Currently 33 million people are living with human immunodeficiency virus type 1 (HIV-1) worldwide. In 2009 2.6 million people became newly infected and 1.8 million people died in the course of AIDS. During the last decades several efforts to induce HIV-1 defending neutralizing antibodies (Abs) have failed but also promising results were reported. One of the most potent neutralizing HIV-1 Abs isolated so far is the monoclonal Ab (mAb) 2F5 which binds to the membrane proximal external region (MPER) of the virus envelope glycoprotein gp41. The potency of such neutralizing Abs alone and in combination was demonstrated by passive immunization and viral challenge in non-human primate models.
Therefore the specific induction of likewise broadly neutralizing Abs against the MPER, 2F5-like Abs, is a major goal for Ab-based HIV-1 vaccine strategies. Despite a strong humoral response to gp41 during the course of HIV-1 infection is evident, approaches to elicit cross-clade neutralizing Abs against the MPER region were difficult to achieve.
An alternative method to induce neutralizing Abs is the anti-idiotypic (Id) approach. This approach is based on the idiotype network theory postulated by Jerne about the Ab (Ab1) - anti-Id Ab (Ab2) – anti-anti-Id Ab (Ab3) cascade stimulation, whereby specific anti-Id Abs can serve as an "internal image" of the target antigen and can be used to induce Ab3s that can bind to the cognate antigen. Anti-Id Abs have been proposed as vaccines for cancer immunotherapy and significant success has been achieved using anti-Id vaccines mimicking tumor-associated antigens in animal studies as well as in clinical trails. The anti-Id Ab Ab2/3H6 was developed at the Department of Biotechnology and is directed against mAb 2F5. The chimeric as well as humanized version of Ab2/3H6 significantly inhibits the binding of mAb 2F5 to its synthetic epitope ELDKWA in an equimolar ratio and also decreases the in vitro neutralization potency of mAb 2F5 in a dose-related manner. Ab2/3H6 is therefore estimated to mimic the epitope of mAb 2F5 and would be of great therapeutic interest as an anti-Id HIV-1 vaccine. To improve the potency of the anti-Id Ab we designed fusion proteins consisting of Ab2/3H6 Ab fragments (Ab2/3H6Fab) and C-terminally attached polypeptides to induce T-cell responses against the virus.
One molecule with a wide range of biological activities is the immune stimulatory cytokine interleukin 15 (IL15). It is involved in the activation and proliferation of CD8+ T-cells and natural killer T-cells, the maintenance of CD8+ memory cells, and the differentiation and maturation of B cells. Previous studies have shown that the incorporation of IL15 into vaccinia-based smallpox vaccine or tuberculosis vaccine induces high avidity, long lived antigen specific memory T-cells as well as persistent antigen specific Ab responses.
Other interesting immunostimulatory peptides are the so-called “promiscuous” T-cell epitopes from tetanus toxin (TT), measles virus, or E6 transforming protein. It has been proposed that T-cells provide “help” to B cells under genetic control which can be provided by incorporation into an effective vaccine. Previous studies showed that co-immunization of the consensus cavealin-1 binding domain peptide with the T-cell epitope from TT increased the production of HIV-1 neutralizing Abs in a macaque prime/boost study.
Therefore we recombinantly expressed fusion proteins of Ab2/3H6Fabs with the IL15 and alternatively an epitope of TT, respectively. In this study we immunized rabbits with the different Ab2/3H6Fabs and evaluated the humoral immune response as well as the neutralization potency of the obtained Ab3s to those generated with Ab2/3H6Fab only.