Lipid nanoparticles (LNPs) have enabled RNA therapeutics by transforming inherently unstable RNA molecules into clinically viable treatments. The COVID-19 pandemic showcased their transformative potential, with LNPs enabling the rapid global deployment of highly effective messenger RNA (mRNA) vaccines and setting a powerful precedent for RNA-based therapeutics. However, their widespread success was accompanied by a critical challenge: heightened reactogenicity that elicited more frequent adverse reactions compared with traditional vaccines. More recently, reactogenicity has been observed in two ongoing clinical trials evaluating LNPs in the context of prophylactic HIV vaccines. These HVTN302 and IAVI-G002 studies have reported unusually high rates (7% and 18%, respectively) of chronic hives that last more than 12 months in some patients1,2.

This increased reactogenicity has prompted scrutiny of individual LNP components. Structurally, LNPs are designed to encapsulate and protect RNA, promote cell uptake, and facilitate endosomal escape — all key steps for achieving efficient gene delivery. Their performance hinges on four core components. These include ionizable lipids, which electrostatically bind RNA and enable endosomal escape through pH-dependent charge switching, and helper lipids and cholesterol, which stabilize the nanoparticle structure and enhance membrane fusion. In addition, polyethylene glycol (PEG)-lipids control particle size by limiting vesicle fusion and extend circulation time by reducing uptake by phagocytes3.

Of these components, the synthetic PEG-lipid has become a primary focus of investigation in LNP-associated immune responses. This is partly because up to 43% of the population possesses pre-existing anti-PEG antibodies4, probably due to widespread exposure to PEG in consumer products, pharmaceuticals and processed foods. Furthermore, administration of PEGylated LNPs can elevate anti-PEG antibody levels, which correlate with increased systemic reactogenicity symptoms, including fatigue, fever and joint pain4,5. However, mounting evidence suggests that immunogenicity is multifaceted, with ionizable lipids and helper phospholipids contributing alongside PEG in shaping innate and adaptive responses. While PEG seems to play a key role, definitive causation requires a deeper understanding of how PEG and other LNP components drive immunogenicity. Recognizing this complexity points towards a more nuanced approach: rather than eliminating PEG, balancing its benefits with targeted mitigation of immune memory may unlock the full potential of RNA therapeutics.

How PEG activates the immune system

Immunologically, PEG is a hapten. Haptens are molecules that are inert in their free form, but they become immunogenic when displayed on a macromolecular carrier such as a protein, liposome or LNP. For protein immunogens, anti-PEG responses are dependent on T cell help4. However, in the context of LNPs, PEG behaves as a thymus-independent type 2 antigen that crosslinks B cell receptors on naive B cells, particularly within the splenic marginal zone4. This crosslinking triggers a rapid, T cell-independent surge of cross-reactive anti-PEG immunoglobulin M (IgM) antibodies that become problematic upon repeated exposure. For example, it has been shown that after intravenous administration, these pre-existing antibodies can reduce efficacy by inducing LNP lysis or redirecting nanoparticle tropism towards Kupffer cells (macrophages) in the liver6, leading to premature clearance and diminished bioavailability (Fig. 1). In addition, intramuscular or subcutaneous re-exposure can trigger the complement cascade at the injection site and/or cell lysis and resultant basophil degranulation (Fig. 1), which probably contribute to hypersensitivity reactions observed post-vaccination.

Fig. 1: Mechanism of T-independent anti-PEG response.
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Upon the first dose, PEG on LNPs directly crosslinks B cell receptors (BCRs) on naive B cells in the spleen and lymph nodes, in a T-independent manner, leading to plasma cell differentiation and production of anti-PEG IgM antibodies. Following a second dose, circulating anti-PEG antibodies bind to LNPs, triggering the complement cascade at the injection site, which triggers LNP lysis and basophil degranulation. The anti-PEG IgM–LNP complexes also undergo accelerated blood clearance, primarily through Kupffer cell-mediated phagocytosis in the liver.

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Some of these adverse effects may be attenuated by spacing out dosing schedules. Although accelerated blood clearance and reduced efficacy tend to persist, reactogenic anti-PEG responses are typically short-lived due to the absence of T cell involvement. Nevertheless, the clinical implications of anti-PEG immunity remain an active area of investigation. While severe adverse events are relatively rare, mounting awareness of PEG’s immunogenic potential has prompted more stringent surveillance of anti-drug responses in clinical trials and stimulated interest in alternative surface chemistries.

New designs to circumvent adversities

Given the mounting concerns surrounding anti-PEG immunity, alternative polymers are being developed to replace PEG. These surrogates aim to minimize adverse immune responses while retaining PEG’s favourable properties, including solubility in both water and organic solvents, facile integration into the LNP matrix, biocompatibility and ability to prolong LNP circulation.

One class of potential PEG replacements are poly(2-oxazoline)s, which have exhibited stealth-like behaviour, tunable hydrophilicity and low immunogenicity. Unlike PEG, which is linear, poly(2-oxazoline)s contain tertiary amide side chains that impart chemical versatility7. Accordingly, these PEG substitutes can be tuned for thermal responsiveness, degradability and drug compatibility while maintaining water solubility and biocompatibility. One study reported that Moderna-mimic LNP formulations containing poly(2-ethyl-2-oxazoline) demonstrated comparable efficacy to PEGylated LNPs in mice7. Importantly, the poly(2-ethyl-2-oxazoline) formulation induced lower levels of anti-polymer IgM upon repeat dosing and mitigated the efficacy loss that is sometimes observed with PEGylated LNPs.

