Using Experiments of Nature to Assist in HIV Vaccine Design
Authors: Justen Hoffman Russell BSc, Joseph Mathew Antony PhD, and Kelly S. MacDonald MD FRCPC
Affiliation: Director, HIV Research Program, Mount Sinai Hospital and University Health Network; Departments of Medicine and Immunology, University of Toronto, IMS Member
With several million patients showing new signs of Human Immunodeficiency Virus type 1 (HIV) infection globally, continued progress in the development of a vaccine against this pathogen is a priority. Canada is not immune to HIV infection and the consequent acquired immune deficiency syndrome (AIDS)-associated mortality. Indeed, new infections continue to occur in Canada. The groups at highest risk in Canada are intravenous drug users, gay men who have unprotected sex (especially if they have concurrent sexually transmitted diseases), select ethnic groups originating from countries with high endemic rates of HIV, and certain aboriginal groups in Canada (in some First Nation reserves in Saskatchewan the HIV rate is 11 times the national average at 63 cases per 100,000.1
There is no question that the long-term concurrent use of highly active anti-retroviral medications has considerably lengthened the lifespans of infected patients and enormously improved their general well-being. However the unwanted side effects of these medications and the incomplete suppression of all viral effects can result in toxicity and pathological changes in a variety of organs including the brain. It has become clear that even on intense treatment, the immune system does not completely normalize and there is persistent innate immune activation to a variable extent even among aggressively treated patients, hastening aging. Understanding why this occurs, the consequences, and how to avoid it, has joined the list of research topics on the HIV research agenda. It seems that pharmacotherapy may not be the Holy Grail and immune-mediated approaches directed at both prevention of HIV infection and disease are important. Not unlike the fields of rheumatology and oncology, we find ourselves endeavouring to enhance certain therapeutic immune responses and suppress other less desirable immune responses. The fields of prevention and therapeutics have merged as vaccine approaches that require broadly neutralizing antibodies and persistent cellular immune responses against conserved T cell protein epitopes.
Unlike diseases such as influenza or malaria that rely on mouse models to understand the basic biology of infections, we have had to rely on a nonhuman primate models to study HIV because it belongs to a retrovirus family that exclusively infects primates. Macaque monkeys infected with Simian Immunodeficiency Virus (SIV), the macaque counterpart of HIV, have been preferred over chimpanzees for ethical and cost reasons. Rhesus macaques were used initially because of availability in North America but now the long-tailed or cynomolgus macaque is increasingly used, and together, they remain the best available animal models to study SIV.2 It should be remembered that SIVcpz gave rise to HIV-1 and SIVsm gave rise to HIV-2, which actually are very similar at a genomic level. Both SIVs are endemic in West Africa and do not normally infect macaques from Asia. The macaque model closely recapitulates HIV infection, with opportunistic infections occurring if the animals are observed without intervention over a year or two. The MacDonald laboratory works by engaging in research studies with collaborators in epidemiological research, particularly where interesting observations are made that generate ideas incorporating rational approaches in the design of a vaccine against HIV. Dr. MacDonald has had a long-standing collaboration with researchers in Nairobi, Kenya where she and colleagues observed that certain genetic and immune factors3 played key roles in HIV resistance among individuals who were frequently exposed to HIV through sex work despite heavy exposure.4 Dr. MacDonald’s HIV vaccine studies have gone on to try to exploit some of these factors in vaccine design. These factors include the presence of long-term persistent cellular immune responses at the mucosal interface and a lack of excessive immune activation in mucosal tissue. In the opinion of Dr. MacDonald, this ability to carry out human epidemiological studies to generate hypotheses and then test the basic science mechanism responsible in the lab is a key advantage of the clinician scientist research environment in the laboratory.
Current vaccine work in our laboratory focuses on the use of herpesviruses such as Varicella Zoster Virus, Oka vaccine strain (VZV-Oka) and Cytomegalovirus (CMV) as vaccine vectors for SIV vaccination. Both herpesviruses establish lifelong cycles of latency and reactivation. When paired with SIV transgenes, asymptomatic reactivation in normal hosts provides an opportunity for re-exposure to SIV. This effectively boosts the animal on a regular basis leading to a robust and rapid immune response should the animals ever encounter actual SIV.
We have already demonstrated that the VZV-Oka establishes infection and reactivates in cynomolgus macaques5 similar to children getting chickenpox vaccine with progressive boosting in antibody levels. This is in contrast to a typical vaccine where antibody levels drop over time. Then using the VZV-Oka vaccine with SIV transgenes inserted in it (one version with Gag-Pol, and Env, the other version with Nef-Tat-Rev), cynomolgus macaques were vaccinated against SIV. The protection was not perfect, but showed promise above and beyond contemporary vaccine attempts with about a third of vaccine recipients acquiring infection initially but then apparently eradicating the virus and maintaining this state for more than a year and a half. It appears immune responses against viruses in the herpesvirus family (VZV, Herpes Simplex virus, CMV, etc.) are different from those against other viruses in that they are more “agile” and they can attack the infected target immediately instead of requiring a delay. This may have to do with evolution since these viruses are latent and our bodies have evolved to control reactivation quickly.
