STUDIES ON PROCESSING, PARTICLE FORMATION, AND IMMUNOGENICITY OF THE HIV-1 GAG GENE-PRODUCT - A POSSIBLE COMPONENT OF A HIV VACCINE

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 55 gag 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 55 gag was expressed in combination with the protease (p 55-PR) or with the entire polymerase (p55-pol), respectively. Budding of 100-160 nm p55 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 of Spodoptera frugiperda (Sf 9) insect cells with the recombinant Autographa 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 55 gag 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 in Escherichia 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 entire pol gene to the gag gene. Moreover, processing was not found to be dependent on amino-terminal myristylation of the gag 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 55 gag particles represent a noninfectious and attractive candidate for a basic vaccine component.