Lipid Nanoparticle-Formulated Synthetic Messenger RNA Induces Reversible Hematopoietic Bias via Type I Interferon Responses Under Upper-Bound Inflammatory Conditions in Preclinical Animal Models.

Synthetic messenger RNA (mRNA) formulated in lipid nanoparticles (LNPs) is increasingly used for vaccines and therapeutics, yet the consequences of high-dose systemic exposure for bone marrow hematopoiesis remain poorly defined. We previously reported that intramuscular mRNA/LNP administration transiently alters circulating reticulocytes and bone marrow erythroid cellularity in mice; here, we investigated the mechanistic basis and cross-species generalizability of this hematopoietic bias in murine and non-human primate models. Animals received intramuscular LNP-formulated mRNA encoding SARS-CoV-2 spike protein or firefly luciferase at deliberately supra-clinical, upper-bound doses, and type I interferon signaling was interrogated by interferon-α/β receptor blockade. Bone marrow was assessed by histopathology, bulk and single-cell RNA sequencing, and trajectory analysis. LNP-formulated mRNA induced reversible erythroid-lineage suppression with concomitant granulocytic expansion, irrespective of the encoded antigen. These effects were absent after naked mRNA and only modestly induced by LNP alone, indicating that formulation of synthetic mRNA within the LNP was required to elicit the full phenotype. Single-cell analysis showed biased hematopoietic stem and progenitor cell differentiation toward granulopoiesis with broad induction of interferon-stimulated genes, and the bias resolved within two weeks. Interferon-α/β receptor blockade mitigated both changes, demonstrating type I interferon dependence. High-dose LNP-formulated synthetic mRNA can therefore drive reversible, type I interferon-dependent hematopoietic lineage bias in preclinical animal models.