HIV Antibody Breakthrough in Transgenic Mice Could Prove Boon to Vaccine Effort
Formaldehyde frozen fusion
Recombinant HIV envelope vaccines to date have demonstrated the ability to elicit neutralizing antibodies to HIV strains from which they were constructed (as well as against selected laboratory adapted strains), but only rarely have they resulted in the production of effective antibodies against HIV actually isolated from HIV-infected individuals (“primary isolates”). Furthermore, these highly engineered vaccines have, for the most part, been constructed from the syncytia-inducing (“SI”) type of HIV rather than the non-syncytia-inducing (“NSI”) HIV which is believed to be transmitted from person to person-largely because SI HIV is much easier to grow in the lab than is NSI.
These lab viruses also “look” different to the immune system than the viruses derived from HIV-infected individuals. Primary isolates seem to be much better at hiding their neutralizing sites from antibodies by adopting a shape in which the vulnerable antigen sites are buried within the internal folds of the protein complex. Researchers from the University of Montana have applied a creative approach to freeze-framing these crucial viral epitopes during the brief time they are exposed to the immune system as the virus fuses with the host cell, Results in mice (first presented at the Keystone meeting earlier this year) are quite exciting. The next step will be to test a similar approach in monkeys. Gregg Gonsalves reports.
What do we want an HIV vaccine to do? In general, vaccines prevent or abort infections from invading organisms by preparing the immune system to repulse a real attack through a practice exercise in which harmless portions or deactivated whole bits of the offending agent are offered up for the killing. When the true enemy arrives, the immune system has been primed to respond to the interloper and replies with a quick and vigorous defense. We would like any candidate HIV vaccine to be able to gin up the immune response to the virus, so that when a real infection is in progress, it is quickly repulsed.
The two major immune responses that are likely to be important in protecting against infection with HIV are the humoral (or antibody) response, and the cellular (or cytotoxic T-cell response). A strong antibody response might be important in neutralizing virions which float freely through the various fluids that transmit HIV before they have a chance to infect their first CD4+ T-cell or macrophage. A strong cytotoxic T-cell response might be an important weapon in rooting out cells just recently infected with HIV.
One of the key disappointments about the current crop of preventive HIV vaccines is that they are unable to elicit potent neutralizing antibody responses to strains of the virus found in HIV-infected people (“primary isolates”). One of the reasons for their failure in this regard may be the difference between the genetically engineered viral envelope used in these vaccines and the real world viral envelope found in nature. The shape of the recombinant protein is so different from the form of envelope that is present on strains of virus circulating in HIV-infected individuals that an immune response to the recombinant imitation does not generate a response to the real McCoy.
Neutralizing HIV, in any case, is a difficult project. Even antibodies from HIV-infected individuals are generally unsuccessful in neutralizing the virus, although there are some notable exceptions. HIV is a tricky foe. In its natural state, the envelope of the virus is covered in sugary glycoproteins which allow it to stealthily evade recognition by the immune system. Furthermore, the parts of the envelope which might evoke a strong antibody response are buried beneath these sugary molecules and in otherwise inaccessible crevices within the envelope’s spikes.
It would be a great advance if someone could design a vaccine that had the ability to generate neutralizing antibodies to a broad panel of viruses found in HIV-infected individuals. An important step towards this goal was made this winter by Jack Nunberg and his colleagues at the University of Montana (Fusion-Competent Vaccines: Broad Neutralization of Primary Isolates of HIV Rachel A. LaCasse, Kathryn E. Follis, Meg Trahey, John D. Scarborough, Dan R. Littman, and Jack H. Nunberg Science 1999 January 15; 283: 357-362). In work supported by the American Foundation for AIDS Research (and shamefully passed over for funding by the National Institutes of Health), Nunberg was able to devise an immunogen that neutralized 23 out of 24 strains of HIV representing viruses from HIV-infected individuals from North America, Europe, Africa, India and Thailand.
While antibodies directed at recombinant envelope proteins cannot neutralize strains of the virus from HIV-infected individuals, on a few occasions, antibodies from HIV-infected individuals themselves can do the trick. Nunberg and his colleagues, working from these exceptional cases, surmised that the manner in which the viral envelope is presented to the immune system in a natural infection may make the difference in generating these rare, effective neutralizing responses. Unlike the recombinant envelope protein which exists in a static, nonfunctioning state, the viral envelope in HIV-infected individuals undergoes many different changes in its shape as it interacts with its target cell.
