Variations in individuals' immune systems can dramatically affect responses to infection—HIV is no exception. The result generally can be shown as a bell curve, with a group of people whose disease progresses rapidly, a broad middle segment who progress typically, and a small group of "elite controllers" whose immune systems are quite effective at containing HIV viral replication.
The quest to figure out why has focused primarily on the adaptive immune system, because CD4+ and CD8+ T cells have a clearly demonstrated capacity to kill cells infected with HIV. But that response only arises some days, weeks and even months after a person has been exposed to HIV and the virus has integrated itself into cellular DNA, establishing lifelong infection. The adaptive immune response can only contain an established infection, it cannot prevent that infection from occurring at its onset.
B cells are the first line of defense against infection. They attack at the initial exposure to a pathogen, and can prevent the establishment of infection—and HIV is no exception. But there are a number of reasons why it has proved difficult to identify their contribution to neutralizing the deadly virus.
HIV transmission is not very efficient. Exposed persons may avoid infection for a variety of mechanical (barrier) and biological reasons, such as the virus's failure to penetrate to the surface of mucosal tissue or dendritic cell difficulties in latching onto the virus to carry it to a lymph node. So it is challenging to conclusively identify the contribution of a specific immune response that can prevent an initial infection.
Over the years, it has become clear that there are factors other than CD4+ and CD8+ T cells that help to control the virus in at least a portion of those infected with HIV.
Researchers have identified several antibodies that can neutralize the virus. Most of them bind weakly to small, often deep, pockets on the virus. In most instances, once infection becomes established rapidly mutating HIV evolves resistance to those narrowly focused antibodies, often by adding glycans or sugars to its outer envelope, which shields or blocks antibody access to the binding site.
What is needed is an antibody that binds strongly to a surface site on the virus, and which cannot be easily blocked. It is also important that the binding site is greatly conserved across the many strains of HIV.
Researchers at NIH Vaccine Research Center (VRC) decided to look at neutralizing antibodies in the blood of persons who are able to better control HIV infection. Elite controllers were not part of the mix because they seem to control HIV through their adaptive immunological system T cell mechanisms.
Using sophisticated reverse-engineering techniques, the researchers identified three proteins that are broadly neutralizing, which they labeled VRC01, VRC02 and VRC03. They also isolated the B cells that produced them.
The first two antibodies have very similar chemical structures and bind to HIV's gp120 trimer spike on its surface. The virus uses the trimer to link up with a CD4 receptor, which is the first of many steps taken to enter and infect a host cell. The antibody and gp120 spike bind in a way that is, in part, similar to the way that the spike and CD4 receptor bind.
As a result, VRC01 and VRC02 binding is particularly long and strong compared with the bonds formed by other antibodies. Further, the binding site on the gp120 spike is well exposed and not likely to become blocked by the addition of sugars to the viral envelope.
The two antibodies neutralized 91 percent of the 190 different HIV isolates that the team tested. Those isolates represent all of the various clades or strains of HIV present worldwide, says John Mascola, one of the VRC research team leaders. Also, the antibodies were able to neutralize all of the limited number of HIV variants that are transmitted sexually—a key point, because 80 percent of all new infections result from sexual activity.
VRC01 and VRC02 occur naturally and are produced by what are called RSC3 memory-specific B cells, an extremely rare component of the immune system. Using flow cytometry, the NIH team could isolate only 29 of those cells from among the 25 million cells that they screened. Furthermore, the proteins produced by those B cells often are immature and it appears that the proteins must undergo a series of combinations before they become functional VRC01 or VRC02.
X-ray crystallography allowed the researchers "to identify the [antibody's] binding site down to the atomic level structure…. It is a particularly invariant part of the CD4 binding site, which is exposed," Mascola says. He calls that knowledge "a blueprint from which to design new vaccines. It allows us to try to design a protein that mimics and presents that specific site to the immune system" to stimulate B cells "to crank out the antibody."
Mascola acknowledges the complex nature of the VRC01 and VRC02 antibodies and their low naturally occurring numbers may prove to be an obstacle to developing a vaccine. It is too early to understand all of the issues surrounding the stimulation of antibody production and the concentration necessary to afford protection from infection.
The VRC research team has designed vaccine antigens that already are in preclinical study in small animals. If those prove successful, the work may advance into a monkey model, although it is not completely clear how monkeys can control the simian version of HIV and not progress to advanced disease. The identification of VRC01 and VRC02 may also help to advance a better understanding of the disease in monkeys.
Mascola says these discoveries also may lead to development of a "therapeutic vaccine" or immune-based therapy that helps train the immune system of an HIV-infected person to better control the virus without the use of drugs.
It may be possible to mass-produce these antibodies for passive administration as an adjunct or substitute for current small molecule drugs used to treat HIV. And if production costs can be reduced sufficiently there may be a role for them in topical microbicides as a preventative for HIV exposure.