Immune Responses of Alphavirus Vaccination in Patients with HPV-Induced Cancers

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In this issue of Molecular Therapy, the safe application of a cancer vaccine based on self-amplifying Semliki Forest virus (SFV) RNA replicon particles was recently demonstrated by Komdeur et al.¹ in the first-in-human phase I clinical trial for the treatment of human papillomavirus (HPV)-induced cancers. The study showed both CD4⁺ and CD8⁺ T cell responses in all immunized subjects. These findings are encouraging, but, for the “hardcore” alphavirus enthusiast, not surprising and rather frustrating because it took so long. The first engineering of SFV RNA replicon vectors took place in the 1990s, where replacement of the SFV structural genes by the gene of interest allowed immunization studies, with replication-incompetent recombinant SFV particles showing strong immune responses and protection against challenges with viral pathogens and tumor cells in various animal models.² The utilization of RNA replicons based on overexpression of the SFV non-structural proteins nsP1–4 provides a 200,000-fold amplification of mRNA for the gene of interest, which will result in extreme levels of heterologous gene expression and, in the case of vaccine development, cellular antigen production. Moreover, the single-stranded mRNA generates only transient expression for approximately 5 days before its degradation, presenting not even a theoretical possibility of integration into the host genome. Another interesting feature of self-amplifying RNA (saRNA) viral vectors relates to their utilization as RNA replicons or plasmid DNA replicons.² This has allowed improved safety, faster and more flexible engineering of vaccine candidates, and the application of 100- to 1,000-fold lower doses to achieve similar immune responses obtained for conventional mRNA and DNA plasmids, respectively. Therefore, it can be seen as a major step forward that self-amplifying SFV vectors have been subjected to the first-in-human phase I clinical trial for a therapeutic cancer vaccine in patients with established HPV-induced cancers, providing both good safety and strong immune responses in all vaccinated patients.

The phase I trial on the SFV-HPV vaccine candidate was made possible by the positive findings from studies in tumor-bearing mice.³ In that study, the fusion of HPV16 E6 and E7 proteins was expressed under the control of a translational enhancer derived from the SFV capsid protein. Immunization with recombinant SFV-HPV E6/E7 particles resulted in regression and complete elimination of established tumors. Moreover, long-term high-level activity of cytotoxic T lymphocytes (CTLs) and anti-tumor responses lasted up to 340 days. Therefore, it is somewhat peculiar that it took 13 years before the first-in-human clinical trial was conducted for this vaccine candidate. It needs to be pointed out, however, that other phase I clinical trials for alphavirus-based vaccine candidates for the carcinoembryonic antigen (CEA) and the prostate-specific membrane antigen (PSMA) expressed from Venezuelan equine encephalitis (VEE) virus have been conducted as previously reviewed.² In any case, the aim of the SFV-HPV study was to evaluate the safety, tolerability, and immunogenicity of the Vvax001 vaccine candidate. The study was preceded by a thorough review focusing on design, delivery, combination strategies, preclinical efficacy, and product development. Moreover, relevant attention was paid to the design of a good manufacturing practice (GMP)-compliant manufacturing process.⁴ The clinical grade material complied with all specifications and was released for use as an investigational medicinal product.

The phase I trial for the Vvax001 vaccine candidate was conducted on 12 individuals with a history of cervical intraepithelial neoplasia.¹ The patients were divided into four cohorts, receiving doses of 5 × 10⁵, 5 × 10⁶, 5 × 10⁷, and 2.5 × 10⁸ infectious particles (IPs) per immunization. The regimen involved three immunizations with a 3-week interval. The vaccinations generally triggered only mild adverse events, such as injection site reactions and hematomas, peripheral edema, chills, myalgia, back pain, and swelling of lymph nodes. There was no correlation between dose level and adverse events, and all four tested doses were deemed safe and well tolerated. Cellular immune responses were monitored in peripheral blood mononuclear cells (PBMCs) isolated before and after the second and third immunizations by identification of HPV16-specific interferon-γ (IFN-γ)-producing cells applying IFN-γ enzyme-linked immunospot (ELISPOT). No or very few positive SFV-HPV E6/E7-induced T cell responses were detected before the first immunization. However, the lowest dose of 5 × 10⁵ IPs showed positive vaccine-induced T cell responses in all three vaccinated individuals. After two vaccinations, 5 out of 12 participants showed HPV E6/E7-specific IFN-γ-producing T cells, and, after three vaccinations, the number was 10 out of 12. Generally, the response against HPV E6 was stronger than against HPV E7. Furthermore, immunization with the SFV-HPV E6/E7 vaccine showed activation of both CD4⁺ and CD8⁺ T cells specific for HPV E6 and E7. The potential antibody responses against the SFV vector were also investigated. At the lowest dose (5 × 10⁵ IPs), no anti-SFV antibodies were detected, whereas elevated SFV antibody titers were found in two out of three subjects after three immunizations with 5 × 10⁶ IPs. Immunization with the two highest doses generated enhanced levels of anti-SFV antibodies after both the second and third immunizations. Although the elicited antibodies neutralized SFV infection in BHK-21 cells, booster immunizations further increased the immune response, as also previously seen in preclinical studies.¹

The SFV-based HPV vaccine showed similar or even higher levels of HPV16-specific IFN-γ-producing T cell responses, as previously observed in clinical trials for the DNA vaccine candidates GX-188E and VGX-3100.¹ This comparison bodes well for alphavirus-based vaccine development. Additionally, development of vaccine candidates for hepatitis C virus using recombinant SFV particles is in progress. However, it is surprising that approaches of using nucleic acid-based alphavirus replicon vaccine candidates have not been utilized as the potential efficacy of an alphavirus DNA replicon vaccine candidate for HPV E6/E7 has already been demonstrated in a mouse model.⁵ In comparison to a conventional DNA, which did not inhibit tumor growth, a 200-fold lower dose of replicon DNA vaccine resulted in 85% tumor-free mice. Moreover, lipid nanoparticle encapsulated alphavirus saRNA expressing the SARS-CoV-2 spike protein elicited high dose-dependent neutralizing antibody responses in mice at a higher level than seen in coronavirus disease 2019 (COVID-19) patients.⁶

In conclusion, the recombinant SFV particle-based phase I study demonstrated high safety and tolerability profiles in patients with HPV-induced cancers. The robust HPV16-specific antibody responses in 12 out of 12 patients supports further evaluation of the Vvax001 vaccine candidate in phase II trials.