Studies on processing, particle formation, and immunogenicity of the HIV-1gag gene product: a possible component of a HIV vaccine

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Abstract
Antigens in a particulate conformation were shown to be highly immunogenic in mammals. For this reason, the particle forming capacity of derivatives of the HIV-1 group specific core antigen p 55gag was assayed and compared dependent on various expression systems: recombinant bacteria, vaccinia-and baculoviruses were established encoding the entire core protein p 55 either in its authentic sequence or lacking the myristylation consensus signal. Moreover, p 55gag was expressed in combination with the protease (p 55-PR) or with the entire polymerase (p 55-pol), respectively. Budding of 100–160 nm p 55 core particles, resembling immature HIV-virions, was observed in the eucaryotic expression systems only. In comparison to the vaccinia virus driven expression of p 55 in mammalian cells, considerably higher yields of particulate core antigen were obtained by infection ofSpodoptera frugiperda (Sf9) insect cells with the recombinantAutographa californica nuclear polyhedrosis (AcMNPV) baculovirus. Mutation of the NH2-terminal myristylation signal sequence prevented budding of the immature core particles. Expression of the HIV p 55-PR gene construct by recombinant baculovirus resulted in complete processing of the p 55gag precursor molecule in this system. The introduction of an artificial frameshift near the natural frameshift site resulted in constitutive expression of the viral protease and complete processing of p 55, both inEscherichia coli and in vaccinia virus infected cells. Interestingly, significant processing of p 55 resembling that of HIV infected H 9 cells could also be achieved in the vaccinia system by fusing the entirepol gene to thegag gene. Moreover, processing was not found to be dependent on amino-terminal myristylation of thegag procursor molecule, which is in contrast to observations with type C and type D retrovirus. However, complete processing of p 55 into p 24, p 17, p 9 and p 6 abolished particle formation. Purified immature HIV-virus like particles were highly immunogenic in rabbits, leading to a strong humoral immune response after immunization. Empty immature p 55gag particles represent a noninfectious and attractive candidate for a basic vaccine component.