Evaluation of new strategies to elicit neutralizing antibodies against HIV-1

Abstract

The development of an effective HIV-1 vaccine must be considered one of the greatest biomedical challenges of today. The nature of the virus, characterized by immense variability as well as its tropism for the essential CD4+ T cells of the immune system, allows it to establish a chronic infection for years under constant immune selection pressure. Eventually the depletion of CD4+ T cells is so severe that the immune system fails and the host succumbs to AIDS. One of the most variable components of the virus is the external envelope glycoproteins, which mediate receptor binding and fusion. As the sole virally derived components facing the outside of the viral membrane, the envelope glycoproteins also represent the only target for neutralizing antibodies.

Under the scope of this thesis we evaluate novel strategies to elicit neutralizing antibodies against HIV-1, using different envelope glycoprotein-based immunogens and different modes of delivery. By biochemical methods we characterize vaccine candidates for their structural and antigenic properties. In addition, we also investigate the ability of these candidates, administered by selected regimens, to induce immune responses against the HIV-1 envelope glycoproteins in small animals (paper I) and in non-human primates (paper III).

One of the major findings of this thesis is that the priming agent in combined prime-boost regimens can be utilized to shape the immune response in terms of specificity and T-helper bias of the CD4+ T cells without apparent detriment for the elicited neutralizing antibody response. In addition, we design a novel recombinant Semliki Forest virus vector, allowing for a ten-fold increase of HIV-1 envelope glycoprotein-secretion, as compared with the parental vector (paper II). We also show that the increased secretion is translated into increased immunogenicity in mice. This vector may be useful for future HIV-1 envelope glycoprotein vaccine studies.

Finally we define the mechanism by which antibodies against the co-receptor binding site of the HIV-1 envelope glycoproteins are elicited. In a set of controlled experiments in non-human primates, rabbits and transgenic rabbits we definitively conclude that the in vivo presence of primate CD4 is critical for the elicitation such antibodies. This thesis provides new fundamental information of relevance to both natural HIV-1 infection and the development of HIV-1 envelope glycoprotein-based vaccines.