The Power of mRNA Vaccines: Essential Synergy Between Nucleoside-Modified mRNA and LNP Delivery Systems

Nucleoside-modified messenger RNA (mRNA) vaccines elicit protective antibodies through their ability to promote T follicular helper (Tfh) cell differentiation. The lipid nanoparticles (LNPs) of mRNA vaccines possess intrinsic adjuvant activity. However, the extent to which nucleoside-modified mRNA is sensed and contributes to Tfh cell responses remains undetermined.

Recently, the team led by Michela Locci at the University of Pennsylvania published a research paper titled "Distinct components of mRNA vaccines cooperate to instruct efficient germinal center responses" in Cell. The study found that deconvoluting the signals induced by LNPs and mRNA instructs dendritic cells (DCs) to promote Tfh cell differentiation. The study demonstrated that mRNA drives the production of type I interferon, which acts on DCs to promote their maturation and Tfh cell differentiation, and favors plasma cell and memory B cell responses. Meanwhile, LNPs enable DCs within the draining lymph nodes to uptake mRNA and also modulate Tfh cell responses by shaping the positioning of CD25+ DCs. In summary, this research work reveals the unique adjuvant characteristics of mRNA and LNPs required for inducing Tfh cells, which has significant implications for rational vaccine design.

Messenger RNA (mRNA) vaccines against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) have been shown to successfully drive potent neutralizing antibody (nAb) and memory B cell (MBC) responses and prevent severe coronavirus disease 2019 (COVID-19). Long-lived plasma cells (LLPCs) that secrete antibodies and MBCs are typically generated through germinal center (GC) responses.

Previous studies have shown that mRNA vaccines containing nucleoside modifications and purified mRNA encapsulated in lipid nanoparticles (mRNA-LNPs) elicit robust GC responses in mice and humans, which correlate with nAb production and antigen-specific MBC responses. However, the mechanisms by which mRNA-LNPs instruct GC responses remain largely undefined.

GC responses are regulated by a specialized subset of CD4 T cells called T follicular helper (Tfh) cells. Tfh cell differentiation begins when antigen-presenting cells (APCs), typically dendritic cells (DCs), present antigen to naive CD4 T cells within the T cell zone of secondary lymphoid organs. In this interaction, DCs deliver multiple signals to CD4 T cells that promote Tfh cell differentiation, including co-stimulatory molecules and cytokines. Conversely, antagonistic signals driven by interleukin (IL)-2 and its high-affinity receptor CD25 can suppress early Tfh cell differentiation. Activated CD4 T cells also upregulate Epstein-Barr virus-induced gene 2 (Ebi2), which allows these cells to initially localize at the interface of B cell follicles and the T cell zone (T-B border), where they commit to the Tfh differentiation pathway through interactions with ICOSL+CD25+ DCs.

The LNP component of mRNA vaccines has intrinsic adjuvant activity and can promote Tfh cell differentiation by inducing IL-6. However, whether important signals for Tfh cell differentiation also originate from the sensing of nucleoside-modified mRNA is not fully understood. The incorporation of modified nucleosides, such as pseudouridine, can suppress the ability of Toll-like receptors (TLR) 3 and TLR7/8 to induce pro-inflammatory cytokines, including type I interferon (IFN), in response to in vitro transcribed mRNA, which is thought to render mRNA immunologically silent.

However, it remains unclear whether nucleoside-modified mRNA can trigger low-level production of type I IFN that is measurable with sensitive methods and biologically relevant to Tfh cell responses. Mice lacking the TLR adapter MyD88 show impaired Tfh cell responses after immunization with mRNA-LNPs, but not when injected with protein subunit antigen mixed with empty LNPs (eLNPs), prompting the hypothesis that mRNA modification and purification do not completely eliminate the ability of pattern recognition receptors (PRRs) to recognize the mRNA component of mRNA-LNP vaccines.

The study sought to separate the signals promoted by LNPs from the mRNA-driven signals that elicit optimal Tfh and GC responses. The study revealed the importance of type I IFN, which is produced in response to mRNA vaccine components through an IFN regulatory factor 3 and 7 (IRF3-IRF7)-dependent pathway, in promoting DC maturation and Tfh cell differentiation by acting directly on DCs.

Figure 1. mRNA-LNP uptake by DCs mostly occurs in draining lymph nodes. (Castaño D, et al., 2025)

Notably, mRNA influenced the functional profile of Tfh cells, favoring strong elicitation of MBCs and LLPCs. On the other hand, sensing of LNPs instructs transcriptional programs in DCs that favor Tfh cell differentiation, including co-stimulatory molecules, CD25, and the chemokine receptor Ebi2. Furthermore, the study found that most conventional DCs in the draining lymph nodes (dLNs) effectively internalize LNPs and translate mRNA in situ, with only a few LN DCs processing mRNA-LNPs at the vaccine injection site. In summary, the study highlights the dual immunostimulatory mechanism of mRNA-LNP vaccines and reveals the distinct adjuvant properties of mRNA and LNP vaccine components.

Reference

Castaño D, et al. Distinct components of mRNA vaccines cooperate to instruct efficient germinal center responses. Cell, 2025.