Spotlight on antibodies in Boston HIV vaccine sessions

Two sessions at the recent Retrovirus conference in Boston focussed on designing vaccines to induce broadly neutralising antibodies. Four oral presentations reviewed what is now understood about those antibodies, which appear to be directed against strongly conserved parts of the outer proteins of HIV-1, namely gp120 and gp41.

Susan Zolla-Pazner, from the Veterans Affairs Medical Center and New York University, spoke on Wednesday about broadly-neutralising antibodies, of which she reported that there are now between 15 and 20 such antibodies described, which were directed against at least six different regions or conformations of the virus. She expected more antibodies to be found, and more targets identified, as research continued.

In particular, she discussed work on broadly neutralising antibodies which are directed at the V3 loop of the virus.

The V in V3 stands for ‘variable’ and while early vaccine research showed that neutralising antibodies are directed against this part of gp120, it also found that small changes allow the virus to escape from this immune response. Most V3 antibodies are highly strain-specific. This applies particularly to antibodies against short sections of the V3 loop (linear peptides) which are therefore generally considered worthless as vaccines.

Zolla-Pazner presented research which had looked at particular monoclonal antibodies that bind to V3 but do not seem so limited in the number of strains they recognise. She argued that since the V3 loop is involved in the virus-cell interaction, it must in fact have some limits to the shapes it can take, with active sites, at the tip and at the base of the loop, which cannot radically change. Antibodies which recognise these sites – especially those which mimic the cell structures that the virus interacts with - could therefore offer clues to designing more effective vaccine candidates. The fact that such antibodies can be found in some people with HIV shows there is no problem for the immune system in telling the antibody apart from the target cell’s own structure.

On the Friday morning, a panel of three presenters extended this review to other regions of the virus.

Dennis Burton, from the Scripps Institute of La Jolla, California, leading a group that is trying to reverse-engineer vaccines from known broadly-neutralising antibodies, gave a progress report on two lead projects in what he now calls ‘retrovaccinology’. At present, the emphasis is on understanding exactly how these antibodies interact with HIV, although in one case this has led to modified versions of gp120 that can be tested as antigens.

An antibody called b12, described at the previous Retrovirus conference, has a finger-like structure that binds inside a shielded part of gp120. The virus-antibody interaction has been studied in detail and the structures involved are well understood. But how can antibodies like this be generated?

Burton's group have modified gp120 by adding sugar groups to those parts of the surface of the protein which are not related to the antibody. The idea is, to block the formation of anything other than the desired antibodies. The group has shown that they can produce a gp120 to which b12 will still bind, but other antibodies generally will not. They are now immunising small animals with this, to find out if they generate b12-like antibodies.

A second antibody, called 2G12, binds to mannose sugars on a surface of HIV that doesn’t generally stimulate immune responses. This binding can therefore be blocked by mannose, which is not the case for other neutralising antibodies so far tested. The antibody has a structure that has never been seen before, in which the normal arrangement of the main protein chains is switched, and the antibody forms dimers (interlinked pairs of molecules) with three reaction sites that correspond to specific locations of three mannose residues on the surface of many strains of gp120.

It is thought 2G12 could work as a coating on the vaccine particle, protecting vulnerable cells. A vaccine to induce such antibodies would need to present mannose residues to mimic their orientation on the gp120 surface. It is still not clear how likely it is, that similar antibodies could be produced. (Raising antibodies against non-proteins is quite possible, but they need to be shown to the immune system with a protein that generates CD4 T-cell responses, to unlock antibody production.)

Carol Weiss, a vaccine researcher with the US Food and Drug Administration, reported on the development of vaccines based on the regions of the gp41 molecule that make up the entry inhibitor T20 (enfuvirtide) and its active site. These regions of gp41 are well-conserved between different HIV strains. They are mainly exposed during the process of binding between virus and cell, when the two are close together, leaving little room for antibodies to get in and do their work. (It is also possible that they remain on the cell surface, following viral entry.) However, the fact that T20 works at all shows there may be enough space and time for an immune response to operate, but measuring this in the lab is difficult. The assay system needs to copy the interaction between virus and cell in which the key parts of gp41 are exposed; chilling a cell-culture system to 31 degs C seems to help.

The most promising vaccine candidate takes the 'T20' peptide region of gp41 and combines it with the region of gp41 to which the drug T-20 (enfuvirtide) binds. In rabbits this produces antibodies that neutralise HIV in the specialised test-system just described. These are now being studied in more detail, to make monoclonal antibodies which can then be tested against different HIV-1 strains. If the concept works, further development may still be needed to stabilise the vaccine candidate and generate higher levels of antibodies before phase I trials in people. As a subunit vaccine related to a well-tested drug, it is unlikely to raise toxicity concerns and could progress rapidly into trials.

Joseph Sodrowski, of the Dana-Farber Cancer Institute in Boston, led the group that first described the way gp120 binds first to CD4 and then, after changing shape, to its co-receptor targets CCR5 or CXCR4. At this meeting, he discussed a further target region on gp120, namely the binding site for the CCR5 receptor. This is exposed only when gp120 has bound to the CD4 molecule on the surface of a target cell, which means that the virus is very close indeed to the cell surface. Here, the space for an antibody to get in between the virus and the cell seems to be more limited than in the case of antibodies against gp41. While fragments of the antibodies including the binding site are effective in cell-culture systems, there must be serious doubt about how useful this will be as a vaccine strategy.