| |
Experimental mRNA HIV Vaccine Safe, Shows Promise in Animals
NIAID Scientists Developed Vaccine Platform
|
| |
| |
Download the PDF here
December 9, 2021
An experimental HIV vaccine based on mRNA-the same platform technology used in two highly effective COVID-19 vaccines-shows promise in mice and non-human primates, according to scientists at the National Institute of Allergy and Infectious Diseases (NIAID), part of the National Institutes of Health. Their results, published in Nature Medicine, show that the novel vaccine was safe and prompted desired antibody and cellular immune responses against an HIV-like virus. Rhesus macaques receiving a priming vaccine followed by multiple booster inoculations had a 79% lower per-exposure risk of infection by simian-human immunodeficiency virus (SHIV) compared to unvaccinated animals. The research was led by Paolo Lusso, M.D., Ph.D., of NIAID's Laboratory of Immunoregulation, in collaboration with other NIAID scientists, investigators from Moderna, Inc. and colleagues at other institutions.
"Despite nearly four decades of effort by the global research community, an effective vaccine to prevent HIV remains an elusive goal," said NIAID Director Anthony S. Fauci, M.D., chief of the Laboratory and a paper co-author. "This experimental mRNA vaccine combines several features that may overcome shortcomings of other experimental HIV vaccines and thus represents a promising approach."
The experimental vaccine works like mRNA COVID-19 vaccines. However, instead of carrying mRNA instructions for the coronavirus spike protein, the vaccine delivers coded instructions for making two key HIV proteins, Env and Gag. Muscle cells in an inoculated animal assemble these two proteins to produce virus-like particles (VLPs) studded with numerous copies of Env on their surface. Although they cannot cause infection or disease because they lack the complete genetic code of HIV, these VLPs match whole, infectious HIV in terms of stimulating suitable immune responses
In studies with mice, two injections of the VLP-forming mRNA vaccine induced neutralizing antibodies in all animals, the investigators report. The Env proteins produced in the mice from the mRNA instructions closely resembled those in the whole virus, an improvement over previous experimental HIV vaccines. "The display of multiple copies of authentic HIV envelope protein on each VLP is one of the special features of our platform that closely mimics natural infection and may have played a role in eliciting the desired immune responses," said Dr. Lusso.
The team then tested the Env-Gag VLP mRNA vaccine in macaques. The details of the vaccine regimen differed among subgroups of vaccinated animals but involved priming the immune system with a vaccine modified to optimize antibody creation. The prime was followed by multiple booster inoculations delivered over the course of a year. The boost vaccines contained Gag mRNA and Env mRNA from two HIV clades other than the one used in the prime vaccine. The investigators used multiple virus variants to preferentially activate antibodies against the more conserved "shared" regions of the Env-the target of broadly neutralizing antibodies-rather than the more variable regions that differ in each virus strain.
Although the doses of mRNA delivered were high, the vaccine was well tolerated and produced only mild, temporary adverse effects in the macaques, such as loss of appetite. By week 58, all vaccinated macaques had developed measurable levels of neutralizing antibodies directed against most strains in a test panel of 12 diverse HIV strains. In addition to neutralizing antibodies, the VLP mRNA vaccine also induced a robust helper T-cell response.
Beginning at week 60, immunized animals and a control group of unimmunized macaques were exposed weekly, via the rectal mucosa, to SHIV. Because non-human primates are not susceptible to HIV-1, scientists use a chimeric SHIV in experimental settings because that virus replicates in macaques. After 13 weekly inoculations, two out of seven immunized macaques remained uninfected. The other immunized animals had an overall delay in infection, which occurred, on average, after eight weeks. In contrast, unimmunized animals became infected on average after three weeks.
"We are now refining our vaccine protocol to improve the quality and quantity of the VLPs produced. This may further increase vaccine efficacy and thus lower the number of prime and boost inoculations needed to produce a robust immune response. If confirmed safe and effective, we plan to conduct a Phase 1 trial of this vaccine platform in healthy adult volunteers," said Dr. Lusso.
