Chemokine-like MDL proteins modulate flowering time and innate immunity in plants.

Human macrophage migration inhibitory factor (MIF) is an atypical chemokine implicated in intercellular signaling and innate immunity. MIF orthologs (MIF/D-DT-like proteins, MDLs) are present throughout the plant kingdom, but remain experimentally unexplored in these organisms. Here, we provide an in planta characterization and functional analysis of the three-member gene/protein MDL family in Arabidopsis thaliana. Subcellular localization experiments indicated a nucleo-cytoplasmic distribution of MDL1 and MDL2, while MDL3 is localized to peroxisomes. Protein-protein interaction assays revealed the in vivo formation of MDL1, MDL2, and MDL3 homo-oligomers, as well as the formation of MDL1-MDL2 hetero-oligomers. Functionally, Arabidopsismdl mutants exhibited a delayed transition from vegetative to reproductive growth (flowering) under long-day conditions, but not in a short-day environment. In addition, mdl mutants were more resistant to colonization by the bacterial pathogen Pseudomonas syringae pv. maculicola. The latter phenotype was compromised by the additional mutation of SALICYLIC ACID INDUCTION DEFICIENT 2 (SID2), a gene implicated in the defense-induced biosynthesis of the key signaling molecule salicylic acid. However, the enhanced antibacterial immunity was not associated with any constitutive or pathogen-induced alterations in the levels of characteristic phytohormones or defense-associated metabolites. Interestingly, bacterial infection triggered relocalization and accumulation of MDL1 and MDL2 at the peripheral lobes of leaf epidermal cells. Collectively, our data indicate redundant functionality and a complex interplay between the three chemokine-like Arabidopsis MDL proteins in the regulation of both developmental and immune-related processes. These insights expand the comparative cross-kingdom analysis of MIF/MDL signaling in human and plant systems.

Cross-kingdom mimicry of the receptor signaling and leukocyte recruitment activity of a human cytokine by its plant orthologs.

Human macrophage migration-inhibitory factor (MIF) is an evolutionarily-conserved protein that has both extracellular immune-modulating and intracellular cell-regulatory functions. MIF plays a role in various diseases, including inflammatory diseases, atherosclerosis, autoimmunity, and cancer. It serves as an inflammatory cytokine and chemokine, but also exhibits enzymatic activity. Secreted MIF binds to cell-surface immune receptors such as CD74 and CXCR4. Plants possess MIF orthologs but lack the associated receptors, suggesting functional diversification across kingdoms. Here, we characterized three MIF orthologs (termed MIF/d-dopachrome tautomerase-like proteins or MDLs) of the model plant Arabidopsis thaliana Recombinant Arabidopsis MDLs (AtMDLs) share similar secondary structure characteristics with human MIF, yet only have minimal residual tautomerase activity using either p-hydroxyphenylpyruvate or dopachrome methyl ester as substrate. Site-specific mutagenesis suggests that this is due to a distinct amino acid difference at the catalytic cavity-defining residue Asn-98. Surprisingly, AtMDLs bind to the human MIF receptors CD74 and CXCR4. Moreover, they activate CXCR4-dependent signaling in a receptor-specific yeast reporter system and in CXCR4-expressing human HEK293 transfectants. Notably, plant MDLs exert dose-dependent chemotactic activity toward human monocytes and T cells. A small molecule MIF inhibitor and an allosteric CXCR4 inhibitor counteract this function, revealing its specificity. Our results indicate cross-kingdom conservation of the receptor signaling and leukocyte recruitment capacities of human MIF by its plant orthologs. This may point toward a previously unrecognized interplay between plant proteins and the human innate immune system.

Physicochemical and structural insights into lyophilized mRNA-LNP from lyoprotectant and buffer screenings.

The surge in RNA therapeutics has revolutionized treatments for infectious diseases like COVID-19 and shows the potential to expand into other therapeutic areas. However, the typical requirement for ultra-cold storage of mRNA-LNP formulations poses significant logistical challenges for global distribution. Lyophilization serves as a potential strategy to extend mRNA-LNP stability while eliminating the need for ultra-cold supply chain logistics. Although recent advancements have demonstrated the promise of lyophilization, the choice of lyoprotectant is predominately focused on sucrose, and there remains a gap in comprehensive evaluation and comparison of lyoprotectants and buffers. Here, we aim to systematically investigate the impact of a diverse range of excipients including oligosaccharides, polymers, amino acids, and various buffers, on the quality and performance of lyophilized mRNA-LNPs. From the screening of 45 mRNA-LNP formulations under various lyoprotectant and buffer conditions for lyophilization, we identified previously unexplored formulation compositions, e.g., polyvinylpyrrolidone (PVP) in Tris or acetate buffers, as promising alternatives to the commonly used oligosaccharides to maintain the physicochemical stability of lyophilized mRNA-LNPs. Further, we delved into how physicochemical and structural properties influence the functionality of lyophilized mRNA-LNPs. Leveraging high-throughput small-angle X-ray scattering (SAXS) and cryogenic transmission electron microscopy (cryo-TEM), we showed that there is complex interplay between mRNA-LNP structural features and cellular translation efficacy. We also assessed innate immune responses of the screened mRNA-LNPs in human peripheral blood mononuclear cells (PBMCs), and showed minimal alterations of cytokine secretion profiles induced by lyophilized formulations. Our results provide valuable insights into the structure-activity relationship of lyophilized formulations of mRNA-LNP therapeutics, paving the way for rational design of these formulations. This work creates a foundation for a comprehensive understanding of mRNA-LNP properties and in vitro performance change resulting from lyophilization.