Since the primary application of a HIV vaccine will be in Africa, where chickenpox is not encountered as frequently (for reasons unknown), our laboratory has set out to better understand the immune mechanisms associated with varicella in the human genital mucosa in this population. We have initiated a clinical trial in a cohort of healthy women in Nairobi, Kenya. In collaboration with Dr. Walter Jaoko and Dr. Omu Anzala of KAVI (Kenya), Catia Perciani, a PhD candidate in our laboratory, is working to determine some of the cellular and molecular aspects of varicella-mediated mucosal immunity so as to apply them when designing a vector based on varicella. The goal is to examine the baseline and immune activation and the impact of vaccination on these parameters as well as its immunogenicity. This is a safe approach since there is no HIV transgene in the varicella zoster vaccine.
Concurrent to the work on a varicella zoster vaccine, we have explored the use of a second herpesvirus, CMV. Previously, the only known macaque CMV vector had been derived from a rhesus macaque CMV. Working in the cynomolgus macaque, a monkey model that more closely mirrors the disease progression seen in HIV-infected humans, infectivity studies were precluded by the species specificity of CMV. Recent findings from the laboratory indicate that the viral vector derived from rhesus macaque CMV is unable to infect cynomolgus macaques.6 While human varicella can infect cynomolgus macaques,5 development of vectors based on CMV required the isolation of cynomolgus macaque CMV. We recently isolated two unique strains of CMV from cynomolgus macaques, one of Filipino origin7 and the other imported from the island of Mauritius.8 In fact, studies from the lab comparing genome sequences of macaque CMV indicate an evolutionary relationship between host and virus as well as between viruses, such that CMV might even reflect macaque population structure.8 They have recently completed the significant undertaking of developing an HIV/SIV vaccine based on this newly isolated CMV9 and are looking forward to an upcoming vaccine trial. Comparing two different herpesviruses with different tissue tropism and rates of reactivation and other factors will be very informative in understanding what is more optimal as an HIV immunogen.
The study of vaccines for HIV has had a long history, with only small successes along the way. HIV is a difficult virus for the immune system to fight and has defied conventional knowledge on vaccination. However, we have learned from previous attempts at vaccination and continue to learn from the herpesviruses that the MacDonald laboratory employs. Although herpesviruses are the oldest viruses, they have offered a new approach in vaccination, and through their careful study and design, the MacDonald laboratory hopes to put a stop to the sombre reality that is HIV. Only with a vaccine can we hope to do more than just slow new HIV infections.
- Public Health Agency of Canada.
- Antony JM, MacDonald KS. A critical analysis of the cynomolgus macaque, Macaca fascicularis, as a model to test HIV-1/SIV vaccine efficacy. Vaccine 2015, 33(27): 3073-3083.
- MacDonald KS, Fowke KR, Kimani J, et al. Influence of HLA supertypes on susceptibility and resistance to human immunodeficiency virus type 1 infection. J Infect Dis 2000, 181(5):1581-1589.
- Fowke KR, Nagelkerke NJ, Kimani J, et al. Resistance to HIV-1 infection among persistently seronegative prostitutes in Nairobi, Kenya. Lancet 1996, 348(9038):1347-1351.
- Willer DO, Ambagala AP, Pilon R, et al. Experimental infection of Cynomolgus Macaques (Macaca fascicularis) with human varicella-zoster virus. Journal of virology 2012, 86(7):3626-3634.
- Marsh AK, Ambagala AP, Perciani CT, et al. Examining the Species-Specificity of Rhesus Macaque Cytomegalovirus (RhCMV) in Cynomolgus Macaques. PLoS One 2015, 10(3):e0121339.
- Marsh AK, Willer DO, Ambagala AP, et al. Genomic sequencing and characterization of cynomolgus macaque cytomegalovirus. Journal of virology 2011, 85(24):12995-13009.
- Hoffman Russell JN, Marsh AK, Willer DO, et al. Macaques and their passengers: Exploring the divergence and adaptation of the ubiquitous viral pathogen CMV in fascicularis macaques. In: Microbiology & Infectious Diseases Research Days 2015. University of Toronto, Toronto, ON; 2015.
- Hoffman Russell JN, Marsh AK, Willer DO, et al. Development of cynomolgus macaque cytomegalovirus as a novel SIV vaccine vector. In: Keystone Symposia HIV Vaccines: Adaptive Immunity and Beyond. Banff, Alberta; 2014.