https://www.niaid.nih.gov/news-events/experimental-mrna-hiv-vaccine-safe-shows-promise-animals?utm_campaign=+50339639&utm_content=&utm_medium=email&utm_source=govdelivery&utm_term=
----------------------------------
Nature Medicine Dec 9 2021 - pdf attached
The present study provides a proof of principle that a multiclade HIV-1 env-gag mRNA vaccine is safe and immunogenic, can induce neutralization breadth accompanied by polyfunctional CD4+ T cell responses, and can protect rhesus macaques from infection with a heterologous tier-2 virus. Specifically, the results that we obtained both in mice and macaques validate the further development of a VLP-generating env-gag mRNA platform for the presentation of native HIV-1 Env to the immune system. The use of mRNA as a vehicle may offer significant advantages over immunization with exogenous proteins in terms of the challenges, cost and time required for immunogen manufacturing for human clinical trials. In the specific case of HIV-1, it may also overcome many of the issues that arise with the recombinant production of Env trimer immunogens, especially glycan processing, antigenic modifications associated with trimer stabilization, and the immunodominance of off-target epitopes. Furthermore, many primary HIV-1 Envs that might serve as effective immunogens are not amenable to expression as stabilized soluble trimers.
Despite these encouraging results, there are some limitations with the present study. First, the neutralization titers elicited by our vaccine against the challenge virus, AD8, and other heterologous tier-2 viruses were relatively low and did not reach levels that have previously been reported to be protective in vivo7,74,75,76,77,78. This suggests that protection was unlikely to be due to neutralizing antibodies alone, but rather to a combination of neutralizing antibodies with other immunologic mechanisms. One such mechanism could be ADCC, which was effectively documented in our immunized animals. Moreover, the induction of antibodies against multiple Env antigenic sites by our vaccine raises the possibility of a cooperative effect between antibodies with different specificity and functionality. Another limitation of our study was that the protective effect of the vaccine was partial, with only two animals being fully protected. However, it should be emphasized that we used a heterologous tier-2 strain (SHIV AD8) that is particularly difficult to neutralize, and still the vaccine induced a 79% per-exposure risk reduction upon 13 sequential mucosal challenges, a protocol that best approximates the prevalent modality of sexual transmission in humans. Given that mucosal transmission of HIV-1 in humans is an inefficient event, this level of risk reduction might still have a significant impact on viral transmission. Finally, the vaccine has so far been tested only in a single study, and therefore the results need to be confirmed with additional preclinical and clinical trials.
In conclusion, the env-gag VLP mRNA platform described here offers a promising approach for the development of a preventive HIV-1 vaccine. Although the intensive immunization schedule with high mRNA doses that we used was remarkably well tolerated, a vaccination regimen encompassing seven or more sequential immunizations would be difficult to implement in humans. Refinements of the current protocol, including improvements in both the yield and quality of VLP generation, may increase the efficacy of immunization and thereby require fewer vaccine boosts. Specific modifications in the choice of Env immunogens and mRNA formulations are being evaluated, in parallel with the original protocol, in a second vaccine-challenge study in rhesus macaques that is currently under way.
A multiclade env-gag VLP mRNA vaccine elicits tier-2 HIV-1-neutralizing antibodies and reduces the risk of heterologous SHIV infection in macaques
Dec 9 Nature Medicine
Abstract
The development of a protective vaccine remains a top priority for the control of the HIV/AIDS pandemic. Here, we show that a messenger RNA (mRNA) vaccine co-expressing membrane-anchored HIV-1 envelope (Env) and simian immunodeficiency virus (SIV) Gag proteins to generate virus-like particles (VLPs) induces antibodies capable of broad neutralization and reduces the risk of infection in rhesus macaques. In mice, immunization with co-formulated env and gag mRNAs was superior to env mRNA alone in inducing neutralizing antibodies. Macaques were primed with a transmitted-founder clade-B env mRNA lacking the N276 glycan, followed by multiple booster immunizations with glycan-repaired autologous and subsequently bivalent heterologous envs (clades A and C). This regimen was highly immunogenic and elicited neutralizing antibodies against the most prevalent (tier-2) HIV-1 strains accompanied by robust anti-Env CD4+ T cell responses. Vaccinated animals had a 79% per-exposure risk reduction upon repeated low-dose mucosal challenges with heterologous tier-2 simian-human immunodeficiency virus (SHIV AD8). Thus, the multiclade env-gag VLP mRNA platform represents a promising approach for the development of an HIV-1 vaccine.
Discussion
The present study provides a proof of principle that a multiclade HIV-1 env-gag mRNA vaccine is safe and immunogenic, can induce neutralization breadth accompanied by polyfunctional CD4+ T cell responses, and can protect rhesus macaques from infection with a heterologous tier-2 virus. Specifically, the results that we obtained both in mice and macaques validate the further development of a VLP-generating env-gag mRNA platform for the presentation of native HIV-1 Env to the immune system. The use of mRNA as a vehicle may offer significant advantages over immunization with exogenous proteins in terms of the challenges, cost and time required for immunogen manufacturing for human clinical trials. In the specific case of HIV-1, it may also overcome many of the issues that arise with the recombinant production of Env trimer immunogens, especially glycan processing, antigenic modifications associated with trimer stabilization, and the immunodominance of off-target epitopes. Furthermore, many primary HIV-1 Envs that might serve as effective immunogens are not amenable to expression as stabilized soluble trimers.