Poly(ethylene glycol) methyl ether methacrylates (PEGMAs) have also been explored as PEG substitutes with potentially lower immunogenicity. Like poly(2-oxazoline)s, PEGMA lipids feature branches that enhance steric hindrance and increase hydrodynamic volume, which may reduce recognition by anti-polymer antibodies. Compared with PEGylated LNPs, some PEGMA-LNP formulations elicit lower anti-polymer antibody responses and higher protein expression upon repeat dosing8. In addition, PEGMA-LNPs demonstrate therapeutic benefits in protein replacement and enhanced gene editing in mouse models requiring multiple doses8.

Another PEG surrogate, polysarcosine (pSar), is a hydrophilic and biodegradable polypeptoid generated from the endogenous amino acid sarcosine (N-methylglycine). Its chemistry eliminates hydrogen-bond donors, which minimizes protein adsorption while resisting enzymatic degradation and evading immune detection. An initial study from BioNTech in 2020 demonstrated that polysarcosine-functionalized LNPs improved mRNA delivery in vivo compared with PEG while reducing cytokine and complement responses in vitro9. Another study showed that pSar LNPs produced fewer anti-polymer antibodies in vivo compared with PEGylated LNPs, although repeat dosing was not addressed, which might limit its evaluation of long-term tolerability10.

In addition, a recent study highlighted poly(carboxybetaine) as a potential PEG replacement. This zwitterionic polymer contains terminal acyl chains to ease incorporation into the LNP matrix11. Compared with PEGylated lipids, poly(carboxybetaine) lipids enhanced endosomal escape in vitro, improved protein expression and gene editing efficiency in vivo, and offered a tolerable safety profile11. Moreover, no anti-polymer antibodies were detected, and efficacy was maintained upon repeat LNP dosing.

Reframing the problem

While these PEG alternatives show promise, they represent tactical solutions to a strategic problem. PEG has a compelling regulatory track record: first approved in the 1990s, it has since been incorporated into over a dozen LNPs, biologics and protein-based therapies approved by the US Food and Drug Administration. Its proven clinical performance suggests that its use as an excipient will endure.

Moreover, PEG surrogacy addresses only part of a larger immunological puzzle, given that other LNP components can also contribute to immunogenicity. For example, helper phospholipids engage B cell receptors12, potentially leading to the production of detrimental anti-phospholipid antibodies. Furthermore, we and others have shown that ionizable lipids activate innate immune pathways, including Toll-like receptor 4 (TLR4)13,14, interferon regulatory factor14 and endosomal damage pathways15, resulting in inflammatory cytokine production.

Given this likely persistence of PEG in future formulations, it will be prudent to develop solutions that harness existing materials while simultaneously pursuing next-generation immunologically silent delivery systems. This reframing suggests three complementary approaches (Fig. 2).

Fig. 2: Strategies to mitigate PEG-driven immune responses and enable next-generation LNP design.
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a, Targeted modulation of pattern recognition receptors and innate immune pathways can be selectively leveraged to suppress anti-PEG IgM production and improve the tolerability of repeat dosing. b, Integrative approaches combining multi-omic analyses of individuals with anti-PEG hypersensitivity and high-throughput in vivo mouse screens can enable systematic identification of molecular and cellular drivers of LNP reactogenicity. c, Knock-in mouse models will allow precise tracking and evaluation of anti-PEG antibodies and B cell dynamics.

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First, rather than eliminating PEG, we must mitigate its adverse events. For this, strategic modulation of the immune response can be leveraged to reduce PEG-driven adversities. In our recent work, we demonstrated such an approach by activating select innate immune pathways. Specifically, stimulation of TLR4, CD1d and lipid raft-associated mechanisms enabled certain ionizable lipids to elicit key cytokine responses that suppressed anti-PEG IgM production and supported repeated dosing13. Additional modulatory strategies are expected to emerge as we investigate further mechanisms of LNP immunogenicity. However, the effectiveness of these approaches is probably application-specific. For instance, complement inhibition using antihistamines is leveraged to suppress infusion-related reactions arising from intravenous administration, but it might inhibit germinal centre responses for slow-release vaccines that rely on complement deposition to traffic antigen to follicular dendritic cells in lymph nodes. Thus, careful consideration will be required to balance immunogenicity and reactogenicity across different therapeutic contexts.

In parallel, we must identify the molecular and cellular correlates of reactogenicity. Adverse responses to PEGylated LNPs probably result from complex interactions between formulation components and the host immune system. Multi-omic analyses of clinical data from people with anti-PEG hypersensitivity, combined with high-throughput in vivo screens in mice, offer a powerful way to map these interactions, isolate the immunogenic drivers and design new lipids guided by newfound insights. Such insights will also be key to building predictive frameworks for LNP tolerability and understanding why some people are more susceptible to anti-PEG responses.

Finally, new assays driven by fundamental immunology of anti-drug responses can be used to benchmark novel materials and formulations. Anti-PEG immunity, in particular, can be the foundation behind these assays. Now that crystal structures and gene sequences of human anti-PEG antibodies are available, we have the tools to engineer knock-in mouse models that express PEG-specific B cell receptors. These models enable adoptive transfer studies that involve taking B cells from one animal and transferring them into another to track specific cell populations in a controlled environment. Such approaches would allow precise tracking of anti-PEG B cell behaviour in lymphoid organs, enabling researchers to assess how different LNP formulations activate B cells, understand the cellular origins of anti-polymer immunity and test whether these responses can be selectively modulated. These models can also address key open questions: how does anti-PEG immunity influence systemic versus local reactogenicity? Does it behave differently in the context of immunodominant vaccines versus stealthier protein-replacement or gene-editing therapies?

Answering these questions is essential for de-risking existing PEGylated formulations — and for guiding the design of next-generation platforms that are more immunologically silent. Together, these scientific advances will transcend binary decisions about PEG and yield delivery systems that anticipate and circumvent immune memory.