The first change in the shape of the envelope occurs when it binds to the CD4 molecule on a T-cell. The envelope shifts its shape at that point to bring it into closer proximity with the other molecules — generally the CC chemokine receptor 5 (CCR5) or the CXC chemokine receptor 4 (CXCR4) — it needs to facilitate entry into its target lymphocyte. Then the envelope changes shape again, pulling together the membranes of the virus and the host cell where they can fuse and allow for the entry of the virus into the cell.
Nunberg speculated that these transitional forms of the envelope protein might be more susceptible to attack by antibodies — either by exposing previously hidden targets or offering new parts of the envelope for neutralization. Nunberg’s team then crafted what they have called a fusion-competent (FC) immunogen by taking cells expressing an envelope protein from a T-cell tropic strain of virus from an HIV-infected person in a cohort study in Amsterdam and another set of cells expressing both the CD4 and CCR5 coreceptor, letting them begin to fuse together and then freezing them in this transitional state with formaldehyde.
These scientists from Montana then tested the ability of the FC immunogen to generate neutralizing antibody responses to two dozen different strains of HIV from HIV-infected individuals with amazing success. The antibodies to the FC immunogen were raised in mice which had been genetically engineered to express human CD4 and the CCR5 coreceptor. (Nunberg’s team took this step to ensure that the antibodies that they would be testing for virus neutralization weren’t simply reacting against the CD4 or CCR5 molecules and thereby interfering with viral fusion and entry by blocking these receptors.)
In fact, Nunberg and his associates took several other steps to rule out the possibility that antibody responses to cellular proteins were responsible for the effectiveness of their FC immunogen. Nunberg’s team also made sure that antibodies to their immunogen were not providing some non-specific protection against viral infection by trying, unsuccessfully, to neutralize SIV and an HIV virus which had been engineered to express the envelope of the murine leukemia virus (MLV).
In a commentary on Nunberg’s Science paper, neutralization gurus David Montefiore from Duke University and John Moore from the Aaron Diamond AIDS Research Center here in New York, while charmed by the Rocky Mountain researchers’ work, still weren’t satisfied that antibodies to cellular targets weren’t involved in the antiviral effect seen in the study. They were particularly concerned about the potential antiviral role of antibodies directed at new targets induced by changes in the cells expressing CD4 and CCR5 during fusion or changes in the shape of the CD4 and CCR5 molecules themselves during this process. (see “HIV Vaccines: Magic of the Occult?” David C. Montefiori and John P. Moore Science 1999 283: 336-337. (in Perspectives)).
More work on FC immunogens now has to be carried out in primates to follow up on Nunberg’s pioneering success. Research in macaques will help to define the true targets of the antibodies raised by FC immunogens. Challenge experiments will then be necessary in order to assess the utility of this vaccine concept for testing in humans. While there are concerns about the safety of vaccines based on tumor cell lines and substantial difficulties in production of cell-based immunogens, there may also be ways to mimic Nunberg’s success by co-administering recombinant viral vectors respectively expressing envelope and CD4/CCR5 or subunits vaccines that offer up the specific protein complexes that are targeted by Nunberg’s antibodies.
Until now, we haven’t had an immunogen that could neutralize strains of HIV from HIV-infected individuals — let alone divergent strains of the virus from all over the globe. While the crop of current vaccine candidates looks gloomy, there are indeed rays of hope on the horizon.
Status of Current HIV-1 Vaccine Strategies
|Safety Concerns||Potency of Elicited Immune Responses||Other Undefined Protective Mechanisms|
|Live attenuated virus||Yes||Poor||Emerging evidence||Some evidence|
|Killed virus||Limited||Poor||Negligible||No evidence|
|Envelope subunits||None||Poor||Negligible||No evidence|
|Vaccinia or avipox prime/boost||None||Poor||Weak||No evidence|
|DNA prime/boost||Limited||Poor||Weak||No evidence|
|Source: Nature Medicine (vaccine supplement), May 1998|