The vaccine platform that we designed combines a series of unique features, all of which may have contributed to its efficacy. One of the most notable is the co-formulation of mRNAs encoding membrane-anchored Env and Gag proteins, which leads to the endogenous generation of VLPs similar to the viral particles produced during natural infection. Although in vivo-generated VLPs are difficult to visualize, in vitro transfection experiments documented a robust and consistent VLP formation with diverse HIV-1 Env immunogens. The results obtained in mice by direct comparison of co-formulated env-gag mRNA with env mRNA alone provided supporting evidence for the advantages of this design. Other factors that were presumably critical were the initial priming with a transmitted-founder HIV-1 Env lacking the N276 glycan, which is in line with lineage-based vaccine strategies aimed at the early recruitment of germline bNAb precursors, and the intensive multiclade heterologous boosting. Corroborating the importance of such intensive multiclade stimulation, neutralization breadth started to appear only after the third heterologous immunization.
As often occurs in preclinical and clinical vaccine studies, the immunological mechanisms responsible for the risk reduction conferred by our vaccine remain uncertain. However, analysis of protection correlates conducted at the time of virus challenge provided some clues about the potential protective mechanisms. A significant correlation was observed with antibodies against the CD4-BS, a key supersite of HIV-1 vulnerability, which supports a role of the humoral immune response in mediating protection. In rhesus macaques, the lineages of anti-CD4-BS bNAbs are not yet clearly defined and, therefore, it was not possible to test the ability of our priming immunogen to engage germline macaque antibodies against this site. However, we can postulate that removal of the N276 glycan, which effectively shields the CD4-BS from germline bNAb precursors in humans, may have succeeded in recruiting the relevant macaque B cell precursors at the onset of the immunization protocol. The multiple booster immunizations with closed, tier-2 Envs from three different clades may have then focused the B cell responses on shared tier-2 epitopes, thereby fostering the maturation of broad-spectrum neutralizing antibodies. Consistent with a role of humoral immune responses in protection, our vaccine induced robust polyfunctional Env-specific CD4+ T cells at the time of virus challenge. As previously reported with mRNA vaccines41,70,71,72, we also documented the induction of virus-specific Tfh cells, a key subpopulation that was shown to support B cell differentiation into affinity-matured long-lived plasma cells and promote bNAb development against HIV-1 (ref. 73).
Despite these encouraging results, there are some limitations with the present study. First, the neutralization titers elicited by our vaccine against the challenge virus, AD8, and other heterologous tier-2 viruses were relatively low and did not reach levels that have previously been reported to be protective in vivo7,74,75,76,77,78. This suggests that protection was unlikely to be due to neutralizing antibodies alone, but rather to a combination of neutralizing antibodies with other immunologic mechanisms. One such mechanism could be ADCC, which was effectively documented in our immunized animals.
Moreover, the induction of antibodies against multiple Env antigenic sites by our vaccine raises the possibility of a cooperative effect between antibodies with different specificity and functionality. Another limitation of our study was that the protective effect of the vaccine was partial, with only two animals being fully protected. However, it should be emphasized that we used a heterologous tier-2 strain (SHIV AD8) that is particularly difficult to neutralize, and still the vaccine induced a 79% per-exposure risk reduction upon 13 sequential mucosal challenges, a protocol that best approximates the prevalent modality of sexual transmission in humans. Given that mucosal transmission of HIV-1 in humans is an inefficient event, this level of risk reduction might still have a significant impact on viral transmission. Finally, the vaccine has so far been tested only in a single study, and therefore the results need to be confirmed with additional preclinical and clinical trials.
In conclusion, the env-gag VLP mRNA platform described here offers a promising approach for the development of a preventive HIV-1 vaccine. Although the intensive immunization schedule with high mRNA doses that we used was remarkably well tolerated, a vaccination regimen encompassing seven or more sequential immunizations would be difficult to implement in humans. Refinements of the current protocol, including improvements in both the yield and quality of VLP generation, may increase the efficacy of immunization and thereby require fewer vaccine boosts. Specific modifications in the choice of Env immunogens and mRNA formulations are being evaluated, in parallel with the original protocol, in a second vaccine-challenge study in rhesus macaques that is currently under way.
|
|
| |
| |
|
|
|
