Recently mRNA vaccines have received particular attention based on their effectiveness during the COVID-19 pandemic. Additionally, mRNA vaccines have achieved some promising clinical responses in treating a variety of solid tumours . However, other vaccine technologies have been under investigation for some time in immuno-oncology, too, including cell-based vaccines, dendritic cell-based vaccines, peptide vaccines, viral particle derived vaccines and DNA vaccines. . This article provides an overview of the various technologies (Figure 1) and highlights why mRNA vaccines are particularly well suited for this endeavour.
Cell-based cancer vaccines include vaccines in which live cells or cell extracts are administered to the patient to elicit an immune response against the tumour. Tumour cell lysate vaccines are either prepared from inactivated tumour cells taken directly from a patient or from allogeneic cell lines. With tumour cell vaccines, where many common cellular antigens along with tumour antigens are presented to the immune system, a broad, but not very specific immune response is induced. One of the most promising candidates was Canvaxin, developed by the company CancerVax, which appeared favourable in a phase II trial but did not pass phase III testing . A fundamental problem with tumour cell vaccines is central tolerance which prevents a strong immune response against many tumour antigens . Thus, to date, this approach has not led to an approved drug.
Dendritic cell- (DC-) based vaccines are also considered to be cell-based vaccines, although their production differs significantly from that of tumour cell-based vaccines and they usually target tumour antigens with higher specificity. DCs, which are professional antigen presenting cells (APCs), are key in the initiation of the adaptive immunity activation cascade. They are potent activators of CD4+ (helper) and CD8+ (cytotoxic) T-cells and stimulate the development of memory T-cells to induce long-lasting immunity. They also activate B-cells, natural killer cells and natural killer T-cells , all of which are important to mount a strong anti-tumour response. To manufacture a DC vaccine, DCs are harvested from the patient, cultivated, and loaded with the desired antigen or antigens, which can be patient-derived tumour antigens, specific antigens or mRNA encoding the antigens. The cells are then administered back to the patient to activate an immune response against cells expressing the antigen(s). In 2010, the DC vaccine Sipuleucel-T was approved for use against metastatic castration-resistant prostate cancer. Sipuleucel-T showed activity against tumour cells and a moderate improvement in overall survival (OS) in a pivotal trial . The vaccine consists of DCs that are loaded with a fusion protein of prostatic acid phosphatase (PAP) and granulocyte-macrophage colony stimulating factor (GM-CSF) for APC activation. PAP is a tumour associated antigen (TAA) that is highly expressed in metastatic prostate cancer cells, while the GM-CSF part activates the DCs synchronously .
However, the most commonly pursued approach is the transfection of DCs with mRNA. DCs can be transfected with selected antigen-coding mRNAs or total mRNA obtained from patients’ tumour cells. Both approaches have demonstrated the potential to improve patient survival in clinical trials . To further enhance T-cell activation, the DCs can additionally be transfected with suitable signalling molecules. In this context, the combination of CD40L, CD70 and caTLR4 mRNA (TriMix) proved to be particularly effective . Although DC vaccines were shown to be safe and moderately effective in various clinical trials, Sipuleucel-T remains the only FDA-approved DC vaccine so far. A possible explanation could be that the process of cell collection, cultivation and transfection is complex and costly.
Another technology that has also been used for several decades against infectious diseases are peptide vaccines. In contrast to inactivated vaccines, or tumour cell vaccines, only one or a few key antigens are used for vaccination. The performance of this technology largely depends on the length of the peptide chain. Short peptide fragments have a short half-life in vivo. They are generally not processed by antigen presenting cells (APCs), but directly presented on MHC class I molecules. The resulting lack of co-stimulation leads to only transient activation of CD8+ cytotoxic T-cells (CTCs) and sometimes even tolerance to the antigen. Long peptides are processed by APCs following endocytosis, presented on MHC class II molecules and used to elicit a CD4+ T-helper (TH) cell mediated, strong activation of CTCs. .
In a randomised phase III trial, a peptide vaccine based on the melanoma-associated protein gp100 was able to produce a slight but significant prolongation of progression-free survival. A similar trend was seen, although not statistically significant, for overall survival (OS). Moreover, it was shown that the effect of the vaccine could be enhanced with high-dose interleukin-2, which is known to promote lymphocyte proliferation and differentiation . Thus, peptide-based cancer vaccines, e.g., against gp100, continue to be investigated in combination with other proteins to boost the immune response. Yet, none of these approaches have advanced to clinical practice so far , probably due to the lack of immunogenicity of peptides in general and the resulting limited effectiveness. Peptide vaccines rely on mass production of peptides via chemical synthesis for short peptide fragments or biological expression platforms for long peptides, limiting the pool of antigens to common TAAs, which occur in tumour cells as well as healthy cells. TAAs, however, are subject to central tolerance, as CTCs with reactivity to self-antigens are generally rare and their activity is often limited by immune-regulatory mechanisms. This limits the strength of immune activation and additionally harbours the risk of autoimmunity in response to vaccination .
