A paper in the Sept 4, 2009 issue of Science describes not only two broadly neutralizing antibodies to HIV, but also a strategy to discover more of them – both of which give new hope to the quest for an AIDS vaccine.
Because the AIDS virus mutates rapidly, an effective vaccine must induce antibodies that are broadly neutralizing, meaning that they are effective against many different types of HIV strains. And those broadly neutralizing antibodies must be effective against so-called non-clade B variant of the virus – the type of infection that is most prevalent outside of the U.S. and Europe.
It might not seem like much, but the challenges to finding such antibodies have turned out to be formidable: only four such antibodies had been described prior to the report in last week's Science. And those four – each of which was discovered more than a decade ago – came from people infected with versions of HIV primarily found in Australia, the U.S. and Europe, which collectively account for fewer than 5% of global new infections.
The new discoveries, Wayne Koff told Medical Device Daily's sister publication, BioWorld Today, were made possible by a combination of several factors. The scientists began with the establishment of "partnered research centers" in the developing world that allowed screening of comparatively large numbers of donors – roughly 1,800 – in Asia and Africa, where most new infections occur and where vaccines are needed the most. Koff is senior VP of research and development at the nonprofit International Aids Vaccine Initiative (IAVI), which led the current efforts.
IAVI combined this donor screening, Koff said, with identifying companies that were not currently engaged in vaccine research, but have technologies for advancing the way such research is done. The work now published in Science was jointly performed by researchers from IAVI, the Scripps Research Institute, the Ragon Institute of Massachusetts General Hospital (Boston), and two such companies: Monogram Biosciences (South San Francisco), and Theraclone Sciences (Seattle).
The investigators identified the two antibodies by using a new assay, a microneutralization assay, which was jointly developed by Monogram and IAVI. The microneutralization assay, Koff said, consists essentially of "mixing serum and virus, incubating it, and seeing whether it can infect cells. If it can't, it means that that sample has neutralizing antibodies." How potent those antibodies are can be further studied by repeatedly diluting and testing the serum.
After identifying sera that contained potent broadly neutralizing antibodies, the researchers proceeded to identify the specific antibody using high-throughput technology developed by another startup, Theraclone.
Getting a single antibody out of the serum, Theraclone CEO David Fanning told BioWorld Today, was like getting the proverbial "needle out of a haystack." Theraclone's technology is able to do two things: first, to isolate the specific B cells that are producing the antibody of interest from the tens of millions of antibodies that are in any one serum sample; and secondly, to "trick those cells into producing enough antibody in vitro" to use in several assays, enabling a thorough assessment of its potential.
In combination, the approach yielded two new antibodies that are both broadly neutralizing and highly potent, both from the same African donor. The authors write that the two antibodies, PG9 and PG16, "demonstrated a remarkable combination of neutralization breadth and potency. PG9 neutralized 127 out of 162 and PG16 119 out of 162 viruses with a potency that frequently exceeded that noted for the four control" broadly neutralizing antibodies.
In further studies, researchers found that the antibodies bind to conserved regions of the viral spike, which is formed by the proteins gp41 and gp120 and which HIV uses to enter cells. However, the epitope, or specific part of the virus recognized by the virus, spans several different regions of gp120 – parts of the proteins that are distant from each other in its two-dimensional amino acid sequence, but close together once the protein is folded up.
Koff said that the team is currently working on obtaining the crystal structure of the epitope, and plans to "move rapidly into the immunogen design phase." They also continue to use the strategy to search for other broadly neutralizing antibodies.
More generally, Koff thinks the findings will change the strategy of looking for antibodies. "When we searched for antibodies in the past," he said, "we, as a field, did a lot of binding assays." That approach has not yielded any new broadly neutralizing antibodies in more than a decade – and it would not have identified either PG9 or PG16, both of which bound poorly to soluble gp41 and gp120.
Christos Petropoulos, chief scientific officer and vice president of virology research and development at Monogram Biosciences, said in a prepared statement that if you think of the search for broadly neutralizing antibodies "as a fishing expedition ... we and the rest of the field were previously using the wrong bait."