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 inflamma--tory cytokine and chemokine, but also exhibits enzymatic activity. Secreted MIF binds to cell-surface immune recep-tors 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-DT-like proteins or MDLs) of the model plant Arabidopsis thaliana. Recombinant Arabidopsis MDLs (AtMDLs) share similar secondary structurecharacteristics 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 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 trans-fectants. Notably, plant MDLs exert dose-dependent chemotactic activity towards 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 plant orthologs. This may point towards a previously unrecognized interplay between plant proteins and the human innate immune system.