Concepts in DNA immunization overcoming viral diversity and enhancing plasmid immunogenicity

Abstract

On April 23, 1984, the prominent scientist Robert Gallo held a historical press conference at the Department of Health and Human Services, Washington D.C., USA. He announced that his laboratory at the National Institutes of Health had over the last months isolated a retrovirus named Human T-cell Leukemia Virus type III (HTLV-III). The virus came from 48 patients in the homosexual community in San Francisco. The city had just been hit by the mysterious epidemic of acquired immunodeficiency syndrome (AIDS). HTLV-111 was later renamed human immunodeficiency virus (HIV). At the same meeting, Gallo further explained that his laboratory was able to grow large quantities of the virus in cell cultures and as a consequence it was stated that "we believe that the new process will enable us to develop a vaccine to prevent AIDS in the future ... we hope to have such a vaccine ready for testing in approximately two years."

This thesis is printed on the very same day, twenty years later and the now mature field of HIV/AIDS vaccine development has still not discovered what exactly mediates protection against HIV infection, while we are still far away from a clinically useful vaccine. Why is this? What makes HIV so special when other virus diseases, like polio, can be recognized and eliminated by the immune system, and where vaccination works very well?

The focal points of this thesis are two major problems in modem vaccine development. Many viruses exist in multiple subtypes or serotypes, a phenomenon that has serious implications for the choice of vaccine antigen. It is especially critical when trying to vaccinate against HIV, the surface structure (gpl20) of which presents immense antigenic variability. Moreover, modem genetic vaccines based on smaller units of the virus or consisting of multiple genes (combination genetic vaccine) are weak immunogens, this is the second problem that this thesis explores.

More specifically, we have shown that removal of inhibitory elements in a DNA immunogen is of importance for efficient induction of immunity. Further, antibody responses to a DNA immunogen can be substantially enhanced if the genetic immunogen is coupled to a carrier, in our case the polyomavirus VP1 capsid. In combination with generally immunoactivating agents, for instance recombinant granulocyte macrophage colony stimulating factor (GM-CSF), the HIV envelope genes from multiple subtypes (A, B and C) can change the envelope DNA immunogen into a potent entity that induces high titers of broadly reactive antibodies as well as cellular responses. We also show that immunization with proteins followed by DNA immunogens, a strategy tentatively called "reverse prime-boost immunization" induces strong immunity. These findings will be further validated in human clinical trials within the near future. Last but not least, we have developed an HIV murine challenge model based on pseudotyped viral particles; combining the HIV genome and the murine leukemia virus (HIV/MuLV) envelope. This model resembles human acute primary HIV infection. Protection in this model can be ascribed to cellular immunity, in the complete absence of antibodies. Using this model, we have shown that primeboost immunization induces better protection against subtype homologous HIV challenge, than against heterologous exposure.

The immunization strategies covered in this thesis describe the biological problems that face vaccine development in general and HIV vaccinology in particular. The problems and concepts illustrate why the statement by several scientists in the 1980s has proven to be somewhat premature.