Evaluating New Approaches to Developing AIDS Vaccines by Creating Virus-like Particles that Display HIV Antigens or by Using Live, Attenuated (Weakened), non-HIV Viruses to Express HIV Proteins
Principal Investigator: Ira Berkower, MD, PhD
Office / Division / Lab: OVRR / DVP / LI
Our laboratory is working to solve some of the difficult problems facing scientists who are trying to develop a safe and effective vaccine against HIV, the virus that causes AIDS.
One challenge is to trigger the immune system to produce durable and "broadly neutralizing" antibodies that can protect against the broad diversity of HIV strains in circulation. These antibodies are produced in response to HIV infection; but they have never been elicited reliably by a vaccine. Another challenge is to elicit strong T cell immunity to HIV proteins. This type of immunity is important for control of viral replication inside cells and for eradicating an infection once it has started.
Neutralizing antibodies target viral structures that perform essential viral functions. Inhibition of these functions is lethal for the virus. Since the function is shared by all HIV strains, the antibody targets are likely to be shared as well. For example, the envelope glycoprotein contains a structure called the CD4 binding site (CD4bs) that is found on all HIV strains identified to date. The CD4 binding site enables HIV to bind its receptor on the cells that it subsequently infects. Our lab identified this site as a target for broadly neutralizing antibodies. We are working to expose the CD4 binding site to the immune system and trigger antibody production. If an AIDS vaccine could elicit antibodies of this type, it might protect people from the many different circulating varieties of HIV.
We are using two proven strategies for increasing vaccine potency: the first is to present HIV antigens as part of a virus-like particle (VLP). (Antigens are molecules that are targeted by antibodies.) The hepatitis B surface antigen (HBsAg) readily assembles with other viral proteins to form particles. By itself, HBsAg is already a successful vaccine. Our lab developed a way to incorporate HIV envelope proteins into VLPs by linking them to HBsAg. We are studying ways to use VLPs to reliably trigger production of broadly neutralizing antibodies.
Many successful vaccines are based on live attenuated (weakened) versions of the parental virus. A similar approach, using live, attenuated HIV would be too risky for human use. Instead, our lab has developed the attenuated vaccine strain of rubella virus (that causes German measles) as a vector to carry foreign genes. The vector replicates normally while expressing HIV antigens.
The first monkey trials of rubella/Gag vectors showed vaccine potency: antibody titers to the vectors were as high as to natural infection with SIV. The vaccine elicited a strong T cell response to SIV Gag protein. Besides rubella vectors, macaques are also the animal model of choice for SIV challenge studies. The overlapping host range will allow us to immunize with rubella vectors, measure the immune response, and then test protection against a live viral challenge.
These studies provide new scientific insight and tools for developing and evaluating vaccines for HIV/AIDS. The experience with HIV vaccine antigens will provide CBER with the expertise it needs to evaluate proposed HIV vaccines and anticipate their likely side effects.
For many vaccine antigens, such as polio, rabies, and hepatitis B, vaccine potency depends on particle formation. Despite advances in recombinant DNA technology, only a handful of recombinant vaccines have been approved so far, and these are particle formers: hepatitis B surface antigen (HBsAg) assembles 22 nm particles with a strong lipid component. L1 protein of human papilloma virus (HPV) assembles into 60 nm particles. To enhance the immune response to biologically important targets on HIV, we have incorporated HIV antigens into virus-like particles (VLP).
We have used HBsAg as a carrier protein: when linked to another viral protein, it assembles particles and incorporates the other viral protein into the particle as well. This effectively displays HIV viral envelope proteins, such as gp120 and gp41, on the VLP surface, where they mimic HIV virions. In addition, we discovered that the transmembrane domain of HIV gp41 can anchor the envelope proteins to the particles naturally. Gp120 and gp41 contain important antigenic determinants that are targeted by human antibodies with broad neutralizing activity against HIV isolates from around the world.
Our second approach is to incorporate HIV viral genes into a live attenuated viral vector. We are using the rubella vaccine strain as a vector because safety and potency have been demonstrated in millions of children: one or two doses protect for life against rubella. Rubella vaccine elicits mucosal immunity, and it is safe for use in immunocompromised children.
We have found that rubella vectors can accommodate and express a surprising range and size of vaccine inserts. These include: HIV and SIV Gag and Env proteins, as well as other viral Env proteins (hepatitis C and VEE) and non-viral proteins (malaria CSP). We have completed the first successful vaccine trial of rubella vectors in rhesus macaques. The vectors grew well in 6 out of 6 macaques and elicited high titered antibodies in all of them. Antibody titers elicited by vaccine equaled natural SIV infection. The antibodies were durable, and they were boosted by re-exposure to the vectors. Antigen-specific T cells reached high levels, as measured by tetramer staining.
