HIV-1, how llamas help us fight the AIDS pandemic

Human Immunodeficiency Virus type 1 (HIV-1) is one of the major health problems worldwide and has been for over thirty years. Most (67%) of the people infected with HIV-1 are living in sub-Saharan Africa. Here, the access to treatments is limited and most women are not in a position to protect themselves from getting infected. The aim of this study was to identify VHH that are both broad and potent HIV-1 neutralizers, which can be used in the fight against HIV-1. A VHH is an antibody fragment containing the antigen binding domain derived from a heavy chain only antibody. This special type of antibody can be found in members of the camel family. The advantage of VHH is that they are relatively small, stable, cheap to produce and easy to work with. In order to raise VHH against HIV-1, two llamas were immunized with a mix of its envelope proteins. After cloning of the llama’s VHH repertoire into a phagemid expression system, multiple different strategies were followed to identify neutralizing VHH. In total, twelve families of neutralizing VHH were found and characterized in more detail. These VHH target four independent areas on the HIV-1 envelope protein. The best VHH of them being J3, which neutralized 96 out of a 100 tested viruses with a median IC50 of 0.9 µg/ml and is targeting the CD4 binding site. The CD4 binding site on the envelope protein is considered an important area as the first interaction between virus and its target cell occurs via CD4. Previous studies have shown that linking two VHH together into so-called biheads may increase their breadth and potency. For this reason we have linked two different VHH, targeting independent epitopes, together to form bi-specific biheads. Several combinations of VHH have been tried, the best (J3 linked to 2E7) yielded upto 1400 times increase in potency compared to a mix of the individual VHH. Another bihead (J3 linked to 1F10) was capable of neutralizing a viral strain J3 alone could not neutralize, possibly improving the breadth of this molecule above 96%.