The class of virus-based vaccines includes three major groups representing different approaches and modes of action. The first and very successful group comprises vaccines against oncogenic viruses, e.g., Human Papilloma Virus (HPV)16/18 or Epstein Barr Virus (EBV), that promote tumourigenesis. Vaccines against the HPV virus family are widely used  and could prevent millions of deaths from cervical cancer in the future , . The second group are oncolytic viruses, attenuated viruses which infect and destroy tumour cells specifically . This leads to a reduction in tumour mass and release of tumour antigens and has the potential to activate the immune system’s response against remaining tumour cells . T-VEC, a modified herpes simplex virus, became the first US and EU-approved oncolytic virus after it was demonstrated to achieve durable remissions in patients with inoperable melanoma and to enhance effectiveness of anti-PD-1 antibodies . Thirdly, vector viruses can be used to introduce DNA or RNA, encoding tumour antigens, into patient cells. This technology makes use of modified, non-replicating viruses. This is similar to the anti-SARS-CoV-2 vaccines developed by AstraZeneca or Johnson & Johnson which are adenovirus vector vaccines. Their inherent immunogenicity may serve as a mechanism for immune boosting to potentiate the response to the tumour antigens . Different agents based on e.g. adenovirus, vaccinia virus and lentiviruses are currently under investigation .
Nucleic acid-based vaccines
Nucleic acid vaccines can be subdivided into DNA vaccines and mRNA vaccines. DNA vaccines, e.g., in the form of antigen-coding plasmids, can be injected intradermally, where they are taken up by APCs. DNA is relatively stable but must first enter the nucleus for subsequent transcription of the antigen encoding genes or genetic adjuvants. The transcribed mRNA is then exported into the cytoplasm and translated into a specific peptide, which is processed intracellularly and presented on MHC class I and II, thereby inducing both CD4+ T-cells for humoral response and CD8+ T-cells for cellular response to the tumour , . DNA vaccine technology has for example led to an approved drug against melanoma in dogs . A drug against precancerous cervical lesions, VGX‑3100, is currently investigated in a phase III study (NCT03185013). The vaccine is designed to direct the immune system against the viral oncoproteins E6 and E7, which are introduced into cells by HPV16/18. The inherent immunogenicity of DNA vaccines is low and must be optimised by introducing CpG motifs into the genetic code or adjusting the vaccine formulation . A major safety concern regarding DNA vaccines is the potential for genomic integration, possibly leading to the risk of genetic alterations, dysregulation of gene expression or cancer. This risk, however, is considered to be very low , .
mRNA-based vaccines work similar to DNA vaccines, encoding tumour antigens rather than containing parts of tumour cells or proteins. Fully synthetic mRNAs are introduced into patient cells, instructing the translational machinery to produce and present the desired antigens on the surface of APCs to activate anti-tumour response , . Free mRNA is extremely unstable, degraded in the body within very short time and strongly immunogenic; consequently, it must be protected by appropriate packaging . For this purpose, lipid envelopes are widely used since their formulation can be optimised to favour uptake of the mRNA by DCs . Unlike exogenously loaded DCs or DNA vaccines, the mRNA contained in the vaccine can elicit an interferon-dependent antiviral response by binding to specific receptors such as the Toll-like receptors 3, 7, and 8 . This in turn allows the mRNA itself to act as an adjuvant. Conversely, as a consequence of the antiviral response, translation of the mRNA is dampened, allowing less of the desired antigen to be produced . The immunogenicity of mRNA can be reduced by inserting modified nucleotides, e.g., pseudouridine, N1-methylpseudouridine, or N6-methyladenosine, into the mRNA template . Both effects must be weighed against each other to achieve a well-balanced immune response. Since mRNA is less likely than plasmid DNAs to enter the nucleus and is chemically incompatible with genomic DNA, genomic integration is extremely rare and not considered a relevant risk. Nonetheless, there has been a recent controversy about the possibility of reverse transcription and genomic integration of viral RNA or vaccine mRNA , .
The development of mRNA cancer vaccines is an important breakthrough in treating different tumour types. The technology has several advantages such as the simultaneous encoding of multiple antigens in full length, thereby inducing broader humoral and cellular immune responses. Due to their promising future, it is not surprising that numerous clinical trials on mRNA vaccines are being conducted. Several studies with vaccines encoding common tumour specific antigens are ongoing, including vaccines against mutated KRAS variants (NCT03948763) or oncogenes introduced by viruses (NCT04534205). As with tumour lysate-based vaccines, it is also possible to incorporate specific, patient-derived tumour antigens into the vaccine. Since the vaccine is fully synthetic and not made from whole cells, it is much easier to select and combine the most suitable antigens, such as tumour-specific neoantigens, to create a tailored product. The antigens can be identified by deep sequencing of tumour samples and comparison with healthy patient DNA . Corresponding programmes are being developed and tested by BioNTech (Individualized Neoantigen Specific Immunotherapy = iNeST) and Moderna (Personalized Cancer Vaccine = PCV). In this context, Moderna’s PCV mRNA-4157 is currently undergoing phase II testing in melanoma patients (NCT03897881), while BioNTech’s BNT-122 is investigated in patients with pancreatic cancer in a phase I trial (NCT04161755). Moreover, studies indicated that the combination of cancer vaccines with checkpoint inhibitors can further enhance the immune response against tumour cells . This could circumvent the problem of self-tolerance of tumour-antigens to some degree.
Thanks to the great success of the COVID-19 vaccines, funding for future mRNA vaccine research is secured for years to come. This momentum has opened doors for exploring the potential of mRNA technology in oncology. Advances in recent years enabled to create mRNA vaccines with the favourable properties of DC vaccines and the flexibility of using nucleic acids, combining strong immune activation with low manufacturing cost. Several mRNA-based cancer vaccines are now in advanced clinical testing. However, even with positive results, it will likely take a few more years for mRNA cancer vaccines to get approved by the FDA and EMA either as single treatment or in combination with other drugs. Advanced technologies are often associated with exaggerated expectations, especially in cancer research. Nonetheless, we remain cautiously optimistic that mRNA vaccines will mark an important step in the fight against cancer, and that their potential is unlikely to be exhausted in the near future.