Besides rubella, rhesus macaques are also the animal model of choice for SIV challenge studies. The overlapping host range will allow us to immunize with the vectors, measure the immune response, and then test protection against a live SIV viral challenge.
An alternative design is to infect first, followed by early ART drug therapy and vaccination with our vectors. If successful, the immune response to vaccine may control infection and allow us to stop ART therapy without viral rebound. Experiments are underway to determine if this combined approach can produce a "functional cure". The outcome would be important for 20,000 babies born infected each month.
The dual approaches of live vectors and virus like particles can be combined in a prime/boost strategy to achieve high levels of T cell and B cell immunity that could prevent HIV infection. They address important issues concerning antigenicity, antigen presentation, and vaccine potency.
Vaccine 2017 May 31;35(24):3272-8
Expression of complete SIV p27 Gag and HIV gp120 engineered outer domains targeted by broadly neutralizing antibodies in live rubella vectors.
Virnik K, Nesti E, Dail C, Hockenbury M, Ni Y, Felber BK, Schief WR, Berkower I
Hum Vaccin Immunother 2015 Aug 3;11(8):2005-11
Dose-dependent inhibition of Gag cellular immunity by Env in SIV/HIV DNA vaccinated macaques.
Valentin A, Li J, Rosati M, Kulkarni V, Patel V, Jalah R, Alicea C, Reed S, Sardesai N, Berkower I, Pavlakis GN, Felber BK
Vaccine 2015 Apr 27;33(18):2167-74
Recombinant rubella vectors elicit SIV Gag-specific T cell responses with cytotoxic potential in rhesus macaques.
Rosati M, Alicea C, Kulkarni V, Virnik K, Hockenbury M, Sardesai NY, Pavlakis GN, Valentin A, Berkower I, Felber BK
Retrovirology 2013 Sep 16;10(1):99
Live attenuated rubella vectors expressing SIV and HIV vaccine antigens replicate and elicit durable immune responses in rhesus macaques.
Virnik K, Hockenbury M, Ni Y, Beren J, Pavlakis GN, Felber BK, Berkower I
Vaccine 2013 Apr 19;31(17):2119-25
Enhanced expression of HIV and SIV vaccine antigens in the structural gene region of live attenuated rubella viral vectors and their incorporation into virions.
Virnik K, Ni Y, Berkower I
Vaccine 2012 Aug 10;30(37):5453-8
Live attenuated rubella viral vectors stably express HIV and SIV vaccine antigens while reaching high titers.
Virnik K, Ni Y, Berkower I
J Virol 2011 Mar;85(5):2439-48
Hepatitis B virus surface antigen assembly function persists when entire transmembrane domains 1 and 3 are replaced by a heterologous transmembrane sequence.
Berkower I, Spadaccini A, Chen H, Al-Awadi D, Muller J, Gao Y, Feigelstock D, Virnik K, Ni Y
Vaccine 2010 Feb 3;28(5):1181-7
Stable expression of a foreign protein by a replication-competent rubella viral vector.
Spadaccini A, Virnik K, Ni Y, Prutzman K, Berkower I
Virology 2008 Aug 1;377(2):330-8
Targeted deletion in the beta20-beta21 loop of HIV envelope glycoprotein gp120 exposes the CD4 binding site for antibody binding.
Berkower I, Patel C, Ni Y, Virnik K, Xiang Z, Spadaccini A
Virology 2008 Mar 30;373(1):72-84
Analysis of the human immunodeficiency virus type 1 gp41 membrane proximal external region arrayed on hepatitis B surface antigen particles.
Phogat S, Svehla K, Tang M, Spadaccini A, Muller J, Mascola J, Berkower I, Wyatt R
J Infect Dis 2007 Oct 1;196(7):1026-32
Antibodies to the A27 Protein of Vaccinia Virus Neutralize and Protect against Infection but Represent a Minor Component of Dryvax Vaccine-Induced Immunity.
He Y, Manischewitz J, Meseda CA, Merchlinsky M, Vassell RA, Sirota L, Berkower I, Golding H, Weiss CD
Virology 2004 Mar 30;321(1):75-86
Assembly, structure, and antigenic properties of virus-like particles rich in HIV-1 envelope gp120.
Berkower I, Raymond M, Muller J, Spadaccini A, Aberdeen A