Messenger RNA (mRNA) is a single-stranded ribonucleic acid that carries the genetic information from DNA to the ribosomes, where it is translated into proteins. mRNA has emerged as a promising platform for the development of vaccines and therapeutics for various diseases, such as infectious diseases, cancer, and genetic disorders. mRNA-based drugs have several advantages over conventional drugs, such as high specificity, versatility, safety, and efficacy. However, mRNA-based drugs also face several challenges, such as instability, degradation, delivery, and immunogenicity. Therefore, it is essential to understand the molecular and immunological aspects of mRNA and its interactions with the host cells and tissues. The innate immune system is the first line of defense against foreign invaders, such as viruses and bacteria, and it plays a crucial role in shaping the adaptive immune response. The recognition and sensing of mRNA by the innate immune system can have both positive and negative effects on the immunogenicity and tolerability of mRNA-based drugs, depending on various factors such as mRNA structure, sequence, modification, and delivery. The mRNA recognition and sensing by the innate immune system is the key aspect of mRNA as a natural adjuvant.
The innate immune system is the first line of defense against foreign invaders, such as viruses and bacteria, and it plays a crucial role in shaping the adaptive immune response. The innate immune system consists of various types of cells, such as macrophages, dendritic cells, natural killer cells, and epithelial cells, that express different pattern recognition receptors (PRRs) on their surface or in their cytoplasm. PRRs are specialized proteins that recognize and bind to specific molecular patterns associated with pathogens, known as pathogen-associated molecular patterns (PAMPs), or with cellular damage, known as damage-associated molecular patterns (DAMPs). The recognition and binding of PAMPs or DAMPs by PRRs trigger a series of signaling cascades that activate the expression and secretion of inflammatory cytokines, chemokines, and type I interferons (IFNs), which in turn recruit and activate other immune cells and initiate the adaptive immune response.
mRNA is a type of PAMP that can be recognized and sensed by the innate immune system, as it is a common feature of many RNA viruses, such as influenza, coronavirus, and HIV. However, mRNA can also act as a DAMP, as it can be released from damaged or dying cells or from exogenous sources, such as mRNA-based vaccines and therapeutics. Therefore, the innate immune system has evolved various mechanisms and pathways to discriminate between self and non-self mRNA and to modulate the immune response accordingly. The main PRRs involved in the recognition and sensing of mRNA are Toll-like receptors (TLRs), RIG-I-like receptors (RLRs), and the cGAS-STING pathway, which are located in different cellular compartments and have distinct roles and functions.
The immunogenicity and tolerability of mRNA-based drugs are influenced by various factors, such as mRNA structure, sequence, modification, and delivery. These factors can affect the recognition and sensing of mRNA by the innate immune system and thus modulate the immune response accordingly. mRNA structure can affect the accessibility and binding of mRNA to PRRs, as well as the efficiency and fidelity of mRNA translation. For example, dsRNA regions can enhance the binding of mRNA to TLR3, RIG-I, and MDA5 and thus increase the production of type I IFNs and pro-inflammatory cytokines. mRNA sequence can affect the immunogenicity and tolerability of mRNA-based drugs by influencing mRNA stability, translation efficiency, antigen presentation, and immune recognition. For example, mRNA sequences can contain immunostimulatory motifs, such as CpG dinucleotides, U-rich sequences, or poly(A) tails, that can enhance the binding of mRNA to TLR7, TLR8, or RIG-I and thus increase immune activation and antigen presentation. mRNA modification can affect the immunogenicity and tolerability of mRNA-based drugs by influencing mRNA degradation, translation fidelity, and immune recognition. For example, mRNA modification can reduce the recognition and binding of mRNA to PRRs, such as TLR3, TLR7, TLR8, RIG-I, and MDA5, and thus decrease innate immune activation and adverse reactions. mRNA delivery can affect the immunogenicity and tolerability of mRNA-based drugs by influencing the mRNA distribution, uptake, release, and localization. For example, mRNA delivery can enhance the protection and delivery of mRNA to target cells and tissues and thus increase the expression and presentation of the encoded protein.
There are different types of mRNA that can be used for mRNA-based drugs, such as conventional non-replicating mRNA, self-amplifying mRNA (sa-RNA), and modified mRNA (modRNA). These types of mRNA differ in their structure, sequence, modification, and expression level, which can affect their immunogenicity and adjuvanticity.
Conventional non-replicating mRNA is the simplest form of mRNA, which consists of a 5' cap, a 5' untranslated region (UTR), an open reading frame (ORF) encoding the protein of interest, a 3' UTR, and a poly(A) tail. Conventional non-replicating mRNA can be translated once by the host ribosomes and then degraded by cellular enzymes. Conventional non-replicating mRNA has moderate immunogenicity and adjuvanticity, as it can induce both innate and adaptive immune responses but also cause some adverse reactions, such as inflammation and fever. Conventional non-replicating mRNA can be further modified by adding nucleoside analogs, such as pseudouridine or 1-methylpseudouridine, to reduce its recognition by PRRs and increase its stability and translation efficiency. Modified mRNA (modRNA) has lower immunogenicity and adjuvanticity than conventional non-replicating mRNA, as it can induce less innate immune activation and pro-inflammatory cytokine production but also less adaptive immune response and vaccine efficacy.
Self-amplifying mRNA (sa-RNA) is a more complex form of mRNA, which contains a 5' cap, a 5' UTR, an ORF encoding the protein of interest, a 3' UTR, a poly(A) tail, and an additional ORF encoding the viral replicase. Self-amplifying mRNA can be replicated by the viral replicase in the host cytoplasm, resulting in a higher expression level and a longer duration of the protein of interest. Self-amplifying mRNA has a higher immunogenicity and adjuvanticity than conventional non-replicating mRNA, as it can induce stronger innate and adaptive immune responses but also more severe adverse reactions, such as cytotoxicity and autoimmunity. Self-amplifying mRNA can be modified by deleting or mutating some immunostimulatory motifs, such as 5'-ppp ssRNA or dsRNA regions, to reduce its recognition by PRRs and increase its safety and tolerability.
mRNA as a natural adjuvant has both benefits and drawbacks for vaccine development and immunotherapy. On one hand, mRNA as a natural adjuvant can enhance vaccine efficacy by inducing strong and broad immune responses against the encoded antigen, such as neutralizing antibodies, cytotoxic T cells, and memory cells. mRNA as a natural adjuvant can also induce durable immunity by stimulating the expression and presentation of the antigen for a prolonged period of time, as well as the activation and expansion of long-lived plasma cells and memory cells. On the other hand, mRNA as a natural adjuvant can cause adverse reactions by activating the innate immune system and triggering the production of pro-inflammatory cytokines, chemokines, and type I interferons. mRNA as a natural adjuvant can also cause immune-mediated toxicity or autoimmunity by inducing cross-reactive or self-reactive immune responses against the encoded antigen or the host cells and tissues. Therefore, mRNA as a natural adjuvant needs to be carefully balanced and modulated to achieve the optimal immunogenicity and tolerability of mRNA-based drugs.
In summary, mRNA can act as a natural adjuvant, activating innate immune receptors and enhancing adaptive immune responses. However, the immunogenicity of mRNA can be influenced by its type, modification, and purity, which require careful optimization and balancing. In addition, mRNA can act as a delivery vehicle for both antigens and adjuvants, increasing the vaccine's efficacy and safety. mRNA can encode and express various antigens, which can induce specific immune responses against the target pathogens or cells. mRNA can also encode and express adjuvants, which can modulate the immune response and enhance its quality and durability.
