Stabilised viral fusion proteins
Viral fusion proteins mediate the fusion of the viral envelope with host cell membranes, allowing viruses to enter host cells. Upon contact with host cells, fusion proteins transition from a higher energy pre-fusion confirmation to a lower energy post-fusion conformation. Recombinant production of fusion proteins favours production in the post-fusion conformation, especially in the absence of the cell membrane. However, epitopes which stimulate the production of neutralising antibodies are often contained within the pre-fusion confirmation and are absent in the post-fusion conformation. Immunisation using a protein in the pre-confirmation is clinically desirable, and by stabilising viral proteins in this conformation on the surface of virions the protein’s efficacy as a vaccine can be improved.
Viral fusion proteins
Viral fusion proteins are classified into Classes I, II or III. Stabilisation of several Class I proteins in the pre-fusion conformation has been reported. However, due to structural differences between Class I and Class III fusion proteins, methods for stabilising Class I proteins cannot be extrapolated to Class III proteins.
To date, only one pre-fusion structure of a Class III fusion protein has been described, and there are no published reports of the successful design of stabilised pre-fusion configurations of any Class III fusion protein in a format suitable for vaccination.
Clinical significance of Class III fusion proteins
Class III fusion proteins are expressed on clinically significant viruses and represent important vaccine immunogens. Class III fusion proteins of the herpesvirus family and the rabies virus glycoprotein (RVG) are the targets of neutralising antibodies with a protective effect. However, some of these antibodies are known to bind only under neutral pH conditions (when protein is in the pre-fusion conformation) and lose binding under acidic pH (when protein adapts post-fusion conformation).
Therefore, there is a need for stabilised forms of the pre-fusion conformation of Class III fusion proteins which can be recombinantly expressed, displaying pre-fusion conformation epitopes capable of stimulating a neutralising antibody response.
Researchers at Oxford University have identified a region, common to all Class III fusion proteins, that when mutated prevents Class III proteins from transitioning from the pre-fusion to post-fusion conformation. The stabilised pre-fusion conformation Class III fusion proteins retain and display pre-fusion conformation-specific epitopes found on the corresponding wildtype protein, and have been recombinantly expressed successfully. By stabilising the pre-fusion conformation, the invention allows enhanced yields of proteins bearing pre-fusion conformation-specific epitopes which elicit the production of neutralising antibodies in-vivo. This enhanced yield, together with long term stability and antigen quality is advantageous for vaccine production.
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