HPLC) and three immunochemical assays (Fluorescent Focus-Forming Unit (FFU); tissue culture expression dose 50 (TCED50); Western blot) were developed for potential use as in vitro assays to monitor AdFMD vaccine lot-to-lot consistency and other potential applications. These results demonstrate the feasibility of using a traditional modified-live vaccine virus infectivity assay in combination with a set of physiochemical and immunochemical tests to build a vaccine product profile that will ensure the each AdFMD vaccine lot released is similar to a reference vaccine of proven clinical safety and efficacy.

Epitope-specific antibody and suppression of autoantibody responses against a hybrid self protein.

This study addresses the relationship of epitope-specific Ab responses and alternative autoantibody responses in a model system in which an antigenized self protein serves as the carrier for a defined heterologous B cell epitope. Ubiquitin, a nonimmunogenic self protein, was engineered to present heterologous B and T cell epitopes in the recombinant molecule. Fusion to the C terminus introduced a universal T cell epitope from a Mycobacterium tuberculosis Ag. The B cell epitope was created by inserting a 12-residue loop sequence of HIV-1 gp120 at a surface-exposed position of ubiquitin. These modifications preserved the ubiquitin fold, allowing a new conformational epitope to be presented among native self epitopes. Mice immunized with the hybrid protein bearing only the mycobacterial T cell epitope elicited a strong autoantibody response to native ubiquitin. In contrast, antisera elicited against hybrid ubiquitin presenting the HIV B cell epitope reacted specifically with the foreign epitope but not with native ubiquitin. Absence of autoantibody in the response was attributed to poor competition of autoreactive B cells for limiting T cell help. Both types of responses were associated with Th responses to defined epitopes of the ubiquitin hybrid protein. These results may have implications for a tolerance mechanism dependent on B-T cell cooperation.

Ubiquitin fusion technology: bioprocessing of peptides.

Ubiquitin fusion technology represents an emerging method for economically producing peptides and small proteins in the bacterium Escherichia coli. Our focus is on peptide production where the need for cost-effective, scaleable processes has recently been highlighted by Kelley (1996). There are two principal features: (1) the expression system consists of a suitable E. coli host strain paired with a plasmid that encodes the ubiquitin fusion and (2) an ubiquitin-specific protease, UCH-L3, which cleaves only C-terminal extensions from ubiquitin. In this work, multigram yields were obtained of four ubiquitin fusions derived from cell paste generated in single 10-L fermentations. All were expressed intracellularly and remained soluble at extremely high levels of expression. Bacterial freeze--thaw lysates contained over 95% pure ubiquitin fusion protein. All four fusions were efficiently cleaved to ubiquitin and the peptide products. In one case, the final yield of peptide was 1.08 g from 3 L of low cell density bacterial culture. The combination of exceptional overexpression of the ubiquitin--peptide fusion proteins and a robust and specific protease are unique advantages contributing to a cost-effective, scaleable, and generic bioprocess for peptide production.

High-level expression and efficient recovery of ubiquitin fusion proteins from Escherichia coli.

The transition in growth and induction of bacterial cultures expressing recombinant proteins from the laboratory bench to the pilot scale for production has been performed successfully for several ubiquitin fusion-expressing clones. Increased protein turnover and decreased metabolic efficiency of Escherichia coli at high cell densities are often responsible for failures in fermenter cultures. Current data indicate that (1) yields in shaker flask cultures are directly scalable to a 10 L fermenter, (2) higher cell densities actually augment the specific yield of ubiquitin fusion proteins, (3) an in vivo heat shock during fermentation increases the ubiquitin fusion yield, which provides an initial separation step in the fermenter, and (4) the ubiquitin fusion expression clone makes at least 3-fold more total protein, including host proteins, than the parent strain of E. coli. A series of three fermentations was performed, using the model strain, with varied temperature shift protocols. These fermentations showed that a maximal heat shock of 12 degrees C (from 30 to 42 degrees C), initiated simultaneously with induction, gave a maximal specific yield (over 90% of the total soluble protein by densitometry, 709 mg/L by protein assay), in which the recoverable ubiquitin fusion product comprised 16% of the wet weight of the cell paste. These results illustrate the enormous potential of ubiquitin fusion technology for the economical production of peptides, even in a 10 L fermenter.

Identification and characterization of the ribosomal RNA-encoding genes in Clavibacter xyli subsp. cynodontis.

Clavibacter xyli subsp. cynodontis (Cxc) is a xylem-inhabiting bacterial endophyte of Bermudagrass. This organism is classified with Gram-positive, high G + C content, coryneform-actinomycete bacteria. Southern-blot analysis showed that Cxc contains only one copy of the ribosomal RNA-encoding genes (rRNA). A clone containing the rRNA genes was isolated from a genomic library of Cxc DNA cloned in the lambda EMBL3 vector. The gene cluster was partially sequenced, revealing the gene order 5'-16S-23S-5S-3', similar to that found in other prokaryotes. Low-resolution S1 mapping suggested multiple transcription start points of the rRNA operon.