Regulating Immune Responses Induced by PEGylated Messenger RNA-Lipid Nanoparticle Vaccine

Abstract

Messenger RNA (mRNA)-based therapeutics have shown remarkable progress in the treatment and prevention of diseases. Lipid nanoparticles (LNPs) have shown great successes in delivering mRNAs. After an mRNA-LNP vaccine enters a cell via an endosome, mRNA is translated into an antigen, which can activate adaptive immunity. mRNAs can bind to various pattern recognition receptors (PRRs), including toll-like receptors (TLRs), and increase the production of inflammatory cytokines. This review summarizes mechanisms of innate immunity induced by mRNAs. Polyethylene glycol (PEG) has been employed as a component of the mRNA-LNP vaccine. PEGylated nanoparticles display enhanced stability by preventing aggregation of particles. However, PEGylation can cause adverse reactions, including blood clearance (ABC) of nanoparticles via complement activation and anaphylaxis. Mechanisms of PEG-induced ABC phenomenon and anaphylaxis are presented and discussed. There have been studies aimed at reducing immune responses associated with PEG to make safe and effective vaccines. Effects of modifying or replacing PEG in reducing immune responses associated with PEGylated nanoparticles are also discussed. Modifying mRNA can induce immune tolerance, which can prevent hypersensitivity reactions induced by PEGylated mRNA-LNP vaccines. Current progress of immune tolerance induction in association with mRNA-LNP is also summarized. This review might be helpful for developing safe and effective PEGylated mRNA-LNP vaccines.

Keywords: ABC phenomenon; PEG; allergies; complement; immune tolerance; innate immunity; mRNA-LNPs.

Figure 1

Immune responses induced by mRNA vaccines. mRNA-LNP vaccine is endocytosed by antigen presenting cells. Following endosomal escape, mRNAs are then translated into corresponding proteins. Proteins undergo proteasomal degradation. Peptides are then presented on MHC-I to induce activation of CD8⁺ T cells. Activated CD8⁺ T cells can secrete IFNγ and TNF-α. Activated CD8⁺ T cells can kill tumor cells by granzyme B (GzmB). Secreted proteins are recognized and engulfed by antigen presenting cells such as B cells. Antigenic peptides are presented on the MHC-II of B cells to induce activation of CD4⁺ T cells. Activated CD4⁺ T cells, such as TH2 cells and TFH cells, can activate B cells to induce production of antigen-specific antibodies. Arrows denote the direction of reaction. TFH denotes follicular T helper cells. IVT denotes in vitro transcription.

Figure 2

Innate immunity and translational inhibition induced by mRNA. (A) Endosomal TLR (TLR3, 7, or 8) can bind to ssRNA or dsRNA. Endosomal TLR7/8 can increase the production of type I IFNs and various proinflammatory cytokines including IL-6, CCL2, and CXCL10 by activating MYD88-NF-kB signaling. DsRNA produced during IVT can activate PKR or OAS pathway, resulting in degradation of mRNA. Endosomal TLR3 can bind to dsRNA and activate TRIF pathway. RLRs and MDA5 can bind to cytosolic RNAs and increase production of proinflammatory cytokines by activating IRF3 and N-kB. (B) DsRNA can bind to PKR and OAS. PKR can suppress mRNA translation by inducing phosphorylation of eIF2. OAS can activate RNase L, which in turn can induce translational arrest. Hollow arrows denote the direction of reaction. IRAK, interleukin-1-receptor-associated kinase; MAVS, mitochondrial antiviral-signaling protein; TAB, Mitogen-activated protein kinase kinase kinase 7-interacting protein 1; TAK, mitogen-activated protein kinase kinase kinase 7; TRAF6, tumor necrosis factor receptor-associated factor 6; TRAM, translocating- chain-associated membrane protein; ZAK, mixed lineage kinase. Hollow arrows denote the direction of reaction. T bars denote inhibition of reaction.

Figure 3

PEG induces complement activation-associated anaphylaxis and ABC phenomenon. PEGylated mRNA-LNP vaccine induces cross linking of B cell receptors. B cells are then differentiated into plasma cells. Plasma cells produce anti-PEG antibodies. Anti-PEG IgE activates immune cells such as mast cells and basophils, resulting in anaphylaxis through Fcε receptor in a complement-independent manner. PEG-anti-PEG-IgM complex can activate complement via classical pathway. Activation of complement produces anaphylatoxins such as C3a and C5a. These anaphylatoxins can stimulate mast cells to release various mediators and induce degranulation of mast cells. Anaphylatoxins can also induce immune cells to release PAF. PAF can bind to mast cells to cause degranulation of mast cells. PEG-mRNA-LNP vaccine can induce the production of anti-PEG IgM, which in turn can bind PEG-LNP. This binding can induce complement activation to produce complement fragments such as C3b and C5b. C3 can activate C5 convertase to produce C5a and C5b. These fragments can bind to complement receptors (CR3 and CR4) to induce ABC phenomenon by macrophages. Arrows denote the direction of reaction. ↓ denotes decreased expression/activity and ↑ denotes increased expression/activity. LTR, leukotriene receptor; TxA2, thromboxane A2; PGE, poly glutamic acid ethylene oxide.

Figure 4

Induction of immune tolerance by mRNA-LNP. After endocytosis of modified PEG-mRNA-LNP vaccine by APC, mRNA undergoes endosomal escape. mRNA is then released and translated into antigen. Antigenic epitopes can be presented on MHC-II and bind to TCR on CD4⁺ T cells. CD4⁺ T cells are then differentiated into FoxP3⁺ Treg cells. Treg cells can secrete immune suppressive cytokines such as IL-10 and TGF-β. These Treg cells can induce immune tolerance. This tolerogenic effect is accompanied by decreased expression TH2 cytokines and IgE. CTLA-4, cytotoxic T-lymphocyte antigen 4; IVT, in vitro transcription; LAG3, lymphocyte activation gene-3; PD-1, programmed death-1; RA, rheumatoid. Arrows and hollow arrows denote the direction of reaction. T bars denote inhibition of reaction. ↑ denotes increased expression/activity.