Advanced genetic screening to speed vaccine development

biological function of interest.

The release notes that this simple, yet powerful technique uses the host’s immune system itself to rapidly reduce any pathogenic genome (viral, fungal, bacterial or parasitic) to a handful of antigens capable of conferring protection in the host.

The advantage of this in vivo technique is that it offers a means of rapidly screening entire genomes, with the results of the search displaying desired immunogenic traits. The mode of entry of vaccines designed in this way closely resembles the natural infection process of host cells — an improvement over live attenuated vaccines.

This promising approach has been used effectively to engineer a vaccine against hepatitis and may provide a new avenue for the development of protective agents against pathogens that have thus far eluded traditional vaccine efforts, including HIV and ebola.

“We had developed a method for screening for protective subunits against a specific disease,” Sykes says. “However this type of safer vaccine design is notoriously less potent than the whole pathogen designs. What we needed was a method to find  generally useful vaccine components that would serve to enhance and control immunity.” 

The group chose the pathogen parapoxvirus ovis (known as the Orf virus) for the current set of experiments, in which expression library immunization techniques were used to screen for an immunogenic factor buried in the pathogen’s genome.

Parapoxvirus ovis causes a highly infectious disease known as Orf, which is prevalent in sheep and goats and may be transmitted cutaneously to humans handling these animals, causing pustular lesions and scabs.

Once the group had sequenced the full genome of parapoxvirus, PCR was used to amplify all the viral open reading frames, which code for all of the viruse’s proteins. Each ORF, comprising a library of genomic components, was compiled into a unique high throughput expression construct, and these were randomly distributed into sub-library pools. These pools were directly delivered into sets of mice for in vivo expression. Functional testing for the activity desired identified B2 as the immune cell accumulator.

In further experiments, the team co-delivered B2L as an additive or adjuvant for an influenza gene vaccine, to see if it could improve survival rates in mice challenged with the influenza virus. The co-immunized mice indeed displayed full protection against influenza compared with 50 percent protection of the control group, immunized with influenza vaccine alone.

In addition to infectious agents like Orf, non-infectious diseases including cancer may be amenable to vaccine defense. Thus far however, the discovery of tumor-specific antigens has been frustrating. One approach may lie in using non-specific immunogenic factors like B2.

In the current study, two forms of cancer were investigated in a mouse model, following the administering of B2 alone, in the absence of a disease antigen. The experiments evaluated B2’s ability to enhance survival and shrink tumor size. In the case of an aggressive melanoma, tumor size was significantly reduced and survival rate improved.  Administration of B2 to an infection induced by a breast cancer cell line also showed a modest but measureable reduction in tumor size.

With the growing popularity of sub-unit vaccines, the need arises for more effective adjuvants, which may be used to compensate for the reduced immunogenicity of such vaccines compared with their whole-pathogen counterparts. Techniques similar to those applied here to isolate and evaluate B2 could potentially permit the screening of virtually any genome for any gene-encoded activity testable in an organism.

— Michael J McGuire et al., “Novel immune-modulator identified by a rapid, functional screen of the parapoxvirus ovis (Orf virus) genome,” Proteome Science 2012 10, no. 4 (13 January 2012) (doi:10.1186/1477-5956-10-4)