Empty lipid nanoparticles trigger maturation and activation of antigen-presenting cells

By Tarun Sai LomteNov 9 2022Reviewed by Danielle Ellis, B.Sc.

In a recent study posted to the Research Square* preprint server and under review at the Nature Portfolio journal, researchers reported activation and maturation of antigen-presenting cells (APCs) by lipid nanoparticles (LNPs).

Study: Lipid nanoparticles (LNP) induce activation and maturation of antigen presenting cells in young and aged individuals. Image Credit: Kateryna Kon / Shutterstock

This news article was a review of a preliminary scientific report that had not undergone peer-review at the time of publication. Since its initial publication, the scientific report has now been peer reviewed and accepted for publication in a Scientific Journal. Links to the preliminary and peer-reviewed reports are available in the Sources section at the bottom of this article. View Sources

Two coronavirus disease 2019 (COVID-19) vaccines are based on nucleoside-modified mRNA LNPs. The mRNA vaccine platform has been widely tested and shown to elicit protective humoral responses effectively. The immuno-stimulatory effect of mRNA-LNP vaccines is due to the intrinsic adjuvant effect of the ionizable lipid that can trigger an inflammatory response. Nevertheless, the mechanisms underlying these responses remain poor.

Age influences immune responses to vaccines. Age-related decreases in immunity against pathogens may lead to higher mortality rates following infection among individuals aged 65 or above. This is reflected in the COVID-19 pandemic, where older populations (> 65 years) are significantly more likely to succumb to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection. 

The study and findings

In the present study, researchers investigated the effects of empty LNPs (eLNPs), viz., LNPs lacking mRNA cargo, on antiviral pathways and immune function of cells from younger and older individuals. First, the effect of eLNPs on dendritic cell (DC) maturation was evaluated in vitro. Monocytes from healthy participants were treated with granulocyte macrophage-colony stimulating factor (GM-CSF) for 48 hours, followed by eLNP treatment for 24 hours.

The frequency of surface costimulatory and human leucocyte antigen (HLA) marker-expressing cells was assessed in eLNP-treated monocyte-derived DCs (MDDCs) relative to unstimulated cells. eLNP treatment significantly upregulated the proportion of cluster of differentiation 40 (CD40)-positive MDDCs. The markers of MDDC maturation were also elevated following eLNP treatment. In addition, the production of cytokines, such as interleukin (IL)-12, IL-21, interferon (IFN)-α, and IFN-γ, increased significantly after eLNP treatment.

Next, the authors tested whether eLNP can induce phosphorylation and activation of IFN regulatory factor 7 (IRF7) and TANK-binding kinase 1 (TBK1). To this end, human peripheral blood mononuclear cells (PBMCs) were stimulated with eLNP for 15 mins, 45 mins, six hours, and 25 hours, followed by flow cytometry to evaluate whether eLNP can induce and activate IRF7 in monocytes and DCs, measured by IRF7 phosphorylation (pIRF7).

There was a significant upregulation of pIRF7 at 45 mins of stimulation in conventional DC 1 (cDC1). Besides, the authors observed TBK7 phosphorylation following eLNP stimulation in both cDC1 and cDC2, indicating that eLNPs can activate IRF7 and TBK1 signaling pathways. Next, PBMCs from healthy adults were stimulated with eLNPs for 24 hours to determine whether phagocytosis would occur using an in vitro assay measuring fluorescence.

cDC1 and cDC2 subsets showed increased fluorescence upon stimulation with eLNPs relative to unstimulated cells, suggesting a superior phagocytic function. PBMCs stimulated with eLNPs for 6 to 24 hours showed the induction of transforming growth factor beta 1 (TGFB1) after 24 hours, whereas the induction of TGFB3 started at six hours of stimulation and increased by 24 hours. Notably, MDDCs treated with eLNPs for 24 hours showed downregulation of TGFB2.

Next, the researchers evaluated how eLNPs affect an aged immune system, and therefore, the 18 subjects were grouped as young and old. Older adults were healthy and non-frail. The average age of older and younger individuals was 73 and 30. The authors studied the effects of eLNPs on the maturation and activation of the innate immune system in this cohort of younger and older adults.

The frequencies of eLNP-stimulated cell types were not significantly different between younger and older individuals. The proportions of CD40- and CD83-positive MDDCs were significantly upregulated in both older and younger donors following stimulation with eLNPs. Intergroup comparisons revealed increased CD40 expression in younger donors, albeit statistically insignificant.

While eLNPs induced MDDC maturation in younger and older donors, the maturation markers were significantly lower in older individuals, but pro-inflammatory cytokines were elevated. Further investigations indicated that eLNP was immuno-stimulatory in older individuals and could yield a sufficient vaccine response when combined with antigens.

A 24-hour stimulation of MDDCs with eLNPs caused a significant decrease in TGFB2 production in younger donors than in older individuals. Moreover, there was a substantial decline in all TGFB isoforms in older adults compared to unstimulated MDDCs in the same age group, contrasting the observation of comparable TGFB levels in younger individuals, regardless of eLNP stimulation.

Conclusions

In summary, the study demonstrated that eLNPs cause maturation and activation of monocytes and DCs and showed age-specific differences in the maturation and activation of PBMC subsets and MDDCs. The findings also revealed the phosphorylation/activation of IR7 and TBK1 upon eLNP stimulation. TGFB expression was significantly elevated upon eLNP stimulation in PBMCs from older donors, whereas in younger individuals, it was reduced. Overall, the study provided valuable insights into the mechanisms of LNPs used in COVID-19 vaccines.

This news article was a review of a preliminary scientific report that had not undergone peer-review at the time of publication. Since its initial publication, the scientific report has now been peer reviewed and accepted for publication in a Scientific Journal. Links to the preliminary and peer-reviewed reports are available in the Sources section at the bottom of this article. View Sources