Macrophage migration inhibitory factor (MIF) is a pleiotropic cytokine that has been implicated in the pathogenesis of inflammatory disorders such as infection, sepsis, and autoimmune disease. MIF exists preformed in cytoplasmic pools and exhibits an intrinsic tautomerase and oxidoreductase activity. MIF levels are elevated in the serum of animals and patients with infection or different inflammatory disorders. To elucidate how MIF actions are controlled, we searched for endogenous MIF-interacting proteins with the potential to interfere with key MIF functions. Using in vivo biotin-tagging and endogenous co-immunoprecipitation, the ribosomal protein S19 (RPS19) was identified as a novel MIF binding partner. Surface plasmon resonance and pulldown experiments with wild type and mutant MIF revealed a direct physical interaction of the two proteins (K D ‫؍‬ 1.3 ؋ 10 ؊6 M). As RPS19 is released in inflammatory lesions by apoptotic cells, we explored whether it affects MIF function and inhibits its binding to receptors CD74 and CXCR2. Low doses of RPS19 were found to strongly inhibit MIF-CD74 interaction. Furthermore, RPS19 significantly compromised CXCR2-dependent MIF-triggered adhesion of monocytes to endothelial cells under flow conditions. We, therefore, propose that RPS19 acts as an extracellular negative regulator of MIF.A large body of evidence now shows that macrophage migration inhibitory factor (MIF) 2 activates a range of intracellular pathways and plays a key role in host immune and inflammatory responses (1, 2). Certain of the MIF inflammatory functions also have been proposed to be the result of the unusual enzymatic properties of the protein, namely tautomerase and oxidoreductase activities (3-6). Inhibition or deletion of MIF attenuates disease progression in experimental models such as atherosclerosis, arthritis, glomerulonephritis, sepsis, autoimmune encephalitis, and autoimmune diabetes (7-13). A pivotal step in the inflammatory response is the chemokine-governed adherence of monocytes to the endothelial lining which is then followed by their egress from the vasculature at the affected site. Earlier data from MIF Ϫ/Ϫ mice illustrate a role of MIF in leukocyte recruitment that was recently substantiated by the finding that MIF serves as a chemoattractant for monocytes and T cells by directly binding to the chemokine receptors CXCR2 and CXCR4 (14, 15). On the cell surface MIF also associates with CD74 (invariant chain of major histocompatibility complex class II) which colocalizes with CXCR2 (14, 16). Interaction with different surface molecules is thought to partly explain the wide impact of MIF on cellular pathways.Despite its role as a key mediator in immune and inflammatory diseases, very little is known of how MIF action is regulated and terminated. Accordingly, we searched for endogenous molecules with the ability to control key steps of MIF signaling (i.e. receptor binding and/or receptor-associated functions). In this study, we identified ribosomal protein S19 (RPS19), a component of the small...

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